Bash IF statement is used to execute a set of instructions or commands based on whether a certain condition is satisfied or not. This post should serve you as a cheat sheet to most of the scenarios that you might encounter in your everyday scripting tasks that uses conditional execution.
Simple ‘IF’ statement Example
Let’s assign a value of ‘1’ to variable ‘x’ and use ‘IF’ statement to check the value of x.
x=1 if [ x == 1 ] then echo "value of x is 1" elif [ x == 2 ] then echo "value of x is 2" else echo "value of x is something else" fi
value of x is 1
‘x == 1’ is the condition. Below are a few common examples of using different conditions for evaluation.
# if string is empty, then..
if [ -z $str ]
# if string is Not empty, then..
if [ -n $str ]
# if two numbers are equal, then..
if [ $num1 -eq $num2 ]
# if two numbers are not equal, then..
if [ $num1 -ne $num2 ]
# if num1 is lesser than num2, then..
if [ $num1 -lt $num2 ]
# if num1 is lesser than OR equal to num2, then..
if [ $num1 -le $num2 ]
# if num1 is greater than num2, then..
if [ $num1 -gt $num2 ]
# if num1 is greater than OR equal to num2, then..
if [ $num1 -ge $num2 ]
# If str1 matches the regex then..
# The regex used in the below example checks if $str is a valid email address.
if [[ $str =~ ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,4}$ ]]
# note the double square brackets is used here for reqular or conditional expressions.
# if $num1 is lesser than $num2, then..
if (( $num1 < $num2 ))
# note that double round brackets is used for arithmatic operation.
# noclobber option prevents overwriting existing files with the > operator.
if [ -o noclobber ] If OPTIONNAME is enabled, then..
# If the condition (name is vijay) is not true
if [ ! name == "vijay" ]
# If both conditions are true, then..
if [ name == "vijay" ] && [ age -gt 18 ]
# if one of the conditions is true, then..
if [ name == "vijay" ] || [ age -gt 18 ]
File conditions
# if file Exists.. [ -e file.txt ] # if file is readable [ -r file.txt ] # if the file is a Symlink [ -h file.txt ] # if the file name specified is a Directory [ -d files ] # if file is Writable [ -w file.txt ] # if file Size is > 0 bytes [ -s file.txt ] # if the file name specifued is actually a file and not a directory [ -f file.txt ] # if file is Executable [ -x file.txt ] if file1.txt is newer than file2.txt [ file1.txt -nt file2.txt ] if file1.txt is older than file2.txt [ file1.txt -ot file2.txt ] if file1.txt and file2.txt are the same [ file1.txt -ef file2.txt ]
Nesting IF statements
IF statements in bash supports nesting. (i.e one if statement inside another)
a=4 if [ "$a" -gt 0 ] then if [ "$a" -lt 6 ] then echo "The value of a is in range 0-6" fi fi
The value of a is in range 0-6
However, a better way to check the above condition is by using Conditional Logic.
if [ "$a" -gt 0 ] && [ "$a" -lt 6 ] then echo "The value of a is in range 0-6" fi
The value of a is in range 0-6
