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Shell Scripting (Bash shell only)
by - Akshay
Agenda
In this part of presentation, We will cover…
• Introduction to shell programming,
• How to read/write/execute a script.
Assumptions
Before starting with this you should know-
• How to use text editor such as vi/vim,
• Basic Linux commands.
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What is SHELL ?
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• Shell is just an interface to access an operatingsystem
• Shell reads command from user and tells Linux OSwhat users want.
Shell
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• Simply, the shell is a program that takesyour commands from the keyboard andgives them to the operating system toperform.
• In the old days, Shell was the only userinterface available on a Unix/Linuxcomputer. Nowadays, we have GUIs inaddition to the shell.
Some commands to play with shell
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Tip: To find all available shells in your system type following command:
$ cat /etc/shells
Note that each shell does the same job, but each understand a different command syntax and provides different built-in functions.
****In MS-DOS, Shell name is COMMAND.COM which is also used for same purpose, but it's not as powerful as our Linux Shells are!
.
What is my current SHELL
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Tip: To find your current shell type following command$ echo $SHELL
Hint: Bash which stands for Bourne Again SHell, an enhanced version of the original Bourne shell program, sh, written by Steve Bourne
Tip: To find process-id of current shell.
$ ps -p $$
Further information
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To read more information –
$man bash
$info bash
Mr. Shell
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What is Shell Script & Why should I learn scripting ??
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Shell Script
basically scripts are collections of commands that arestored in a file. The shell can read this file and act on thecommands as if they were typed at the keyboard.
We have thousands of commands available for thecommand line user, Can we remember them all? Theanswer is, “No”. The real power of the computer is itsability to do the work for you. To get it to do that, weuse the power of the shell to automate things. We writescripts.
What are scripts good for?
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A wide range of tasks can be automated. Here are some of the things I automate with scripts:
• On every 1st day of a month, linux-servers backup theirconfigurations and copy it to a “Window/Storage machine“, anddelete old backups on successful execution so that we always havelatest backup with us. This is performed by a script.
• A script on a single click automatically does health check-up onmultiple servers and reports the status with colors (Red-> Critical,Yellow-> Warning , Green -> OK) from all servers. A sample report isattached .(Click here to see)
• Script can sends us an email message if a process is down.
Choice is yours
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OR
How to write shell script
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Following steps are required to write shell script:
(1) Use any editor like vi to write shell script.(2) After writing shell script set execute permission for your script as follows-
Syntax:chmod permission your-script-name
Examples:$ chmod +x your-script-name$ chmod +x abcd.sh
How to execute your script.
Execute your script asSyntax:bash your-script-namesh your-script-name
ksh your-script-name./your-script-name
Examples:$ bash abc.sh$ sh abc.sh$ ./abc.sh
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First Script
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Before starting any thing , I will recommend every one to create a Linux Virtual machine on your laptop/PC . VM will be safe for testing . Do not try any script on production servers (If you face any difficulty while creating VM , let me know after this session )
First Script
• Login to your Linux machine, create a test directorywhich will hold all of our test scripts.
• In my case, I have created a directory called “test” ,which will hold my test scripts .
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First Script
• To create a shell script, you use a text editor.
$ vi my_script
• Press “i” to enter in “insert mode” of vi editor
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First Script
• Write logic of your script .
• Press “Esc” key twice to exit from “insert mode”.
• Type :wq! and hit the entre key to save your scriptand exit from vi-editor.
• Type “chmod +x my_script” to make it executable.
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Explanation
#!/bin/bash
# My first script
echo "Hello World!"
echo "How are you.“
• The first line of the script is important. This is a special clue given to theshell indicating what “shell” is used to interpret this script. In this case, itis /bin/bash.
• The second line is a comment. Everything that appears after a "#" symbol isignored by bash.
• The last two lines are echo commands. This command simply prints what itis given on the display.
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Explanation
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We just covered “echo” command ; It is used to print given text on screen , along with this you can also use echo command to make Alarm which will makes a BEEP sound, you can also use echo command to print text in attractive colors.
Since , this session is limited to only introduction , we will learn this options in another session .
Colored output
Alarm with echo command
How to give your keyboard inputs to script ??
Read command is used to pass your keyboard inputs to script .
• Syntax:read your-variable-name
• Examples:$ read my_variable
Now to read the value stored in a variable use “echo” command.
• Syntax:echo $your-variable-name (Note: “$” before variable name)
• Examples:$ echo $my_variable
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Second Script
• Now we will make a second script which will ask userto give some inputs and then it will print those inputsto screen.
#!/bin/bash
# My second script
echo “What is in your mind…."
read your_input
echo
echo "Your input was : $your_input"
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How to hide your inputs ??
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In the same direction , suppose ,script has asked you to enter your password
and you do not want it to be visible on screen while typing…. In this kind of
situation , everything will be same and you just need to put “-s” (suppress)
option with read command. .
#!/bin/bash
# My 3rd script
echo "Please enter your password"
read -s my_password
echo
echo "Yeah ! i have hacked your password : $my_password"
• In same direction , you can also set a timeout forusers to give there inputs, i.e. if your does not give hisinput in given time then , script will stop its executionafter timeout.
• Change command read your_input to read -t 10 your_input inthe 2nd script we made in this session , and see thedifference.
$ read –t time_in_second name_of_varibale
Example read -t 10 abc (timeout after 10 seconds)
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Echo + read = (read –p)
• As we saw in previous 2 scripts , while giving inputs toscript from keyboard we used read and echocommand in pair. We can save one line if use “-p”(small “P”) with the read command.
$echo “What is in your mind”
$read my_input
$read –p “What is in your mind” my_input
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Thank You
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