Shell Script

Shell script defined as:
"Shell Script is series of command written in plain text file. Shell script is just like batch file is MS-DOS but have more power than the MS-DOS batch file."

Why to Write Shell Script ?

• Shell script can take input from user, file and output them on screen.
• Useful to create our own commands.
• Save lots of time.
• To automate some task of day today life.
• System Administration part can be also automated.

How to write shell script

Following steps are required to write shell script:
(1) Use any editor like vi or mcedit 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 755 your-script-name
Note: This will set read write execute(7) permission for owner, for group and other permission is read and execute only(5).
(3) Execute your script as
syntax: 
bash your-script-name
sh your-script-name
./your-script-name
Examples:
$ bash bar
$ sh bar
$ ./bar

if...else...fi

If given condition is true then command1 is executed otherwise command2 is executed.
Syntax:

           if condition

           then

                       condition is zero (true - 0)

                       execute all commands up to else statement

           else

                       if condition is not true then

                       execute all commands up to fi

           fi

for Loop

Syntax:

            for { variable name } in { list }

            do

                     execute one for each item in the list until the list is

                     not finished (And repeat all statement between do and done)

            done

while loop

Syntax:

           while [ condition ]

           do

                 command1

                 command2

                 command3

                 ..

                 ....

            done