bash scripting

LINUX+ LAB SERIES (LX0-102)

Lab 11b: BASH Scripting

Document Version: 2015-04-30

This work by the National Information Security and Geospatial Technologies Consortium (NISGTC), and except where otherwise noted, is licensed under the Creative Commons Attribution 3.0 Unported License.

Development was funded by the Department of Labor (DOL) Trade Adjustment Assistance Community College and Career Training (TAACCCT) Grant No. TC-22525-11-60-A-48; The National Information Security, Geospatial Technologies Consortium (NISGTC) is an entity of Collin College of Texas, Bellevue College of Washington, Bunker Hill Community College of Massachusetts, Del Mar College of Texas, Moraine Valley Community College of Illinois, Rio Salado College of Arizona, and Salt Lake Community College of Utah.

This workforce solution was funded by a grant awarded by the U.S. Department of Labor's Employment and Training Administration. The solution was created by the grantee and does not necessarily reflect the official position of the U.S. Department of Labor. The Department of Labor makes no guarantees, warranties or assurances of any kind, express or implied, with respect to such information, including any information on linked sites, and including, but not limited to accuracy of the information or its completeness, timeliness, usefulness, adequacy, continued availability or ownership.

http://creativecommons.org/licenses/by/3.0/deed.en_US

http://creativecommons.org/licenses/by/3.0/deed.en_US

Linux+ Lab Series (LX0-102) Lab 11b: BASH Scripting

1


Contents

Introduction ........................................................................................................................ 2

Objective ............................................................................................................................. 2

Linux+ LX0-102 Exam Objectives ........................................................................................ 3

Lab Topology ....................................................................................................................... 4

Lab Settings ......................................................................................................................... 5

1 Shell Scripting Basics ................................................................................................... 6

2 Conditional Execution ............................................................................................... 13


2


Introduction

This lab provides guidance on performing Lab 11b: BASH Scripting of the Linux+ LX0-102 course, using a NETLAB+ system. By performing this lab, students will learn how to work with shell scripts. Objective

The following tasks will be performed:

1. Create simple shell scripts. 2. Create shell scripts with conditional execution.


3


Linux+ LX0-102 Exam Objectives

This lab will cover the topics for the following LX0-102 objectives: 105.2 Customize or write simple scripts

1. Use standard sh syntax (loops, tests). 2. Use command substitution. 3. Test return values for success or failure or other information provided by a

command. 4. Perform conditional mailing to the superuser. 5. Correctly select the script interpreter through the shebang(#!) line. 6. Manage the location, ownership, execution and suid-rights of scripts.

The following is a partial list of the used files, terms, and utilities:

a. for b. while c. test d. if e. read f. seq


4


Lab Topology


5


Lab Settings

The information in the table below will be used to complete the lab. Additional details will be provided within the task sections as required.

System Username/Password

CentOS Server sysadmin/netlab123

Ubuntu Server sysadmin/netlab123

Fedora Workstation sysadmin/netlab123

Ubuntu Workstation sysadmin/netlab123

All Machines root/netlab123


6


1 Shell Scripting Basics

Shell scripting allows you to take a complex sequence of commands, place them into a file and then run the file as a program. This saves you the time of having to type a long sequence of commands that you routinely use. This lab will focus on how to create simple shell scripts. For the purpose of this lab we will assume that you know how to use an editor. Feel free to use the editor of your choice: vi, nano, gedit or any other editor that you like.

1. Click on the Fedora Workstation icon in the pod topology to launch the virtual machine.

2. The virtual machine will display a login screen. Make sure sysadmin is in the user field. Enter the password netlab123 and press Enter.


7


3. Once you have logged in, a terminal window may appear. If it does not, click on the Kickoff Application Launcher, the “f”, in the lower-left corner of the desktop. In the search bar, type konsole and click on Konsole, which will launch a terminal window:


8


4. To create a simple shell script, you just need to create a text file and add commands. Create a file called sample.sh and add the following lines:

echo "Hello there! Here is the calendar for this month:"

cal

Your output should be similar to the following:

5. To make it clear that this is a BASH shell script, we need to include a special line at the top of the file called a "shbang" (or "shebang"). This line starts with #! and then contains the path to the BASH shell executable. Add the following line at the top of the sample.sh file:

#!/bin/bash


9


6. One way that we can run this program is by typing bash before the filename. Execute the following:

bash sample.sh


7. You can avoid having to type bash in front of the filename by making the file "executable". Run the following commands:

chmod a+x sample.sh

./sample.sh



10


8. A common feature used in scripting is called "backquoting". With this technique, you can run a shell command "within" another shell command. The outcome of the internal command is returned as an argument to the external command. Add the following to the bottom of the sample.sh command and execute it:

echo "Today is" `date +%a`

Now execute the script:

./sample.sh

9. We have been using ./ in front of the sample.sh filename to indicate that the file is in the current directory. Execute the following to see how the shell would fail to find the file if we don't use the ./:

sample.sh

Your screen should look like the following:


11


10. Recall that the $PATH variable is used to search for commands that you type. Execute the following to see the $PATH variable for the sysadmin account:

echo $PATH


11. Note that /home/sysadmin/bin is one of the directories in the PATH. This is a great place to put your shell scripts:

mkdir /home/sysadmin/bin

mv sample.sh /home/sysadmin/bin

sample.sh

Your screen should look like the following:


12


12. When a script executes, each command runs using the file access rights of the person who runs the script. Using a file editor of your choice, create file called access.sh and add the following to it:

cat /etc/shadow

Now execute the following commands to add execution privileges to that script and then execute it.

chmod a+x access.sh

./access.sh


Note that the sysadmin account does not have the permission to view the /etc/shadow file.

While you can create setuid scripts, allowing access to files that the user would not normally have access to, this is generally not considered a good idea due to security issues.


13


2 Conditional Execution

Note that during this section more complex examples will be demonstrated. When doing so, we will be using a technique to describe what is happening in the program. The technique will look like the following:

Enter this column into drive.sh This column describes the code (don’t enter into the file)

echo “Please enter your age”

read age #read user input and place in #$age variable

When following the instructions provided, you are to enter the bold text from the left column into the specified file (drive.sh in the example above). The right column is used to describe specific lines in the program. The pound sign (#) character is used because in a shell script, you can place comments within your program by using a # character.

1. More complex scripts may make use of conditional execution. A conditional expression, like the "if" statement, can make use of the outcome of a command called "test". The "test" statement compares two numbers (or two strings) for things like "equal to", "less than", etc. Create the following file (drive.sh) and make it executable to see how the "if" and "test" statements work. Begin by placing the following in drive.sh:

Enter this column into drive.sh This column describes the code (don’t enter into the file)

echo “Please enter your age”

read age #read user input and place in #$age variable

if test $age –lt 16 #test $age -lt 16 returns "true" if $age #is numerically less than 16

then

echo “You are not old enough to drive.”

else

echo “You can drive!”

fi #This ends the if statement

Then make the file executable and run it:

chmod a+x drive.sh

./drive.sh


14



Verbally, you could read the "if" statement as "If $age is less than 16, then echo 'You are not old enough to drive', else echo 'You can drive!'". The "fi" ends the "if" statement.

Note: $age must be an integer value. If not, the program will crash.


15


2. The test statement is automatically called when you place its arguments within square brackets ( [ ] ). Modify the if line of drive.sh so it looks like the following:

if [ $age -lt 16 ]

Then, run the program again:

./drive.sh


To see a full list of test conditions, run the command man test.


16


3. You can also use the outcome of other shell commands as they all return "success" or "failure". For example, create and run the following program, which can be used to determine if a user account is on this system. Add the following to check.sh:

echo "Enter a username to check: "

read name

if grep $name /etc/passwd > /dev/null

then

echo "$name is on this system"

else

echo "$name does not exist"

fi

Then, run the following commands:

chmod a+x check.sh

./check.sh



17


4. Another common conditional statement is called the "while" loop. Begin by placing the following in num.sh:

Enter this column into num.sh This column describes the code (don’t enter into the file)

echo “Please enter a number greater than 100”

read num

while [ $num –le 100 ] #Execute code from “do” to “done" if #test condition is true

do

echo “$num is NOT greater than 100.”

echo “Please enter a number greater than 100”

read num

done #This ends the done statement

echo “Finally, $num is greater than 100”

Then make the file executable and run it:

chmod a+x num.sh

./num.sh



18


If the conditional check for the while statement ($num -le 100) returns true, then the statements between do and done are executed. Once those statements have completed executing, the conditional check for the while statement is checked again. If true, then again the statements between do and done are executed. This is repeated until the while condition returns false.

5. Scripting code is part of the BASH shell, which means you can use these statements on the command line just like you use them in a shell script. This can be useful for a statement like the for statement, a statement that will assign a list of values one at a time to a variable. This allows you to perform a set of operations on each value. For example, run the following:

for name in /etc/passwd /etc/hosts /etc/group

do

wc $name

done


Note that the wc command was run three times: once for /etc/passwd, once for /etc/hosts and once for /etc/group.


19


6. Often the seq command is used in conjunction with the for statement. The seq command can generate a list of integer values, for instance from 1 to 10. For example, run the following to create 12 files named test1, test2, test3, etc. (up to test12):

ls

for num in `seq 1 12`

do

touch test$num

done

ls



20


7. Sometimes you will want to send a message to a user, like the superuser, when an event takes place. Create the following program, filecheck.sh, which will test to make sure the /tmp/check file exists and send the superuser an email message if it the /tmp/check file does not exist:

if [ -f /tmp/check ]

then

echo "all is well"

else

echo "/tmp/check file is missing" | mail root

fi



21


8. Make filecheck.sh executable, run the script and verify that it functions correctly:

chmod a+x filecheck.sh

touch /tmp/check

./filecheck.sh

rm /tmp/check

./filecheck.sh

su - root

netlab123

mail

Your output is similar to the following:

You will see a new message from sysadmin. You can read this message by typing the message number (the number before sysadmin, 1 in this case) at the & prompt. To quit the mail utility, type q. Return to the sysadmin account by typing exit.

9. Click the X in the upper-right corner to close the terminal window. 10. Close the Fedora Workstation remote pc window. 11. Click the I’m Done button to end the reservation.

bash scripting

Documents

linux lab series lx0

lab topology

lab settings

linux lx0

national information

shell scripting basics

bellevue college of

unported license