How to Compare Strings in Bash: Syntax and Examples

In bash, you compare strings using the [ ] or [[ ]] test commands with operators like == or !=. For example, [ "$str1" = "$str2" ] returns true if the strings are equal.

Syntax

Use [ ] or [[ ]] to compare strings in bash. Inside these brackets, use = or == to check if strings are equal and != to check if they are different. Always quote your variables to avoid errors with spaces or empty strings.

• [ "$str1" = "$str2" ]: True if strings are equal.
• [ "$str1" != "$str2" ]: True if strings are not equal.
• [[ $str1 == $str2 ]]: Similar but more flexible and safer for pattern matching.

bash

[ "$str1" = "$str2" ]
[[ $str1 == $str2 ]]

Example

This example shows how to compare two strings and print a message based on whether they match or not.

bash

#!/bin/bash
str1="hello"
str2="world"

if [ "$str1" = "$str2" ]; then
  echo "Strings are equal"
else
  echo "Strings are not equal"
fi

# Using [[ ]] with pattern matching
pattern="hel*"
if [[ $str1 == $pattern ]]; then
  echo "str1 matches the pattern"
fi

Output

Strings are not equal
str1 matches the pattern

Common Pitfalls

Common mistakes include not quoting variables, which can cause errors if strings have spaces or are empty. Also, [ ] does not support pattern matching like [[ ]]. Avoid using single = inside [ ] for equality; use = or == instead.

bash

# Wrong: no quotes, can cause errors
str1=hello world
if [ "$str1" = "hello world" ]; then
  echo "Match"
fi

# Right: quotes prevent errors
str1="hello world"
if [ "$str1" = "hello world" ]; then
  echo "Match"
fi

Output

Match

Quick Reference

Operator	Meaning	Example
=	Equals	[ "$a" = "$b" ]
==	Equals (in [[ ]])	[[ $a == $b ]]
!=	Not equals	[ "$a" != "$b" ]
<	Less than (lexicographically)	[[ $a < $b ]]
>	Greater than (lexicographically)	[[ $a > $b ]]

Key Takeaways

Always quote string variables when comparing to avoid errors.

Use [[ ]] for safer and more flexible string comparisons.

Use = or == for equality and != for inequality in string tests.

Avoid unquoted variables inside [ ] for string comparison.

Pattern matching works only with [[ ]] and not with [ ].