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

NOTE In the last syntax ./ means current directory, But only . (dot) means execute given command file in current shell without starting the new copy of shell, The syntax for . (dot) command is as follows
Syntax:
. command-name

Example:
$ . foo

Now you are ready to write first shell script that will print "Knowledge is Power" on screen. See the common vi command list , if you are new to vi.

$ vi first # # My first shell script # clear echo "Knowledge is Power"

After saving the above script, you can run the script as follows:
$ ./first

This will not run script since we have not set execute permission for our script first; to do this type command
$ chmod 755 first
$ ./first

First screen will be clear, then Knowledge is Power is printed on screen.

Script Command(s)	Meaning
$ vi first	Start vi editor
# # My first shell script #	# followed by any text is considered as comment. Comment gives more information about script, logical explanation about shell script. Syntax: # comment-text
clear	clear the screen
echo "Knowledge is Power"	To print message or value of variables on screen, we use echo command, general form of echo command is as follows syntax:  echo "Message"

 How Shell Locates the file (My own bin directory to execute script)

Tip: For shell script file try to give file extension such as .sh, which can be easily identified by you as shell script.

Exercise:
1)Write following shell script, save it, execute it and note down the it's output.

$ vi ginfo # # # Script to print user information who currently login , current date & time # clear echo "Hello $USER" echo "Today is \c ";date echo "Number of user login : \c" ; who | wc -l echo "Calendar" cal exit 0