How to Check if String Contains Substring in Bash

In bash, you can check if a string contains a substring using the [[ $string == *substring* ]] syntax inside an if statement. This tests if substring appears anywhere within $string.

Syntax

The basic syntax to check if a string contains a substring in bash is:

• if [[ $string == *substring* ]]; then — checks if substring is anywhere in $string.
• then — starts the block of code to run if the condition is true.
• fi — ends the if block.

bash

if [[ $string == *substring* ]]; then
  echo "Substring found"
fi

Example

This example shows how to check if the word hello is inside a string variable and prints a message if it is found.

bash

#!/bin/bash
string="hello world"
if [[ $string == *hello* ]]; then
  echo "The string contains 'hello'"
else
  echo "The string does not contain 'hello'"
fi

Output

The string contains 'hello'

Common Pitfalls

Common mistakes include:

• Using single brackets [ ] instead of double brackets [[ ]], which does not support pattern matching with *.
• Not quoting variables can cause word splitting or globbing issues.
• Using == inside single brackets [ ] is not POSIX compliant; prefer = there.

bash

# Wrong way (does not work as expected):
string="hello world"
if [ "$string" == *hello* ]; then
  echo "Found"
fi

# Right way:
if [[ $string == *hello* ]]; then
  echo "Found"
fi

Output

Found

Quick Reference

Usage	Description
[[ $string == *substring* ]]	True if substring is anywhere in string
[[ $string != *substring* ]]	True if substring is NOT in string
if [[ $string == *sub* ]]; then ... fi	Run code if substring found
Use double brackets [[ ]] for pattern matching	Required for wildcard matching with *

Key Takeaways

Use double brackets [[ ]] with wildcards * to check substring presence in bash.

Always quote variables to avoid word splitting and globbing issues.

Single brackets [ ] do not support pattern matching with * for substring checks.

Use == inside [[ ]] for pattern matching, not inside [ ].

Test your script with different strings to ensure correct substring detection.