chapter 7: advanced shell programming

Chapter 7:Advanced Shell Programming

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Ensuring the Correct Shell Runs the Script

Each UNIX/Linux user can choose which shell they use by default Ensure the correct shell is used to run a

script Not all shells support the same

commands and programming statements

The first line of a script should specify which shell to use

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Setting the Default Shell

System administrator establishes the default shell for a user account User shell set in /etc/passwd file

File can only be edited (carefully!) by system administratorSome systems provide management software to assist in setting default shells

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Using Bash Login and Logout Scripts

When Bash is your default shell, scripts run automatically upon login or re-entry .bash_profile .bashrc (also runs in a subshell)

Administrator controls /etc/bashrc and /etc/profile.bash_logout runs when user logs out Often used to clear the screen

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Defining the Shell (Sha-Bang)

A shell script is little more than a list of commands that are run in sequence. Conventionally, a shellscript should start with a line such as the following: #!/bin/bash

This indicates that the script should be run in the bash shell regardless of which interactive shell the user has chosen. This is important, since the syntax of different shells can vary greatly.

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A Simple Example

Here's a very simple example of a shell script with comments. It just runs a few simple commands

#!/bin/bash # Tell the user what is happeningecho "hello, $USER. Here is a directory listing” # list files echo "listing files in the current directory, $PWD" ls

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More on Shell Variables#!/bin/bash X=ABC

echo "$Xabc" This gives no output. What went wrong ? The answer is that the shell thought that we were asking for the variable Xabc, which is un-initialized. The way to deal with this is to put braces around X to separate it from the other characters.

The following gives the desired result: #!/bin/bash X=ABC

echo "${X}abc"

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Setting Defaults for Using the vi Editor

To use the vi editor for code development, configure .exrc in your home directoryAutomatically sets up the vi environment Set the number of tab spaces to use

when nesting lines of code Display line numbers

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Some vi tips for .exrc

To view the configurations of the parameters in your current vi session, go into the command mode and issue the command set all .:set all or:setall

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Some vi tips for .exrc

Setting syntax on/off syntax off

Precede each line with a number set number

Set word wrap 3 characters from edge of screen set wm=3

Create an abbreviation/shortcut ab wwr William W. Richards

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Spell Checking with vi

:map V :w^M:!ispell % ^M:e!^M^M

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Reading in the Date

:ab DATE ^M^[:.!date '+\%a \%d \%h \%Y'^MkJA

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Clearing the Screen

The clear command is useful for clearing the screen, but there is a faster way Store the output of the clear command

in a variable and then echo the contents of the variable on the screen

About ten times faster than the actual command since the system does not have to locate and execute the clear command

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Clearing the Screen

How do you do this?Simple…

CLEAR=`/usr/bin/clear`export CLEARecho $CLEAR

Chapter 8:Exploring UNIX/Linux Utilities

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Objectives

Understand many of the UNIX/Linux utilities that are available and how they are classifiedUse the dd utility to copy and convert filesMake a bootable removable diskMonitor hard disk usage

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Objectives (continued)

Use system status utilitiesMonitor and manage processesCheck the spelling of text in a documentUse the cmp command to compare the contents of two filesFormat text to create and use a man page

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Understanding UNIX/Linux Utilities

UNIX/Linux utilities let you Create and manage files Run programs Produce reports Monitor and maintain the system Recover from a range of errors

New utilities are continually being added in order to make UNIX/Linux run more efficiently

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Understanding UNIX/Linux Utilities (continued)

Classified into eight major areas: File processing System status Networking Communications Security Programming Source code management Miscellaneous

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Classifying UNIX/Linux Utilities

cat Display files fmt Formats text

cp Copy files grep Matches patterns in files

cpio Copy/back files fgrep Matches patterns in files

cut Selects char/fields gzip Zip/compress files

dd Convert/copy a file/image

gunzip

Unzip/uncompress files

dump Backs up files head Displays 1st part of files

file Displays file type ispell Spell Checks

find Finds files less Displays files

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Classifying UNIX/Linux Utilities (continued)

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Using the dd Command

Allows you to copy a file and change the format of the destination fileHas a rich set of options to handle copies when other methods are inappropriateAn advantage to using the dd command over cp is that all users, not just the administrator, can copy files to and from the floppy drive

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Checking Hard Disk Usage

To maintain adequate hard disk free space, use these strategies: Be vigilant against running dangerously

low on free space by using the df command

Watch for conspicuous consumption using the du command

Follow a routine schedule for “garbage” collection and removal by using the find and rm commands

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Using the df Utility

The df utility reports on the status of1024-byte blocks that are allocated, used, and available

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Using the du Utility

The du utility summarizes disk usage, expressed in 512-byte blocks (default) or by the number of bytes(-b option)

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Removing Garbage Files

Garbage files are temporary files that lose their usefulness after several daysTwo examples of garbage files are core files (named core) and a.out filesUse the find command to assist you in locating these files and the rm command to remove them

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Using System Status Utilities

System status commands reflect the system’s performanceSystem engineers primarily use the data related to system statusGood to know how to obtain and store relevant information to send to system administrator and tune-up specialists

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Using the top Command

One of the most effective utilities for auditing system performance is the top command

The top command displays a listing of the most CPU-intensive tasks in real time

Updates every five seconds by default

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Using the top Command (continued)

The top utility run without any options specified

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Using the uptime Command

uptime tells you how long a system has been running since the last time it was booted

Displays current time How long the system has been up Number of users on the system Load average for 1, 5, and 15 minutes

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Using the free Command

The free utility displays the amount of free and used memory in the system

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Managing Processes

A process is identified through a unique number called a process id (pid)Unix/Linux offer utilities to run, monitor, and kill processes using pids

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Monitoring Processes

The ps command with the -A option shows a list of all system processes currently running

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Killing Processes

Administrator with root privileges can kill any user’s processesUser can kill owned processesUse kill command with the pid of the processUse kill –9 to stop a process that doesn’t respond to an initial kill command

See man page for descriptions of the signals (remember the man trick)

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Running Processes in the Background

Can run a process in the background while working with another program in the foregroundTo run a program in the background, append the & character to end of the startup command, e.g., top&

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Checking the Spellingof a Document

ispell scans a document, displays errors on the screen and suggests alternative spellings

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Comparing Files

Use the cmp utility to compare the contents of two files, and report the first difference between themThe cmp command displays the position and line number of this differenceIf there are no differences, the cmp command displays nothing

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Formatting Text in UNIX/Linux

Text formatting in UNIX/Linux involves preparing a text file with embedded typesetting commands and then processing the fileUNIX’s nroff and troff commands were the early standard in formatting programsAn embedded code is a special sequence of characters that is included with the regular text of the file

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Formatting Text in UNIX/Linux (continued)

Linux introduced groff, which implements the features of both nroff and troff

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Groff can be used to produce a man page that contains the standard man page sections

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Man pages are made available to others by having a privileged user copy it to one of the man page directories

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Chapter Summary

UNIX/Linux utilities are classified into eight major functional areasUtility programs are called commands: executed by entering names on the command linedd command options allow it to handle copies when other copying methods failTo make a bootable removable disk, use provided utilities such as mkbootdisk

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Chapter Summary (continued)df checks and reports on free disk spacedu checks for disk usageUse find to retrieve temporary files and use rm to remove themtop and free provide detailed views of the “internals” of the system that can be redirected to a file for system tune-up

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Chapter Summary (continued)Run a program in the background by appending & to the end of a commandps displays all running processeskill terminates a specific processispell scans for spelling errorsText formatting involves Embedding typesetting commands in a

file Processing the file with a program that

generates commands for the output device

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Chapter Summary (continued)

Linux introduced groff, which implements the features of both nroff and troffText formatted with groff can be used to create new man pages

chapter 7: advanced shell programming

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