Conditionals statement in bash or shell script 

conditional statements is used for making correct decisions and perform the right actions on it.

if statement in shell script

1) if…fi statement
2) if…else…fi statement
3) if…elif…else…fi statement

Switch case in Bash script

case…esac statement

Bash if Statement

if [conditional expression]; then
Statement or command if ‘conditional expression’ is true.
fi

#!/bin/bash
echo "Enter numeric value: "
read value

if [ $value -gt 10 ]; then
   echo "Value is greater than 10"
fi

Bash if else

if [conditional expression]; then
Statement or command if ‘conditional expression’ is true.
else
Statement or command if ‘conditional expression’ is false.
fi

#!/bin/bash
echo "Enter numeric value: "
read value

if [ $value -gt 10 ]; then
    echo "Value is greater than 10"
else
    echo "Value is less than 10"
fi

Bash if bash elif

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

#!/bin/bash
Num=55
if [ $Num-eq 60 ];then
    echo "Number is 60"
elif [ $Num-gt 60 ]; then
    echo "Number is greater than 60"
else
   echo "Number is less than 60"
fi

Bash case statement

case $variable in
pattern-1)
commands
;;
pattern-2)
commands
;;
pattern-3|pattern-4|pattern-5)
commands
;;
pattern-N)
commands
;;
*)
commands
;;
esac

Example

#!/bin/bash
echo "Please Enter Number between 1 to 3 "
read Num

case $Num in
1)
   echo "You Have enter One"
;;
2)
   echo "You Have enter Two"
;;
3)
    echo "You Have enter Three"
;; 
*)
   echo "Sorry you have enter wrong nmber"
;;
esac

Some rules to be follow while using conditions in bash script

i) Always keep space between square brackets.

if [$num -gt 25]; then # this will not work

# correct syntax is 
if [ $num -gt 25 ]; then # this is right syntax

ii) It is good habit to use quote for string variable as string variable may contain space

if [ "$num1" -ge "num2" ]; then

Understanding some conditional statement syntax

#!/bin/sh

file="/home/file.txt"

if [ -e $file ]; then
   echo "File exists"
 else
   echo "File does not exist"
fi

here we used -e option for file exists ,you may check some more option in our previous lessen.

Different between Double bracket and Single bracket in bash conditional statement

[ ] Single Bracket is POSIX shell to to enclose a conditional expression

[[ ]] Double Brackets is an enhance version of POSIX, which is supported by bash and other shells(zsh,ksh).

[ is a bash Builtin

[[ ]] are bash Keywords

if [ a < b ]; then # it will give an error

if [[ a < b ]]; then # this will work fine

For numeric comparison we use eq, ne,lt and gt.
But for double brackets for comparison we can use ==, !=, <, and > .

if [ Num1 lt Num2 ]; then #No Error
if [ Num1 < Num2 ]; then #Error: Num2 No such file or directory 
if [ Num1 \< Num2]; then #No Error ,It work with Escape
if [[ Num1 < Num2]]; then #No Error