Bash IF

Bash IF statement is used for conditional branching in the sequential flow of execution of statements.

We shall learn about the syntax of if statement and get a thorough understanding of it with the help of examples.

Options for IF statement in Bash Scripting

If statement can accept options to perform a specific task. These options are used for file operations, string operations, etc. In this topic, we shall provide examples for some mostly used options.

  • Example – if -z (to check if string has zero length)
  • Example – if -s (to check if file size is greater than zero)
  • Example – if -n (to check if string length is not zero)
  • Example – if -f (to check if file exists and is a regular file)

Syntax of Bash If

Bash If statement syntax is

if [ expression ];
# ^ ^          ^      please note these spaces
then
    statement(s)
fi

Note : Observe the mandatory spaces required, in the first line, marked using arrows. Also the semicolon at the end of first line. And if conditional statement ends with fi

The syntax to include multiple conditions with AND operator is

if [ expression ] && [ expression_2 ];
then
    statement(s)
fi

The syntax to include multiple conditions with OR operator is

if [ expression ] || [ expression_2 ];
then
statement(s)
fi

For compound expressions, following if syntax is allowed. Please observe that the condition has double square brackets.

if [[ expression_1 && expression_2 || expression_3 ]];
then
    statement(s)
fi
Bash If statement

Example 1 – Bash IF

In the following example, we demonstrate the usage of if statement with a simple scenario of comparing two strings.

Bash Script File

#!/bin/bash

# if condition is true
if [ "hello" == "hello" ];
then
	echo "hello equals hello"
fi

# if condition is false
if [ "hello" == "bye" ];
then
	echo "hello equals bye"
fi

Note : In bash, respect each token/literal. Observe the spaces provided after  if   [   string literal “hello” and  ==

When you run the above bash if example script file in a shell program like Terminal, the result would be

Output

~$ ./bash-if-example 
hello equals hello

Example 2 – Bash IF – Compare Numbers

In the following example, we will compare numbers using if statement.

Bash Shell Script

#!/bin/bash

# if condition (greater than) is true
if [ 8 -gt 7 ];
then
	echo "is 8 greater than 7 : true "
fi

# if condition (greater than) is false
if [ 7 -gt 8 ];
then
	echo "is 7 greater than 8 : false "
fi

# if condition (less than) is true
if [ 7 -lt 8 ];
then
	echo "is 7 lesser than 8 : true "
fi

# if condition (lesser than) is false
if [ 8 -lt 7 ];
then
	echo "is 8 lesser than 7 : false "
fi

# if condition (equal to) is true
if [ 8 -eq 8 ];
then
	echo "is 8 equals 8 : true "
fi

# if condition (equal to) is false
if [ 7 -eq 8 ];
then
	echo "is 7 equals 8 : false "
fi

When you run the above bash script file in shell program like Terminal, the result would be

Output

~$ ./bash-if-example-2 
is 8 greater than 7 : true 
is 7 lesser than 8 : true 
is 8 equals 8 : true

Example 3 – Using AND in IF Expression

In this example, we shall learn to use AND operator && to combine multiple conditions and form an expression (compound condition).

Bash Script File

#!/bin/bash

# TRUE && TRUE
if [ "hello" == "hello" ] && [ 1 -eq 1 ];
then
	echo "if 1"
fi

# TRUE && FALSE
if [ "hello" == "hello" ] && [ 1 -gt 2 ];
then
	echo "if 2"
fi

Output

~$ ./bash-if-example-3
if 1

Example 4 – Using OR in IF Expression

In this example, we shall learn to use OR operator || to combine multiple conditions and form an expression (compound condition).

Bash Script File

#!/bin/bash

# TRUE || FALSE
if [ "hello" == "hello" ] || [ 1 -eq 3 ];
then
	echo "if 1"
fi

# FALSE || FALSE
if [ "hello" == "hi" ] || [ 1 -gt 2 ];
then
	echo "if 2"
fi

Output

~$ ./bash-if-example-4
if 1

Example 5 – Bash IF with Multiple Conditions

In this example, we shall learn to include multiple conditions combined with AND and OR forming a single expression.

Bash Script File

#!/bin/bash

# FALSE && TRUE || FALSE || TRUE evaluates to TRUE
if [[ 8 -eq 11 && "hello" == "hello" || 1 -eq 3 || 1 -eq 1 ]];
then
	echo "if 1"
fi

# FALSE && TRUE || FALSE evaluates to FALSE
if [[ 8 -eq 11 && "hello" == "hello" || 1 -eq 3 ]];
then
	echo "if 2"
fi

Output

~$ ./bash-if-example-5
if 1

Example 6 – Bash IF -z

If statement when used with optionz , returns true if the length ofthe string is zero. Following example proves the same.

Bash Script File

#!/bin/bash

if [ -z "" ];
then
	echo "zero length string"
fi

if [ -z "hello" ];
then
	echo "hello is zero length string"
else
	echo "hello is not zero length string"
fi

Example 7 – Bash IF -s

Bash If statement when used with options , returns true if size of the file is greater than zero.

Bash Script File

if [ -s /home/tutorialkart/sample.txt ];
then
    echo "Size of sample.txt is greater than zero"
else
    echo "Size of sample.txt is zero"
fi

Example 8 – Bash IF -n

Bash If statement when used with optionn , returns true if the length ofthe string is greater than zero.

Bash Script File

#!/bin/bash

if [ -n "learn" ];
then
	echo "learn is non-zero length string"
fi

if [ -n "hello" ];
then
	echo "hello is non-zero length string"
else
	echo "hello is zero length string"
fi

Example 9 – Bash IF -f

Bash If statement when used with optionf , returns true if the length ofthe string is zero. Following example proves the same.

Bash Script File

#!/bin/bash

if [ -f /home/tutorialkart/sample.txt ];
then
    echo "sample.txt - File exists."
else
    echo "sample.txt - File does not exist."
fi

Conclusion

In this Bash Tutorial, we learned conditional branching in the sequential flow of execution of statements with bash if statement. We learned the syntax and usage of Bash IF with example shell scripts.