Conditional Statements | Shell Script

Conditional Statements: There are total 5 conditional statements which can be used in bash programming

1. if statement
2. if-else statement
3. if..elif..else..fi statement (Else If ladder)
4. if..then..else..if..then..fi..fi..(Nested if)
5. switch statement

Their description with syntax is as follows: if statement This block will process if specified condition is true. Syntax:

if [ expression ] then statement fi

if-else statement If specified condition is not true in if part then else part will be execute. Syntax

if [ expression ] then statement1 else statement2 fi

if..elif..else..fi statement (Else If ladder) To use multiple conditions in one if-else block, then elif keyword is used in shell. If expression1 is true then it executes statement 1 and 2, and this process continues. If none of the condition is true then it processes else part. Syntax

if [ expression1 ] then statement1 statement2 . . elif [ expression2 ] then statement3 statement4 . . else statement5 fi

if..then..else..if..then..fi..fi..(Nested if) Nested if-else block can be used when, one condition is satisfies then it again checks another condition. In the syntax, if expression1 is false then it processes else part, and again expression2 will be check. Syntax:

if [ expression1 ] then statement1 statement2 . else if [ expression2 ] then statement3 . fi fi

switch statement case statement works as a switch statement if specified value match with the pattern then it will execute a block of that particular pattern When a match is found all of the associated statements until the double semicolon (;;) is executed. A case will be terminated when the last command is executed. If there is no match, the exit status of the case is zero. Syntax:

case in Pattern 1) Statement 1;; Pattern n) Statement n;; esac

Example Programs Example 1: Implementing if statement PHP

#Initializing two variables a=10 b=20 #Check whether they are equal if [ $a == $b ] then  echo "a is equal to b" fi #Check whether they are not equal if [ $a != $b ] then  echo "a is not equal to b" fi 

Output

$bash -f main.sh a is not equal to b

Example 2: Implementing if.else statement PHP

#Initializing two variables a=20 b=20 if [ $a == $b ] then  #If they are equal then print this  echo "a is equal to b" else  #else print this  echo "a is not equal to b" fi 

Output

$bash -f main.sh a is equal to b

Example 3: Implementing switch statement php

CARS="bmw" #Pass the variable in string case "$CARS" in  #case 1  "mercedes") echo "Headquarters - Affalterbach, Germany" ;;    #case 2  "audi") echo "Headquarters - Ingolstadt, Germany" ;;    #case 3  "bmw") echo "Headquarters - Chennai, Tamil Nadu, India" ;; esac 

Output

$bash -f main.sh Headquarters - Chennai, Tamil Nadu, India.

Note: Shell scripting is a case-sensitive language, which means proper syntax has to be followed while writing the scripts.