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We are providing Linux scripting services to our clients, through this article we are trying to provide some of the basics of Linux scripting.

What is a Shell ? :- Shell is a command interpreter – Expands and interprets the command entered before passing to kernel. With GUI, most of the tasks can be done, but not all.

/etc/profile – Ready by all shells and not shell specific

Prompts – A string of characters like $ or # that shell displays when it can accept your command.

Job control – Multitasking. Running many programs in bg and fg.

Wildcard Expansion – *, ?, [ ], !

ls * – all
ls ? – one character
ls [ab]* – starting with a or b
ls [!ab]* – Starting with any except a and b
ls [a-dx-z] – Starting with a, b, c, d, x, y, z

Piping and I/O Redirection – | , <, > , >> , 2>, 1>

Standard Input/Output/Error  :-

File Descriptor STDIN – < or 0
File Descriptor STDOUT – > or 1 or 1>
File Descriptor STDERR – 2> or 2

#ls > output_file
#cat < input_file – > overwrites
#ls >> output_file – >> appends

Redirecting only the errors to a error_file  :-

#ls non_existent_file 2> error_file – clears existing errors and add a fresh error entry
#ls non_existent_file 2>> error_file – appends the new error to existing entry

#ls existent_file non_existent_file > output_file 2>&1

displays the output and error of ls to output_file

#ls existent_file non_existent_file 2>&1 > output_file

displays error to console and output to output_file

By default, stderr is not passed on to pipes. i.e a command such as ls non_existent_file | grep error won’t show any output, however a small change to command like –> ls non_existent_file 2>&1 | grep -i error will show the error, if any.

Line continuation or Escape character – \

Variable expansion – $PWD, $VARIABLE1

Alias expansion – ll –> ls -l –color=tty

Command auto completion and substitution

# rm -f `ls /tmp` – will delete files inside /tmp, but not directories

# rm -f $(ls /tmp) – is same as above

# cat file ; date – Both commands date and cat file is executed even if there is an error with date

# cat file && date – date executes only if cat file is executed without any errors

# cat file || date – date executes only if cat file is executed with an error

Scripting :- A simple language with only a few reserved words like case, esac, if, fi, eif, done, for, do, function, select in, then, until, while etc.

Shebang line – For a perl script it is “#!/usr/bin/perl” – Location of perl (ofcouse without the double quotes), while in a bash script the file line is “#!/bin/bash”

#vi script.sh

# My first script

#!/bin/bash
echo “Hello. This is my first script”
echo “I am running this script from $(pwd) directory”

To run this script,

#chmod +x script.sh
# ./script.sh OR . script.sh OR source script.sh

$1, $2, $3 refer to arguments.

$@ is “$1”, “$2”, “$3” .. ie array of 3 arguments.
$* is “$1 $2 $3” .. ie considered as 1 argument seperated with space
$# is number of parameters
$? exit status
$$ represents the PID of the running shell
$! represents the PID value of the last child process

man test to see the expressions to test file or string status or values. Common options  are
-f – file exists and is ordinary
-d – file exists and is directory
-r – file exists and is readable
-w – file exists and is writable
-x – file exists and is executable
-n – string is not empty
-z – string is empty
== – string comparison and proved equal
!= – string comparison, and not equal

Conditional Statements

if, elif, fi

#!/bin/bash
if [ -f /etc ]
then
echo “/etc is a directory”
else
echo “/etc is what?”
fi

case , esac

echo “Enter your option from 1-5 : ”
read i;

case $i in
1) ls -l ;;
2) ps -aux ;;
3) date ;;
4) who ;;
5) exit
esac

while do done

#!/bin/bash
COUNTER=0
while [  $COUNTER -lt 10 ]; do
echo Count is $COUNTER
let COUNTER=COUNTER+1
done

for do done

Arithmetic operations  :-

!/bin/bash
echo “Please input your first number”
read  first
echo “Now input your second”
read second
echo “Adding first and second numbers gives me” $(($first+$second)) “as result.”

expr and let can be used to calculate in the bash shell also. let x=1+2; echo $x will show the result as 3

Debugging mode in bash  :-

!#/bin/bash -x