This document provides information on various aspects of shell scripting in Linux, including: - How to create, save, make executable, test, and deploy shell scripts using text editors like vi and the bash shell. - How the shebang line (#!) indicates the shell interpreter to use and ensures the correct shell will interpret the script. - Using chmod to set permissions on scripts so they can be executed. - Adding comments to scripts to document what they do. - Debugging techniques like using bash's -x option or the set builtin command to troubleshoot scripts.

1. Lesson 5
• bash Shell Introduction
• Saving and Running Your Script
• Starting a Script With #!
• example of /bin/sh script
• shell Comments
• Permissions on the script
• Debug a script exemples

2. bash shell introduction
To create a shell script:
1. Use a text editor such as vi. Write required Linux commands and logic in the file.
2. save and close the file (exit from vi).
3. make the script executable.
4. test the script, move it to the production environment.
5. simplest program in Bash consists of a line that tells the computer a command.
vi hello.sh
#!/bin/bash
echo "Hello, World!"
echo "Knowledge is power."
Save and close the file

3. Saving and Running Your Script
run the script:
./hello.sh
command ./hello.sh displayed an error message on the screen.
It will not run script since there’s no execute permission for the script hello.sh.
To execute program, change permissions:
bash: ./hello.sh: Permission denied
chmod +x hello.sh
./hello.sh
Hello, World!
Knowledge is power.

4. Starting a Script With #!
The #! syntax used in scripts to indicate an interpreter for execution under
UNIX / Linux operating systems. Most Linux shell and perl / python script
starts with the following line:
#!/bin/bash
#!/usr/bin/perl
!/usr/bin/python
1. It is called a shebang or a "bang" line.
2. It is nothing but the absolute path to the Bash interpreter.
3. It consists of a number sign and an exclamation point character (#!), followed by the full path to
the interpreter such as /bin/bash.
4. All scripts under Linux execute using the interpreter specified on a first line
5. Almost all bash scripts often begin with #!/bin/bash (if Bash has been installed in /bin)
6. Ensures that Bash will be used to interpret the script, even if it is executed under another shell
7. The shebang was introduced by Dennis Ritchie between Version 7 Unix and 8 at Bell
Laboratories.
It was then also added to BSD line at Berkeley.

5. Ignoring An Interpreter Line (shebang) and using sh
If you do not specify an interpreter line, the default is /bin/sh.
But, it is recommended that you set #!/bin/bash line.
#!/bin/sh
For a system boot script, use /bin/sh
Is the standard command interpreter for the system.

6. example of /bin/sh script : /etc/init.d/policykit
#! /bin/sh
### BEGIN INIT INFO
# Provides: policykit
# Required-Start: $local_fs
# Required-Stop: $local_fs
# Default-Start: 2 3 4 5
# Default-Stop:
# Short-Description: Create PolicyKit runtime directories
# Description: Create directories which PolicyKit needs at
runtime,
# such as /var/run/PolicyKit
### END INIT INFO
# Author: Martin Pitt <martin.pitt@ubuntu.com>
case "$1" in
start)
mkdir -p /var/run/PolicyKit
chown root:polkituser /var/run/PolicyKit
chmod 770 /var/run/PolicyKit
;;
stop|restart|force-reload)
;;
*)
echo "Usage: $SCRIPTNAME {start|stop|restart|force-reload}" >&2
exit 3
;;
esac
:

7. Shell Comments
#!/bin/bash
# A Simple Shell Script To Get Linux Network Information
# Vivek Gite - 30/Aug/2009
echo "Current date : $(date) @ $(hostname)"
echo "Network configuration"
/sbin/ifconfig
• A word or line beginning with # causes that word and remaining characters on
that line to be ignored.
• These notes are called comments. It is nothing but explanatory text about
script.
• It makes source code easier to understand. These notes are for humans and
other sys admins.
• It helps other sys admins to understand your code, logic and it helps to modify
the script.

8. permissions on a script
lesson 4
chmod +x script.sh #Allowing everyone to execute the script
The chmod is a shell command in Linux.
It can change file system modes of files and directories. The modes include permissions
and special modes. Each shell script must have the execute permission.
chmod 0755 script.sh #Allowing everyone to execute the script
chmod 0700 script.sh #Only allow owner to execute the script
chmod u=rwx,go= script.sh #Only allow owner to execute the script
chmod u+x script.sh #Only allow owner to execute the script
ls -l script.sh #view the permissions
chmod ug=rx script.sh #user and the group to read and execute only
chmod ug= script.sh #Remove read execute permission for group and user
When script is executed using either bash cmd or dot (.) cmd, you do not have
to set executable permissions on script but read permissions need to be set.

9. Make a script
$ cat trash.sh
#!/bin/bash
# this script deletes some files
cp * trash
rm -rf trash
mkdir trash
echo “Deleted all files!”
We write a program that copies all files into a directory, and then deletes the
directory along with its contents.
This can be done with the following commands:
$ mkdir trash
$ cp * trash
$ rm -rf trash
$ mkdir trash
Instead of having to type all that interactively on the shell, write a shell program instead:

10. Debug a script
You need to run a shell script with -x option from the command line:
bash -x script-name
#!/bin/bash -x
echo "Hello ${LOGNAME}"
echo "Today is $(date)"
echo "Users currently on the machine, and their processes:"
w
bash -xv script-name
can also modify shebang line to run an entire script in debugging mode:
Use of set builtin command
Bash shell offers debugging options which can be turned on or off using set command.
set -x : Display commands and their arguments as they are executed.
set -v : Display shell input lines as they are read.
set -n : Read cmds but do not execute them. Used to check a shell script for syntax errors.

11. Debug a script Exemple
#!/bin/bash
### Turn on debug mode ###
set -x
# Run shell commands
echo "Hello $(LOGNAME)"
echo "Today is $(date)"
echo "Users currently on the machine, and their processes:"
w
### Turn OFF debug mode ###
set +x
# Add more commands without debug mode

12. Debug a script Exemple
#!/bin/bash
set -n # only read command but do not execute them
set -o noexec
echo "This is a test"
# no file is created as bash will only read commands but do
not executes them
>/tmp/debug.txt
example using set -n and set -o noexec: