unix shell scripting basics

UNIX Shell Scripting Basics

Understanding UNIX shell scripts

1. Command-line processing

The UNIX shell is a command-line interpreter that processes each command or combination of commands on a command line when you press Enter.

This example shows only a single command on the command line.

$ sort -r userlist

You can combine multiple commands on the same command line by creating a composite command.

This example shows a composite command comprising the ls and less commands.

$ ls -l | less

You can use a number of special characters in the command line. A semicolon (;), for example, allows you to place more than one command statement in the same command line. When you enter the code, the shell executes the preceding commands only when it reaches the semicolon.

In this example, the shell executes the cp command statement and then the cat command

statement.

$ cp list list.bak; cat list.bak

Special characters you can use to manipulate commands in the command line include

backslash (\) greater than (>) less than (<) pipe (|) ampersand (&)

backslash (\)

The backslash (\) character prevents the shell from treating another character as a special

character through a process called backslash escaping.

This allows you to split a command statement across multiple lines. When you place the

backslash at the end of a line and then press Enter, you can continue the statement on the

next line. The backslash prevents the shell from treating the Enter keystroke – or new line

character – as a special character.

This example shows a long echo statement carried across three lines.

The code for this is

$ echo Long pieces of text may not always fit onto a single \

> line of the command line interface, so it becomes \

> necessary to split them across multiple lines using \

> backslashes.

greater than (>)

The greater-than character (>) allows you to direct the standard output of a command to a

file or a device such as a printer instead of to the terminal screen.

This example will send the output of the ls command to a file called userdirs.


$ ls -l /usr/home > userdirs

less than (<)

The less-than character (<) allows you to send the contents of a file to a command as its

standard input.

This example sends input from a file called list to the sort command.


$ sort -d < list

pipe (|)

The pipe character (|) allows you to direct the output of one command to the input of

another command.

This example pipes the output from the cat command as input to the grep command for

further processing.


$ cat EasyNomad | grep 'South America'

ampersand (&)

An ampersand (&) character at the end of a command statement allows you to run

commands in the background.

This example specifies that the find command will run a long search process in the

background.


$ find 'EasyNomad' &

[1] 48748

$ EasyNomad

If you want to use special characters in command-line text without the shell recognizing them as special characters, you have to enclose them in quotes or precede them with a backslash (\).

This example shows an echo command in which the echo text contains an ampersand. There's a backslash in front of the ampersand, which prevents the shell from treating it as a special character.

$ echo Tours \& AccommodationTours & Accommodation$

Question

How do you think the shell processes the contents of a command line in order to execute it?

Options:

1. By analyzing commands first and then proceeding to options and arguments2. By dividing the command line into segments and processing each segment3. By processing the command line from beginning to end, one character at a time4. By processing the command line from beginning to end, one word at a time

Answer

When you execute a command line, the shell looks for spaces and special characters and splits the command line into segments wherever these characters occur. It then processes each segment in turn.

The segments into which the shell divides a command line are called tokens. To execute a command line, the shell processes the first token and then each subsequent token in turn.

To begin processing a token, the shell checks whether it's a keyword, an alias, or an ordinary word.

If the token is a keyword that opens a substructure such as a function, conditional statement, or bracketed group of commands, the shell processes the substructure before moving on to the next token.

If a token is an alias, the shell replaces it with the command to which the alias is mapped.

If a token is an ordinary word such as a command or a filename, the shell processes it directly.

After comparing a token against the list of known keywords and aliases, the shell processes it using several stages of expansion and substitution.

Expansion and substitution takes place in the following sequence:

brace expansion tilde expansion parameter substitution command substitution arithmetic substitution word splitting pathname substitution

brace expansion

In brace expansion, the shell looks for braces ({}) – also called curly brackets – in the token.

If braces are present, it expands their contents.

For example, the token b{all,ook} expands into ball book.

tilde expansion

In tilde expansion, the shell looks for tildes (~) in the token. If a tilde is present, it replaces

the tilde with the location of the current user's home directory.

For example, depending on the system configuration, the token ~vincep/file2 might

expand into /usr/home/vincep/file2.

parameter substitution

In parameter substitution, the shell checks whether the token is a variable name preceded

by a dollar sign ($). If it is, the shell replaces the token with the current value of the

corresponding variable.

For example, if the value of the SHELL parameter is /bin/ksh, the token $SHELL is

replaced with /bin/ksh.

command substitution

In command substitution, the shell checks whether the token is a command enclosed in

brackets and preceded by a dollar sign ($). If it is, the shell processes the command and

replaces the token with the command's output.

For example, the token $(type username) might be replaced with vincep.

arithmetic substitution

In arithmetic substitution, the shell checks whether the token is an arithmetic expression

enclosed in double brackets and preceded by a dollar sign. If it is, the shell evaluates the

expression and replaces the token with the result.

For example, the shell replaces the token $((72/9)) with 8.

word splitting

In word splitting, the shell examines those parts of the command line that have resulted

from previous stages of expansion and substitution. If any of these contain spaces or

special characters, it splits them into tokens for processing.

pathname substitution

In pathname substitution, the shell looks for wildcard characters in the token. If it finds

asterisks (*), question marks (?), or double slashes (//), it searches the current directory for

filenames that match these wildcards and substitutes them for the token.

For example, depending on the files in the current directory, the token f*.txt might

expand into fares.txt flights.txt funding.txt.

After performing expansion and substitution, the shell processes subsequent tokens until it reaches the end of a command, denoted by a semicolon or a new line character.

Then it matches the command against its list of known functions, built-in commands, and pathnames.

Once the shell has identified which command it needs to execute, it executes the command to produce output.

It then moves on to the next command, processing its tokens in the same way.

Question

Which special character is used to run a command in the background?

Options:

1. &2. |3. ;4. \

Answer

The ampersand symbol (&) is used to run a command in the background.

Option 1 is correct. The ampersand symbol (&) is used at the end of a command, to run a job in the background. Jobs in the background are assigned a job id number. You can you can use this number to foreground the job again.

Option 2 is incorrect. The pipe (|) special character allows you to use the output of the command on the left of the pipe as input for the command on the right of the pipe.

Option 3 is incorrect. The semi-colon (;) is used to combine separate commands on the same line. Commands on the right of the semi-colon are only interpreted once commands on the left have been interpreted and executed.

Option 4 is incorrect. The backslash (\) is used to prevent special characters from being interpreted in such a way that their literal values are used in stings. Also, you can continue your command on a new line by typing a backslash before pressing Enter.

Question

Choose the code that contains an example of command substitution.

Options:

1. cat logfile | grep $(cat /etc/hostname)2. cd ~/documents3. echo server{1,2,3).easynomad.com > hosts4. ls * | grep "easynomad"

Answer

The code $(cat /etc/hostname) is an example of command substitution. The contents of the hostname file are used as a search term by the grep command.

Option 1 is correct. In this example of command substitution, the command cat /etc/hostname is processed before the grep command is executed, so that its output can be substituted into the grep command.

Option 2 is incorrect. This is an example of tilde substitution. The path ~/documents expands to the documents folder in the home directory of the current user.

Option 3 is incorrect. This is an example of brace expansion. The code server{1,2,3).easynomad.com is expanded as: server1.easynomad.com; server2.easynomad.com; server3.easynomad.com.

Option 4 is incorrect. This is an example of filename substitution. The code ls * lists every file in the current directory.

2. Command grouping

You can join commands on a command line in such a way that the second command executes only if the first command has executed successfully.

For example, you can use a first command to check whether a file exists and a second command to perform an operation on it if it exists.

Question

How do you think the shell knows whether a command has executed successfully?

Options:

1. Because the command terminates2. Because the command's exit status is zero3. Because the command's standard error output is null4. Because the command's standard output contains no error messages

Answer

The shell knows that a command has executed successfully when the exit status of the command is zero.

To make one command conditional on another, you join the commands using a double ampersand (&&). The command after the && symbols executes only if the command before

the && symbols produces a zero exit status – in other words, if it executes successfully.

In this example, the ls command checks whether the userlist file exists. Because it does

exist, the ls command executes without errors ( so its exit state is zero. This causes the

sort command to execute.

$ ls userlist && sort userlistuserlistBAKER, DanielCARUSO, MariaGARZA, TeresaLOGAN, GregMANEROWSKI, SarahNOVAK, NicholasNOVIALLO, GlenOSWALD, Sam

PASCUCCI, VinceREILLY, MollySTROTHER, TanyaWADE, Debora$

If you delete the userlist file and run the command again, the ls command encounters an

error – so its exit state is non-zero. Because the sort command is conditional, the shell

doesn't attempt to execute it.

$ ls userlist && sort userlistls: userlist: No such file or directory$

You use a double pipe (||) to make a command conditional on the unsuccessful execution of the previous command.

In such a case, the second command executes only if the first command has a non-zero exit state.

In this example, the ls command looks for a file called userlist. If it fails to find the file, the

touch command creates it.

$ ls userlist || touch userlistls: userlist: No such file or directory$

If the ls command executes successfully, this means that the file already exists. In this

case, the touch command doesn't execute.

$ ls userlist || touch userlistuserlist$

You can group commands using braces ({}). The shell treats any command block enclosed in braces as if it were a single command.

This allows you to redirect input and output to and from a group of commands.

In this example, the braces group the sort and grep commands into a code block so that

the shell sorts input and then extracts any lines containing the word Mexico.

$ {sort | grep 'Mexico'}

You can redirect input and output to a command block as if it were a single command. In this example, the code specifies the flights file as input and the mex_flights file as output.

$ {sort | grep 'Mexico'} < flights > mex_flights$

You can group commands using round brackets – often called parentheses – instead of braces. This causes the shell to spawn a subshell and execute the command block in the subshell.

Commands that execute in a subshell do not affect what's happening in the main shell.

This allows you to define variables that exist only for the lifetime of the subshell, and to change the working directory within the subshell without affecting the parent shell.

$ (sort | grep 'Mexico') < massivefile > mex_info$

Question

You want to create a file named hostname and containing the text easy1.easynomad.com. However, you don't want to overwrite any existing file by that name.

Which line of code will enable you to do this?

Options:

1. cat hostname || echo easy1.easynomad.com > hostname2. cat hostname && echo easy1.easynomad.com > hostname3. cat hostname >> echo easy1.easynomad.com > hostname4. cat hostname | echo easy1.easynomad.com > hostname

Answer

The use of the || ensures that the code that writes the output from the echo command to the hostname file will only execute if the attempt to list the hostname file fails.

Option 1 is correct. You use the double pipe to make a command conditional on the unsuccessful execution of a previous command.

Option 2 is incorrect. The && conditional execution symbol ensures that if the attempt to list the hostname file succeeds, it will get overwritten.

Option 3 is incorrect. The >> redirector is used to append output to a file.

Option 4 is incorrect. The I symbol pipes the output from one command into another command as input.

3. Storing commands in scripts

Command grouping is useful for executing relatively short command-line code that you need to run only once.

However, you may need to run larger pieces of code that include several lines or to use the same piece of code many times.

In such cases, it's advantageous to store the code in a file.

You can store blocks of shell commands in shell scripts.

The contents of shell scripts are stored as ordinary ASCII text.

Question

What do you think distinguishes shell script files from ordinary ASCII text files?

Options:

1. They contain commands2. They have a specific filename suffix3. They have an introductory line of code that defines them as scripts4. You can execute them

Answer

Unlike ordinary ASCII text files, shell scripts contain commands, are executable, and have an introductory line of code that defines them as scripts.

You can read and edit ordinary text files, but you cannot execute them. However, you need to be able to execute shell scripts.

Therefore, you have to assign executable permissions on script files.

The first line in any shell script has to be a special line of code that specifies the particular shell program in which the script must run.

This is necessary because some commands run differently in different shell programs.

The shell identifier at the beginning of a shell script consists of a hash followed by an exclamation point (#!) – commonly called a shebang – and the absolute pathname of the shell program.

This example shows the first line of a script that uses the Korn shell.

#! /bin/ksh

This simple example of a script tests whether the directory /usr/shared/tours exists. If it doesn't, the script creates it. Then it creates a file called tourlist inside this directory and returns a message.

#! /bin/kshls /usr/shared/tours || mkdir /usr/shared/tourstouch /usr/shared/tours/tourlistecho tour directory and tourlist file created.

Once you've created a script and made it executable, you can use it as many times as you like. You can execute it directly from the command line or you can invoke it from inside other scripts.

Question

Identify the statements that correctly describe shell scripts.

Options:

1. Shell scripts are ASCII text files2. Shell scripts need to be compiled prior to execution3. Shell scripts need to have executable permissions set4. The first line of a shell script is used to identify the command interpreter

Answer

Shell scripts are ASCII text files that need to have executable permissions set. The first line of a shell script identifies the command interpreter.

Option 1 is correct. Because shell scripts are simple ASCII text files, you can easily create them in a text editor such as vi or emacs.

Option 2 is incorrect. Shell scipts are interpreted by the command interpreter, so they don't contain binary code and aren't compiled.

Option 3 is correct. Because shell scipts are executed, either one or more of the owner, group, or other executable permissions must be set.

Option 4 is correct. The first line of a shell script consists of a hash symbol followed by an exclamation mark and the absolute path to the command interpreter that will be used to execute the script. For example:#!/bin/bash

Summary

You can use special characters to join commands on a single command line, to redirect input and output, to run commands in the background, and to continue a command over multiple lines. You can prevent the shell from recognizing a special character by preceding it with a backslash. When you execute a command line, the shell splits it into tokens and processes each token in turn.

You can group commands using braces or brackets, which cause the shell to treat the commands as a single command. You can join two commands so that the second command will execute only if the first command executes successfully or only if it executes unsuccessfully.

You can store blocks of commands in a text file called a shell script and make this file executable. You can execute shell scripts directly from the command line and reuse them as often as necessary.

Table of Contents

| Top of page |

| Learning objective |

| 1. Command-line processing |

| 2. Command grouping |

| 3. Storing commands in scripts |

| Summary |

Copyright © 2003 SkillSoft. All rights reserved. SkillSoft and the SkillSoft logo are trademarks or registered trademarks

of SkillSoft in the United States and certain other countries. All other logos or trademarks are the property of their respective owners.

| Print | Contents | Close |

Creating an executable file in UNIX

Learning objective

After completing this topic, you should be able to create and run a simple shell program.

1. Creating a shell script

To create a shell script, you open a text file in a text editor such as vi or emacs.

For example, let's say that you decide to write a shell script for the Korn shell that automates the repetitive task of creating a file and folder structure for a new travel package.

You open a new text file in vi and – in this case – name it add_pkg_files.

Question

You need to invoke the Korn shell at the beginning of the script.

Assuming that the path to the Korn shell is /bin/ksh, see if you can complete the code that does this.

MISSING CODE

Answer

You type #!/bin/ksh to invoke the Korn shell at the beginning of the script.

The names you need to use for the new files and directories depend on the names of the travel packages. Therefore, the shell script needs to prompt the user to supply the package name. It then needs to read the name and store it as a variable.

#!/bin/kshclearecho "Please enter the name of the new travel package: "read PNAME

You need to create a directory for the new package in /usr/shared, and you need to create "bookings" and "info" subdirectories in the package directory.

You do this using a series of mkdir statements.

#!/bin/kshclearecho "Please enter the name of the new travel package: "read PNAMEls /usr/shared/$PNAME || mkdir /usr/shared/$PNAMEcd /usr/shared/$PNAMEmkdir bookingsmkdir info

You need to create three empty text files in the new folder hierarchy – one for flights, one for accommodation, and one for bookings.

You do this using a series of touch statements.

#!/bin/kshclearecho "Please enter the name of the new travel package: "read PNAMEls /usr/shared/$PNAME && mkdir /usr/shared/$PNAMEcd /usr/shared/$PNAMEmkdir bookingsmkdir infotouch /bookings/$PNAME_bookingstouch /info/$PNAME_accomtouch /info/$PNAME_flights

Finally, you want the script to display a message to the user that confirms that the files and folders have been created successfully.

#!/bin/kshclearecho "Please enter the name of the new travel package: "read PNAMEls /usr/shared/$PNAME && mkdir /usr/shared/$PNAMEcd /usr/shared/$PNAMEmkdir bookingsmkdir infotouch /bookings/$PNAME_bookingstouch /info/$PNAME_accomtouch /info/$PNAME_flightsecho "Files and folders for $PNAME created in /usr/shared."

After you've saved and closed the text file containing your new script, you can confirm its existence using the ls -l command. In this example, you've called the script

add_pkg_files.

This command reveals that the file doesn't have execute permissions set for it. Therefore, you can't run it as a script.

$ ls -ltotal 2-rw-r--r-- 1 vincep vincep 326 Feb 11 11:56 add_pkg_files$

When you specify a set of permissions, you need to specify the permissions that apply to

the file's owner users in the file owner's group everyone else

You can specify permissions in either symbolic or numeric form.

In symbolic form, you use the letters r, w, and x to denote read, write, and execute

permissions respectively.

To make a script executable using symbolic form, you use the chmod +x command.

$ chmod +x add_pkg_files$

In numeric form, you specify permissions using a three-digit octal code. The numbers from 0 to 7 represent increasingly open permissions, with 0 specifying no access and 7 specifying read, write, and execute permissions.

The first digit applies to the file's owner, the second to the owner's group, and the third to all other users.

Let's say that you need to allow everyone to execute a script, but you don't want anyone else to be able to write to the script.

You can do this by specifying the script's permissions as 755.

$ chmod 755 add_pkg_files$

Let's say that you're a member of a group of developers and you want other members of the group to be able to edit your script. On the other hand, you don't want general users to be able to use the script at all.

In this case, you would need to specify the script's permissions as 770.

$ chmod 770 add_pkg_files$

Running ls -l again reveals that the script file is now executable.

$ ls -ltotal 2-rwxrwx--- 1 vincep vincep 326 Feb 11 11:56 add_pkg_files$

Question

If the octal mode value of a shell script's permissions is 740, who will be able to run the shell script?

Options:

1. Any user2. Only member of the group that owns the file3. Only the user that owns the file4. Only users who are logged on locally

Answer

In this example, only the file owner has the executable permission assigned – so only they can run the shell script.

Option 1 is incorrect. In this example, general access to the file – as represented by the 0 in the permissions octal – is denied.

Option 2 is incorrect. In this example, the permissions of members of the group that owns the file is represented by a 4 so they can read the file, but can't write to it or execute it.

Option 3 is correct. The file owner's permissions are represented by the 7, so they have full permissions to the file and can execute it.

Option 4 is incorrect. File system permissions aren't affected by whether users are logged in locally or remotely.

Question

Identify the levels at which you can assign permissions.

Options:

1. Everyone2. File owner3. Group4. Local login

Answer

You can assign permissions to the individual user that owns the file, to the group that owns the file, and to everyone else.

Option 1 is correct. You can set permissions for anyone that tries to access the file. If you want everyone to be able to read a memo in a shared directory, you would set the read permission in the third permission set.

Option 2 is correct. By default, the user who creates a file owns it, and usually has full permissions to the file.

Option 3 is correct. By default, the user who creates a file is part of a primary group that also owns the file You use group permissions to control who can access a file and what they can do with it.

Option 4 is incorrect. You can't assign permissions at local login level. Instead you use the group ownership of files to control access.

2. Running a shell script

You can execute a shell script by

executing it directly invoking the shell using your environment

executing it directly

Direct execution is the simplest way to execute a script. You navigate to the directory in

which the script is stored and type its name at the shell prompt, preceded by ./.

Instead of navigating to the script's directory, you can specify its full file path.

invoking the shell

Invoking the shell to execute a script involves running an instance of the shell, and

specifying the path to the script as well as the script name. This allows you to specify shell

options.

using your environment

Using your environment to execute a script requires that the script is located in a directory that is found in the $PATH environmental variable. This allows you to execute the script by

simply typing its name, regardless of your current working directory.

To run the add_pkg_files script using direct execution, you type ./add_pkg_files at

the command prompt.

This causes the shell to spawn a child shell process, also called a subshell. The script executes in the subshell, which returns you to the main shell once the script has finished running.

$ ./add_pkg_files

You can run the script in the background, which allows you to use the main shell while the subshell executes the script.

$ ./add_pkg_files &

You can run the script by invoking the shell. To do this in the Korn shell, for example, you type /bin/ksh add_pkg_files. As with direct execution, the script runs in a subshell.

$ /bin/ksh add_pkg_files

If you have added your script directory to the $PATH statement in your .profile file, you

don't need to use ./ or the shell to run the script.

In this case, you only need to type the name of the script. This causes the script to run in a subshell.

$ add_pkg_files

If you don't want to run the script in a subshell, you can precede it with the keyword source. This causes the current main shell to run each command in the script in

sequence.

You can't do anything else with the main shell while the script is executing.

In some UNIX shells the keyword source is replaced by a period (.)

$ source add_pkg_files

Question

You may not want to execute a script immediately.

What do you think you can use the shell to do?

Options:

1. Run a script at a specified future time2. Run a script regularly at a specified interval3. Run a script when system load levels are low4. Run a script when the system is offline

Answer

You can use shell commands to run a script at a specified future time, at regular specified intervals, or whenever system load levels are low.

You use the at command to run a script at a specified future time.

In this example, you specify that the script must run at 6:00 pm on October 12.

You can also use more informal time specifications like 1pm tomorrow or even

teatime, which means 4:00 pm.

$ at 6pm Oct 12 add_pkg_files

You use the cron command to run a script at regular intervals.

You need to make an entry in the /etc/crontab file, specifying the name of the script and the interval at which to run it.

Then you run the cron command, which starts a process that checks the /etc/crontabs

file each minute and executes any jobs scheduled for the current minute.

$ cron

You use the batch command to run a script as soon as system load levels are low.

When you run a script with the batch command, the shell queues it as a pending job

until the system load average falls below 1.5.

$batch add_pkg_files

Question

Match the commands to their descriptions.

Options:

1. at2. batch3. cron

Targets:

A. Runs scripts when system resource usage is lowB. Runs scripts at a specified intervalC. Runs scripts at a specified time

Answer

You use the at command to run commands at a schedule time, the cron command to run commands at a regular interval, and the batch command to run scripts when the system is relatively idle.

The at command allows you to specify the time and date at which a script will run. You can use a variety of formats, including keywords such as "noon" and "today".

The batch command is useful if you want to run scripts at times when they won't interfere with system performance. It runs commands or scripts when system load levels drop below a specific amount. The amount will vary according to your system.

The cron command allows you to specify the interval at which a command will repeat. It can be set to recognize minutes, hours, days of the week, days of the month, or specific months.

Question

You have logged in as gregl and navigated to /home/gregl/bin, which contains a shell script named addfile.sh. You want to run the script in the current shell.

Which command should you use?

Options:

1. addfile.sh2. ./addfile.sh3. /bin/bash addfile.sh4. source addfile.sh

Answer

The source command executes the commands in a target file as if they were entered into the current shell.

Option 1 is incorrect. This method executes the script in a subshell. It is used when the location of the addfile.sh file is listed in the $PATH variable

Option 2 is incorrect. The ./ syntax only allows you to execute the addfile.sh file

in a subshell. It is used when the location of the file is not listed in the $PATH variable.

Option 3 is incorrect. This method launches a subshell in which the script is executed. It's useful if you need to set options for the subshell.

Option 4 is correct. The source command reads the contents of the referenced file directly into the current shell. It's useful if you want to apply changes to your .profile file during your current session.

Summary

To write a shell script, you begin by invoking a shell. Then you add lines of commands that perform the required tasks. You save the file and make it executable by assigning execute permissions to it.

You can run a shell script by invoking it directly or by invoking it through the shell. Both these processes cause it to run in a subshell. Alternatively, you can run a script in the main shell by using the source keyword. If you include your script directory in the $PATH

statement of the .profile file, you can run a script simply by typing its name at the command prompt.

Exercise: Writing a UNIX shell program

A note about exercises

This exercise is designed for practice use and does not contain new learning content. If your computer doesn't have an application or applications required to carry out the exercise tasks, or if you would prefer to perform the exercise at another time, you can proceed to the next topic.

Scenario

You currently use a text file as a signature file for your e-mail messages. It contains your name, job description, and contact details. You decide to liven it up a little by writing a script that customizes it with a different message every day.

Exercise

Your assignment for this exercise is to write a script that customizes your e-mail signature file – called mysig – with a random message.

You generate the messages using the fortune command, which outputs a random snippet

of wisdom each time you run it. The fortune program is located in /usr/games. You don't

need to supply any options or arguments to the fortune command.

You need to incorporate the fortune message in your signature file. You need to use the

cp command to make a temporary copy of mysig called dailysig. This file prevents a buildup

of previous fortune messages by storing a single message for each day only. Finally, you

append the output from fortune to dailysig by redirecting to dailysig.

The shell you're using for this script is the Korn shell, which is located at /bin/ksh.

Task list

Number Instructions

1 Write a script for the Korn shell that uses the fortune and cp

commands – as well as output redirection – to customize your e-mail signature file with a random message.

Review your solution and, when you are satisfied with it, open the solution page.

Table of Contents

| Top of page |

| Scenario |

| Exercise |


UNIX command execution

Learning objective

After completing this topic, you should be able to manage the execution of UNIX shell commands.

1. Using the command line

In UNIX, shell programs provide an interface between users and the kernel – the core of the operating system.

You interact with the kernel by entering commands at the shell prompt.

It's possible to enter several commands at once. In this case, you separate each command using a semicolon (;).

The code shown here redirects the output of the ls command to a file called listing, and

then uses the cat command to display the contents of the file. Because the two commands

are separated by a semicolon, they run consecutively.

$ ls -l /home > listing ; cat listinglrwxrwxrwx 1 root wheel 9 Oct 30 13:53 /home -> /usr/home$

If you end a line of code with a backslash (\), pressing Enter will move you to a new line and allow you to continue entering commands. You can continue to do this over multiple lines.

When you run the commands, the shell will execute your code as if it were one long line, ignoring the backslash characters.

You use backslash characters for improved readability.

$ echo This is a very long line of \> text, wrapped with a backslashThis is a very long line of text, wrapped with a backslash$

Question

Most shell commands behave as filters – they can take data as input, process it, and provide output. Because of this behavior, you can chain commands together, piping the output of one command into the input of another command.

You do this using the pipe character (|).

See if you can pipe the output of the ls -l command to the less command.

$ MISSING CODE

Answer

You type ls -l | less to pipe the output of the ls -l command to the less

command.

Sometimes you may want to run commands in the background, especially if they're likely to take a long time to complete. Background commands use idle CPU time only, allowing you to get on with other things.

You can background any command by suffixing it with an ampersand character (&). The code shown here will run a script called myscript.sh in the background.

$ ./myscript.sh &[1] 34030$

Question

Which statements about using the command line are true?

Options:

1. You use a backslash (\) to wrap commands across multiple lines2. You use a forwardslash (/) to prevent special characters being interpreted3. You use an ampersand (&) to run commands in the background4. You use a semi-colon (;) to separate commands that execute consecutively

Answer

You use the backslash to make long commands more readable, the semi-colon to enter consecutive commands on the same line, and the ampersand to run commands in the background.

Option 1 is correct. Provided you precede it with a backslash, you can use the Enter key to continue long commands on a new line. The shell will run your command as one long line, ignoring the backslashes.

Option 2 is incorrect. The forwardslash does not prevent special characters from being intepreted. It is used in the syntax for describing the filesystem.

Option 3 is correct. The ampersand allows you to run commands in the background on idle processor cycles, leaving you free to continue working in your current session.

Option 4 is correct. You can use the semi-colon to combine commands that execute consecutively on a single command-line. This is useful when the first command will take a long time to complete.

2. Conditional execution

Conditional execution is a programming strategy that allows you to specify different actions based on the outcome of an initial operation.

For example, you could create a shell script that checks the size of a log file and then backs it up only if it is larger than a certain size.

Regardless of their type or function, most UNIX commands return an exit status number to the shell when they've completed executing. The exit status indicates whether a command executed successfully.

If a command executed successfully, it returns a value of 0 to the shell. Greater values indicate an error of some sort.

Although you never see exit statuses at the shell prompt, you can use them to include conditional decisions in your shell scripts.

Most shells provide at least the following two pairs of operators for performing conditional execution based on exit status values:

double pipe (||) double ampersand (&&)

double pipe (||)

If you separate two commands with two pipe characters (||), the second command will

execute only if the first command returns an error – a non-zero exit status.

double ampersand (&&)

If you separate two commands with two ampersand characters (&&), the second command

will execute only if the first command exits successfully – returning a zero exit status.

It's possible to combine conditional execution operators to produce complex code.

For example, in the syntax shown here, the second command executes only if the first command fails. And if the second command fails, the third command will not run.

Question

Identify the statements that correctly describe conditional execution.

Options:

1. A non-zero exit status indicates that a command did not execute successfully2. An exit status of 0 indicates that a command completed successfully3. Conditional execution allows you to choose an action based on the exit status of a

command4. Conditional execution allows you to choose what code to run based on the output of

a command

Answer

A non-zero exit status indicates that a command did not execute successfully, while an exit status of 0 indicates that it did complete successfully. Conditional execution allows you to choose an action based on the exit status of a command.

Option 1 is correct. Any non-zero exit status is a failed command. The range of possible non-zero values for the exit status depends on the command that you execute, and the shell you use.

Option 2 is correct. If the last command to execute completed successfully, the value of the $? Variable, in which the exit status is stored, will be 0.

Option 3 is correct. Depending on the conditional operator you use, code will execute only if the previous command executes successfully, or only if the previous command fails.

Option 4 is incorrect. You can use the test command to return an exit status after testing for a particular value, but conditional execution only operates on the exit status of commands, not their results.

Question

You are creating a script that writes to the filesystem, and you don't want to alter any existing files.

Which commands are safe to use?

Options:

1. ls hosts && touch hosts

2. ls hosts || touch hosts

3. ls hosts ; touch hosts

4. mv hosts oldhosts && cat oldhosts && touch hosts

Answer

The || operator ensures that the hosts file is not overwritten if it already exists. You

use the && operator to ensure that a new hosts file is only created if the command to

rename the original succeeds.

Option 1 is incorrect. The ls hosts command will have an exit status of 0 if the host

file exists, and the && operator will cause the hosts file access and modification times

to be updated.

Option 2 is correct. The || operator will execute if the hosts file doesn't exist and the

ls command returns a non-zero exit status. This will cause the touch command to

execute and create a new hosts file.

Option 3 is incorrect. The ; operator merely allows consecutive execution of

commands. The execution of the command to overwrite the hosts file is not dependant on the exit status of the command listing the host file.

Option 4 is correct. It is safe to overwrite the hosts file if the command that renames the original file, as well as the command that prints the contents of the renamed file, complete successfully.

.

command 1 || command 2 && command 3

Question

Let's say that you want to create a command that will check for the existence of a log file. If the file exists, the log file should be copied to a backup directory. If the copy operation fails, the system should send an e-mail to the root user.

See if you can type an operator in the first blank space that will back up the log file if it exists.

$ ls -l /var/log/mylog MISSING CODE cp /var/log/mylog /backup/mylog \ MISSING CODE mail -s "Error" root < errmsg.txt

Answer

You use the && operator to run the cp /var/logs/mylog /backup/mylog

command if the ls /var/logs/mylog command locates a log file.

Question

See if you can complete the code shown here to send an e-mail to the root user if the copy operation fails.

$ ls -l /var/log/mylog && cp /var/log/mylog /backup/mylog > MISSING CODE mail -s "Error" root < errmsg.txt

Answer

You use the || operator to execute the mail root@localhost "copy

operation failed" command if the copy operation returns a non-zero exit status.

3. Grouping commands

Conditional execution operations are based on the exit status value of a single command – you can't have more than one command on either side of a conditional operator.

This behavior is sufficient for simple tasks, but often you'll need to base conditional execution on the result of a series of commands. You achieve this using grouping.

You can enclose a series of commands in brackets to create a code block. The shell will treat a code block as if it were a single command ( it will return one exit status value for the entire block, rather than for each individual command that the block contains.

This allows you to create far more complex commands and shell scripts.

You can group commands using the following two types of brackets:

braces ({}) round brackets (())

braces ({})

You can use braces ({}) to create blocks of code that will execute in your current shell.

round brackets (())

You can use round brackets (()) to create blocks of code that will execute in a subshell ( a

new shell program spawned from the shell that you're running.

The advantage of running commands in a subshell ( by grouping them using round brackets ( is that subshells have their own environment variables. As a result, any radical environment changes that take place during the running of a code block don't affect the shell you started with.

Question

Which statement about the line of code shown here is true?(cd /etc ; cat hosts) && ls

$ (cd /etc ; cat hosts) && ls

Options:

1. The code in parentheses executes within a subshell2. The ls command will list the contents of the /etc directory

3. The ls command will execute on condition that the cat hosts command is

successful4. The code in parentheses is a comment

Answer

Code within parentheses is executed in a subshell.

Option 1 is correct. Code within parentheses is run as a separate process in a subshell. You can set local environment variables within the subshell without effecting your main shell session.

Option 2 is incorrect. The variable that stores the current working directory for the shell in which the ls command is executed is not affected by the command to move

to the /etc directory in the subshell.

Option 3 is incorrect. The execution of the ls command is conditional on the

successful completion of the entire block of code within parentheses.

Option 4 is incorrect. You can include comments in shell scripts by preceding each line of comments with the # symbol.

Question

Identify the operators that allow you to group commands in code blocks.

Options:

1. []

2. {}

3. ()

4. ;

Answer

You use the parentheses () and the braces {} operators to group commands in

code blocks that return a single exit status.

Option 1 is incorrect. The [] symbols are used by the shell in pattern matching

operations to specify character ranges.

Option 2 is correct. The {} symbols group commands into a single code block for execution in the current shell.

Option 3 is correct. The () symbols group commands into a code block that is executed in a subshell. This is useful if you want to avoid altering environment variables in your current shell, for instance.

Option 4 is incorrect. The ; symbol is used to enter multiple commands on the same line. However, the commands are executed in sequence, not as a code block.

Question

Suppose that you're in /home/gregl and you need to execute the code shown here. It looks for a specific log file, and if the file exists a message displays.

What do you think your current working directory will be when the code has finished executing?

$ { cd /var/log ; ls mylog } /&& echo "Found the log file"

Options:

1. /var/log2. /home/gregl3. /usr/home

Answer

After the command runs, your current working directory will still be /home/gregl.

Option 1 is incorrect. The command that changes the directory to /var/logs runs in a subshell. To run the code in the current shell, thereby changing the current directory, you need to remove the braces, as shown here:cd /var/log ; ls mylog && echo "Found the log file"

The command that changes the current working directory to /var/log is part of a

code block enclosed in brackets (()), so it will execute in a subshell. As a result, it has no effect on the current shell you're in.

Option 3 is incorrect. At no point does any code change the directory to /usr/home

– either in the current shell or the subshell – so this can't be the current directory.

Summary

The shell's command line provides an interface between users and the UNIX kernel. You can separate commands using a semicolon (;), continue them on new lines using a back slash (\), and plug them into each other using a pipe (|).

You can use conditional operators to specify that commands execute based on the exit status of other commands. You use two ampersand characters (&&) to specify that a command run only if a previous command was successful, and two pipe characters (||) to specify that a command run only if a previous command failed.

You can group commands into code blocks to specify that the shell return one exit status for the entire block. You use braces ({}) to specify that code execute in the current shell, and round brackets (()) to specify that code execute in a subshell.

Table of Contents

| Top of page |


| 1. Using the command line |

| 2. Conditional execution |

| 3. Grouping commands |

| Summary |




Redirection in UNIX

Learning objective

After completing this topic, you should be able to redirect standard input and output, and use file descriptors.

1. Using standard I/O redirection

Most UNIX commands behave as filters – they can take input, process it in some way, and provide output. A shell usually sends this output to the screen, but you can redirect it to provide advanced functionality.

A common example of redirection is the use of the pipe operator (|) to plug the output of one command directly into the input of another command. However, redirection most often involves writing to and reading from files.

You can perform redirection with files by preceding a filename with one of the following operators:

greater than (>)

less than (<) two greater than symbols (>>)

greater than (>)

You use the > operator with a filename to redirect the output of a command to a file. If the

file doesn't exist, the shell will create it. If it exists, the shell will overwrite it.

less than (<)

You use the < operator with a filename to redirect standard input from a file. For example,

when using the mail command to send e-mail, you can redirect input from a file you have

previously prepared, rather than having to type your message at the shell prompt.

two greater than symbols (>>)

You use the >> operator with a filename to redirect the output of a command to a file,

appending the new data if the file already exists. This is useful for creating log files.

If you use a lot of file redirection in your scripts, it's quite easy to overwrite existing files accidentally. To prevent this, most shells implement the noclobber environment variable.

When set, noclobber prevents file overwriting through redirection – but not through the

execution of commands such as cp or rm. You enable noclobber by typing set -o

noclobber, as shown here.

If you want to overwrite output files protected by noclobber, you can use the >| operator

to do this.

$ set -o noclobber$

Question

Let's say that you want to send e-mail to a user called vincep. You've used the vi editor to type the body of the e-mail, and you've saved it to a file called email.txt.

See if you can complete the code shown here to redirect input from the email.txt file rather than from standard input.

$ mail –s "Guess what" vincep@localhost MISSING CODE email.txt

Answer

You use the < operator to redirect input from a file rather than from standard input.

You send the mail, and the shell reads the e-mail body from the text file you've specified.

$ mail –s "Guess what" vincep@localhost < email.txt$

Question

Let's say you're using the man command to find out about the exec command. You'd

like to dump the information to a file so that you can print it later.

See if you can complete the command shown here to dump the output of the command to a file called exec.txt.

$ man exec MISSING CODE exec.txt

Answer

You use the > operator to redirect standard output to a file rather than to the screen.

The shell dumps the output of the man exec command to a text file. Because you've

redirected the output, you don't see it echoed to the screen.

$ man exec > exec.txt$

Question

Match the redirection operators to the descriptions of their function.

Options:

1. >2. <3. >>

Targets:

A. Used to append output from a command to a fileB. Used to redirect output from a command to a fileC. Used to specify a file as input for a command

Answer

You use the > operator to redirect the output from a command to a fle, the < operator to use a file as input for a command, and the >> operator to append the output from a command to an existing file.

You can use redirection to document your system by redirecting the output from commands to a file, as in this example, which records the network interfaces configured for a system: ifconfig –a > network.txt

Several UNIX commands accept files as input. For instance, you can redirect a text file called report.txt to the mail utility and send it to Fred with the following command: mail Fred < report.txt

Appending command output to an existing file is useful for troubleshooting. Scheduling regular execution of the following command enables you to monitor which kernel modules are loaded on a Linux system: lsmod >> /var/modules.log

2. File descriptors

File descriptors are files – usually with numeric filenames ( on disk that point to input/output (I/O) objects, such as the keyboard, the screen, or other files. You can think of them as placeholders.

By using file descriptors, processes don't have to interact mechanically with the I/O objects that the descriptors represent – the kernel handles these mechanics.

Any running process can expect at least the three following default file descriptors to be available:

0 1 2

0

Descriptor 0 is also known as standard input. It's normally mapped to the keyboard.

Processes look to this descriptor for user input.

1

Descriptor 1 is also known as standard output. It's normally mapped to the screen, although

when UNIX was created it was mapped to a printer.

Processes write command output to this descriptor, and the output then appears on your

screen.

2

Descriptor 2 is also known as standard error. It's normally mapped to the screen, but when

UNIX was invented it was mapped to a dedicated printer.

Processes write error messages to this descriptor, and the messages then appear on your

screen.

Processes can also create their own descriptors, usually for interacting with files.

You can redirect file descriptors in a similar way to performing ordinary redirection, using the following syntax – where n is a file descriptor:

<&n

>&n

n< filename

n> filename

n>> filename

n>| filename

<&n

You use <&n to redirect standard input from another descriptor. For example, you could type <&2 to feed standard error into standard input.

>&n

You use >&n to redirect standard output to another descriptor. For example, you could type >&2 to send standard output into standard error.

n< filename

You use n< with a filename to redirect a descriptor from the specified file.

n> filename

You use n> with a filename to redirect a descriptor to the specified file. As with ordinary

redirection, this will overwrite an existing file. To change this default behavior and not have the file overwritten, set the noclobber variable, using the set noclobber command.

n>> filename

You use n>> with a filename to redirect a descriptor to the specified file. This will redirect to

a file but, as with ordinary redirection, this will append to an existing file.

n>| filename

You use n>| with a filename to redirect a descriptor to the specified file, overriding the noclobber variable if this has been set.

Question

Identify the true statements about performing descriptor redirection.

Options:

1. You use 2>| to ensure that existing files aren't overwritten by redirected standard

error2. You use 2> to redirect standard error to a file

3. You use >&2 to redirect standard output to standard error

4. Use 1> to redirect standard output to a file

Answer

You use >&2 to redirect standard output to standard error, you redirect standard error

with 2>, and you redirect standard output to a file with 1>.

Option 1 is incorrect. In fact, this option allows you to overwrite existing files with standard error.

Option 2 is correct. This is useful for for printing error messages.

Option 3 is correct. If your standard error is connected to a printer instead of the display, this would be a quick way of printing command output.

Option 4 is correct. Instead of the output going to the screen it can be redirected to a file using 1>

Question

Let's say that you want to use the find command to locate reports, and you want to

redirect both standard output and standard error to text files.

See if you can complete the first part of the code shown here to redirect standard output to a text file called output.txt.

$ find / -name "report*" MISSING CODE output.txt MISSING CODE error.txt

Answer

You type > to redirect standard output to a file.

Question

See if you can complete the code shown here to redirect standard error to a file called error.txt.

$ find / report* 1> output.txt MISSING CODE error.txt

Answer

You type 2> to redirect standard error to a file.

Question

Match each standard file descriptor value to its name.

Options:

1. 02. 13. 2

Targets:

A. Standard errorB. Standard inputC. Standard output

Answer

The file descriptor for standard input has a value of 0, the descriptor for standard output has a value of 1, and the descriptor for standard error has a value of 2.

Standard input is associated with file descriptor 0, and is usually mapped to the keyboard. Typing commands with a keyboard is the most common way of interacting with the shell.

Standard output is usually mapped to the screen. Most programs use standard output to print usage messages or the results of commands.

Standard error is associated with file descriptor 2, and is usually mapped to the screen – but it is sometimes useful to map it to a printer, to record errors.

3. Scripting with redirection

Sometimes, for convenience, you may want to pass multiple lines of input to a command. You do this using a here document.

Here documents make it possible to give batch input to a command – to pass the command a list of things you want it to do.

You can use here documents using either of the following two syntax forms:

command << terminator [input …]

command <<- terminator [input …]

command << terminator [input …]

When you use the syntax command << terminator [input], the shell executes the command

and passes it the input you specify ( line by line, until it reaches the word or character

you've specified as a terminator.

command <<- terminator [input …]

When you use the syntax command <<- terminator [input], the shell executes the command

and passes it the input you specify ( line by line, until it reaches the word or character you

specified as a terminator.

The trailing dash in the syntax causes all leading tabs in the subsequent input to be

stripped.

.

Let's say that you want to automate a telnet session to e-mail yourself some data off a remote machine. You begin by setting up the telnet command with a here document, using the word END as a terminator.

You press Enter, and the shell waits for further input.

$ telnet << END>

The first thing that a telnet session needs is the address of the target machine ( 190.100.2.3 in this case. So you enter the full command to open a session to that machine and then press Enter.

$ telnet << END> open 190.100.2.3

You know that the remote machine will require a username and password, so you type them on the next line.

In this case, your username is "gregl" and your password is simply "password".

Then you press Enter.

$ telnet << END> open 190.100.2.3> gregl password

The remote system should have logged you on by this point in the script, so you enter the commands you need to fetch the file you want. You also enter the quit command to end

the telnet session.

You press Enter after each command.

$ telnet << END> open 190.100.2.3> gregl password> mail –s "remote userlogs" [email protected] < /var/log/userlog> quit

To finish and execute the here document, you enter the terminator that you specified to begin with ( in this case, the word END – and you press Enter.

The shell executes the telnet command and feeds it the here document line by line.

$ telnet << END> open 190.100.2.3> gregl password> mail –s "remote userlogs" [email protected] < /var/log/userlog> quit > END

Question

Suppose that you're setting up a here document to run an ssh command, using the

word "END" as a terminator.

See if you can complete the code shown here to ensure that any leading tabs in the here document will be stripped out.

$ ssh MISSING CODE Finish

Answer

You use the <<- operator to ensure that leading tabs are stripped out of here

documents.

Question

Which statements about here documents are correct?

Options:

1. The <<- operator allows you to strip leading tabs from a here document2. Here documents can be used to automate login processes3. The first line of a here document is always a command.4. The terminator in here documents is END

Answer

You use the <<- operator to strip leading tabs from lines in a here documents. Here documents are often used to automate interactive logins and the first line of a here document is always a command.

Option 1 is correct. Automatically stripping tabs from your here document allows you to improve its legibility.

Option 2 is correct. Most interactive command line programs, such as telnet or ftp, can be automated using here documents

Option 3 is correct. Here documents start with a command interpreter that loads a command to read and execute the rest of the shell script.

Option 4 is incorrect. Although END is frequently used as a terminator in here documents, you can use any string as a terminator.

Summary

Most UNIX commands behave as filters, accepting input and providing processed output. You can redirect command output to a file, and you can redirect command input from a file.

File descriptors are numeric placeholders for input/output (I/O) objects. The default file descriptors present in any shell environment include standard input (0), standard output (1), and standard error (2). You can redirect file descriptors.

Here documents provide a convenient way of sending batch input to a command. They are most commonly used for setting up scripts for interactive commands such as telnet.

Table of Contents

| Top of page |


| 1. Using standard I/O redirection |

| 2. File descriptors |

| 3. Scripting with redirection |

| Summary |




Substitution and patterns in UNIX

Learning objective

After completing this topic, you should be able to use substitution and quoting to expand and match files and commands.

1. Substitution and matching

You can use filename substitution when entering commands to specify groups of files that you want to work with.

Filename substitution involves using wildcard characters either on their own, or with a pattern fragment. The shell resolves a wildcard to a list of files, which it then passes to the command you're trying to execute.

The wildcard characters that UNIX supports include

asterisk (*) question mark (?)

asterisk (*)

You use the * wildcard to match a string of any length.

For example, if you specify w*, the shell will return a list of every file in your current working

directory with a name beginning with "w".

question mark (?)

You use the ? wildcard to match a single character.

For example, if you specify file0?, the shell might return a list containing file01 through

file09 – if these files existed.

You can specify limited lists of characters for filename substitution using square brackets ([]), using one of the forms shown here:

[abc] [a-d] [a-ce-g]

[abc]

You use the form [abc] to perform substitution with any character that appears between the

square brackets.

For example, if you specify file0[147], the shell will return a list containing file01, file04,

and file07 – if these files exist.

[a-d]

You use the form [a-d] to perform substitution with any character that appears in the range

between the square brackets.

For example, if you specify data_[a-c], the shell will return a list containing data_a,

data_b, and data_c – if these files exist.

[a-ce-g]

You use the form [a-ce-g] to perform substitution with any character that appears in any of

the ranges between the square brackets. You can include any number of ranges in this

way.

For example, if you specify log[1-36-8], the shell will return a list containing log1, log2,

log3, log6, log7, and log8 – if these files exist.

Question

See if you can complete the code shown here to retrieve a list of all the files in the current directory that begin with a lowercase letter or a digit.

$ ls MISSING CODE

Answer

You type [a-z0-9]* or [0-9a-z]* to retrieve a list of all files that begin with a

lowercase letter or a digit.

In the filename substitution shown here, the square brackets hold two ranges – a to z and 0 to 9. The brackets are followed by an asterisk (*), which allows any type of string to follow the specified ranges.

$ ls [a-z0-9]*120415_log report1.txt report3.txt staffdata

120603_log report2.txt searchdata$

You can perform reverse matching using the exclamation mark (!) character. For example, the code shown here lists all files in the current directory that don't end in a digit.

$ ls –l *[!0-9]120415_log report1.txt report3.txt staffdata120603_log report2.txt searchdata$

Occasionally, you may need to refer to a user's home directory in a script. You can refer to a user's home directory without knowing their username using the tilde (~) character.

This enables you to write scripts with path references that any user can run.

$ echo ~/home/vincep$

You can use the tilde (~) character in substitution in the following forms:

~ ~user

~- ~+

~

The shell resolves a lone tilde to the value of the current user's $HOME environment

variable.

~user

The shell resolves ~user to the path of a specific user's home directory.

~-

The shell resolves ~- to the path of the previous directory you were working in.

~+

The shell resolves ~+ to the path of the current directory.

Question

Which commands use the tilde (~) correctly?

Options:

1. cd ~

2. cd ~$HOME

3. cd ~-

4. cd ~Fred

Answer

These commands all use tilde substitution correctly

Option 1 is correct. In this case, the command changes the directory to the home directory of the current user.

Option 2 is incorrect. The tilde substitutes for the $HOME variable, so this command wouldn't work.

Option 3 is correct. Using the tilde command in this context changes the current directory to its previous value.

Option 4 is correct. In this circumstance, the tilde command changes the current directory to the home directory of the user Fred.

Question

Match each filename substitution format with its description.

Options:

1. [abc]

2. [a-e]

3. *

4. ?

Targets:

A. Substitutes for any character in a listB. Substitutes for any character in a rangeC. Substitutes for any single characterD. Substitutes for zero or more characters

Answer

To substitute from a list of characters into a filename, you use the [abc] format. You

can also specify a range using the [a-e] format. The * substitutes for any character

zero or more times, while the ? matches any single character.

The command ls file[abc] would use filename substitution to match and list files

named filea, fileb, and filec if they exist.

The command ls text.[a-e] matches and lists every file named text with a suffix

consisting of a period followed by a character in the range of letters from a to e. In other words, text.a, text.b, text.c, text.d, text.e will be listed.

The command ls file* will match and list any file with a filename that starts with

file, including file123.

The command ls file? will match and list any file with a filename that starts with

file and has one additional character, such as file1 or filea if they exist.

2. Command substitution

Command substitution allows you to use the output of one command as an argument for another command.

You can perform command substitution using the syntax

$(command) $(< filename)

$(command)

When you use $(command) as an argument for another command, the shell executes the

bracketed command first. It then passes this command's entire output to the unbracketed

command.

$(< filename)

When you use $(< filename) as an argument for a command, the shell reads the specified

file and passes its entire contents to the command.

Question

See if you can complete the command shown here to display a list of files that belong to you, without knowing what your username is.

$ ls –l | grep MISSING CODE

Answer

The $(whoami) command returns your current username, which the grep

command uses to filter the output of the ls –l command.

The code shown here shows a list of the files in the current directory that you own.

$ ls –l | grep $(whoami)-rw-r--r-- 1 vincep vincep 9880 Feb 11 15:40 120415_log-rw-r--r-- 1 vincep vincep 9372 Feb 11 15:41 120603_log-rw-r--r-- 1 vincep vincep 73234 Jan 8 09:22 report1.txt-rw-r--r-- 1 vincep vincep 247416 Jan 8 09:23 report2.txt-rw-r--r-- 1 vincep vincep 451008 Jan 8 09:24 report3.txt

When you include a special character such as a wildcard or a pipe as part of an argument for a command, the command does not interpret that character. Instead, the shell resolves it and passes the output to the command as an argument.

If you intend a command to use a special character as an argument, you must prevent the shell from interpreting it by quoting the character.

The code shown here displays an asterisk on the screen. Without the quotation marks, the shell would have resolved the asterisk to a list of all the files in the current directory.

$ echo '*'*$

Instead of quote marks, you can use backslash escaping to perform quoting. To do this, you place a backslash (\) immediately before a special character. This instructs the shell to pass the character literally to the command instead of trying to interpret it.

Backslash escaping is also useful for situations in which you need to pass an actual quote mark as an argument.

The code shown here uses backslash escaping to print out the message, "'*' is a special character."

$ echo \'\*\' is a special character'*' is a special character$

Question

If you need to pass special characters to a command in an argument, you enclose them in quotes to prevent them from being interpreted.

Which commands use a suitable alternative method for escaping special characters?

Options:

1. echo \$USER

2. echo $($)USER

3. echo $(< $)USER

Answer

You use the backslash to escape special characters.

Option 1 is correct. The backslash prevents the $ from being interpreted as a special character, so $USER will be printed onscreen, rather than the value of the $USER

variable.

Option 2 is incorrect. The $(command) syntax allows you to pass the output from a

command as a parameter to another command.

Option 3 is incorrect. The $(< filename) syntax allows you to pass the contents of

a file as a parameter to a command.

3. Using substitution in a script

Let's say that you're a system administrator, and you're preparing a startup script that will run each time a user logs in. You start the vi editor, go into insert mode, and begin typing.

You begin the script by clearing the screen and you use command substitution to greet the user using the whoami command.

#!/usr/local/bin/ksh# Logon welcome scriptclearecho Welcome $(whoami)

You add a line that uses command substitution to display the current date.

#!/usr/local/bin/ksh# Logon welcome scriptclearecho Welcome $(whoami)echo The current date is $(date "+%d-%M-%Y")

Question

You want to display a list of all the reports in the user's home directory, but you don't know what the user's username is.

Complete the code shown here to navigate to the user's home directory.

#!/usr/local/bin/ksh# Logon welcome scriptclearecho Welcome $(whoami)echo The current date is $(date "+%d-%M-%Y")MISSING CODE

Answer

You type cd ~ to navigate to a user's home directory without knowing the user's

username.

Question

You use the ls command to list the contents of the user's home directory.

See if you can complete the code shown here to use the grep command to filter the output of the ls command to show only those files whose names begin with "report".

#!/usr/local/bin/ksh# Logon welcome scriptclearecho Welcome $(whoami)echo The current date is $(date "+%d-%M-%Y")cd ~ls -l | grep "MISSING CODE"

Answer

You type report* to have the command return all files that begin with "report".

You test the script, and it runs successfully.

Welcome vincepThe date is 26-08-2003You have the following reports:-rw-r--r--1 vincep vincep 73235 Jan 8 09:22 report1.txt-rw-r--r--1 vincep vincep 247416 Jan 8 09:23 report2.txt

Question

Identify the true statements about command substitution and wildcards.

Options:

1. Command substitution allows you to use the output from a command as standard input for a second command

2. Command substitution allows you to use the output from a command as part of the argument for another command

3. Wild cards are used to perform command substitution4. Wild cards are used to perform filename substitution

Answer

You use command substitution to include the output from a command in an argument to another command and you perform filename substitution with wildcards.

Option 1 is incorrect. Redirecting standard output into the standard input of a command is performed using the UNIX pipe facility.

Option 2 is correct. The command echo It is $(date), for instance, provides

the following output: It is Fri Aug 18 11:25:55 BST 2003.

Option 3 is incorrect. The shell only performs filename substitution with wildcards. The command echo It is $(dat*) will yield a "command not found" error

message.

Option 4 is correct. Filename substitution provides a quick way of searching large directories for files conforming to a specific pattern.

Summary

Filename substitution involves specifying groups of files to use as arguments for commands. Most shells let you use wildcards like asterisk (*) or a question mark (?) in filename substitution. The shell resolves a wildcard into a list of files that match specified search criteria.

Similarly, you can perform command substitution to use the output of one command as an argument for another command. The shell resolves the argument command first and passes its output to the main command.

You can use any combination of filename substitution and command substitution when you write shell scripts.

Table of Contents

| Top of page |


| 1. Substitution and matching |

| 2. Command substitution |

| 3. Using substitution in a script |

| Summary |



UNIX command syntax reference

Abstract

This article serves as a quick reference for the most commonly used shell scripting syntax.

Introduction

There are hundreds of UNIX commands that you can execute at the shell prompt. Shells have their own built-in syntax that helps you to work more effectively with existing commands by allowing you to perform functions like plugging commands into each other and controlling the flow of execution.

Conditional execution operators

||

You use the double pipe operator in the form

command1 || command2

In the above syntax, the second command executes only if the first command fails.

&&

You use the double ampersand operator in the form

command1 && command2

In the above syntax, the second command executes only if the first command executes successfully.

Command grouping operators

{ }

You can enclose multiple statements in braces ({}) to create a code block. The shell returns one exit status value for the entire group, rather than for each command in the block.

( )

You can enclose multiple statements in round brackets to create a code block. This code block functions in the same way as a code block enclosed in braces, but runs in a subshell.

I/O redirection operators

>

You use this operator to redirect command output to a file. If the specified file doesn't exist, the shell creates the file. If the file does exist, the shell overwrites it with the command output unless the noclobber environment variable is set.

>|

You use this operator to redirect command output to a file. If the specified file doesn't exist, the shell creates the file. If the file does exist, the shell overwrites it with the command output even if the noclobber environment variable is set.

>>

You use this operator to redirect command output to a file. If the file doesn't exist, the shell creates the file. If it does exist, the shell appends the new data to the end of it.

<

You use this operator to redirect command input from a file.

File descriptor redirection operators

<&n

You use this operator to redirect standard input from file descriptor n.

>&n

You use this operator to redirect standard input to file descriptor n.

n< filename

You use this operator with a filename to redirect descriptor n from the specified file.

n> filename

You use this operator with a filename to redirect descriptor n to the specified file. Unlike ordinary redirection, this will not overwrite an existing file.

n>| filename

You use this operator with a filename to redirect descriptor n to the specified file, overriding the noclobber environment variable if it is set.

n>> filename

You use this operator with a filename to redirect a descriptor to the specified file. This will redirect to a file but, unlike ordinary redirection, this will append to an existing file.

Filename substitution

*

You use the * wildcard to match a string of any length.

?

You use the ? wildcard to match a single character.

[abc] , [a-c] , [a-c1-3]

You use square brackets to match only characters that appear inside the specified set. For increased convenience, you can specify multiple ranges.

!pattern

You use the ! operator with a pattern to perform a reverse match. The shell returns only filenames that don't match the pattern.

Command substitution

$(command)

You use this form of command substitution to resolve a command and pass its output to another command as an argument.

$(< filename)

You use this form of command substitution to pass the entire contents of a file to a command as an argument.

Tilde substitution

~

You use the ~ operator to instruct the shell to return the value of the $HOME

variable.

~username

You use the ~ operator with a username to instruct the shell to return the full path of a specific user's home directory.

~+

You use the ~+ operator to instruct the shell to return the full path of the current working directory.

~-

You use the ~- operator to instruct the shell to return the full path of the previous working directory you used.

Miscellaneous syntax

;

If you enter several commands on the same line, you need to separate the commands with semicolons. The shell will execute each command successively once you press Enter.

\

You use a backslash to allow you to press Enter and continue typing commands on a new line. The shell will only begin executing your commands when you press Enter on a line that doesn't end in a backslash. Using a backlash in this way is known as backslash escaping.

&

You add a single ampersand at the end of a command to run that command as a background process. This is useful for tasks that are likely to take a long time to complete.

Summary

Shell programs can execute a wide range of UNIX commands, but they also have built-in functions to help you use shells more effectively.

Most shells support standard operators for conditional execution, input/output (I/O) redirection, file descriptor redirection, and command grouping. They also allow you to perform filename, tilde, and command substitution.

Table of Contents

| Top of page |

| Abstract |

| Introduction |

| Conditional execution operators |

| Command grouping operators |

| I/O redirection operators |

| File descriptor redirection operators |

| Filename substitution |

| Command substitution |

| Tilde substitution |

| Miscellaneous syntax |

| Summary |

Copyright © 2003 SkillSoft PLC. All rights reserved. SkillSoft and the SkillSoft logo are trademarks or registered trademarks

of SkillSoft PLC in the United States and certain other countries. All other logos or trademarks are the property of their respective owners.


Using UNIX commands in a script

Learning objective

After completing this topic, you should be able to write a shell script that uses the shell's command execution and substitution capabilities.

Exercise overview

In this exercise, you're required to complete a script that will perform a search for certain types of files, output the search results to a file, and e-mail that file to the root user.

If an error occurs, the script should display a message on the screen.

This involves the following tasks:

using code blocks using substitution using redirection and conditional execution

Task 1: Using code blocks

Let's say that you're the root user and that you're editing a script in the vi editor.

Step 1 of 1

You want the first two lines of the script to act as a distinct code block that returns one exit value for both commands. The commands should execute in the current shell.

Which type of brackets should you use to achieve this?

MISSING CODE ls -l report MISSING CODE MISSING CODE localreports ;mail -s "local reports" root MISSING CODE localreports ; MISSING CODEMISSING CODE echo An error occurred during search operation

Options:

1. Round brackets (())2. Square brackets ([])3. Braces ({})4. Less than and greater than signs (<>)

Result

You use braces ({}) to create a code block that will execute in the current shell.

Option 1 is incorrect. You use round brackets to perform command substitution, as in the variable assignment command shown here:DATE=$(date)

Option 2 is incorrect. You use the square brackets to enclose conditional expressions, such as:if [ $FEEDBACK = y ] thenecho "proceed"elseexitfi

Option 3 is correct. The syntax for a function – a sequence of code that is available to the shell into which it is read – is:function_name() {commands}

Option 4 is incorrect. The <> special characters are used to perform redirection. They

are also used as conditional operators (less-than and greater-than) within conditional expressions.

Task 2: Using substitution

Step 1 of 1

Suppose that you want to use the ls command to search for all files that begin with

the word "report" and end in a number between 0 and 9.

Use substitution to complete the search criteria shown here.

{ ls -l report MISSING CODE MISSING CODE localreports ;mail -s "local reports" root MISSING CODE localreports ; }MISSING CODE echo An error occurred during search operation

Result

You specify report[0-9] with the ls command to list all files that begin with the

word "report" and end in a number between 0 and 9.

Task 3: Using redirection and conditionals

Step 1 of 3

Suppose that you want to redirect the output of the ls command to a file named

localreports.

Type the symbol that will achieve this.

{ ls -l report[0-9] MISSING CODE localreports ;mail -s "local reports" root MISSING CODE localreports ; }MISSING CODE echo An error occurred during search operation

Result

You use the greater than symbol (>) to redirect output to a file.

Step 2 of 3

In the second line of the script, you want to redirect the contents of a mail from the localreports file.

Type the correct symbol to do this.

{ ls -l report[0-9] > localreports ;mail -s "local reports" root MISSING CODE localreports ; }MISSING CODE echo An error occurred during search operation

Result

You use the less than symbol (<) to redirect input from a file.

Step 3 of 3

You want the last line of the script to execute only if the code block produces an error.

Type the symbol that will do this.

{ ls -l report[0-9] > localreports ;mail -s "local reports" root < localreports ; }MISSING CODE echo An error occurred during search operation

Result

You use the double pipe symbol (||) to execute a command only if the preceding command ( or, in this case, a command block ( fails.

You've now successfully completed a script that will perform a search for specific files, output the search results to a file named localreports, and e-mail the localreports file to the root user. It will also display an error message if the code fails.

{ ls -l report[0-9] > localreports ;mail -s "local reports" root < localreports ; }|| echo An error occurred during search operation

Table of Contents

| Top of page |


| Exercise overview |

| Task 1: Using code blocks |

| Task 2: Using substitution |

| Task 3: Using redirection and conditionals |




Working with UNIX variables

Learning objective

After completing this topic, you should be able to use variables in UNIX programming.

1. Variables and data types

Variables are named spaces used to store data.

Local variables are available only in the function or subshell in which they were created or declared.

Global variables are available throughout the shell, including all its subshells and functions.

You can turn local variables into global variables by exporting them using the export

command.

In this example, the code exports the local variable DEST from a subshell, thus making it a

global variable.

$ export DEST$

Like most programming languages, UNIX shells allow you to define variables using data types.

Data types define the types of data contained in variables. For example, you can define a variable as an integer or as a character string.

This can change the way the shell uses the variables. For example, the shell performs faster arithmetic with integer variables.

The more advanced UNIX shells support four data types:

strings, which are sequences of alphanumeric characters integers, which are whole numbers floats, which are floating-point decimal numbers arrays, which are sequences of variables

Each of the following popular UNIX shells supports variables and data types in different ways:

Bourne shell Bash shell TC shell Korn shell

Bourne shell

The Bourne shell supports string variables only. All variables in the Bourne shell are global.

Bash shell

The Bash shell supports string and integer variables. You can only declare local variables in

the Bash shell.

TC shell

The TC shell supports string, integer, float, and array variables. TC shell variables can be

either local or global.

Korn shell

Like the TC shell, the Korn shell supports string, integer, float, and array variables. Korn

shell variables can be either local or global.

Question

Match the UNIX shell type to its description.

Options:

1. Bash shell

2. Bourne Shell3. Korn Shell

Targets:

A. Supports string and integer variablesB. Supports string, integer, float, and array variablesC. Supports string variables only

Answer

The Bourne shell only supports string variables, the Bash shell supports string and integer variables, while the Korn shell supports string, integer, float, and array variables

The Bash shell is based on the Bourne shell. Although you can export variables to make them global, they can only be declared locally.

The Bourne shell is the most universally deployed shell and, although it has a limited feature set, it's useful for writing platform-independent shell scripts.

The Korn shell provides good support for variable typing and other useability features. It's more likely to be deployed on proprietary UNIX systems than on open source systems.

2. Using variables

The shell defines variables automatically when you assign a value to them for the first time.

You can assign a value to a variable using the equal sign (=).

In this example, the code assigns the value 4070 to a variable called distance.

$ distance=4070$

You can use the typeset command to assign values to variables. Using typeset allows

you to specify command-line options for the assignment operation.

$ typeset distance=4070$

If you don't specify a value for a variable in an assignment statement, the shell assigns a null value to the variable.

In this example, distance is set to null.

$ distance=$

To refer to the value of a variable – rather than to the variable itself – you precede the variable name with a dollar sign ($).

To view the value of a variable, for example, you use the print command and precede the

variable name with a $ sign.

In this example, the code allows you to view the value of the distance variable.

$ print $distance4070$

You can assign the value of one variable to another variable, also using the dollar sign.

In this example, the code assigns the value of the distance variable to another variable

called temp.

$ temp=$distance$

To remove a variable entirely, you use the unset command.

In this example, the code removes the temp variable. The print statement allows you to

confirm that the variable no longer exists.

$ unset temp$ print $temp$

Question

What do you think is the output of the code shown here?

$ t=17h45$ departure=$t$ print $departure

Options:

1. 17h452. t3. departure

Answer

The output of the code is 17h45.

Option 1 is correct. The value of the $departure variable is set to the value of the $t variable. This variable is assigned a value of 17h45 in the first line of code.

Option 2 is incorrect. The command departure = $t assigns the value of the $t variable to the $departure variable, not the character t.

Option 3 is incorrect. The command print $departure prints the value assigned to the departure variable – in this case the value of the $t variable set in the first line of code. It does not interpret $departure as the literal string departure.

3. Quoting

If you want to prevent the shell from processing spaces and special characters such as &

and *, you can enclose them in quotes.

For example, this line of code displays a heading that includes special characters.

Because the character string is enclosed in quotes, the shell displays it without processing the * and & characters.

$ echo '***** Spain & Portugal *****'***** Spain & Portugal *****$

If you omit the quotes, the shell processes the special characters, returning a list of files and an error message.

$ echo ***** Spain & Portugal *****[1] 55412ksh: Portugal: not found$ add_pkg_files Spain

You can use the following kinds of quotes in UNIX shell scripts:

single quotes ('')

double quotes ("") back quotes (``)

single quotes ('')

Single quotes ('') hide the meaning of all special characters. They allow you to use special

characters – including double and back quotes – and spaces and newline characters in

strings.

Single quotes also prevent variable and command substitution because they hide the meaning of the $ character.

double quotes ("")

Double quotes ("") hide the meaning of special characters, spaces, and newline characters, with the exception of back quotes (``) and the $ and \ characters.

This means that command and variable substitution still take place in strings enclosed by

double quotes. Therefore, you can use double quotes to combine variable values with a text

string.

back quotes (``)

Back quotes (``) are a method of command substitution that the Bourne shell uses. The

Korn shell retains them for compatibility with this shell.

When you enclose a command in back quotes, the shell executes them and returns the

value of their output. You can assign this value to a variable.

Say you want to define a variable called pricetag and to specify that this variable must

contain the string $50. You need to do this using single quotes to prevent the shell from treating the characters after the $ sign as a variable name.

$ pricetag='$50'$ print $pricetag$50$

Let's say that you want the shell to read the value of the item and pricetag variables and

to include these values in the value of another variable called priceline.

You need to use double quotes so that you can combine the variable values with text.

$ pricetag='$50'$ item='trombone'$ priceline="The cost of a $item is $pricetag."$ print $pricelineThe cost of a trombone is $50.$

Say you need to declare a variable that contains a full directory listing of the scripts directory. Because the listing needs to be up to date, you need to use the ls command to

obtain it.

Using back quotes, you can assign the output of the ls command to the drlist variable.

$ drlist=`ls -a scripts`$ print $drlist. .. add_pkg_files$

Question

In which variable assignment statement is the $ symbol not interpreted as a special character?

Options:

1. Var=&$100

2. Var=\$100

3. Var="$100"

4. Var='$100'

Answer

The backslash and single quotes prevent the $ symbol from being interpreted as a special character.

Option 1 is incorrect. Placed at the end of a command, the ampersand causes the command to run as a background job, but it does not escape special characters.

Option 2 is correct. The backslash escapes all special characters, including itself. So the expression \\ will be interpreted by the shell as \.

Option 3 is incorrect. Double quotes prevent most special characters from being interpreted, except $, \, and `.

Option 4 is correct. Single quotes prevent all special characters from being interpreted, but they can't be nested.

Question

What do you think is the output of the code shown here?

$ airline='PA'$ flightno="$airline 771"$ print $flightno

Options:

1. airline 771

2. flightno 771

3. PA 771

4. PA771

Answer

The output of the code is PA 771 because the double quotes allow variable

substitution and the single quotes include the space character.

Option 1 is incorrect. The special character $ symbol used to reference variables is retained within double quotes, so in the command flightno="$airline 771" ,

$airline refers to the variable set in the previous command line.

Option 2 is incorrect. The print command prints the flightno variable, not the flightno string.

Option 3 is correct. The first command assigns the value PA to the airline variable.

The second command substitutes the variable at the start of a string that consists of a space and the number 771, and assigns the resulting string to the flightno variable,

which is then printed.

Option 4 is incorrect. Double quotes allow you to create variables that include spaces because everything within the double quotes is interpreted as a single word by the shell.

Summary

Variables are named spaces for storing data. The value of a variable can be one of several data types, such as an integer or a character string. Some shell environments support more data types than others.

You can set the value of a variable using an assignment statement or the typeset

command. You can unset them using the unset command. To access the value of a

variable, you precede the variable name with a dollar sign ($).

You can use quotes when assigning a value to a variable. Single quotes and double quotes allow you to include special characters in strings. Back quotes allow you to assign the output of a command to a variable.

Table of Contents

| Top of page |


| 1. Variables and data types |

| 2. Using variables |

| 3. Quoting |

| Summary |




UNIX shell script variables

Learning objective

After completing this topic, you should be able to describe and use script variables and parameters.

1. Defining variable attributes

Any variable can have one or more attributes.

An attribute is a setting that defines how a variable is represented, accessed, or displayed.

The integer attribute, for example, defines a variable as an integer, and the left justify attribute formats a string variable so that it's left-justified.

The attributes you can set on a variable include

integer (-i) float (-E, -F)

lowercase and uppercase (-u, -l)

right and left justify (-R, -L)

right and left justify with zeroes (-RZ, -LZ)

autoexport (-x)

integer (-i)

The integer attribute (-i) defines a variable as having the integer data type. This allows the

shell to process integer values faster, but it prevents you from assigning any non-integer

value to the variable.

float (-E, -F)

The float attributes (-E and -F) define a variable as a floating-point decimal number.

When you use the -E attribute, you can specify the number of digits in the number,

regardless of the position of the decimal point. For example, 3.1415 and 314.15 both have

five digits.

When you use the -F attribute, you can specify the number of digits after the decimal point.

For example, 196.543 and 2.160 both have three digits.

lowercase and uppercase (-u, -l)

The uppercase attribute (-u) automatically converts any alphabetical characters in a

variable's value into uppercase characters.

The lowercase attribute (-l) converts any alphabetical characters in a variable's value into

lowercase characters.

right and left justify (-R, -L)

The right justify (-R) and left justify (-L) attributes allow you to format data. You need to

specify a field width when you set either of these attributes.

Variables with the -R attribute will display at the right end of the specified field width,

whereas variables with the -L attribute display at the left end of the specified field width.

right and left justify with zeroes (-RZ, -LZ)

The right justify with zeroes (-RZ or -Z) attribute right-justifies a variable to a specified field

width and fills the remainder of the field with leading zeroes.

The left justify with zeroes (-LZ) attribute left-justifies a variable to a specified field width and

strips all leading zeroes.

autoexport (-x)

The autoexport (-x) attribute causes the shell to export a local variable whenever its value is

set. This is useful if you want to ensure that the new value is available throughout the shell.

You use the typeset command to set and unset attributes.

Paradoxically, you use a minus sign (-) to set an attribute and a plus sign (+) to unset it.

In this example, the code sets the integer attribute (-i) on the dist variable.

$ typeset -i dist$

In this example, the code unsets the integer attribute on the dist variable.

$ typeset +i dist$

You can set multiple attributes at the same time. In this example, the code sets the uppercase attribute (-u) and the autoexport attribute (-x) on the name variable.

$ typeset -ux name$

Question

At what stage do you think you can set attributes on a variable?

Options:

1. After you assign a value to it2. After you've unset it3. Before you assign a value to it4. While you assign a value to it

Answer

You can set attributes on a variable either before, during, or after assignment. However, you can't set attributes on a variable that has been unset.

To assign an attribute and a value to a variable at the same time, you use the typeset

command and specify the attribute before the value.

In this example, the code sets the integer attribute (-i) on the dist variable and assigns it

the value 489.

$ typeset -i dist=489$

Attributes take precedence over a variable's current value.

For example, if the value of name is Novak and you set the uppercase attribute (-u) on

name, its value becomes NOVAK.

$ name=Novak$ typeset -u name$ print $nameNOVAK$

You can use the readonly attribute (-r) to prevent any further changes to the value or attributes of a variable.

Unlike other attributes, you cannot unset the readonly attribute on a variable once you've set it.

You can use the typeset command to display a list of all shell variables that currently have

a particular attribute.

For example, this command displays a list of all variables with the integer attribute (-i).

$ typeset -iMAILCHECK=600OPTIND=1PPID=55394RANDOM=22348SECONDS=700TMOUT=0dist=489$

Question

Match the variable attribute to its function.

Options:

1. typeset-i

2. typeset -l

3. typeset -L

4. typeset -x

Targets:

A. Ensures that the variable contains all lowercase charactersB. Ensures that the variable is exported to subshellsC. Ensures that the variable is left justifiedD. Defines the variable as a whole number or its negative

Answer

The –i attribute defines a variable as an integer, the –x attribute exports it, the –l

attribute ensures all its characters are lowercase, and the –L attribute ensures that

the variable is left justified.

The –i attribute is useful when you are using variables in calculations. It enables you

to make sure that numbers are assigned to the variables, rather than letters.

The –l attribute is useful if you need to format the case of a variable consistently,

irrespective of its value.

The –L attribute strips any leading spaces from a variable, which is useful when you

want to avoid errors in shell scripts in which variables are set interactively by the user.

If you want to make a variable available to subshells, you must export it using the –x

attribute. However if you want the variable to be available between logins, or to other users, you will need to add it to a shell configuration file.

2. Working with parameters

Parameters are reserved variables that the shell uses. They allow you to control the behavior of the shell or to obtain information about current processes.

Environment variables are parameters that control the shell's behavior. They are global variables declared in each user's .profile file.

User-defined variables can't use the same names as the environment variables because the system has reserved these names. However, users can change the value of environment variables.

Examples of environment variables are

the SHELL variable, which defines a user's default shell

the HOME variable, which defines a user's home directory

the PATH variable, which defines the search path for commands

The shell has some special parameters that allow you to interact with current processes, including

? - ! $ PPID ERRNO

?

The ? parameter contains the exit status of the last command that the shell has executed. If

the command executed successfully, the exit status is 0. If it didn't execute successfully, the

exit status is non-zero.

-

The - parameter contains the shell options that are set on the current shell process.

!

The ! parameter contains the process ID (PID) of the last background command or co-

process that the shell has executed.

$

The $ parameter contains the process ID (PID) of the current shell process.

PPID

The PPID parameter contains the process ID (PID) of the current shell's parent shell

process.

ERRNO

The ERRNO parameter contains the error number returned by the most recent failed system

call.

This parameter is system-dependent, so it may not be available on all systems.

Question

What do you think you can do to the values of special parameters such as ? and $?

Options:

1. Reassign them2. Set them3. Unset them4. View them

Answer

You can view the values of special parameters, but you can't reassign them, set them, or unset them.

Positional parameters contain arguments that have been passed to a command, script, or function. They allow you to use these arguments in subsequent lines of a script.

Positional parameters are named according to the positions of the arguments they contain.

Parameter 0 contains the name of the command or function to which the arguments apply,

parameter 1 contains the first argument, parameter 2 contains the second argument, and so

on.

In this command, for example, the value of parameter 0 is ls, the value of parameter 1 is -

a, and the value of parameter 2 is the specified directory path.

$ ls -a /usr/home

Positional parameters are particularly useful in functions.

This example of a function searches the current directory for a specified file and displays the contents of the file one screen at a time.

When you call the function and specify a filename as an argument, the filename is stored in the positional parameter $1. This allows the commands in the function to work with the

filename.

disf() {> ls -al | grep $1> cat $1 | more> }$

You can call positional parameters only up to $9. If you want to use ten or more positional

parameters, you need to enclose the identifying numbers in the parameter names in braces ({}).

For example, the tenth positional parameter is ${10}.

The expression $* returns the values of all current positional parameters, separated by

spaces.

The expression $# returns the number of current positional parameters.

Question

Identify the true statements regarding positional parameters.

Options:

1. Although commands can take any number of arguments, you can only view nine positional parameters

2. Positional parameter 2 contains the second argument to be passed to a command3. Positional parameter 0 always contains the value of a command or name of a

function4. You can use positional parameters within a function to manipulate its arguments

Answer

Positional parameter 0 refers to the command or function itself, while parameter 2 refers to the second argument passed to the command. One of the main uses of positional parameters is to call the arguments to a function from within the function.

Option 1 is incorrect. You can view positional parameter 10 or greater, but you must enclose it in braces, as in ${10}.

Option 2 is correct. For the command cat file1 file2, the value of $2 is file2.

Option 3 is correct. For the command cat file1 file2, the value of $0 is cat.

Option 4 is correct. You can access an argument to the special_cat function as

shown here:special_cat () {cat $1 | less}

Summary

You can set attributes that apply to a variable. Attributes can modify the way the shell processes a variable – for example by defining it as an integer or floating-point decimal number. They can also modify the way the shell displays a variable value – for example by specifying that it display as uppercase or left-justified characters.

Parameters are predefined system variables. They include environment variables, which define characteristics of the shell environment, special parameters, which allow you to obtain data about current processes, and positional parameters, which allow you to work with arguments.

Table of Contents

| Top of page |


| 1. Defining variable attributes |

| 2. Working with parameters |

| Summary |




Variable expansion and arrays in UNIX shell scripts

Learning objective

After completing this topic, you should be able to explain variable expansion and use array variables.

1. Variable expansion

Variable expansion allows you to access the values of variables. The simplest form of variable expansion involves referring to the value of a variable directly. To do this, you place a dollar sign ($) in front of a variable name.

Other forms of expansion allow you, for example, to refer to portions of a variable's value or to the length of a variable's value, or to specify default or alternative variable values.

You can use braces ({}) to delimit the value of a variable from any characters following the variable name. This allows you to use the variable name in assignment statements that change the value of the variable.

In this example, the variable en contains the name "EasyNomad." Using braces, you can

change its value to "EasyNomadTravel" by invoking its original value.

$ en=EasyNomad$ en=${en}Travel$ print $enEasyNomadTravel$

If you precede a variable name with a hash sign (#), you can determine the length of its value – the number of characters it contains.

In this example, the output of the command specifies that the value of the en variable

contains 15 characters.

$ print ${#en}15$

The following variable expansion formats allow you to work with default and alternative values:

${variable:-word} ${variable:=word} ${variable:+word}

${variable:?word} ${variable:?}

${variable:-word}

The ${variable:-word} syntax returns the value of the variable if it's set and non-null. If it's not

set or if it's set to null, the expression returns the text string word.

For example, ${name:-Name not found} returns the value of the name variable. If name

is null or not set, it displays the message "Name not found".

${variable:=word}

The ${variable:=word} syntax returns the value of the variable if it's set and non-null. If it's

not set or if it's set to null, the expression assigns the text string word to the variable and

then returns the variable's new value.

For example, ${name:=anonymous} returns the value of the name variable. If name is null

or not set, it assigns the value "anonymous" to name and returns the value "anonymous".

${variable:+word}

The ${variable:+word} syntax returns the text string word if the variable is set and non-null. If

it's not set or if it's set to null, the expression returns nothing.

For example, ${name:+Name already exists} returns the string "Name already

exists" if the value of the name variable is set and non-null. If name is null or not set, nothing

is displayed.

${variable:?word}

The ${variable:?word} syntax returns the value of the variable if it's set and non-null. If it's

not set or if it's set to null, the expression returns the text string word and the shell

terminates.

You can use this format to ensure that essential variables are set before a script executes

further.

${variable:?}

The ${variable:?} syntax returns the value of the variable if it's set and non-null. Otherwise,

the expression returns the default error message "parameter null or not set" and the shell

terminates.

The following variable expansion formats allow you to extract substrings from variable values:

${variable:start} ${variable:start:length} ${variable#pattern} ${variable##pattern}

${variable%pattern} ${variable%%pattern}

${variable:start}

The ${variable:start} syntax returns part of the value of a variable, from the character

position specified by the integer start to the end of the value.

For example, if the value of the en variable is EasyNomad, the expression ${en:4} returns

"Nomad".

${variable:start:length}

The ${variable:start:length} syntax returns part of the value of a variable. The selected

substring begins at the character position specified by the integer start and contains as

many characters as specified by length.

For example, if the value of the en variable is EasyNomad, the expression ${en:4:3}

returns "Nom".

${variable#pattern}

The ${variable#pattern} syntax finds the shortest substring that matches pattern, beginning at

the left of the variable's value. It then removes this substring and returns the remainder of

the value.

For example, say the value of the tour variable is "landing ground on island". The

expression ${tour#land*} finds the first and shortest instance of land* and removes it. It

therefore returns "ing ground on island".

${variable##pattern}

The ${variable##pattern} syntax finds the longest substring that matches pattern, beginning

at the left of the variable's value. It then removes this substring and returns the remainder of

the value.

For example, if the value of the tour variable is "landing ground on island", the expression

${tour##land*} finds the first and longest instance of land*, which is the entire string. It

removes this and returns nothing.

${variable%pattern}

The ${variable%pattern} syntax finds the shortest substring that matches pattern, beginning

at the right of the variable's value. It then removes this substring and returns the remainder

of the value.

For example, if the value of the tour variable is " landing ground on island", the expression

${tour%*land} finds the last and shortest instance of land* and removes it. It therefore

returns "landing ground on is".

${variable%%pattern}

The ${variable%%pattern} syntax finds the longest substring that matches pattern, beginning

at the right of the variable's value. It then removes this substring and returns the remainder

of the value.

For example, if the value of the tour variable is "landing ground on island", the expression

${tour%%*land} finds the last and longest instance of land*, which is the entire string.

Removing this, it returns nothing.

The following variable expansion formats allow you to find and replace substrings in variable values:

${variable/pattern1/pattern2} ${variable//pattern1/pattern2} ${variable#pattern1/pattern2} ${variable%pattern1/pattern2}

${variable/pattern1/pattern2}

${variable/pattern1/pattern2} replaces the first instance of pattern1 in a variable's value with

pattern2 and returns the result.

For example, say the variable heading contains the string "Last-minute flights and cut-

price flights". In this case, the expression ${heading/flight/tour} returns the string

"Last-minute tours and cut-price flights".

${variable//pattern1/pattern2}

The ${variable//pattern1/pattern2} syntax replaces all instances of pattern1 in a variable's

value with pattern2 and returns the result.

For example, if the variable heading contains the string "Last-minute flights and cut-price

flights", the expression ${heading//flight/tour} returns the string "Last-minute tours

and cut-price tours".

${variable#pattern1/pattern2}

The ${variable#pattern1/pattern2} syntax replaces the first instance of pattern1 in a variable's

value with pattern2 if the value begins with pattern1.

For example, say the variable col3 contains the string "Cost of flight." In this case, the

expression ${col3#Cost/Price} returns the string "Price of flight".

${variable%pattern1/pattern2}

The ${variable#pattern1/pattern2} syntax replaces the last instance of pattern1 in a variable's

value with pattern2 if the value ends with pattern1.

For example, if the variable col3 contains the string "Cost of flight", the expression $

{col3%flight/cruise} returns the string "Cost of cruise".

Question

You have set the value of the file variable to file1.

Select the statements that correctly describe variable expansion of the file variable.

Options:

1. The command en=${file}2 changes the value of the variable to file12

2. The command echo ${#file} returns the integer 5

3. The command echo ${file:4} returns the integer 1

4. The command echo ${file:?} returns a null value and exits the shell

Answer

Variable expansion allows you to reuse the current value of a variable when assigning a new value, return the number of characters in a variable, and return just part of a variable.

Option 1 is correct. You can expand the current value of a variable and include it as part of a new string, or perform an arithmetic operation on it if it is an integer to create a new value.

Option 2 is correct. When using braces to expand a variable, preceding the variable name with a # symbol returns the number of characters in a variable, not the string or integer itself.

Option 3 is correct. The number in this syntax identifies where the returned string starts. In this case the variable has been assigned a five-character string, so only the last character is returned.

Option 4 is incorrect. This only happens if the file variable has either not been

assigned, or has been assigned a null value.

Question

What do you think you can do using variable expansion formats?

Options:

1. Display a custom error message if a variable is not set2. Extract a substring from the value of a variable3. Replace a substring in the value of a variable with another substring4. Replace the value of one variable with the value of another variable

Answer

Using variable expansion formats, you can display a custom error message if a variable is not set, extract a substring from the value of a variable, and replace a substring in the value of a variable with another substring.

Option 1 is correct. If the file variable is not set, or has a null value, this command

will output the message "File not found":echo ${file:-File not found}

Option 2 is correct. If the file variable has been assigned the value file1, you can

return just the 1 using the command:echo ${file:4}

Option 3 is correct. If the file variable has been assigned the value file1, you can

replace the 1 with a 2 using the command:echo ${file/1/2}

Option 4 is incorrect. Variable expansion isn't used for this. You simply assign the variable as shown:newvar=$file

2. Arrays

Arrays are variables that can contain multiple values, stored as a sequence of elements.

The name of an array variable denotes the entire array. You refer to an individual element in an array using a subscript that identifies its position in the sequence of elements in the array. This subscript takes the form of a number enclosed in square brackets.

Numbering begins at zero. Therefore, the subscript of the first element is [0], the subscript of the second element is [1], and so on.

Question

How do you think you create an array variable?

Options:

1. By applying the array attribute to an existing variable2. By assigning a value to one of its elements3. By assigning a value to the array4. By creating an ordinary variable with the typeset command using the array attribute

–A

Answer

You create an array variable by assigning a value to one of its elements.

If a variable already has an existing value when you convert it into an array, that value becomes the value of the array's first element.

In this example, the code assigns the value "January" to the mth variable. It then assigns

the value "February" to the mth[1] element, thereby making it an array. The value of

mth[0] is still "January."

$ mth=January$ mth[1]=February$ print ${mth[0]}January$

If you use arithmetic expressions or variables in the subscript of an array element, the shell evaluates the expressions or variables and then references the array.

In this example, the code assigns a value to element 4 + 9 of the items array, which lists

souvenirs for sale. The next line of code displays the element whose subscript is contained in the variable count. Assuming that the value of count is 3 and the fourth element in the

array is "wooden giraffe", the resulting output is "wooden giraffe."

$ items[4+9]="safari hat"$ print ${items[$count]}wooden giraffe$

The following variable expansion formats allow you to work with array variables:

${array[*]} ${!array[*]} ${!array[*]:n} ${!array[*]:n:x} ${#array[*]} ${#array[n]}

${array[*]}

${array[*]} lists all elements of an array.

${!array[*]}

${!array[*]} lists all initialized subscript values of an array.

${!array[*]:n}

The ${!array[*]:n} syntax lists all array elements, beginning with the subscript n.

For example, ${!items[*]:5} lists all elements of the items array from the sixth

element onwards.

${!array[*]:n:x}

The ${!array[*]:n:x} syntax lists x array elements, beginning with the subscript n.

For example, ${!items[*]:5:3} lists the sixth, seventh, and eighth elements of the

items array.

${#array[*]}

The ${#array[*]} syntax returns the number of elements in an array.

${#array[n]}

The ${#array[n]} syntax returns the length of the array element specified by the subscript [n].

Let's say that you want to write a function that accepts an element number as an argument and displays a specified element, as well as the number and length of this element, in the items array. The items array is an existing array that lists souvenirs for sale.

You decide to name the function get_item.

$ get_item() {>

You begin by printing the subscript specified in the argument. You do this using a positional parameter.

$ get_item() {> print "Item no. $1"

Then you print the length of the specified element, using the appropriate array variable expansion format.

$ get_item() {> print "Item no. $1"> print "Length ${#items[$1]}"

Finally, you print the value of the specified element.

$ get_item() {> print "Item no. $1"> print "Length ${#items[$1]}"> print ${items[$1]}> }$

When you call the get_item function for element 2, the data for the third element of the

items array displays.

$ get_item() {> print "Item no. "$1+1> print "Length "${#items[$1]}> print ${items[$1]}> }$ get_item 2Item no. 2Length 14leopard poster$

Question

In this question, the variable var is an array.

Match the expansion statements for the var variable to their corresponding

functions.

Options:

1. ${var[*]}2. ${!var[*]}3. ${#var[*]}

Targets:

A. Returns all the elements of the arrayB. Returns all the initialized subscript values of the arrayC. Returns the number of elements in the array

Answer

The statement ${var[*]} returns all the elements in the array, ${!var[*]}

returns every initialised subscript, and ${#var[*]} returns the number of elements

in an array.

The * metacharacter is used as a wildcard. You use it to reference every element in an array.

You use the ! metacharacter to reference the subscript values in an array. A subscript consisting of a number equal to the total number of elements in the array is initialized whenever you add an element. You use an element's subscript to reference the element.

The value returned when you use the # metacharacter will be equivalent to the value of the last subscript to be initialized for the array.

Summary

The process of variable expansion allows you to work with the values of variables. Using variable expansion formats, you can extract or replace part of a variable's value. You can also provide default and alternative values.

You can store multiple values in a variable by turning it into an array. You assign and access the elements of an array using a subscript in square brackets.

Table of Contents

| Top of page |


| 1. Variable expansion |

| 2. Arrays |

| Summary |




UNIX arithmetic operators and precedence

Learning objective

After completing this topic, you should be able to use arithmetic operators in shell scripts.

1. Performing simple arithmetic

You can incorporate arithmetical and logical expressions in UNIX command-line statements and shell scripts. The shell evaluates these expressions and substitutes the results for the expressions themselves.

However, the shell works with positive integers only.

To declare an integer variable, you can simply assign an integer value to it. For example, the code itemID=5 declares the integer variable itemID.

However, you can also declare an integer variable explicitly, which improves performance and allows type checking. To do this, you use the integer command.

$ integer itemID=5

Alternatively, you can declare variables explicitly using the typeset command with the -i

option to set the integer attribute on them.

$ typeset -i itemID=5

Integer variables in the UNIX shell don't need to use the base-10 decimal system, although this is the default. To specify a base number other than 10 for an integer variable, you use a hash symbol (#) between the base and the value. In this example, the code assigns the binary value 1101 to the nibble variable.

$ nibble=2#1101

You can specify a base number after the -i option of the typeset command. In this

example, the code assigns the hexadecimal value of B1F7 to the position variable.

$ typeset -i16 position=B1F7

Note

You can use any base between 2 and 36.

In early versions of UNIX, the expr command was used to evaluate expressions. This

example shows the variable i being incremented by 1.

$ i = `expr $i + 1`

More recent versions of UNIX support the let command.

The let command doesn't require that you use a dollar sign ($) when referring to a

variable's value. This bypasses the process of variable expansion, which makes it up to 60 times faster than the expr command.

$ let i=i+1

If you include spaces or special characters in an arithmetical expression, you need to enclose it in quotes.

$ let "i=i + 1"

As an alternative to using the let command to evaluate expressions, you can use double

brackets as shown here. The shell treats the code in the brackets as an arithmetical expression and evaluates it.

$ ((i=i + 1))

When the shell evaluates an arithmetical expression, it follows the conventional order of precedence for arithmetical operators.

Multiplication (*) and division (/) operators take precedence over addition (+) and subtraction (-) operators, unless you use brackets to override this precedence.

Question

What do you think is the result value of the following expression?12 * (48 / 6 + 2)

Options:

1. 362. 723. 1204. 144

Answer

The expression 12 * (48 / 6 + 2) evaluates to 120.

The UNIX shell supports a modulo operator (%), which returns the remainder that results from the division of one expression by another.

For example, ((18%8)) evaluates to 2, because 18 divided by 8 is 2, leaving a remainder

of 2.

$ ((rem = 18 % 8))$ print $rem4$

You can assign negative values to variables by placing a minus sign (-) immediately in front of the value.

However, you need to use the print - command to display negative values, because

otherwise the print command interprets the minus sign in a variable value as an

argument.

$ let "num = -9"$ print - $num-9$

Question

Identify the true statements concerning declaring integer variables.

Options:

1. You can declare integers that aren't base 102. You can use binary numbers as integers3. You use the typeset option to explicitly declare an integer variable4. You need to explicitly declare integer variables

Answer

You can use the typeset option to explicitly declare integer variables that aren't base 10, such as variables containing binary numbers, which are base 2.

Option 1 is correct. Integer variables can use any base between 2 and 36, although the default is 10. You can declare hexadecimal integers like this:Variable=16#b6

Option 2 is correct. Binary numbers are base 2, and you can use variables with base 2 integers as values.

Option 3 is correct. To explicitly declare an integer variable, you use the typeset –

i option.

Option 4 is incorrect. You can make a variable an integer variable by simply assigning it an integer value.

2. Logical and bitwise operations

The UNIX shell supports logical operators that perform Boolean logic operations on expressions.

The logical operators that the shell supports are

logical AND (&&)

logical OR (||) logical negation (!)

logical AND (&&)

The && operator compares two expressions logically.

If both are true or evaluate to zero, the entire expression evaluates to zero.

If either or both are false or evaluate to one, the entire expression evaluates to one.

logical OR (||)

The || operator compares two expressions logically and evaluates the entire expression to

zero if either of them are true.

The entire expression evaluates to one only if both expressions are false.

logical negation (!)

The ! operator evaluates an expression and then reverses it logically.

If an expression evaluates to zero, the ! operator changes it to one, and vice versa.

The UNIX shell supports several bitwise operators.

Whereas logical operators compare and manipulate entire expressions, bitwise operators perform Boolean logic on individual bits.

This is chiefly useful for working with binary numbers.

The bitwise operators are

bitwise AND (&) bitwise OR (|) bitwise XOR (^) bitwise negation (~)

bitwise AND (&)

The & operator compares two expressions bit by bit. If both bits are 1, it returns a 1. It

returns a 0 if either of the bits is 0.

For example, ((2#1101 & 2#1001)) evaluates to 2#1001.

bitwise OR (|)

The | operator compares two expressions bit by bit. If either of the bits is 1, it returns a 1. It

returns a 0 only if both bits are 0.

For example, ((2#1101 | 2#1001)) evaluates to 2#1101.

bitwise XOR (^)

The ^ operator compares two expressions bit by bit. If one of the bits is a 1 and the other is

a 0, it returns a 1. It returns a 0 if both bits are 1 or if both bits are 0.

For example, ((2#1101 ^ 2#1001)) evaluates to 2#100.

bitwise negation (~)

The ~ operator acts on one expression to turn all zeros into ones and vice versa.

For example, ((~ 2#1001)) evaluates to 2#110.

The UNIX shell supports bit shift operators. These act on binary number values, shifting their bits either to the left or to the right by a specified number of places.

The left shift operator (<<) shifts bits to the left.

For example, ((2#1001 << 1)) evaluates to 2#10010 and ((2#11 << 2)) evaluates to

2#1100.

The newly created bits on the right are zeros.

The right shift operator (>>) shifts bits to the right.

For example, ((2#1001 >> 1)) evaluates to 2#100. Bits at the right end of the original

values are lost.

Question

Which operator performs bitwise reverses on the value of each bit in a binary expression?

Options:

1. ~2. &3. ^4. |

Answer

The ~ operator performs bitwise negation on the value of each bit in a binary expression.

Option 1 is correct. If you perform bitwise negation on the base2 integer 10001, the result will be 1110.

Option 2 is incorrect. You use the & symbol to perform a bitwise AND on two binary expressions.

Option 3 is incorrect. You use the ^ symbol to perform a bitwise XOR on two binary expressions.

Option 4 is incorrect. You use the | symbol to perform a bitwise OR on two binary expressions.

Question

What do you think is the output of the following piece of code?((x = 2#1101 & 2#110))((y = ~x))print - $y

Options:

1. 01002. 113. 04. 100

Answer

The output of this code is 2#1011. The first statement sets x to 2#0100 and the

second statement performs bitwise negation on this value.

Option 1 is incorrect. This is not valid output for a binary integer. If it were, the leading zero would be stripped.

Option 2 is correct. The first statement compares the two binary numbers, bit by bit. Where both bits are set, the returned bit is also set. Where one bit isn't, the returned bit is also not set. The second statement performs bitwise negation, unsetting set bits and setting unset bits.

Option 3 is incorrect. This would be the result if the first statement were a bitwise OR, yielding a result of 1111.

Option 4 is incorrect. This would be the result if the first statement were a bitwise XOR, yielding a result of 1011.

3. Additional operators

The UNIX shell supports a range of comparative operators that compare the values of two expressions. These are useful when you need to set conditions for conditional statements.

The comparative operators that the UNIX shell supports are

less than (<) greater than(>) less than or equal to (<=) greater than or equal to (>=) equal to (==) not equal to (!=)

less than (<)

The < operator returns 0 (true) if the expression before it evaluates to a lower value than the

expression after it. If not, it returns a 1 (false).

greater than(>)

The > operator returns 0 (true) if the expression before it evaluates to a higher value than

the expression after it. Otherwise, it returns a 1 (false).

less than or equal to (<=)

The <= operator returns 1 (false) if the expression before it evaluates to a higher value than

the expression after it. Otherwise, it returns a0 (true).

greater than or equal to (>=)

The >= operator returns 1 (false) if the expression before it evaluates to a lower value than

the expression after it. Otherwise, it returns a 0 (true).

equal to (==)

The == operator returns 0 (true) if the expression before it evaluates to the same value as

the expression after it. Otherwise, it returns a 1 (false).

not equal to (!=)

The != operator returns 1 (false) if the expression before it evaluates to the same value as

the expression after it. Otherwise, it returns a 0 (true).

One of the most common arithmetic tasks consists of incrementing or decrementing a variable.

The UNIX shell simplifies this task with increment and decrement operators.

The increment operator (+=) sets a variable to a new value that's equal to its previous value plus any number you specify after the operator.

For example, ((count+=1)) increments the count variable by 1.

Question

What do you think is the output of the following piece of code?((417 <= 28))x=$?((x += 2))echo $x

Options:

1. 4192. 23. 3

Answer

The output from this code is 3.

Option 1 is incorrect. The first line does not return a value of 417. The first line evaluates a conditional expression and returns a value of 0 or 1, depending upon whether the expression evaluates as true or false, respectively.

Option 2 is incorrect. When a conditional expression evaluates as false, a value of 1 is returned. So the value of $x that is incremented by 2 is 1, and when $x is printed it

has a value of 3.

Option 3 is correct. The $? variable is assigned the exit status of the previous

command. In this case, the previous command is the conditional expression that evaluates as false, so the value that is assigned to $x is 1. The next expression

increments the value of $x by 2, and the print statement prints the new value of $x –

3 – to the screen.

The decrement operator (-=) sets a variable to a new value that's equal to its previous value minus any number you specify after the operator.

For example, ((remainder-=1)) decrements the remainder variable by 1.

Question

You have issued the following command to assign values to variables:x=4 ; y=2

Which statements will not yield an error to this command?

Options:

1. $x >= $y2. $x < $y+=4

3. $x == $y4. $x != $y

Answer

$x < $y+=4, $x >= $y, and $x != $y all use comparative and increment operators correctly to construct valid statements.

Option 1 is correct. The >= operator means that the value on the left is equal to or greater than the value on the right of the operator.

Option 2 is correct. The +=4 operator increments the y variable by 4, making it larger than the x variable, so the statement is correct.

Option 3 is incorrect. The x variable is greater than variable y. The == operators means "is equal to", so this statement would result in an error.

Option 4 is correct. The != operator means "is not equal to", so this statement is correct.

Summary

You can evaluate arithmetical expressions using the let command or the ((…)) construct.

The basic arithmetical operators include addition, subtraction, multiplication, division, and modulo operators. These follow the conventional precedence order – with multiplication and division taking precedence over addition and subtraction – unless parentheses override this convention.

The UNIX shell supports logical and bitwise operators that perform Boolean logic on entire expressions or on individual bits of binary numbers.

The UNIX shell also supports comparative operators that compare the values of two expressions.

Table of Contents

| Top of page |


| 1. Performing simple arithmetic |

| 2. Logical and bitwise operations |

| 3. Additional operators |

| Summary |




Using calculations in a UNIX script

Learning objective

After completing this learning object, you should be able to use variables and arithmetic in a shell script.

Exercise overview

In this exercise, you're required to write a piece of code that performs calculations using variables.


creating and assigning variables performing arithmetical calculations

Let's say that you're working for the EasyNomad travel company and you're required to write a script that calculates the amount of commission that agents earn on each package tour they sell.

Task 1: Assigning variables

For the commission script, you need a variable to store the commission percentage that agents receive and another variable to store the amount received.

Step 1 of 2

The commission percentage will be stored as a positive integer.

Type the command that declares variables explicitly and the attribute that specifies a variable to be an integer.

#!/usr/local/bin/kshMISSING CODE

Result

The typeset -i command declares integer variables explicitly.

Step 2 of 2

Let's say that agents receive a commission of 10 percent.

Complete the line of code that creates a variable called commrate and assigns the

value of 10 to it.

#!/usr/local/bin/kshtypeset -i MISSING CODE

Result

You type commrate=10 to create and set the variable.

You repeat the same task to create the amount variable. You don't assign a value to

amount yet because it still needs to be calculated.

#!/usr/local/bin/kshtypeset -i commrate=10typeset -i amount

Task 2: Performing calculations

The script you are writing needs to accept the value of a tour package that an agent sells as its first argument.

Step 1 of 3

Type the command that evaluates arithmetical expressions.

#!/usr/local/bin/kshtypeset -i commrate=10typeset -i amountMISSING CODE

Result

The let command evaluates arithmetical expressions.

Step 2 of 3

Type the code that assigns the value of the script's first argument to the amount

variable.

#!/usr/local/bin/kshtypeset -i commrate=10typeset -i amountlet MISSING CODE

Result

You type amount=$1 to assign the value of the script's first argument to the amount

variable.

Step 3 of 3

Type the arithmetical expression that applies the commission percentage to the value of the amount variable.

#!/usr/local/bin/kshtypeset -i commrate=10typeset -i amountlet amount=$1let amount= MISSING CODE

Result

You type amount*commrate/100 to apply the commission percentage to the value

of the amount variable.

You add a print statement to output the value of the amount variable. The script is now

complete.

#!/usr/local/bin/kshtypeset -i commrate=10typeset -i amountlet amount=$1let amount=amount*commrate/100print amount

Table of Contents

| Top of page |



| Task 1: Assigning variables |

| Task 2: Performing calculations |



| Print | Contents |

UNIX text tools

Abstract

This article discusses the history of UNIX text and format processing tools.

Plain text

Although computer processors act on binary data, binary output is not legible to human beings. Output needs to be in plain text for processed data to be accessible to both the computer and its human operator.

Early text processing

One of the original purposes of UNIX was to develop, maintain, and track documents and memos in the AT&T/Bell Telephone Company in the late 1960s.

The text editors at this time were referred to as line editors because they required all changes to a file to be made on a line-by-line basis. Each command to a line editor specified a line number, and the changes to be applied to that line. The original standard UNIX line-mode text editor – named "ed" – was first compiled in assembly language. It was later rewritten in C, although it retained the same functionality.

Text formatting

To improve the readability of text created in a text editor on UNIX systems – and to print this text, you need a separate program to format it. Text formatters allow you to specify features such as fonts, spacing, paragraphs, margins, and footnotes when displaying or printing documents. The earliest UNIX text formatter was runoff, which was named after print runs in the publishing industry. You can run this formatter using the command roff. Since runoff, next runoff (nroff) and typesetting runoff (troff) have been developed to provide additional features and greater flexibility.

Other text formatters such as SCRIPT for IBM mainframes, Scribe, Digital Standard Runoff (DSR), and TeX have also been developed.

TeX and LaTeX

TeX is a text processor used mainly in scientific and mathematical environments to produce papers, books, and theses. It is especially suited to material with a high mathematical content. TeX provides fonts and font sizes, page layout, mathematical equations, mathematical characters, diacriticals, and alternative character sets. For example, it provides Greek symbols, as well as support for indexing, footnotes, line drawing, diagrams and boxes, multicolumn printing, and tables.

LaTeX is a set of macros based on TeX macros that facilitates the structuring of large documents. It provides automated section and chapter, cross-referencing, and bibliography macros.

Emacs

The Emacs text editor was written mostly in the programming language Lisp. At the core of Emacs, there is a full-featured Lisp interpreter written in C. This built-in programming language allows you to customize and extend Emacs.

Emacs is a visual editor in that it displays a representation of an entire document on screen. This allows you to navigate through the document to edit pieces of text at random. You can also open more than one document at a time and use Emacs windows to move between documents.

You can start up a UNIX shell from inside Emacs. In this case, you receive a UNIX prompt inside the current Emacs window. You can then run commands – including compile commands – as you would outside of Emacs and save the shell output to a file.

Shell programming tools

In addition to the runoff text formatter, Emacs, and TeX, there are a number of text tools that system administrators commonly use in shell programming. The most commonly used tools are

the sed utility the awk language the PERL language

sed

The sed utility is a stream editor that allows you to search for and replace text strings in a file. This allows you to delete lines of text by "replacing" them with nothing, which is especially useful in the case of lengthy system logs and e-mails.

You can execute sed commands at the command line or you can run a command file to replace characters and strings in a specified data file.

In the following example, the sed command deletes the first two lines of the easynomad.txt file:

sed -e '/^line.*one/s/line/LINE/' -e '/line/d' easynomad.txt

awk

Named after its originators, Aho, Weinberger, and Kernighan, awk is a complete pattern-scanning and processing language. It allows you to extract parts of a large body of text, to format text, or to extract information for other programs. However, it is most commonly used as a UNIX command-line filter to reformat the output of other commands, such as PERL commands.

You can run awk commands from the command line as a simple text-processing tool, or you can create a command file containing awk statements.

In the following example, the awk command will remove the second field from each line in a file called profile.

awk {$2="";print} profile

PERL

PERL (Practical Extraction and Report Language) is a portable text, process, and file-manipulation tool that bridges the gap between shell and C programming. Its syntax resembles the syntax you use in shell programs and in C programming. PERL also includes awk, sed, and grep as built-in functions.

PERL was developed originally as a data reduction language capable of processing large quantities of data efficiently.

Summary

Developed to support a documentation project in the AT&T/Bell Telephone Company, ed was the first text editor for UNIX text files. Because the early line editors lacked text-formatting capability, runoff was subsequently developed to improve the readability of printed documents.

More comprehensive text-processing tools such as TeX – which is used in scientific environments – and Emacs – which is used for programming – were developed to improve on the runoff tool.

The three most commonly used text-processing tools are awk, sed, and PERL. Awk allows you to perform pattern-scanning and processing, sed allows you to perform search and replace operations, and PERL is a text, process, and file-manipulation tool.

Table of Contents

| Top of page |

| Abstract |

| Plain text |

| Early text processing |

| Text formatting |

| TeX and LaTeX |

| Emacs |

| Shell programming tools |

| Summary |




UNIX editing with sed

Learning objective

After completing this topic, you should be able to explain how to use the sed line editor.

1. Introducing sed

Unlike text editors that allow you to alter a file by moving the cursor on the screen, sed is a noninteractive line editor. It processes one line of standard input at a time and returns the results as standard output.

In most cases, it processes input via a keyboard and returns output to the screen. It doesn't alter original files.

In its simplest operation, sed works as a filter that searches for a specified pattern and takes a specified action if it matches the pattern.

You often use sed to perform search and replace operations on the content of specified text files. However, you can also use it to append, display, delete, or insert text.

You can perform many of these functions using the vi text editor. However, sed provides a better solution than the vi text editor for working with multiple text files.

Sed applies each editing command to the first line in a file and then moves on to subsequent lines one at a time.

As a result, an entire file is never loaded into memory.

Question

Because sed works with open format files – or plain text files – it is portable and supports a wide range of text applications.

What do you think you can use sed for?

Options:

1. Converting double and triple spacing in a file2. Converting DOS newlines to UNIX newlines3. Deleting leading or trailing spaces and consecutive blank lines4. Performing substitutions on all or specific lines5. Sorting information in a text file

Answer

You can use sed to convert line spacing in a file and to convert DOS newlines to UNIX newlines. You can also use sed to delete leading or trailing spaces and consecutive blank lines, and to perform substitutions on all or specific lines.

You can execute sed operations by

using the -f flag

creating a sed executable file

using the -f flag

You can create a script containing only editing commands, which you can then pass as a file for execution by sed using the -f option.

For example, the code sed -f sample.sed memo.txt causes sed to execute the

commands in a file called sample.sed on a file called memo.txt.

creating a sed executable file

An executable sed script contains the line #!/usr/bin/sed -f, which allows you to run it

through the shell using the ./ command.

However, you need to use the chmod command to give the file execute permissions before

you can run it.

Question

You have stored sed editing commands in a text file named edit.sed.

Which sed command do you execute at the command line to edit the file

reports.txt?

Options:

1. cat edit.sed | sed reports.txt

2. sed –f edit.sed reports.txt

3. sed –f reports.txt edit.sed

4. ./edit.sed reports.txt

Answer

You use sed –f edit.sed reports.txt to edit the reports.txt file using the sed

commands in edit.sed.

Option 1 is incorrect. It is possible to pipe input into sed, but to do so, you would use the following command: cat reports.txt | sed –f edit.sed

Option 2 is correct. You use the –f option to specify the file containing the sed

commands, and then you provide one or more files for sed to operate on.

Option 3 is incorrect. The file containing the sed commands is provided after the –f

option, and not after the file on which sed is to operate on.

Option 4 is incorrect. This command would work if edits.sed were an executable file, but in this example it just contains sed commands.

2. Matching expressions with sed

To specify a sed command in a script or on the command line, you follow the command with

a string that includes the following components:

editing command regular expression and pattern string replacement text

For instance, the code sed 's/^night/day/' substitutes the word "night" with the word

"day" when the word night occurs at the beginning of a line.

Note

The sed utility supports most of the regular expressions that other UNIX text utilities ( such as awk, gawk, and grep ( and UNIX shells support.

The most commonly used regular expressions with the sed command are

^ $ * ? [ ]

^

The ^ expression attempts to match a pattern string with the beginning of a line in a

specified file. If a ^ replaces a line number, for example, it denotes the first line in a file.

For instance, the command sed 's/^Thu /Thursday/' easynomad will substitute Thu

with Thursday only when Thu occurs at the beginning of a line.

$

The $ expression attempts to match a pattern string with the end of a line in a specified file.

For instance, the command sed 's/ $//' easynomad will replace any space character

( indicated by a forward slash ( that occurs at the end of a line.

*

The * expression attempts to match any occurrences of a specified character with

characters in the content of a specified file.

For instance, the command sed 's/[ ]*//' easynomad will delete any spaces in the

file.

?

The ? expression attempts to match one instance of a specified character with the content

of a specified file.

For instance, the command sed 's/?D/4/' easynomad will substitute the first instance

of the letter D with the number 4.

[ ]

The [ ] expression attempts to match the content of the square brackets with a specified

pattern string.

For instance, the command sed 's/[Ee]asy[Nn]omad/EasyNomad/g' easynomad

will substitute uppercase and lowercase instances of the letters "e" and "n" with the word

"EasyNomad."

Question

Sed allows you to use combinations of regular expressions.

See if you can match each sed command with the function it performs.

Options:

1. sed 's/ *$//' test1

2. sed 's/[ ][^ ]*$//' test2

3. sed 's/^$//' test3

Targets:

A. Deletes all trailing spaces from each lineB. Finds all blank lines by identifying each line end that follows immediately after a line

beginningC. Deletes the last word on each line

Answer

The command sed 's/ *$//' test1 deletes all trailing spaces from each line,

sed 's/[ ][^ ]*$//' test2 deletes the last word on each line, and sed

's/^$/ blank line/' test3 finds all blank lines by identifying each line end

that follows immediately after a line beginning.

The -s option to sed substitutes anything matched by the first expression with the

contents of the second expression. In this case, the metacharacter * matches the

preceding character – a space – zero or more times. The $ matches the end of a

line. Because nothing is specified in the second expression, the effect is to delete all trailing spaces from each line.

The -s option to sed substitutes anything matched by the first expression with the

contents of the second expression. The expression [.][^ ]*$ matches any string

that starts with a space, but which contains anything other than a space thereafter, and is at the end of a line. The effect of this is to match every word at the end of a line.

When it is not preceding a class or range, the circumflex (^) matches the start of a line or string. In this example, ^$ matches the start and end of an empty line. To

insert a paragraph separator such as --- you would adapt the expression as follows:sed 's/^$/---/' test3

In addition to the substitute command (s), you can use the insert (i), append (a), and delete

(d) commands with the sed command.

For instance, the command sed '$d' text.txt deletes the last line in a file named

text.txt.

You can preface sed commands with line numbers to specify which lines in the input file are

to be modified by the sed command.

For instance, 2 s/tourist/tourists/ looks for the string "tourist" in line 2 of a specified

file and replaces it with the string "tourists."

In the example sed 9 a\Copyright EasyNomad 2003, the sed command appends the

string "Copyright EasyNomad 2003" after line 9 in a specified file.

If you wanted the string inserted before line 9, you would specify the command sed 9 i\

Copyright EasyNomad 2003.

You can use two advanced sed commands – branching (-b) and testing (-t) – to rearrange

the order in which portions of a script execute.

These commands transfer control to a line containing a specified label. If the label does not appear in the script, control passes to the end of the script.

Branching passes control to the end of the script automatically, but testing requires an s

(substitution) command to change the current line before control is passed.

Note

The UNIX man pages provide a full listing of the sed options, with examples of their usage.

Question

Identify the true statements regarding sed commands.

Options:

1. You can specify the line on which a sed command will operate2. You use the a command to append text in sed

3. You use the i command to insert text in sed

4. You use the –n option to insert new lines

Answer

The a command appends text to the specified location, while the i command inserts

it. You preface sed commands with the specific line on which you want them to

operate.

Option 1 is correct. You can precede your sed command with the number of the line

on which it is to operate.

Option 2 is correct. You use the a command to append text in sed. For example, this

command adds the text "circa 1848" to the end of line 10:sed 2 a\circa 1848

Option 3 is correct. You use the i command to insert text in sed. For example, this

command adds the text "circa 1848" to the start of line 10:sed 2 i\circa 1848

Option 4 is incorrect. You use the –n option to suppress printing of all output except

that explicitly matched by the sed command.

Question

Match the sed regular expression characters with their descriptions.

Options:

1. ^

2. $

3. *

4. [ ]

Targets:

A. Matches a class of charactersB. Matches any number of the preceding characterC. Matches the beginning of a line

D. Matches the end of a line

Answer

You match the start of a line with ^ and the end of a line with $. You use [ ] to

match a class of characters and * to match the preceding character, zero or more times.

The ^ command matches the beginning of a line. In this example, the sed command

will delete any new line that starts with a space:sed '/^ /d'

You match the end of a line with the $ command. For example, this command

changes DOS newlines to the UNIX format:sed 's/.$//'

The * command matches the preceding character, zero or more times. The following sed command will replace the character 2 with 1:sed 's/2*/1/g'

The [ ] command matches a class of characters. For example, the following sed

command will delete any line that starts with a number:sed '/^[1-9]/d'

3. Editing text with sed

Let's say that you have a simple phone list that you need to extract data from using sed

commands.

Suppose that you begin by replacing instances of 100- with (100) so that you can distinguish the area codes from the rest of phone numbers more easily.

To do this, you enter the code shown here.

The output shows that only the first instances of "100-" in each line were changed.

bash -2.05a$ sed 's/100-/(100) /' phonelist.txtName Home_Number Cell_Number E-MailGreg (100) 100-555-1111 100-555-1112 [email protected] (100) 555-2222 100-555-1113 [email protected] 101-555-1111 101-555-2222 [email protected] (100)555-3333 101-555-1114 [email protected] (100)555-4444 101-555-1115 [email protected] -2.05a$

To change all instances of "100-" in each row, you use the g option.

For instance, both Greg and Tanya's cell numbers are now prefixed with "(100)".

bash -2.05a$ sed 's/100-/(100) /g' phonelist.txtName Home_Number Cell_Number E-MailGreg (100) 100-555-1111 (100) 555-1112 [email protected] (100) 555-2222 (100) 555-1113 [email protected] 101-555-1111 101-555-2222 [email protected] (100) 555-3333 101-555-1114 [email protected] (100) 555-4444 101-555-1115 [email protected] -2.05a$

Note

Even though you've used s (substitution) options on the phone list file, its original

content doesn't change. The command to change "100-" to "(100)" , for instance, changes only the data output and not the original text.

Let's say that you want to replace the string "2222" with "5555" and to instruct sed to print

only the lines affected by the change.

Using the p option prints Tanya and Sam's records because these are the only records that

contained the string "2222".

bash -2.05a$ sed 's/2222/5555/p' phonelist.txtName Home_Number Cell_Number E-MailTanya 100-555-5555 100-555-1113 [email protected] 101-555-1111 101-555-5555 [email protected] -2.05a$

The example here shows how you can use the d (delete) option to display a specific number

of lines – in this case, the first three lines ( in a file.

bash -2.05a$ sed 's/4,$d/' phonelist.txtName Home_Number Cell_Number E-MailGreg 100-100-555-1111 100-555-1112 [email protected] 100-555-2222 100-555-1113 [email protected] -2.05a$

You can use the q (quit) option to display lines up to and including a specified line – line 5,

in this example.

bash -2.05a$ sed '5q' phonelist.txtName Home_Number Cell_Number E-MailGreg 100-100-555-1111 100-555-1112 [email protected]

Tanya 100-555-2222 100-555-1113 [email protected] 100-555-3333 101-555-1114 [email protected] 100-555-4444 101-555-1115 [email protected] -2.05a$

Let's say that you want to print all records in the phone list except Sam's record.

To do this, you use the d (delete) option with the specification that all lines beginning with

the word "Sam" be excluded from the output.

bash -2.05a$ sed 's/^Sam/d' phonelist.txtName Home_Number Cell_Number E-MailGreg 100-100-555-1111 100- 555-1112 [email protected] 100-555-2222 100-555-1113 [email protected] 100- 555-3333 101-555-1114 [email protected] 100-555-4444 101-555-1115 [email protected] -2.05a$

You can direct the output of a sed command to a new file.

In this example, the code replaces a 3-digit string with a 2-digit string and saves the output in a new text file named newphone.txt.

bash -2.05a$ sed 's/555/55/g' > newphone.txtName Home_Number Cell_Number E-MailGreg 100-100-55-1111 100-55-1112 [email protected] 100-55-2222 100-55-1113 [email protected] 101-55-1111 101-55-2222 [email protected] 100-55-3333 101-55-1114 [email protected] 100-55-4444 101-55-1115 [email protected] -2.05a$

If you want to execute a number of sed commands on the same text file, it's advisable to

write a script that you can then run against the file.

In this example, the script contains a number of s (substitution) commands.

cat sedscripts/555/55/gs/100-/(100) /gs/101-/(101) /gs/easynomad/EasyNomad/g

When you run a sed script against the phone list text file, it outputs the table with all the

substitutions in the script implemented.

bash -2.05a$ sed -f sedscript phonelist.txtName Home_Number Cell_Number E-MailGreg (100) 55-1111 (100) 55-1112 [email protected] (100) 55-2222 (100) 55-1113 TanyaS@ EasyNomad.comSam 101-55-1111 (101) 55-2222 SamO@ EasyNomad.comDaniel (100) 55-3333 (101) 55-1114 DanielB@ EasyNomad.comNick (100) 55-4444 (101) 55-1115 NicholasN@ EasyNomad.combash -2.05a$

Summary

Sed is a noninteractive line editor, which processes one line of standard input at a time and returns the results as standard output.

To specify a sed command in a script or on the command line, you follow the command with

regular expressions, the pattern string that you want the command to locate, and replacement text for this string.

Sed allows you to perform a range of text edits on plain text files, either as single or multiple command-line instructions or by passing multiple instructions in a script.

Table of Contents

| Top of page |


| 1. Introducing sed |

| 2. Matching expressions with sed |

| 3. Editing text with sed |

| Summary |




Creating a configuration script

Learning objective

After completing this topic, you should be able to use sed in a UNIX shell script to edit a file.

Exercise overview

In this exercise, you're required to create a new configuration file and perform a sed substitution.


redirecting sed output substituting a text string

Task 1: Redirecting sed output

Let's say that you want to adapt the configuration file named "hosts" on the primary GLEESON server for use on the secondary server, without changing the original file.

You decide to use sed to search and replace host addresses and to save the output as a new file called "en_hosts".

Step 1 of 2

Let's say that you want to change all instances of "190" to "192".

See if you can type the sed command that will substitute the 190 string.

sed MISSING CODE

Result

You enter 's/190/192/g' to specify the global substitution that will change all instances of "190" to "192."

Step 2 of 2

Let's say that you want to create a new configuration file containing the changed output.

Choose the code that you think will specify hosts as the input file and en_hosts as the output file.

$ sed 's/190/192/g' MISSING CODE

Options:

1. en_hosts > hosts2. hosts > en_hosts3. en_hosts < hosts

Result

The code that specifies hosts as the input file and en_hosts as the output file is hosts > en_hosts

Option 1 is incorrect. This option uses en_hosts as the input file and hosts as the output file.

Option 2 is correct. The data on the left of the > redirector is written to the file on the right of the redirector.

Option 3 is incorrect. This command would redirect the hosts file as an argument to the command on the left of the redirector. Since en_hosts is a file, this command will generate an error.

Task 2: Substituting a text string

Step 1 of 2

Let's say that you need to change all instances of "gleeson" to "gleeson_assoc" in the en_hosts configuration file.

Choose the code that you think will substitute all the text strings that match "gleeson."

$ sed MISSING CODE

Options:

1. 's/^gleeson/gleeson_assoc/g'2. 's/gleeson_assoc/gleeson/g'3. 's/gleeson/gleeson_assoc/p'4. 's/gleeson/gleeson_assoc/g'

Result

You enter 's/gleeson/gleeson_assoc/g' to specify the substitution that will change all instances of "gleeson" to "gleeson_assoc" in the en_hosts configuration file.

Option 1 is incorrect. This would only replace the string "gleeson" if it appeared at the start of a line.

Option 2 is incorrect. This would replace gleeson_assoc with gleeson, instead of vice versa.

Option 3 is incorrect. The g flag is required to ensure the each instance is replaced,

not just the first. The p flag sends the results of each replacement to standard output.

Option 4 is correct. You use the s command with the g flag to perform global

substitutions. The first term is the search term that is replaced with the second term.

Step 2 of 2

See if you can complete the sed command to output the new configuration information to screen.

$ sed 's/gleeson/gleeson_assoc/g' MISSING CODE

Result

You enter en_hosts to specify the filename of the new configuration file.

Table of Contents

| Top of page |



| Task 1: Redirecting sed output |

| Task 2: Substituting a text string |




Programming with awk for UNIX

Learning objective

After completing this topic, you should be able to explain how to use awk.

1. Introducing awk

Like the sed line editor, awk scans and processes patterns in a text file. However, its extensive programming syntax classifies it as a programming language rather than as a simple text utility.

Awk allows you to perform a variety of data processing tasks, including data extraction, analysis, and reporting.

Note

Awk derives its name from the surnames of its developers, Aho, Weinberger, and Kernighan.

Question

Awk uses common programming constructs, such as conditionals, loops, and arithmetic and string operators.

Which of the following operations do you think awk allows you to perform on a text file?

Options:

1. Viewing a text file as a table of records and fields2. Using variables to manipulate the text as if it were data in a database3. Generating formatted reports4. Performing search and replace operations

Answer

Awk allows you to view text files as tables of records and fields, use variables to manipulate the text as if it were data in a database, and generate formatted reports.

To use the awk command, you need to specify

the input data the awk program or script

the input data

The input data is the data that awk is to process.

You can specify this data as one or more files or you can use standard input from the keyboard after executing the awk command.

You can also provide standard input by piping another program or UNIX command into awk.

the awk program or script

The awk script specifies how the input data is to be processed.

You can specify the awk script as a file passed with the awk command or you can specify it

on the command line.

In addition to the input data and the awk script, you can specify options with the awk

command using the following syntax:

-f program file

program source

-F character

-v var=value

-f program file

You can use the -f option with the name of a program file to specify the file containing the

awk program code to execute. This is an alternative to entering the required source code on

the command line.

program source

You can enter the program source – the code that the awk program needs to execute

against – on the command line. This is an alternative to using the -f option to specify the

name of a program file.

When you enter the program source on the command line, you need to enclose the code in

single quotes to prevent it from being interpreted as shell code.

-F character

You use the -F character option to set the field separator in the input data to a character

other than the default white space.

For instance, to set the field separator that the input data uses to a colon, you enter -F:.

-v var=value

You use the -v var=value option to initialize a variable on the command line. This sets the

specified variable prior to execution.

Question

Identify the statements that correctly describe the use of awk.

Options:

1. Awk only operates on a single data file per command2. You can enter the program code for awk on the command line3. You can initialize variables from the awk command line4. You use the –f option to specify a file containing awk program code

Answer

Awk program code can be entered on the command line or you can use the –f

option to read it from a file. You can initialize variables used by awk from the command line.

Option 1 is incorrect. You can specify multiple files for an awk program to use as input.

Option 2 is correct. You can specify the patterns to be matched and the actions to be taken as a single awk command line statement.

Option 3 is correct. You can use the –v option on the awk command line to name a

variable used in your awk code and assign it an initial value.

Option 4 is correct. If you have a complex awk program that you want to reuse, you can store the program instructions in a text file to read in from the awk command line.

2. Awk building blocks

Awk works by matching a specified pattern with data in an input file and taking a specified action on all lines that contain pattern matches. To specify an awk command, you use the

following basic syntax.

awk [-v var=value] [-F field separator] [--] 'pattern {action}' var=value datafile(s)

You enclose the actions that awk must perform in braces ({}) so that awk can differentiate them from their associated patterns.

A string of characters placed between forward slashes (//) is called a regular expression. When awk encounters any occurrence of that pattern within a line, it will select the line.

To select records on the basis of data in a particular field, you can use a matching operator such as the equals (=) sign. For instance, the code fragment deposit = $5 instructs awk

to use the value in column 5 for calculations involving the deposit.

In addition to regular expressions in an awk command, you can search for patterns using

relational expressions such as greater than (>), less than (<), and less than or equal to (<=).

And you can address both regular and relational expressions in compound forms using the logical OR (||) and logical AND (&&) operators.

The BEGIN statement allows you to specify actions – such as resetting variables or creating

column headings for output – that need to occur before awk starts processing input lines.

The END statement allows you to specify actions – such as printing an ending text line or

calculating cumulative variables such as line counts or column totals – that need to occur after awk has processed the last input line.

The BEGIN statement is called once only before awk reads any data lines.

The END statement is called after awk has read and processed all lines.

If the source program for awk includes multiple files, END is called after awk has finished

processing the last file.

You don't need to specify awk variables before you use them. Rather, you can specify awk variables only when they're needed.

For instance, this code prints the string "EasyNomad" to screen.

bash-2.05a$bash: / : is a directorybash-2.05afor i in terry larry marydo mail $i << END $(date) Have a good holiday $i! END

quitexitclear

{en='EasyNomad'print en;}

Awk provides reserved program variables. You can change some of these variables – such as the default field separator. Awk updates other reserved program variables – such as current record number – automatically.

The most commonly used reserved program variables are

NR or $0

NF or the $1-$n syntax

FS and OFS

RS and ORS

NR or $0

The NR variable denotes the number of the current record, whereas the $0 expression

represents the number of the current record as a single variable.

NF or the $1-$n syntax

Both the NF variable and the $1-$n syntax denote the number of fields in the current record.

FS and OFS

The FS and OFS variables represent the input field separator and the output field separator

respectively.

RS and ORS

The RS and ORS variables represent the input record separator and the output record

separator respectively.

By default, both separators are newline characters, which means that awk interprets each

line in an input file as a separate record. It also appends a newline character to the end of

each record that it sends to standard output.

Awk provides a number of functions, which you can use in awk scripts to return values needed by the script.

These are divided into arithmetic functions – such as cos, exp, int, log, sin, and

rand – and string functions – such as index, length, and match.

Awk allows you to define your own functions for use in multiple scripts by using the function command to specify a list of statements.

For instance, you can write a function that returns values sorted in a specified order.

Awk allows you store sets of values in associative arrays. These arrays are known as associative arrays because you can reference the values they include using a string or an index number.

A value with its index number is known as an element.

You use this syntax to create an array.

array[index] = value

You can use a special for structure with an awk array to loop through the array elements,

using this syntax.

for(elem)action

Question

In terms of awk syntax, what does the $0 expression denote?

Options:

1. The first argument to the awk command2. The first field in a line of input3. An entire line of input4. A null variable

Answer

The $0 expression is an awk field variable that matches an entire line of input.

Option 1 is incorrect. In the syntax of the shell command interpreter, the $0 positional

parameter references a command itself. However, it acquires a different meaning in the syntax of the awk command interpreter.

Option 2 is incorrect. The first field in a line of input is referred to with the $1 field descriptor.

Option 3 is correct. The $0 field descriptor matches every field in a line of input. The default field delimiter is a space, but you can use the –F option to define the field

delimiter.

Option 4 is incorrect. In awk, all variables have a null value until a value is assigned to them.

Question

Which statements accurately describe awk syntax?

Options:

1. If multiple files are called, the END statement is called after each is processed

2. You must declare any variables used in your awk program on the command line3. You use the BEGIN statement to specify actions that take place before processing

any input4. You use the END statement to specify actions that are performed after input is

processed

Answer

The BEGIN statement specifies actions to be taken before processing all input, while

the END statement specifies actions taken after processing all input.

Option 1 is incorrect. Irrespective of the number of files on which the awk code operates, the END statement is called only once, after all the input has been

processed.

Option 2 is incorrect. As is generally true of shell scripting, awk variables can be initialized as you need them.

Option 3 is correct. You can use the BEGIN statement to perform initial operations,

such as creating columns or assigning values to variable, before processing any input.

Option 4 is correct. Once all input has been processed, you can use actions defined in the END statement to manipulate the processed data.

3. Programming with awk

One of the main uses of awk is to find out about the content of a text file.

For instance, you may want to know whether a configuration file contains comments – indicated by a hash sign (#) – and, if so, where they are located in the file.

The code awk '/#/ {print "Comment found "}' /etc/hosts will print "Comment

found" for each line that contains at least one hash sign anywhere in the line.

Let's say that you want to print a text file to a screen that is limited to a width of 80 characters per line. You don't want the lines to wrap.

Before entering complex formatting instructions, you want to check whether there are any lines in the file that exceed 80 characters.

To do this, you use the code awk 'length > 80' hosts.

You can use awk to reformat text files awk for improved readability or for export into other applications.

For instance, the code awk '{print ; print ""}' nomad.txt > nomad1.txt

changes the single line spacing in the file nomad.txt to double-line spacing in a file named nomad1.txt.

In this example, the single quotes allow the use of double quotes in the awk expression.

You use single quotes in this way to hide special characters from the shell.

Awk can process text that has been broken into multiple logical fields. And you can reference individual fields in an awk script.

The code awk -F":" '{ print $1 }' /etc/passwd, for example, prints a list of all

user accounts on the local system.

In the code, the -F option changes the field separators between multiple fields in the

passwd file from the default spaces to colons. When awk processes the print $1

command, it will print out the first field that appears on each line in the input file.

Question

Awk allows you to perform integer and floating point mathematical operations on input files.

What do you think the output of the following script will be?

BEGIN { x=0 }/^$/ { x=x+1 }END { print "I found " x " blank lines. :)" }

Options:

1. The number of blank lines in a file2. The number of non-blank lines in a file3. A printout of all blank lines in a file

Answer

The code will calculate the number of blank lines in a file and return the result to the screen.

Option 1 is correct. Each time a blank line is detected, the awk program initialises the x variable with a value of 0, and increments it by one. The print command sends the

value of x to standard output, accompanied by the text strings enclosed in double

quotes.

Option 2 is incorrect. The regular expression that increments the x variable - ^$ -

matches every blank line, not lines containing characters.

Option 3 is incorrect. The print command only prints the value of the x variable and

some helpful text. The pattern that is matched – in this case blank lines – is not automatically printed.

You can use awk with other UNIX commands, such as cat and grep.

In the example cat text1 | awk '$1 < $2 {print $0, $1/$2}' > text2, the

content of the text1 file is piped to awk for pattern matching. The output of the command will be stored in a new file named text2.

Summary

Awk allows you to perform a variety of data processing tasks, including data extraction, analysis, and reporting.

Awk works by matching a specified pattern with data in an input file and taking a specified action when it locates this pattern.

One of the main uses of awk is to find out about or change the content of a text file. For instance, you can use it to determine whether a file includes comments or to reformat a file for improved readability.

Table of Contents

| Top of page |


| 1. Introducing awk |

| 2. Awk building blocks |

| 3. Programming with awk |

| Summary |




UNIX shell programming principles

Learning objective

After completing this topic, you should be able to explain the principles of logical flow in a shell program.

1. Programming with the shell

The shell is an interface between the user and a UNIX operating system.

You can create shell scripts – consisting of ASCII text files – that the shell interprets and the operating system executes.

The shell supports a set of functions implemented as keywords that provide a simple programming language. You can combine these functions with UNIX command-line programs.

Shell programming is useful for

automating tasks prototyping system administration

automating tasks

A shell program – or script – allows you to specify operations in a certain sequence.

You can use shell scripting to automate tasks that need to be performed in sequence, such

as transferring files using the File Transfer Protocol (FTP) and then e-mailing users once

the files have been downloaded.

prototyping

You can use shell programming to prototype complex applications so as to provide proof-of-

concept implementations and to test logical functions.

Once you've prototyped an application, you can rewrite it in languages such as C++ and

Java to improve its performance and extend its functionality.

system administration

Shell scripts run at system bootup to restore system configurations and to set up services.

They manage the initialization and shutting down of essential system services during

system startups and shutdowns, as well as the transition from one run-level to another.

System administrators can write shell scripts to analyze the behavior of a system and to

modify it, as well as to automate repetitive tasks.

Question

In certain cases, you should use compiled languages such as C, C++, or Java, rather than shell scripting.

For which of the following purposes do you think a compiled language is more appropriate than shell scripting?

Options:

1. Creating applications with cross-platform portability2. Designing proprietary applications3. Improving system performance4. Manipulating graphics or GUIs5. Prototyping an application

Answer

You should use more powerful scripting languages than shell scripting for improving system performance, designing proprietary applications, and manipulating graphics or GUIs.

Question

Match the action to the example.

Options:

1. Automation2. Prototyping3. System Administration

Targets:

A. Developing a proof-of-concept applicationB. Optimization of system initilization scriptsC. Scripting tasks that are performed repeatedly

Answer

Automation involves scripting repetitive tasks, developing proof of concept applications is an example of prototyping, and optimization of shell scripts used to initialize a system is an important part of system administration.

Repetitive tasks can be automated by shell scripts. For instance, if every project you work on has a similar directory structure, you can automate the process of generating the directories for a particular project.

Because shell programs are relatively simple to write, it's often useful to test procedures in the shell before incorporating them into programs written in more sophisticated languages.

Services and system configurations on UNIX systems are usually initialized by shell scripts, so understanding shell programming techniques is essential to optimizing your system.

2. Programming fundamentals

Programming effectively requires that you first determine the solution that a program must provide. You then plan the components that the program requires to do this and determine how the components will fit together to execute the required tasks.

General programming principles that make it easier to write and interpret programs include

writing pseudocode debugging during code construction indenting code for readability

writing pseudocode

To identify the steps that a program needs to include, you should use pseudocode.

Pseudocode consists of an informal written plan that describes what each item of code must

do, together with the programming language components required to achieve the specified

tasks.

debugging during code construction

You should debug a program as you work on it – rather than after completing it – to verify

that each logical section of code works correctly. Ensuring that each section of code works

correctly before you continue programming simplifies the debugging process because it

allows you to isolate incorrect code more easily.

indenting code for readability

You should enter spaces between commands and indent code to make it more readable.

For example, indenting an if then fi statement or a do while loop makes it easier to

verify that you've completed the required code correctly so that subsequent code will

execute.

Core tools available in most programming languages – including shell scripting – include

conditional statements functions loops variables

conditional statements

You use conditional statements to control the flow of statement execution in a program.

You use if then else statements to check a condition and then to perform an action

based on whether the condition evaluates to true or false.

If the condition specified in the if statement in this example evaluates to true, the then

statement executes. If the if statement evaluates to false, the else statement executes to

repeat the request that the user enter a valid number.

functions

Functions – also known as procedures or subroutines – consist of code that implements a

program that you can embed and reuse in a shell script. They allow programmers to create

modular code that is easier to maintain and extend than code implemented as a single

routine.

Information is passed to functions in the form of arguments. A function can perform

operations based on the arguments passed to it, and return values for use in the code from

which the function is called.

In the example shown here, the check_value function ensures that a variable set from

standard input corresponds to an appropriate range of values.

The following code is shown on screen:

echo "Please enter a value less than 10:"

read VALUE

check_value ()

while [ $VALUE –gt 10 ]

do

echo "Please enter a value less than 10:"

read VALUE

done

echo "The value you have chosen is $VALUE"

}

check_value VALUE

loops

You use loops to count through a list and to apply a certain action to each element in that

list. You can also use a loop to test a condition and repeat the process until the condition

returns an exit status of 1 to indicate that it is no longer true.

In this statement, the loop will print "Hello World" the number of times that the user

specifies. If the user enters a value of 5, for example, the code will print "Hello World" while

the count is smaller than or equal to 5.

The following code is shown on screen:

echo "Enter a number"

read NUM

COUNT=1

while [ $COUNT -le $NUM ]

do

print "Hello World"

COUNT=$((COUNT+1))

done

variables

Variables associate a name with a location in memory that contains data. You can use the

name to access and change the data.

Modern shells – such as the Korn and Bash shells – provide for the use of variables with both a local and global scope in shell scripts. You can use the local keyword in a function

to define a variable that is available only to that function. This is useful for maintaining

modularity in complex programs.

When naming a variable, you should ensure that the name isn't used by a function. You

should choose a descriptive name. And a useful convention for shell scripts is to capitalize

variable names.

Question

Identify the statements that describe sound programming practice.

Options:

1. Debug programs once you've completed the code2. Indent code and use whitespace to improve readability3. Use functions to create modular code4. Use pseudocode to plan programs

Answer

It is good practice to plan your programs in psuedocode, to use whitespace to improve the legibility of your code, and to use functions to create modular code.

Option 1 is incorrect. To reduce errors and save time and effort, you should debug code as you write it.

Option 2 is correct. Although use of whitespace and indenting is not a functional requirement, it makes debugging and maintenance considerably easier.

Option 3 is correct. Creating modular code through the use of functions makes your code easier to maintain and reuse, and thereby improving efficiency.

Option 4 is correct. Psuedocode is particularly useful for organizing your approach to complex programming tasks.

Question

Why would you write pseudocode?

Options:

1. To assist with debugging procedures2. To identify the logical steps in a program3. To make your code more readable

Answer

Pseudocode helps you identify the logical steps needed in your program.

Option 1 is incorrect. Pseudocode is not interpreted, so it can't be debugged.

Option 2 is correct. Although pseudocode isn't needed for simple tasks, it is difficult to accomplish complicated programming tasks without first breaking them down into steps.

Option 3 is incorrect. Pseudocode isn't included in your actual program, so it doesn't affect the readability of your code.

Summary

The shell allows users to interact directly with a UNIX operating system. You can use shell scripts to instruct the shell to perform operations in a specific sequence. Shell programming is useful for automating tasks, creating prototype programs, and simplifying system administration.

The core tools for programming include variables, conditional statements, loops, and

functions. General programming principles that you should follow include writing pseudocode, debugging a program as you construct it, and indenting code for readability.

Table of Contents

| Top of page |


| 1. Programming with the shell |

| 2. Programming fundamentals |

| Summary |




Testing conditions in the UNIX shell

Learning objective

After completing this topic, you should be able to use conditional expressions to provide flow control in shell programs.

1. Testing conditions

Programmers frequently need to make the execution of their code dependent on certain circumstances being met.

For instance, if your code needs to append data to a file, it is a good idea to check that the file exists before executing code that tries to append data to the file.

The shell provides a number of tools for testing conditions that simplify the process of building conditional execution into shell scripts.

Question

You perform tests by evaluating conditional expressions.

What do you think are uses of conditional expressions?

Options:

1. Determining specific outcomes2. Evaluating and comparing integers3. Evaluating and comparing strings4. Testing file attributes

Answer

You can use conditional expressions to evaluate and compare integers, evaluate and compare strings, and test file attributes.

You can test conditional expressions using three different forms of syntax:

[ expression ] test expression

[[ expression ]]

In the syntax, expression is the condition that you are testing. You must include a space between the expression and the brackets.

When testing an expression, the command will return an exit status of 0 if the conditional expression is true and an exit status of 1 if it is false.

The [[ … ]] command is preferable to the test and [ … ] commands because it helps

avoid syntax errors.

For instance, when you compare two variables using the test and [ … ] commands and

one of the variables is set to null or is unset, it will return a syntax error.

Furthermore, because wildcard expansion and word splitting aren't automatically applied to expressions in the [[ … ]] command, you're less likely to need to resort to quoting.

For example, you can test whether x is less than or equal to y using the following statements:

[[ $x le $y]]

[ $x le $y ]

test $x le $y

Compound expressions combine two or more expressions using the

double ampersand operator (&&), where && means "and"

double pipe operator (||), where | | means "or"

For example, you can combine two expressions using the syntax shown here.

[[ expression1 && expression2 ]]

Suppose that you want to ensure that two conditions are met before printing a message. In this case, a student's class mark has to be higher than the average mark of the class and higher than 80.

$[[ $class_mark > $class_average && $class_mark > 80 ]] \&& print "Your class mark is very high"

You can use the [[ … ]] syntax together with string operators to test variables.

The following string operators are useful for checking that variables used in a program have been assigned appropriate values:

-n – true if length of string is not zero

-o – true if option is set

-z – true if length of string is zero

The following code fragment executes the set_var function until the $VAR variable is

assigned a value that is not null.

set_var (){echo "Enter a variable"read VAR}while [[ -z $VAR ]]do set_vardone

Question

Identify the statements regarding the test command that are true.

Options:

1. Spaces on either side of the [ symbol are not interpreted by the shell

2. The –n option to the test command returns a value of 1 if the variable being tested

has a null value3. You can use conditional operators within a test command

4. The [[ condition ]] syntax is backwardly compatible with the Bourne shell

Answer

If test –n returns a 1 when testing a variable, then that variable has a null value.

Conditional expressions can form part of conditions evaluated by the test command.

Option 1 is incorrect. The [ and ] symbols are commands, so if you neglect to

include white spaces on either side the shell will return a syntax error.

Option 2 is correct. The shell returns a value of 1 when a condition evaluates as false. The –n option is used to test that a a variable has been assigned a non-null

value.

Option 3 is correct. You can use the && operator to require two conditions to be met, and you use the || operator to require only one of two conditions to be met.

Option 4 is incorrect. The [[ condition ]] syntax is supported by the Korn shell,

but not by the Bourne shell.

2. Conditional statements

You can compare one string with another string using the syntax [[ string1 = string2 ]].

For example, this code tests whether the string "Belize" has been assigned to the $DESTINATION variable and if it has, prints the message "flights available."

[[ $DESTINATION = "Belize" ]] && print "flights available"

You can use the following string operators to compare strings:

= != < >

=

You use the [[ string = pattern ]] syntax shown here to test if a string matches a pattern

constructed using a combination of literals.

Consider the following code:

DOTW=$(date +%A)

[[ $DOTW = S* ]] &&

echo "Weekend!"

In this case, the format options to the date command assign values such as "Saturday" and

"Wednesday" to the DOTW variable. The DOTW variable is then tested to see whether it starts

with an "S", and if it does, the string "Weekend!" is echoed.

!=

You use the [[string! = pattern ]] syntax to test if a string doesn't match a pattern. The syntax

returns a true value if the string fails to match the specified pattern.


DOTW=$(date +%A)

[[ $DOTW != S* ]] &&

echo "Workday!"

In this example, the DOTW variable is tested to see whether it does not start with an "S", and

if it doesn't, the string "Workday!" is echoed.

<

You use the [[ string1 < string2 ]] syntax to test if the characters in string1 comes before

characters in string2 in the dictionary.

The string operators test the lexigraphical order of two strings. A string that appears before

a second string in the dictionary is considered "smaller" than the second string.


SMALLER=$(abc) [[ $SMALLER < bcd ]] && echo "Smaller"

In this code, the string "abc" is smaller than "bcd" and so the code echoes the string

"Smaller."

>

You use the [[ string1 > string2 ]] syntax to test if the characters in string1 comes after

characters in string 2 in the dictionary.


BIGGER=$(bcd) [[ $BIGGER > abc ]] && echo "Bigger"

In this example, the string "bcd" is greater than the string "abc", so the code echoes the

string "Bigger."

You use the following options ( in the syntax [[ option filename ]] ( to test files:

-a ( to test whether a file exists

-d ( to test whether a file is a directory

-f ( to test whether a file is a regular file

-r ( to test whether a file is readable

-w ( to test whether a file is writable

-x ( to test whether a file is executable

-s ( to test whether a file contains any data

-u ( to test whether a file has a user ID bit set for it

To test whether a file named "new" contains data so that you don't overwrite it if it does, for example, you use this statement.

[[ -s new ]] && print "The file contains data"

Additional file options that the Korn shell provides for testing file attributes include

-G -L -O -S

-G

The [[ -G filename ]] command returns an exit status of 0 if the specified file exists and the

group ID of the file matches the effective group ID for the current process.

-L

The [[ -L filename ]] syntax returns an exit status of 0 if the specified file exists and is a

symbolic link. A symbolic link is a directory or file name that you use to change to another

directory without typing its full pathname. Symbolic links can bridge file systems and link to

directories.

-O

The [[ -O filename ]] syntax returns an exit status of 0if the user ID for the specified file

matches the effective user ID for the current process.

-S

The [[ -S filename ]] syntax returns an exit status of 0 if the specified file exists and is a

socket. Sockets are file descriptors and obey file descriptor semantics.

In addition to using options to test file attributes, you can compare files using the following syntax:

[[ filename1 -ef filename2 ]] [[ filename1 -nt filename2 ]] [[ filename1 -ot filename2 ]]

[[ filename1 -ef filename2 ]]

The expression [[ filename1 -ef filename2 ]] evaluates whether two files are the same.

[[ filename1 -nt filename2 ]]

The syntax [[ filename1 -nt filename2 ]] checks if filename1 is newer than filename2.

[[ filename1 -ot filename2 ]]

The syntax [[ filename1 -ot filename2 ]] checks if filename1 is older than filename2.

Question

Suppose that you're writing a program that backs up files. You want to back up a file named "currentfile" and to ensure that the program only overwrites files by this name that are older than the backup version to ensure that the information it contains remains up to date.

What statement do you think you need to use to ensure that only older files are overwritten?

[[ MISSING CODE ]] && print "Overwriting older file"

Options:

1. backupfile -nt currentfile2. currentfile -ef backupfile3. currentfile -ot backupfile4. currentfile -nt backupfile

Answer

You use the statement [[ currentfile -nt backupfile ]] && print

"Overwriting older file" to ensure that the program will overwrite only files

older than the backup version of the file.

To compare expressions involving integers, you use the following integer operators:

-eq -ne -le -ge -gt

-eq

You use the -eq option with the syntax [[ expr1 -eq expr2 ]] to test whether expression1 is

equal to expression2.

To return an appropriate message to users who enter an age of 64, for example, you use

this code.

[[ $age –ne 64 ]] && print " Your age is 64"

-ne

You use the -ne option with the syntax [[ expr1 -ne expr2 ]] to test whether expression1 is

not equal to expression2.

[[ $age –ne 64 ]] && print " Your age is not equal to 64"

You use this code to return an appropriate message to users who enter an age other than

64, for example.

-le

You use the -le option with the syntax [[ expr1 -le expr2 ]] to test whether expression1 is

less than or equal to expression2.

To return an appropriate message to users who enter an age at or below 64, for example,

you use this code.

[[ $age –le 64 ]] && print " Your age is below 65"

-ge

You use the -ge option with the syntax [[ expr1 -ge expr2 ]] to test whether expression1 is

greater than or equal to expression2.

[[ $age –ge 64 ]] && print " Your age is above 64"

You use this code to return an appropriate message to users who enter an age equal to or

above 64, for example.

-gt

You use the -gt option with the syntax [[ expr1 -gt expr2 ]] to test whether expression1 is

greater than expression2.

To return an appropriate message to users who enter an age greater than 64, for example,

you use this code.

[[ $age –gt 64 ]] && print " Your age is above 64"

Question

Suppose that you want to test if a file named "travel" exists.

See if you can identify the command that you think will do this.

#!/usr/local/bin/kshif MISSING CODEthen

Answer

You use the statement [[ -a travel ]] to check if the file named "travel" exists.

Question

Suppose that if the file named "travel" exists, you want to test that it contains data.

See if you can identify the code required to do this.

#!/usr/local/bin/kshif [[ -a travel ]] then print "The file exists" if MISSING CODE then print "The file is not empty" else print "The file is empty" fi else print "The file doesn't exist"fi

Answer

You type [[ -s travel ]] to check that the file contains data.

Read more about conditional testing using the Bash shell.

Footnote

The Bash shell supports the use of single and double square brackets ([] and [[]]) for conditional testing. Double square brackets are a recently added feature, and are slightly more versatile.

The Bash shell supports the following arithmetic test options:

-lt – less than

-le – less than or equal to

-eq – equal to

-ge – greater than or equal to

-gt – greater than

-ne – not equal to

Question

Which operator will return a 0 if you compare two strings with different values?

Options:

1. !=

2. =

3. >

4. <

Answer

The != string operator will cause a test condition to return a value of 0 if the strings being compared are not the same.

Option 1 is correct. If variables X and Y have different values, the test shown here will evaluate as true, as represented by a return value of 0:[[ $X != $Y ]]

Option 2 is incorrect. This operator will return a value of 1 if the two strings have different values.

Option 3 is incorrect. This operator will return a value of 0 if the string on the left of the operator has a greater value than the string on the right of the operator.

Option 4 is incorrect. This operator will return a value of 0 if the string on the right of the operator has a greater value than the string on the left of the operator.

Question

Match the options in the syntax to the description of their function.

Options:

1. [[ -a filename ]]

2. [[ -s filename ]]

3. [[ -x filename ]]

Targets:

A. This syntax tests whether a file contains any dataB. This syntax tests whether a file existsC. This syntax tests whether a file is executable

Answer

The –a option tests a file's existence, the –s option tests whether the file contains data, and the –x option tests that the file is executable.

A test using the –a option on a filename will evaluate as true if the file exists.

A test using the –s option will evaluate as true if the file is not empty.

A test using the –x option will evaluate as true if an executable permission has been

set on the file.

Summary

The Korn shell allows you to test strings, files, and expressions for specific conditions. You can use the [ … ], test, and the [[ … ]] commands to test conditional statements.

The [[ … ]] command is preferable because it results in fewer syntax errors. You can use

compound expressions to test conditions that require the comparison of files, strings, or expressions by using the double ampersand (&&) or the double pipe (||) operators.

You can compare strings, check file attributes and do arithmetic tests with the Korn shell. This allows you to manipulate files, integers, and strings.

Table of Contents

| Top of page |


| 1. Testing conditions |

| 2. Conditional statements |

| Summary |




Using UNIX shell control structures

Learning objective

After completing this topic, you should be able to use flow control commands in shell programs.

1. Flow control

You use conditional statements to control the flow of code execution in UNIX shell scripts.

Conditional statements include statements that use the if then command, the if then

else command, the if then elif command, and the case command.

You use the if then command to execute subsequent commands when a given condition

is true.

You use the if then command with this syntax.

if conditionthencommandsfi

This code, for example, checks that the password that a user enters is "password01". If it is, the conditional statement evaluates to true and the subsequent code executes to display the message "You can proceed".

#!/usr/local/bin/kshecho "Enter password"read passwordif [[ $password = password01 ]] then print "You can proceed"fi

The if then else command allows you to specify that one set of commands must

execute if the condition is true and that another set of commands must execute if the condition is false. You use the if then else command with this syntax.

if conditionthencommandselsecommandsfi

In this example, the statements after the then keyword will execute if the condition for the

if command evaluates to true.

If the condition is false – because a user enters a password other than "password01" – the statements after the else keyword will execute to display the message "Goodbye" and to

exit.

#!/usr/local/bin/kshecho "Enter password"read passwordif [[ $password = password01 ]] then print "You can proceed" else print "Goodbye" exitfi

The if then elif command allows you to specify more than one condition. It executes a

set of commands if the first condition is true and another set of commands if a subsequent condition is true.

If no condition evaluates to true, the statements that follow the else keyword execute.

if condition1thencommandselif condition2thencommandselif conditionthencommandselsecommandsfi

In the example, the elif command allows you to ask the user to re-enter a password if they

typed it incorrectly. The program will then proceed once the correct password is typed.

#!/usr/local/bin/kshecho "Enter password"read passwordif [[ $password = password01 ]] then print "You can proceed" elif [[! $password = password01 ]] then print "Please retype password" read password if [[ $password = password01 ]] then print "You can proceed" else print "Goodbye" exitfiprint "Welcome $USER"

The case command can compare a single value to a number of other values. If the single

value matches one or more specified patterns, the commands associated with the patterns will execute.

case value inpattern1) commandcommand;;pattern2) commandcommand;;patternn) commandcommand;;esac

This Korn shell script illustrates the use of a simple case statement.

The following code prompts the user to supply a value for a variable called password. If the user input for the variable conforms to one of three options, the code performs the specified action. In this case, it prints a suitable message to standard output.

echo "Enter password"read passwordcase $password in admin01 ) print "Administrator rights are assigned" ;; password01 ) print "User rights are assigned" ;; read01 ) print "Read rights only" ;;esac

Question

Suppose that you are writing a menu program that will load a certain program when the user enters a specific number.

See if you can complete the code that allows the user to type a number to execute a program.

#!/usr/local/bin/kshecho "Enter menu number"read menuMISSING CODE 1 ) ./travelpackage ;; 2 ) ./flights ;; 3 ) ./bookings ;;esac

Answer

You type case $menu in to complete the case command that allows the user to

type a number to execute a program.

Question

Suppose that you want to add another menu item to run a new program named "customers".

See if you can type the code that adds this program as number 4 to the menu program.

#!/usr/local/bin/kshecho "Enter menu number"read menu 1 ) ./travelpackage ;; 2 ) ./flights ;; 3 ) ./bookings ;; MISSING CODEesac

Answer

You type 4 ) ./customers to create another menu item that will execute the

program named "customers".

Question

Which conditional statement allows you to execute one set of commands if a condition is true, and another if it is false?

Options:

1. if then fi

2. if then else fi

3. if then elif fi

4. case esac

Answer

To specify the execution of code based on whether a single condition is true or false, you use an if then else fi statement.

Option 1 is incorrect. This statement only executes code if a condition evaluates as true.

Option 2 is correct. This statement executes code after then if the specified

condition is met, and after else if it isn't.

Option 3 is incorrect. This statement is useful if you need to use additional conditions after the elif syntax.

Option 4 is incorrect. This statement is used to execute code based on whether one or more patterns match a particular value.

2. Introducing and using loops

Loops are recursive code that executes repeatedly until a specific condition is met. This process is called iterative execution. Loops can be infinite, in which case they continue to execute indefinitely, so you need to be careful when specifying conditions.

You use loops to perform various tasks in a sequence and to automate system administration.

You use the for command with this syntax to execute commands a specific number of

times.

A word is a piece of a line that the shell has broken up, such as "a", "apple", and "airplane." The commands execute after the do statement and the done statement ends the loop.

for variable in word1 word 2 … wordndo

commandsdone

For example, you can use this program to display a loop number and a loop value for each iteration of the loop.

#!/usr/local/bin/ksh integer loopnum=1for x in a b cdo print "Loop $loopnum: x=$x" (( loopnum+=1 ))done

This is the output of the script.

Loop 1: x=aLoop 2: x=bLoop 3: x=c

You can assign parameters to a loop dynamically, as in this code, which you can use to make one or more files executable.

#!/usr/local/bin/kshfor FILEdo chmod 700 $FILE print "$FILE is executable"done

If you save a file named "exe" and you have a script named "sample" then you can make the sample file executable by typing the command

./mkexe sample

To make one or more files that you own executable, you use the syntax shown here.

./mkexe filename1 filename2 … filenamen

A loop that uses the while command continues to execute until a specified condition is

true.

while condition1docommandsdone

If the exit status of the conditional statement is zero the commands between do and done are executed.

For example, to create directories using the while command you use the code shown here.

#!/usr/local/bin/kshecho "What is the name of the directory"read directoryecho "How many directories do you want to make?"read dirnumbercount = 1while [[ $count -le $dirnumber ]]do mkdir $directory$count (( count+=1 ))done

A loop that uses the until command continues to execute while a specified condition

continues to evaluate to false. In other words, the commands continue to execute until condition1 returns a zero exit status.

until condition1docommandsdone

Question

Suppose that you want to debug a Korn shell script that another person wrote. Tracking the value of variables is an important part of debugging a script. Syntax errors are usually fairly easy to detect, but logical errors may require monitoring how a variable changes during the course of a script's execution.

What do you think the output of this script will be?

#!/usr/local/bin/kshinteger y=3while (( y < 5 ))do print "The value of y is $y" (( y = y + 1 ))done

Options:

1. The value of y is 2The value of y is 3The value of y is 4

2. The value of y is 3The value of y is 4

3. The value of y is 3The value of y is 4The value of y is 5

4. The value of y is 4

Answer

The initial value of y is 3 and the loop executes until the value of y is less than 5. The output is then as follows:The value of y is 3The value of y is 4

Option 1 is incorrect. Although y having a value of 2 meets the conditions of the test on which the loops is performed, the first line of code assigns a value of 3 to y.

Option 2 is correct. The first iteration of the loop prints the current value of y, and then increments it by 1. The second iteration prints the new value, 4, before incrementing it.

Option 3 is incorrect. Only two iterations of the loop take place, because after the second iteration the condition on which the loop takes place is no longer true.

Option 4 is incorrect. The statement that prints the value of y is executed before y is incremented, so the first iteration of the loop prints the initial value of y, which is 3.

Question

Which statements correctly describe the characteristics of loops?

Options:

1. The shell checks the logic of loops to prevent code being executed an infinite number of times

2. Loops execute code while a specified condition is met, or a specified number of times

3. You must specify the number of parameters that you intend to pass to a for loop

4. You use the test command with the while loop

Answer

Loops execute code for each argument supplied, or until a condition is met. If the condition isn't met, the loop will be infinite. You use the test command with the

while loop.

Option 1 is incorrect. Loops can be infinite, so you should take care when specifying conditions on which they take place.

Option 2 is correct. You supply parameters to the for loop on which code is

executed, and you specify a condtion for which while loops take place.

Option 3 is incorrect. You can dynamically assign parameters to the for loop. For

example, you can use command-line arguments to assign them.

Option 4 is correct. You can use the test command to specify a condition, which

while it holds true, will cause a while loop to execute.

Summary

To control the flow of execution in UNIX shell scripts, you use conditional statements. These include statements that use the if then command, the if then else command, the if

then elif command, and the case command.

A loop is a set of commands that executes – or iterates – repeatedly, either while or until a specific condition is met or for a specified number of iterations.

Table of Contents

| Top of page |


| 1. Flow control |

| 2. Introducing and using loops |

| Summary |




Using flow control in a UNIX shell program

Learning objective

After completing this topic, you should be able to use conditional expressions and flow control in a shell program.

1. Exercise overview

In this exercise, you're required to use conditional expressions and flow control in a shell program.


writing a function writing the main script

2. Task 1: Writing a function

Suppose that you want to create a script that will notify a user by e-mail if specific files have been modified during week days.

First, you need to write a Korn shell script that exits if the date is not a week day. To do this, the script must check the date.

If it is a week day, the script must check to see if any files have been modified in a specified directory. If files have been modified, it must e-mail the user. If not files have been modified, the script must terminate.

Question

Let's say that you want to write a function called check_date that determines whether the current date falls on a weekend.

See if you can type the code indicating the function name and the start of the function.

#!/usr/local/bin/ksh#=====================# Function" Check if today is a weekday#====================== MISSING CODE today=$(date '+%A')#======================check_date

Answer

You either enter function check_date { or check_date { to begin the

function definition.

Question

You want to ensure that only files modified on weekdays are mailed to the user.

See if you can supply the operator that will exit the shell script on Saturdays and Sundays.

#!/usr/local/bin/ksh#==================#Function "Check if today is a weekday#======================function today=$(date '+%A') if [[$today == Saturday MISSING CODE$today == Sunday ]] then return=false fi}#=======================check_date

Answer

The operator that will exit the shell script on Saturdays and Sundays is ||.

Question

You want to ensure that the program runs on weekdays

See if you can complete the code that allows you to provide an alternative condition for the if statement.

#!/usr/local/bin/ksh#=====================# Function" Check if today is a weekday#======================function check_date { today=$(date '+%A')

if [[ $today == Saturday || $today == Sunday ]] then return=false MISSING CODE fi}#=======================check_date

Answer

The code that allows an alternative condition, in this case, is else.

Question

The condition you include needs to evaluate to true on weekdays so that the program continues to run on these days.

See if you can complete the code that allows this.

#!/usr/local/bin/ksh#=====================# Function" Check if today is a weekday#======================function check_date { today=$(date '+%A') if [[ $today == Saturday || $today == Sunday ]] then return=false else print "Today is a weekday" MISSING CODE

}#=======================check_date

Answer

The code that allows a true return value iselse return=true

3. Task 2: Using conditional statements

After writing the check_date function to check if the current day is a weekday, you also

write this modify function to check if any files have been modified in the last 24 hours.

#================#Function: checks what files are modified today#================function modify { find /home/vincep –ctime 1 > logfile if [[ ! –s logfile ]] then print "No files were modified" modified=false else print "Files were modified" modified=true fi}

Question

You now want to complete the code so that the program checks to see if any files have been modified in a directory and then outputs any files that have been modified.

See if you can complete the code that allows you to call the modify function.

return=falsecheck_dateif [[ $return == true ]] then print "Finding out if files were modified today" MISSING CODE

Answer

You type modify to call the modify function.

Question

You want to print the list of any files that have been modified using the cat

command.

Choose the flow control statements that will print out the modified files.

return=falsecheck_dateif [[ $return == true ]] then print "Finding out if files were modified today" modify MISSING CODEfi

Options:

1. if [[ ! $modified == true ]]then print "The modified files:"cat logfilefi

2. if [[ $modified == true ]]then print "The modified files:"cat logfilefi

3. while [[ $modified == false ]]doprint "The modified files:"cat logfiledone

4. while [[ $modified == true ]]doprint "The modified files:"cat logfiledone

Answer

The missing code isif [[ $modified == true ]]then print "The modified files:"cat logfilefi

Option 1 is incorrect. The use of the ! operator on the first line means that the

condition evaluates as false if files are modified, whereas you need it to evaluate as true.

Option 2 is correct. The first line tests that files have been modified. If this test evaluates as true, the rest of the if fi statement is executed, printing a list of the

modified statement accompanied by a message.

Option 3 is incorrect. The loop executes if files are not modified, in which case there will be nothing to print. Also, if the condition is true, this loop will be infinite.

Option 4 is incorrect. To avoid infinite looping if the condition is met, you need to include the following command at the end of the code that is executed by the loop:modified=false

You have now created a function that tests whether the current day is a weekday. The shell script then checks if the files in a directory have been modified and outputs these files with the cat command. The shell script exits if it is a weekend or if no files have been modified.

return=falsecheck_dateif [[ $return == true ]] then print "Finding out if files were modified today" modify if [[ $modified == true ]] then print "The modified files:" cat logfile fifi

Table of Contents

| Top of page |


| 1. Exercise overview |

| 2. Task 1: Writing a function |

| 3. Task 2: Using conditional statements |




Functions in UNIX shell programming

Learning objective

After completing this topic, you should be able to use functions to improve the efficiency of program code.

1. Introducing functions

A function is a set of commands grouped together and called by the same name that accepts a number of arguments as input and can return a value.

Functions can be self-contained or contained in shell scripts.

When functions are self-contained, they can be stored in memory and executed at the command line.

For example, this self-contained function will create a directory and then access that directory.

function md { mkdir $1 && 'cd' $1}

When functions are contained within shell scripts, they need to be called in the main body of the shell script.

Functions improve shell programming by

increasing the speed of code execution organizing shell scripts

increasing the speed of code execution

Functions execute faster than scripts because they are loaded in memory. As a result, the

shell doesn't need to read them from disk each time a script references them.

organizing shell scripts

Functions organize shell scripts into modular pieces of code that are easier to develop and

maintain than separate commands. Functions also consolidate redundant sequences of

commands.

For example, you can create a function to print an error message whenever specific errors occur.

function name {commands}

A simple function named quit is called from the main shell program to quit after printing

"Hello world".

#!/usr/local/bin/kshfunction quit { exit}print "Hello world"quit

This code uses a get_time function to return the current time in the format that shell

scripts require.

#!/usr/local/bin/kshfunction get_time { TIME=$(date +%m/%d/%y-%H:%M:%S) printf "$TIME\n"}get_time

You can declare a local variable in a function using the typeset command. All the other

function variables are inherited and shared by the Korn shell script.

Because functions can have local variables, recursion is possible. This reduces the need for extra code.

#!/usr/local/bin/kshx=1function myfunc { x=3 typeset y=4 print "In local function x is $x" print "In local function y is $y"}myfuncprint "Outside local function x is $x"print "Outside local function y is $y"

Note

You can declare functions using the Portable Operating System Interface (POSIX) format shown here to maintain compatibility with the Bash shell. Functions that have this format cannot contain local variables.

The syntax for doing this is

function_name() { commands }

You can use the return command to pass an exit value from a function to the invoking

Korn or Bash shell script.

The return command uses the syntax

return variable

You can use this script to ensure that the user's input is an integer value and then to return this value for further calculation.

This allows you to pass variables to a function and to obtain a return value from the function.

#!/usr/local/bin/kshfunction get_number { print "Input a number between 1 and 9" read numcase $num in +([0-9])) print "Input is numeric" ;; *) print "Non-numeric input, try again" ;;esac}get_numberprint "The number is $num"integer answer=$num*5print "Five times number is $answer"

Question

Why do functions improve the performance of shell programs?

Options:

1. Because they allow you to reuse code2. Because they are loaded into memory3. Because they use control flow structures4. Because they use local variables

Answer

Functions improve the performance of shell programs because they are loaded into memory.

Option 1 is incorrect. While functions do allow you to reuse code, it is the fact that the function's code is copied into memory that improves performance.

Option 2 is correct. When a function is first declared it is copied into memory, from where it is executed whenever a script calls it. This is more efficient than reading the code from disk.

Option 3 is incorrect. Functions don't have to include control flow stuctures.

Option 4 is incorrect. Functions can use local or global variables, and there is no significant performance gain associated with either.

Question

Identify the true statements concerning functions.

Options:

1. Functions always return the exit status of their last command2. Functions can be defined within a shell script3. You can pass arguments to a function4. You can read self-contained functions directly into memory

Answer

Functions can be defined within a shell script or read directly into memory. Usually, functions accept one or more arguments.

Option 1 is incorrect. You can use the return command to specify a return value for

a function.

Option 2 is correct. Functions can be defined at any point in a shell script, as long as it is before they are first called.

Option 3 is correct. It is common for a function to accept arguments, which are supplied after the function name when you call the function.

Option 4 is correct. You can store a function in an ordinary text file and use the .

command to read it into memory.

2. Uses of functions

Functions are a form of command that can be

self-contained used within shell scripts

self-contained

You use self-contained functions to execute a number of commands from the shell.

You need to read these functions into memory so that you can execute them from the

command line.

In the following example, a self-contained function – named get_time – outputs the time in

a specific format.

function get_time {

TIME=$(date +%m/%d/%y-%H:%M:%S)

printf "$TIME\n"

}

used within shell scripts

You use functions within scripts to organize data.

Functions can accept arguments and then return values after processing the data. Because

functions have local variables, they allow recursion.

You normally use functions for commands that are invoked often. For example, you can use

a function to verify that a user inputs the correct data in response to a prompt, as illustrated

in the following code:

#!/usr/local/bin/ksh

function get_number {

print "Input a number between 1 and 9"

read num

case $num in

+([0-9])) print "Input is numeric" ;;

*) print "Non-numeric input, try again" ;;

esac

}

get_number

print "The number is $num"

integer answer=$num*5

print "Five times number is $answer"

You use the . command to read self-contained functions into memory.

For example, to read in the function error_message, you type . error_message.

Once you read a function in, you can invoke it from a shell script using the syntax

function shell_script

In the Korn shell you can export functions using the typeset –fx functionname command.

For example this command will export the get_number function.

function get_number {print "Enter a number from 1-9"read numcase $num in ([0-9]) ) print "Thanks" * ) print "Try again!"esac}typeset –fx get_number

You can autoload functions to improve performance. In this case, the shell reads in the functions when they are first invoked and keeps them in memory, rather than reading them in each time they are invoked.

You generally autoload functions that are not invoked frequently.

To autoload a function, you first need to set the FPATH variable with the name of the

directory that stores it.

The shell searches each directory in the FPATH variable from left to right for a filename that

matches the name of the function. When it finds the appropriate file, it reads and executes it in the current environment.

To autoload a function, you use this syntax.

typeset –fu function_name

Discipline functions manipulate variables.

You can define these functions, although you don't call them specifically in shell scripts. Rather, a script calls a discipline function when it accesses the variable associated with the function.

For example, the shell calls the discipline functions time.get, time.set, and

time.unset when the variable time is accessed, set, or unset respectively.

Question

Suppose that you have written a shell script to ensure that users enter an integer value for use in a calculation.

What do you think the output of this script – excluding interactive prompts – will be if a user first enters a text value and then the value 6?

#!/usr/local/bin/kshfunction get_number {print "Enter a number from 1-9"read numcase $num in ([1-9]) ) print "Thanks" ;; * ) print "Try again!" ;;esac}while [ $num != [1-9] ]do get_numberdone((square=num*num))print "$num squared is: $square"

Options:

1. Thanks6 squared is 36

2. Try again!

3. Try again!Thanks6 squared is 36

4. Try again!6 squared is 36

Answer

The output of the script will be as follows:Try again!Thanks6 squared is 36

Option 1 is incorrect. The user first enters text, which matches the "catch-all" pattern in the case statement:*) print "Try again!"

Option 2 is incorrect. The while loop ensures that the get_number function is

executed repeatedly until the user supplies a value between 1 and 9.

Option 3 is correct. Once the user has supplied a valid value for the num variable, the

while loop exits because its condition is met, and the code that prints the square of

the num variable is executed.

Option 4 is incorrect. The get_number function provides feedback when a suitable

variable has been entered.

Summary

Functions can be self-contained or contained in shell scripts. Functions improve shell programming by executing faster than separate commands and by organizing shell scripts into manageable modules of code. You declare functions using the Portable Operating System Interface (POSIX) format to maintain compatibility with the Bash shell.

You can read a function in and then invoke it from the shell. After autoloading a function, the shell reads it in when it is invoked rather than each time a Korn shell script invokes it. To allow the autoloading of functions, you need to set the FPATH variable with the pathnames

to files for autoloading functions.

Table of Contents

| Top of page |


| 1. Introducing functions |

| 2. Uses of functions |

| Summary |




Using I/O in UNIX shell programs

Learning objective

After completing this topic, you should be able to use the UNIX I/O system in shell programs.

1. Understanding I/O control

Each program you run from the shell opens files that provide the primary means of communication between it and other processes or programs.

These files exist for as long as the program runs.

The files that active programs open are

standard input standard output standard error

standard input

The standard input file provides a way to send data to a program. By default, the standard

input is read from the terminal keyboard.

standard output

The standard output file provides a means for the program to output data. By default, the

standard output goes to the terminal display screen.

standard error

The standard error file is where the program reports any errors encountered during

execution. By default, the standard error goes to the terminal display.

You need to use file descriptors with some redirectors – such as the exec command, which

opens specified files.

The Korn shell automatically assigns the most common file descriptors, which are

0, to identify standard input (stdin) 1, to identify standard output (stdout) 2, to identify standard error (stderr)

If you want input to go to standard error, you use the following syntax:

input 2>standard_error_output

You can control program input/output (I/O) – including the source that a program uses for input and where it sends output and standard error – using redirection.

You can use the following redirection commands:

>, to redirect standard output to a file >>, to append standard output to a file >|, to overwrite an existing standard output file even if noclobber is enabled

<, to redirect standard input from a file <>, to redirect standard error

You can instruct the shell to use the content of a file as input. For instance, you can include the instruction <text1 in a shell command to instruct the shell to read input from a file

called text1 instead of from the terminal keyboard.

In the example more < /etc/passwd, the shell sends the content of the password file to

the more command for display on the screen.

You use a "here" document to redirect multiple lines from standard input to a command. To do this, you redirect the standard input using a marker string ( most commonly, EOF (end-of-

file).

The shell reads the input to the command until it encounters a line that contains only the marker string. All the lines it has read – except for the marker string line – are then fed to the command as its standard input.

In the example cat > script4 << EOF, the shell reads standard input for cat until it

reaches a line containing only the string EOF. It then creates a file called script#4.

The syntax for a here document is

command<<word

For instance, in a script that sends e-mail to a list of users named Terry, Larry, and Mary, the here document feeds instructions to the mail command, line by line, until it encounters

the specified word – in this case, END.

Question

You can include user interactivity in a shell script.

What command do you think transfers the user's response to memory so that the input can be used for processing?

Options:

1. echo

2. print

3. quote

4. read

Answer

The read command transfers the user's response to memory so that it can be used

for processing.

You can modify the behavior of the read command using the following options:

-p -r -s -un

-p

The -p option instructs the shell to read an input line from a coprocess.

-r

The -r option forces the shell not to interpret the backslash (\) as a line continuation

character.

-s

The -s option causes the shell to save a copy of an input line in the command history file.

-un

The -un option causes the shell to read a specified input line from the specified file

descriptor.

The print command displays the content of a specified string – the prompt you want the

user to respond to – on screen.

You can modify its behavior using the following options:

- -n -p -r -R -s -un

-

The - option causes the shell to interpret all strings following the hyphen as an argument,

even when the string itself begins with a hyphen.

-n

The -n option prevents the shell from adding an ending newline to the output.

-p

The -p option causes the shell to redirect arguments to a coprocess.

-r

The -r option causes the shell to ignore the backslash (\) escape conventions.

-R

The -R option causes the shell to ignore the backslash (\) escape conventions, as well as

preventing it from interpreting arguments preceded by a hyphen as options.

-s

The -s option causes the shell to redirect specified arguments to the history file.

-un

The -un option causes the shell to redirect arguments to the specified file descriptor.

You can direct the flow of output from a command to a file instead of to the default terminal screen.

For instance, the code ls /tmp > ~/ls.out creates a file containing the output – in this

case a listing of the /tmp directory – of the ls command into a new file called ls.out.

Note

If the file named ls.out already exists, the old version will be overwritten. To prevent this, you can append content to the end of a file using the >> command.

Instead of displaying errors on screen, you can redirect a program's standard error to a file or command.

For instance, in the Korn shell you could sort the /etc/passwd file, place the results in a file called prog_errors, and trap any errors in a file called log_errors using the command

sort < /etc/passwd > prog_errors 2> log_errors

Question

Match the redirection operator to its specific function.

Options:

1. >>2. >3. >|

Targets:

A. Appends standard output to an existing fileB. Redirects standard output to a fileC. Redirects standard output to a file, overwriting an existing file even if the noclobber

option is set

Answer

The > redirects standard output to a file. If the file already exists and you want to overwrite it despite the noclobber option being set, you use the >| operator. You

append standard output to a file with the >> operator.

You can append the output from the ls command to a file named dirlist.txt

with this command:ls >> dirlist.txt

You can redirect the output from the ls command to a file named dirlist.txt

with this command:ls > dirlist.txt

Even if the noclobber option is set, you can redirect standard ouptut to overwrite

an existing file called dirlist.txt with this command:ls >| dirlist.txt

Question

Which option for the read command do you use if you don't want the backslash (\) to

be interpreted as a special character when supplied as input?

Options:

1. -p

2. -r

3. -s

4. -u

Answer

You use the –r option to prevent the backslash (\) being interpreted as a special

character in input supplied by the read command.

Option 1 is incorrect. The read –p option is used to accept input from a co-process

spawned using the |& operator.

Option 2 is correct. If the option is not inserted, the backslash causes special characters to be treated as literals.

Option 3 is incorrect. The read –s option will cause input to be saved in the

command history file.

Option 4 is incorrect. The read –u option is used to specify the file descriptor used

to provide input. The default is standard input, as in read –u 0.

2. Programming with I/O control

Let's say that you want to write a program to automate file transfer across a network for users.

During planning, you decide to divide the program into two logical sections. The first section will gather user data ( such as a user's username and password ( and the second section will use this data to connect to a server and transfer local files to the server.

A user may need to transfer a file to an anonymous FTP site – which doesn't require the user's username or password – or to a secure site – which does require authentication.

So your code first needs to determine how the user intends to transfer a file.

Using an echo command, you start interaction with the user by printing the question Do

you want to perform an anonymous ftp [y/n]: to the screen.

You then use the read command to create a new variable called ANON to place the user's

reply – yes or no – in memory.

To ensure that the value that a user enters is either y or n, you create an echo … read

loop inside a while … do statement.

Each time a user enters a value other than y or n, the commands in the loop execute to

prompt the user for a correct response. The loop exits only when the user enters a correct value, which the program then reads into memory as the value of the ANON variable.

To cater for the two possible values of the ANON variable – yes or no – you need to use an

if … else statement to create two conditions.

If the value of ANON is yes, the statement will create two new variables – FTPLOGIN and

PASSWD. The value of FTPLOGIN will be "anonymous" and PASSWD won't contain a value.

echo "Do you want to perform an anonymous ftp [y/n]:read ANON

while [[ $ANON != y && $ANON != n ]]doecho "Enter y or n, please: "read ANONdone

if [[ $ANON != y ]]thenFTPLOGIN=anonymousPASSWD=" "elseecho "Enter the username to use: "read FTPLOGINecho "Enter the password to use: "read PASSWDfi

Question

If the FTP session that a user requests is not anonymous, the user needs to provide authentication details to log in to the remote server.You decide to use two echo … read statements to allow the user to supply these

values.

Which variables do you think you need to specify in the statements?

Options:

1. ANON

2. FTPLOGIN

3. PASSWD

4. A new variable to store the user's login details

Answer

You need to specify the FTPLOGIN and PASSWD variables to retrieve the user's

username and password.

Once the program has obtained user authentication details, it needs to prompt the user to supply the address of the server to which a file is to be transferred and the name and location of this file.

To make the program do this, you use two print … read statements, in which you create

two new variables – FTPSERVER and FILE – to contain the new information.

echo "Do you want to perform an anonymous ftp [y/n]:read ANON

while [[ $ANON != y && $ANON != n ]]doecho "Enter y or n, please: "read ANONdone

if [[ $ANON != y ]]thenFTPLOGIN=anonymousPASSWD=" "

print "Enter the address of the FTP server: "read FTPSERVER

print "Enter the file to transfer: "read FILE

To allow the program to establish an FTP session with a server, you need to specify the ftp

commands that the user's local computer must use to make the actual file transfer.

To do this, you write a here document, which terminates itself and the FTP program when it reaches the END marker.

(ftp -n <<- ENDopen $FTPSERVERquote USER $FTPLOGINquote PASS $PASSWDbinarycd /home/greglput $FILEEND)

Let's say that you want to test your program by anonymously sending a file called case.sh to a server at the IP address 190.100.2.2.

You follow the screen prompts and the file is transferred without error.

$ ./ftp.shDo you want to perform an anonymous ftp [y/n]:yEnter the address of the FTP server:190.100.2.2

Enter the file to transfer:case.sh$

Question

You are writing a script to automate the process of adding a user account to the system.

Identify the command that completes the code that obtains an account name from standard input.print "Enter the account name: "Missing_code NAME

Options:

1. echo

2. read

3. typeset -x

4. <<

Answer

You use the read command to obtain the account name from standard input.

Option 1 is incorrect. The echo command will merely print the string NAME to the

screen.

Option 2 is correct. The read command will provide a prompt, and input at the

prompt will be assigned to the NAME variable.

Option 3 is incorrect. typeset –x is used to export a variable, not to receive input.

Option 4 is incorrect. The << operator is used to open a here document.

Summary

Programs that run from the shell open files that provide the primary means of communication between it and other processes or programs.These files are standard input, standard output, and standard error. You need to use file descriptors with some redirectors. You can direct the flow of output and errors from a command to a file instead of to the default terminal screen.

You can write programs that use input/output (I/O) control. For example, you can create a

program that automates file transfer across a network for users. This program gathers data and then uses this data to connect to a server and then transfer local files to the server.

Table of Contents

| Top of page |


| 1. Understanding I/O control |

| 2. Programming with I/O control |

| Summary |




Using menus in UNIX shell programs

Learning objective

After completing this topic, you should be able to create user menus with the select command and use them in shell programs.

1. Introducing the select command

The select command allows you to create a simple menu that users can use to interact

with a program. The menu contains numbered items and displays a prompt message.

Note

The select command is available only in the Korn shell and version 1.14 and later

of the Bash shell.

You specify a select command using the syntax shown here. In the syntax, you need to

supply a list of menu item names, followed by a do … done statement loop. The command

will associate the names you specify with the commands contained in the loop.

select variable in name1 name2 … namendo

commandsdone

Question

The select command generates a menu containing each specified item in a

numbered list.

See if you can place the steps that the select command performs when a user

selects a menu item in the correct sequence.

Options

Option Description

A Executes the statements in the body of the command

B Prompts the user for a number

C Breaks on encountering a break, exit, return, or EOF marker

D Stores the selected option in a built-in variable named REPLY

Answer

Correct ranking

Option Description

B Prompts the user for a number

D Stores the selected option in a built-in variable named REPLY

A Executes the statements in the body of the command

C Breaks on encountering a break, exit, return, or EOF marker

2. Creating menus

Let's say that you want to write a shell script that allows users to view the files in a directory, run a text editor, or exit the script.

Whenever a shell displays a menu prompt, it uses a default prompt character stored in a shell variable named PS3.

You decide to change the default menu prompt ( which is #?, meaning "which number?" ( to a new prompt that you specify.

To do this, you set the PS3 variable to the text you want to appear as the prompt.

#!/usr/local/bin/kshPS3="Select an option [1-3]> "

You name the menu items "Show files", "Run editor", and "Exit", in the order in which they are to appear on screen.

When the user selects an option, its corresponding value is recorded in the user-defined OPTION variable and stored in the built-in REPLY variable.

#!/usr/local/bin/kshPS3="Select an option [1-3]> "select OPTION in "Show files" "Run editor" "Exit"

The do … done statement in the select command needs to specify actions that

correspond to a user's selection of each of the three specified menu items.

In this case, you first specify that if the user chooses the third menu option ( the Exit option for quitting the menu ( the select command will execute the exit command. You do this

using an if ... else statement.

#!/usr/local/bin/kshPS3="Select an option [1-3]> "select OPTION in "Show files" "Run editor" "Exit"doif [[ $REPLY = 3 ]] then exit

Question

Let's say that you want to test the menu at this point before adding functionality to the first two menu items.

What command do you think you can use to insert dummy text for these options?

Options:

1. echo

2. read

3. select

4. quit

Answer

You can use echo statements to print dummy option text to test the functionality of

the menu.

To test the code at this point – without adding functionality for options 1 and 2 ( you add an echo statement that is printed if a user selects any item other than the third menu item.

#!/usr/local/bin/kshPS3="Select an option [1-3]> "select OPTION in "Show files" "Run editor" "Exit"doif [[ $REPLY = 3 ]] then exitelse echo "You chose to $OPTION"fidone

When you execute the select command, it generates a menu with labeled options and the

changed prompt text you've specified.

$ ./select.sh1) Show files2) Run editor3) ExitSelect an option [1-3]>

Let's say that you test the menu by selecting option 1.

This returns the message "You chose to Show files" and returns to the menu.

$ ./select.shShow filesRun editorExitSelect an option [1-3]>You chose to Show filesSelect an option [1-3]>

The example here shows a dummy menu generated by a select command that uses the

case command to structure the menus, instead of an if … else statement.

When you use the case command, you need to number the menu items manually with

echo statements and you need to terminate each command line with a double semicolon

(;;).

#!/usr/local/bin/kshecho "1. Menu item one"echo "2. Menu item two"echo "3. Menu item three"

echo "Choose an option [1-3]"read MENUITEMcase $MENUITEM in 1 ) echo "Do action 1" ;; 2 ) echo "Do action 2" ;; 3 ) echo "Do action 3" ;;esac

The output of the dummy menu using the case command is shown here.

$ ./case.sh1. Menu item one2. Menu item two3. Menu item threeChoose an option [1-3]

Question

Identify the true statements about the select command.

Options:

1. The prompt used by the select command is determined by the PS3 built-in

variable2. The select command can be combined with the case command

3. The select command is available in the Korn and C shells

4. The select command uses the REPLY variable to store user input

Answer

The PS3 variable determines the prompt used by the select command, and

select command input is stored in the REPLY variable. The select and case

commands are often combined.

Option 1 is correct. The shell's interactive prompt is determined by the PS3 variable.

The Korn shell default is #?

Option 2 is correct. The case command can be used instead of if then else

statements to execute code based on the user's menu selection.

Option 3 is incorrect. The select command is supported by the Korn shell and

recent versions of the Bash shell, but not the C shell.

Option 4 is correct. The select command presents the user with a numbered menu

and a prompt. The value entered by the user is stored in the REPLY variable.

Summary

You can use the select command in scripts to create basic menus. The select

command is available in the Korn shell and in version 1.14 and later of the Bash shell.

The case command provides similar functionality, but is more globally applicable. Both the

case and select commands allow you to specify behavior based on the value of a

variable.

Table of Contents

| Top of page |


| 1. Introducing the select command |

| 2. Creating menus |

| Summary |




Writing a UNIX shell program using I/O commands

Learning objective

After completing this topic, you should be able to write an interactive program that makes use of standard input and output.

Exercise overview

In this exercise, you're required to create a shell script that generates a simple menu.


specifying the menu items adding functionality to the menu

Task 1: Specifying menu items

Let's say that you want to create a program that allows users to view files in the current directory, edit a specified file, or quit the program using a simple menu.

$ ./editor.shView filesLaunch EditorQuitChoose a menu item [1-3]>

Step 1 of 2

Let's say that you want to specify the menu prompt "Choose a menu item [1-3]>."

See if you can type the code that will change the prompt.

#!/usr/local/bin/ksh#MISSING CODE "Choose a menu item [1-3]>"

Result

You enter PS3= to change the prompt text.

Step 2 of 2

See if you can type the command that will generate a menu.

#!/usr/local/bin/ksh#PS3="Choose a menu item [1-3]>"MISSING CODE ITEM in "View files" "Launch Editor" "Quit"

Result

You enter the select command to generate a menu.

Task 2: Adding functionality to options

Let's say that you now want to specify the commands that the select command must call when a user selects one of the menu items.

#!/usr/local/bin/ksh#PS3="Choose a menu item [1-3]>"select ITEM in "View files" "Launch Editor" "Quit"do

Step 1 of 4

Choose the most appropriate code to start structuring the menu.

#!/usr/local/bin/ksh#PS3="Choose a menu item [1-3]>"select ITEM in "View files" "Launch Editor" "Quit"doMISSING CODE

Options:

1. case REPLY in

2. case $REPLY in

3. case ITEM in

4. case $ITEM in

Result

You enter case $REPLY in to specify the menu structure.

Option 1 is incorrect. REPLY is a variable returned by the select command, so you

need to use the $REPLY syntax to expand the variable.

Option 2 is correct. The case command executes code based upon the value of a

variable – in this case the REPLY variable returned by the select command.

Option 3 is incorrect. The ITEM variable needs to be expanded using the $ITEM

syntax. Furthermore, it is more efficient to use the REPLY variable.

Option 4 is incorrect. Although this is a possible solution, it requires you to specify each possible value for the ITEM variable listed in the select command. Using the

REPLY variable, which contains a number assigned to each item in the menu that is

created by the select command, reduces the likelihood of typographical errors.

Step 2 of 4

Choose the code that you think will retrieve a listing of the current directory if a user selects the first menu item, and will allow to then choose other menu options.

#!/usr/local/bin/ksh#PS3="Choose a menu item [1-3]>"select ITEM in "View files" "Launch Editor" "Quit"

docase $REPLY inMISSING CODE

Options:

1. 1)echo "The current directory is $(pwd)"ls ;;

2. 1)echo "The current directory is (pwd)"ls ;;

3. 1)echo "The current directory is $(pwd)"lsbreak ;;

Result

The code that will retrieve a listing of the current directory is shown here:1)echo "The current directory is $(pwd)"ls ;;

Option 1 is correct. The pwd command provides the name of the current directory as

output. Command substitution is used to insert the output into the string that is sent to standard output by the echo command. The ls command then lists the contents

of the current directory.

Option 2 is incorrect. To perform command substitution, the brackets that contain the command must be preceded by the $ special character.

Option 3 is incorrect. The break command will exit the case loop, causing the

program to end once the user has viewed the contents of the current directory.

Step 3 of 4

Choose the code that you think will specify that the second menu item is to load a specified file in the vi editor.

#!/usr/local/bin/ksh#PS3="Choose a menu item [1-3]>"select ITEM in "View files" "Launch Editor" "Quit"docase $REPLY in

1) echo "The current directory is $(pwd) ls ;;MISSING CODE

Options:

1. 2)echo "Name the file to edit"read $FILEvi FILE ;;

2. 2)echo "Name the file to edit"read FILEvi $FILE ;;

Result

The code that will load a specified file in the vi editor is shown here:2)echo "Name the file to edit"read FILEvi $FILE ;;

Option 1 is incorrect. This command will open the vi editor with a file named FILE in

the edit buffer. To achieve the desired result of expanding the FILE variable set

using the read command, you need to perform variable expansion using the $FILE

syntax.

Option 2 is correct. The 2) denotes that the subsequent code is executed if

$REPLY=2. The echo command provides a prompt. The read command allows the

FILE variable to be set using standard input. Once it has been set, the value the

FILE variable is used as an argument to the vi editor.

Step 4 of 4

Choose the code that you think will exit the menu if a user selects the third menu item.

#!/usr/local/bin/ksh#PS3="Choose a menu item [1-3]>"select ITEM in "View files" "Launch Editor" "Quit"docase $REPLY in1)

echo "The current directory is $(pwd) ls ;;2) echo "Name the file to edit" read FILE vi $FILE ;;MISSING CODE

Options:

1. 3);;

2. 3break ;

3. 3)break ;;

Result

The code that will exit the menu if a user selects the third menu item is3)break ;;

Option 1 is incorrect. This code will simply result in nothing being executed when the third option is selected – but the loop will not be exited and the user will be prompted to choose a menu item again.

Option 2 is incorrect. Each case statement must end with two semicolons, not one.

This code will result in a syntax error, crashing the program.

Option 3 is correct. The break command is used to exit a case, while, or for

loop.

When you have specified the functionality for each menu item, you end the case and do

commands and print a statement to screen.

#!/usr/local/bin/kshPS3="Choose a menu item [1-3]>"select ITEM in "View files"\ "Launch Editor"\ "Quit"do case $REPLY in 1) echo "The current directory is $(pwd) ls ;;

2) echo "Name the file to edit>" read FILE vi $FILE ;; 3) break ;; esacdone

Table of Contents

| Top of page |



| Task 1: Specifying menu items |

| Task 2: Adding functionality to options |




Writing legible UNIX shell scripts

Abstract

This article describes recommended practices for writing human-readable shell scripts, including the use of comments, white space, and indentation.

Introduction

In addition to ensuring that your shell scripts contain accurate code, it's a good idea to lay them out so that human readers can read them easily.

You can use blank lines and indentation to indicate the structure of the script, and you can give meaningful names to variables and functions so that the reader can understand their purposes. These techniques make it easier for you to debug the script after you've written it, and they allow other shell programmers to read and understand your script.

Variable names

If you name variables and functions after the tasks they perform, it becomes much easier for readers to figure out what's going on in a script. This may require that names be quite long, but they will be more transparent.

Note that dots (.) and underscores (_) don't split a name, so they're a good way to build names that contain multiple words. If, for example, you need a variable to contain the name of a customer, you could call it customer_name. A function that reads customer data could

be read.customer.data. This allows you to create similar variables – such as

package_name and read.flight.data – without causing any confusion.

Comments

The shell disregards any line in a script that begins with a hash (#). This allows you to insert comments in a script. Although comments aren't essential to the functioning of a script, it's a good idea to insert them liberally so that other users can work out what each part of your script is for.

At the beginning of the read.customer.data function, for example, you should insert a

comment like the following:

# This function reads a customer's name and contact details from

user input.

It's also a good idea to insert a comment at the beginning of a script. This comment needs to explain the general purpose and structure of the script. For example, an appropriate comment might be

# This script obtains data about one or more new bookings using input functions and then inserts the data as a new entry in the

bookings table.

When you create a variable, you can add a comment that explains its function. This is especially useful if the variable is an important global variable or if it performs a complex function in a script. For example, an appropriate comment for explaining a specific variable is

bookingID=$2 #unique ID number for the new booking

White space

Except for spaces in individual command lines, the UNIX shell ignores white space in scripts. You can insert empty lines and spaces before lines of code without affecting the running of a script.

This is useful if you want to make a script more readable by separating functions, both from each other and from the main script. You can split the main script at any point you choose – for example when one task has been accomplished and another begins. There's no such thing as a script with too much white space – empty lines can only make a script more readable and reduce eye strain for the reader.

Indentation

Because the shell doesn't respond to initial spaces at the beginning of a line, you can indent lines by a number of spaces. You should do this for any lines enclosed in a loop or an if

statement, so that the beginning and the end of the statement are obvious to the reader.

This example of an if statement shows sensible use of indentation:

if [$bool1 = n] then rmdir /shared/$tour echo "Rerun add_tour to start again." exit else touch /shared/$tour/status$tour chmod 666 /shared/$tour/status$tour

fi

Summary

When writing a script, you should ensure that it's readable to human readers. To help human readers understand the script, you should use variable and function names that reflect the purposes of the corresponding variables and functions. You can also make the structure of the script easier to understand by using blank lines and indentation. You should insert explanatory comments wherever necessary so that readers know what each function, variable, or script is for.

Table of Contents

| Top of page |

| Abstract |

| Introduction |

| Variable names |

| Comments |

| White space |

| Indentation |

| Summary |




Debugging and error-handling UNIX shell scripts

Learning objective

After completing this topic, you should be able to explain how to debug shell scripts.

1. Troubleshooting principles

Once you've written a shell script – unless it's a very simple one – it's likely to contain a few bugs. The more complex a script is, the more time you'll have to spend debugging it.

For this reason, it's very useful to structure the script in a way that makes it easy for you to make sense of it.

You can reduce the effort of debugging by carefully examining your script at the design stage.

Question

What do you think you need to keep track of when you analyze a script at the design stage?

Options:

1. Arithmetic substitution2. Function calls and arguments3. Program flow4. Variable values

Answer

You need to analyze the logical flow of your script. For example, you need to be aware of the functions that should be called at each stage and of which arguments

these functions should use. You also need to keep track of variables you've declared, including where they are used and what values they're likely to have.

Once you've sketched the program flow for a script and have eliminated any logical problems at the design stage, you write the script code.

Then you try running the script. If it requires user input, you should test a wide range of input values ( including completely inappropriate values ( to see how the script responds.

When testing a script, you may encounter the following types of errors:

logical errors resource errors syntax errors

logical errors

Logical errors occur as a result of errors in program flow. For example, a script may fail to

terminate because it's caught in a logical loop, or it may be passing a string to a function

that performs integer arithmetic.

Copying file file @a to backup directory…




resource errors

Reference errors occur when scripts require resources that they can't access. For example,

a script may refer to a file that doesn't exist or it may need to modify a file to which it doesn't

have write permissions.

$ user_backup

cp: file: No such file or directory

$

syntax errors

Syntax errors occur as a result of lexical errors ( such as misspelled variable names and

parentheses or quotes that haven't been closed ( in scripts.

$ user_backup

user_backup[3]: syntax error: 'while' unmatched

$

If one of your test runs of a script produces an error, you should try to reproduce the error by running the script again with similar input.

If you can't get the error to repeat, you can't make any further progress towards solving the problem. However, if it repeats in a predictable way, this indicates a fixable bug.

Once you've established what the effects of a bug are, you can begin looking for its causes. You first need to find the place in the script at which things begin to go wrong.

If the script gets stuck in a logical loop, for example, you need to find out why the exit conditions for the loop aren't being met. Perhaps it's because a variable value is wrongly assigned at some other point in the script.

Once you've located the cause of a bug, you can begin finding a way to fix the bug. This may be as simple as correcting a spelling mistake, or it may involve rewriting sections of code.

Question

Match the examples to the corresponding error type.

Options:

1. The program contains an opening quote that isn't closed2. The program is caught in a loop that fails to finish3. The program tries to write to a directory that doesn't exist

Targets:

A. Logical errorB. Resource errorC. Syntax error

Answer

An infinite loop is an example of a logical error, an attempt to write to a non-existent directory is a resource error, and if a set of quotes isn't complete a syntax error will be generated.

Syntax errors are generated by mistyping variable or command names, and not using the correct format for commands.

Logical errors result from design flaws, such as passing improper arguments to functions or failing to terminate loops.

Resource errors result from a script not being able to access external commands or filesystem locations – either because they don't exist or because the requisite permissions aren't in place.

Question

Which of the following types of error can be minimized at the design stage of writing a program?

Options:

1. Logical errors2. Resource errors3. Syntax errors

Answer

Logical errors can usually be minimised by planning your programs carefully at the design stage.

Option 1 is correct. Logical errors are usually the result of design flaws. Preparing your program in pseudocode helps to eliminate this type of error.

Option 2 is incorrect. Resource errors – such as incorrect permissions, or non-existant filesystem resources – are usually encountered during the writing and testing of a program.

Option 3 is incorrect. Syntax errors are only encountered once you start to write a program, so they aren't encountered at the design stage.

2. Tracking variables

Finding the place in a script that's causing a bug is the most difficult part of debugging. To do this, you need to track changes in variable values and the direction of program flow.

The easiest and simplest way to track variables is by introducing large numbers of echo

statements into a script.

You use the echo statements to output the name and value of a variable at different stages

in the script.

$ echo "customerID = $customerID"customerID = 411$

For example, this echo statement at the beginning of a function displays the values of any

arguments passed to the function.

read.customer.info () {> echo $*> new_entry[0]=$1> new_entry[1]=$2> new_entry[2]=$3> new_entry[3]=$4> }$

This example shows an echo statement in a loop. Each time the loop executes, the echo

statement displays the current value of two variables.

while (($count<$total)) do name=new_entry[0] touch "$name _data" count+=1 echo "iteration $count, name is $name" done

Detailed output from echo statements is necessary for debugging only. Once you've

finished debugging a script, you can disable the echo statements by commenting them out.

This allows you to re-enable them if you need to debug the script again in the future.

while $count<$total do name=new_entry[0] touch "$name _data" count+=1# echo "iteration $count, name is $name" done

You can use traps to track a particular variable or set of variables. Traps are statements that execute if the script receives a particular signal.

Question

What kind of statements do you think you need to execute in traps when you're debugging a script?

Options:

1. Statements that assign values to variables2. Statements that echo an error message3. Statements that echo variable values4. Statements that exit the script

Answer

When you debug a script using traps, you need to use the traps to run statements that echo variable values.

The most relevant traps for debugging are

DEBUG EXIT ERR

DEBUG

The DEBUG trap causes a statement to execute each time a line of code in the script

executes. This allows you to display the value of a variable every time the script executes a

statement.

EXIT

The EXIT trap causes a statement to execute when a script exits. This is especially useful if

you're dealing with an error that causes a script to exit prematurely because it allows you to

display the values of important variables immediately before the script crashes.

ERR

The ERR trap causes a statement to execute whenever a statement in the script causes an

error – in other words, if its exit status is non-zero. This allows you to view variable values at

the point at which errors occur, but it doesn't allow you to trace them through the statements

that lead up to that point in a script.

You use this syntax for trap statements.

trap command signal

In this example, you use the DEBUG trap to run the showvars function after each script

statement executes.

$ trap showvars DEBUG

A trap is active from the position of the trap statement onwards. If you want a trap to remain active throughout a script, you need to enter the trap statement at the beginning of the

script.

To deactivate a trap, you use another trap statement without specifying a command.

$ trap - DEBUG

In this example, you use the EXIT trap to run the exitmsg function if the script receives an

EXIT signal. This function might contain commands that display the values of all variables

and the line number of the last executed statement, for instance.

$ trap exitmsg EXIT

Question

Identify the true statements about the echo command.

Options:

1. You can include echo commands in your code without affecting control flow

2. You can use the echo command to rename variables

3. You use the echo command to print a variable without modifying its value

4. You use the # symbol to turn echo commands into comments

Answer

The echo command does not modify the values of variables and doesn't affect

control flow within scripts. You use the # symbol to turn echo commands into

comments.

Option 1 is correct. The echo command simply prints any arguments to standard

output. It doesn't affect control flow, so is useful for debugging programs.

Option 2 is incorrect. The echo command will print the value of any variable passed

to it as an argument, but it can't be used to rename variables.

Option 3 is correct. The echo command can't modify the value of variables passed to

it in arguments, so it is useful for monitoring variable values in order to debug programs.

Option 4 is correct. Any line in a shell script that is preceded by a # is treated as a

comment, including lines containing the echo command.

Question

You have written a shell program that creates several temporary files. You want to use the trap command to remove these files if the program exits unexpectedly.

Which signal do you specify for the trap command?

Options:

1. DEBUG

2. ERR

3. EXIT

Answer

You use the EXIT signal if you need to execute code when a program exits.

Option 1 is incorrect. The DEBUG signal is used to create a trap that executes code

after every command in the scripts is executed.

Option 2 is incorrect. The ERR signal is used to create a trap that executes code after

every error command, whether or not the error causes the program to exit.

Option 3 is correct. The EXIT command is used to create a trap that executes code –

in this case code that removes temporary files – when a program terminates. The specified code will be executed even if the program terminates unexpectedly.

3. Advanced debugging techniques

To assist in debugging a script, you can set debugging options when you run it.

The following shell options are useful for debugging:

noexec verbose xtrace

noexec

The noexec option causes the shell to read a script without executing any commands. This

is useful when you want to check for syntax errors or when you suspect that a bug may

hang the system.

verbose

The verbose option echoes all input that a script receives to the standard error file. This

can help you to locate a bug by allowing you to identify which input the script received

immediately before an error occurred.

xtrace

The xtrace option echoes each statement in a script as it's executed, and it echoes each

stage of variable, arithmetic, or command substitution. This is a powerful way of detecting

bugs that occur in the substitution stages of a statement.

This example of xtrace output shows how stages of execution are displayed. The xtrace

prompt – in this case a plus sign (+) – indicates how many levels of substitution are

occurring for a particular line. The example shows one level of variable substitution in which the number 3 is substituted for the variable i.

$ ((res = 36/4 + $i))+ let res = 36/4 + 3$

Note

You can customize the xtrace prompt using the PS4 environment variable.

You set debugging options using the set command with the -o option. The code shown

here sets the verbose option.

$ set -o verbose

You can run the set command without the -o option. You do this by using the first letter of

the debugging option. This code, for example, sets the xtrace option.

$ set -x

To unset a debugging option, you use the set +o command. The code shown here unsets

the verbose option.

$ set +o verbose

Note

You can't unset the noexec option because it prevents the set +o command from

executing.

Question

Which debugging option do you think allows you to view the input that a script receives?

Options:

1. noexec

2. verbose

3. xtrace

Answer

The verbose debugging option allows you to view the input that a script receives.

The most powerful tool for debugging a script is a debugging script. A debugging script appends itself to the target script and enables various debugging tools.

A debugging script allows you to

start a script with debugging options cause a script to exit under specified conditions display data if a script crashes change a script so that you can test debugging solutions

You can write your own debugging script, but some shells provide an existing debugger. For example, the Bash shell provides the Bash debugger.

To run the Bash debugger on a script, you run a session of the Bash shell and specify the --debugger option.

$ bash --debugger add_pkg_files

Question

Which debugging option allows you to monitor substitution as commands in a script execute?

Options:

1. noexec

2. verbose

3. xtrace

Answer

The xtrace option allows you to monitor substitution as commands in a script

execute.

Option 1 is incorrect. The noexec option reads commands without executing them.

It's useful for discovering syntax errors.

Option 2 is incorrect. The verbose option causes input to be displayed as it is read.

It's useful for verifying input.

Option 3 is correct. The xtrace option displays each command as it is executed and

is the most appropriate option for general debugging.

Question

Identify the true statements about debugging scripts.

Options:

1. A debugging script appends itself to the target script2. You can use a debugging script to automatically correct logical errors3. You can use a debugging script to display data when a script exits unexpectedly4. You can use a debugging script to enable debugging options

Answer

A debugging script appends itself to the target script. You can use it to enable debugging options and display data when a script exits unexpectedly.

Option 1 is correct. When you execute a debugging script you supply the target script as a parameter. The debugging code is appended to the code of the target script.

Option 2 is incorrect. A debugging script may help you locate the source of a logical error, but it won't correct it.

Option 3 is correct. A debugging script can be used to set traps that print useful data – such as the values of variables – when a program exits unexpectedly.

Option 4 is correct. You can use a debugging script to set shell options – such as the xtrace option – that facilitate debugging.

4. Handling user errors

If you're developing a script that requires input from users, you need to include statements that check the input to make sure it makes sense.

For example, an input function that reads a time shouldn't accept hours lower than zero or higher than 24.

Question

What do you think error-handling code needs to do if a user enters inappropriate input?

Options:

1. Exit the script2. Print an error message3. Print variable values4. Restart the script

Answer

If user input doesn't fit into the range of input that makes sense, the error-handling code in the script should provide a simple error message and exit the script.

This example shows part of a function that reads a date from user input. It checks the input to ensure that it makes sense and displays an error message and exits if it doesn't fit into the required range of values.

read.date () {> echo "Enter month:"> read month> if ((month<0)) | ((month>12))> echo "Invalid month!"> exit> fi

Summary

When you write complex shell scripts, they frequently contain bugs. You can avoid some bugs by checking program logic thoroughly before you write a script, but you need to remove other bugs after writing. To do this, you need to find the place in the script that causes the bug and then formulate a solution that fixes it.

You can locate bugs by inserting echo statements that allow you to trace the values of

variables at different points in the script's execution. You can also use traps to display variable values when certain conditions occur.

You can set debugging options when you run a script. These allow you to investigate how the script runs. You can also use debugging scripts, which incorporate various debugging tools.

When you write scripts that accept user input, you should include statements that test the input to make sure it makes sense.

Table of Contents

| Top of page |


| 1. Troubleshooting principles |

| 2. Tracking variables |

| 3. Advanced debugging techniques |

| 4. Handling user errors |

| Summary |




Exercise: Debugging a UNIX shell program

A note about exercises

This exercise is designed for practice use and does not contain new learning content. If your computer doesn't have an application or applications required to carry out the exercise tasks, or if you would prefer to perform the exercise at another time, you can proceed to the next topic.

Scenario

You've written a script that creates a shared directory. It allows a user to specify the directory's name and to undo the creation process if they've made a mistake. However, you've run the script and discovered that it has bugs.

Exercise

Your assignment for this exercise is to find and eliminate all bugs in the script. In its current state, the script contains two logical errors and three syntax errors.

Task list

Number

Instructions

1 Remove the bugs from the following script:

#!/usr/local/bin/kshDirPath="/home/shared/"Exist=0

while [[ $Exist -ne 0 ]] do echo "*************************************************** Welcome, $USER Please type the Directory Then press Enter:"

Task list

Number

Instructions

read DirName if [[ -d $DirPath]]; then Exist=1 echo "*************************************************** DIRECTORY EXISTS, Please Enter Another Directory" else Exist=0 fi done

clearmkdir -m 777 $DirPath$DirNameecho "The $DirName directory has been created in $DirPath. Are you happy with this?"read -p "[y/n]" BOOL1clear if [ $BOOL1 = n ] then rmdir $DirPathDirName echo "$DirName has been removed" exit else touch $DirPath$DirName/status$DirName chmod 666 $DirPath$DirName/status$DirName

if

Review your solution and, when you are satisfied with it, open the solution page.

Table of Contents

| Top of page |

| Scenario |

| Exercise |



unix shell scripting basics

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