7- BASH Scripting- Advanced uses of If-then conditional statement

Welcome Linux –fanatics, this is 7th tutorial in BASH scripting series. In our previous scripting tutorial, we discussed File, String & Numeric comparisons  (also check out rest of scripting tutorials HERE). We will now be discussing some advanced uses of If-then statement in BASH scripting. So, let’s start….

Using logical operators && (AND operator) and || ( OR operator)

&& is logical AND operator. When used with if-then statement will only trigger when all  the conditions are TRUE.

Syntax used is

[ condition ]  && [ condition ]

 

|| is logical OR operator. When used with if-then statement will only trigger when any of the conditions are TRUE.

Syntax used is

[ condition ]  || [ condition ]

 

Example Script

#!/bin/bash
# Checking AND operator
dir=/home/dan
if [ -d $dir ] && [ -w $dir ]
then
echo “$dir directory exists & its writable”
fi

Similarly we can also use OR operator which works if any of the given conditions is met, then statement will be triggered.

 

Double Parenthesis “  (( ))  “

It is used for handling advanced mathematical expressions. Parameters used with  “ (( )) ” are

  • val ++                                  post increment (increases value by one)
  • val–                                      post decrement (decreases value by one)
  • ++val                                   pre  increment (increases value by one)
  • –val                                      pre decrement (decreases value by one)
  • !                                              logical negation (Not operator)
  • &&                                         logical AND operator
  • ||                                             logical OR operator
  • **                                            exponential
  • ~                                             bitwise NOT operator
  • &                                             bitwise AND operator
  • |                                               bitwise OR operator
  • <<                                           left bitwise shift
  • >>                                           right bitwise shift

Important Note:- When using double parenthesis, we don’t need to escape the parameters i.e. using \ is not required.

Example Script

#!/bin/bash
# Use of DOUBLE PRENTHESIS
num1=15
if (( $num1 ** 2 > 100 ))
then
(( num2=$number ** 2 ))
echo “Square of $num1 is $num2”
fi

 

Double Brackets “[[ ]]”

It is advanced version of “[ ]” & provides us with advanced features for string comparisons.

Example Script

#!/bin/bash
# Script for pattern matching with double brackets
If [[ $USER == r* ]]
then
echo “Welcome $USER”
else
echo “Who are you $USER”
fi

Here “ == ” is used for pattern matching, where == looks for “r” at start of username and if found , condition is TRUE. We can also use “!=” which would be TRUE when “r ” si not found at start of username.

 

So, this completes our tutorial for BASH scripting, in this tutorial we learned advanced uses of if-then conditional statement. In our next tutorial, we will learn about for conditional statement.

Also please mention you feedback/queries down below in comment box. ADIOS !!!

 

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Shusain

Passionate about Linux & open source. Loves to learn, read & write about Linux as well as new technologies.

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