Linux Shell scripting basics and Simple example to add two numbers

Shell scripting is a file which contains the list of commands for executing. What is Shell? Shell is special program or application provided for the user interaction.

There are different kinds of shells in Unix/Linux, they are

• Sh: Bourne Shell
• Csh: C Shell
• Ksh: Korn Shell
• Bash: Bourne again Shell.. etc

Some of the advantages of the shell scripting are,

• Automation of tasks such as backups,log monitoring etc
• To do repetition of of tasks
• Save lots of time..

Unix/Linux Shell scripting also containg the functions, control statements,loops like other programming languages C, C++...

• To create a variable, we use the following syntax,

           variablename=value
Note: there is no semicolon(;) at the end of each line like other languages

variable name can contain alphabet, digits.Remember Spaces and special characters are not allowed except underscore( _ ) like in other programming languages C,C++.

• Comments in the shell script are start with # symbol
• To retrieve the value of a variable use $ symbol
• To do arithmetic operations use ((expression)) syntax 

To start shell scripting program, we need any text editor like vi, vim etc.

Here is the simple Linux Shell Scripting program.

firstscript.sh:

#!/bin/bash

# variable declaration

str="Hello World"

#echo command used to display text 

echo "$str"

echo "From DevelopersFactory"

To execute the shell script we should use any of the following methods,

First Method

• Give executable permissions to the file

              $chmod 0755 filename

• Then execute it

              $./filename

2nd Method

• $sh filename or

       $bash filename

Output:

$chmod 0755 firstscript.sh

$./firstscript.sh

Hello World
From DevelopersFactory

• The following code snippet will describe arithmetic operations in shell script

 add.sh:

#!/bin/bash

sum=0

echo "enter two numbers"

read a

read b

#we can use any of the following 

sum=$(($a+$b))

#sum=`expr $a + $b`

# bc command can calucate accurately with floating point

#here we use division to examine the floating point result

#echo "$a/$b" | bc -l;

echo "sum of the two numbers is $sum"

Output:

$chmod 0755 add.sh

$./add.sh

enter two numbers
3
5
sum of the two numbers is 8