What is and How to do Shell programming ?

Shell scripting skills have many applications, including :

Ability to automate tasks, such as Backups, Administration tasks, Periodic operations on a database via cron, Any repetitive operations on files 

Example 1 : Store the following in a file named simple.sh and execute it

#!/bin/sh

# Show some useful info at the start of the day

date

echo Good morning $USER

cal

last | head -6

Shows current date, calendar, and a six of previous logins.

Notice that the commands themselves are not displayed, only the results.

Example 2 : Storing File Names in Variables

#!/bin/sh

# An example with variables

filename="/etc/passwd"

echo "Check the permissions on $filename"

ls -l $filename

echo "Find out how many accounts there are on this system"

wc -l $filename

Now if we change the value of $filename, the change is automatically propagated throughout the entire script.

Example 3 : Performing Arithmetic

Backslash required in front of '*' since it is a filename wildcard and would be translated by the shell into a list of file names.

You can save arithmetic result in a variable.

#!/bin/sh

# Perform some arithmetic

x=24

y=4

result='expr $x \* $y'

echo "$x times $y is $Result"