How to Check if String Contains Substring in Bash

Quick Answer: Check if String Contains Substring

Use the [[ operator with wildcards: if [[ $string == *"substring"* ]]; then echo "found"; fi. This is the fastest and cleanest method in modern Bash.

Quick Comparison: Substring Checking Methods

Method	Speed	Case-sensitive	Best For
*[[ == …* ]]**	Fastest	Yes	Most cases
Parameter expansion	Very fast	Yes	Avoiding pipes
grep	Medium	Optional (-i flag)	Regex patterns
=~	Very fast	Yes	Complex patterns

Bottom line: Use [[ $str == *"pattern"* ]] for best performance and clarity.

Check if a string contains a substring using various methods in Bash.

Method 1: Using [[ ]] (Recommended)

The [[ operator (double brackets) is Bash-specific and offers the cleanest syntax for substring checking. The wildcards *pattern* match anything before and after the pattern, so the condition becomes “does the string contain this pattern anywhere?”

text="Hello, World!"

if [[ $text == *"World"* ]]; then
  echo "String contains 'World'"
fi

This is fast because it’s built into Bash (no external commands), readable because the syntax is straightforward, and flexible because it supports wildcards and patterns. The asterisks mean “zero or more of any character,” so *World* means “anything, then World, then anything.”

Case-insensitive checking is also straightforward—convert to lowercase with ${text,,}:

if [[ ${text,,} == *"world"* ]]; then
  echo "Contains 'world' (case-insensitive)"
fi

The ${text,,} syntax converts the entire string to lowercase (Bash 4+), then checks against the lowercase pattern. This is much simpler than older workarounds using tr or sed.

When to Use [[ ]] Substring Checking

Use this method when:

You need simple substring checking
Performance matters
You’re using Bash 3.2 or newer
You don’t need complex regex patterns

Method 2: Using Parameter Expansion

text="Hello, World!"

if [ "$text" != "${text#*World}" ]; then
  echo "String contains 'World'"
fi

Method 3: Using grep

text="Hello, World!"

if echo "$text" | grep -q "World"; then
  echo "String contains 'World'"
fi

Case-insensitive:

if echo "$text" | grep -iq "world"; then
  echo "Contains 'world' (case-insensitive)"
fi

Method 4: Using Regular Expression

text="Hello, World!"

if [[ $text =~ World ]]; then
  echo "String contains 'World'"
fi

Practical Examples

Check if Variable is Empty

if [[ -z $var ]]; then
  echo "Variable is empty"
fi

Extract Matches

text="My email is john@example.com"

if [[ $text =~ ([a-z]+@[a-z]+\.[a-z]+) ]]; then
  echo "Found email: ${BASH_REMATCH[1]}"
fi

Search Multiple Patterns

text="error in file.txt"

if [[ $text == *"error"* || $text == *"warning"* ]]; then
  echo "Found error or warning"
fi

Performance Comparison

[[ ]]: Fastest (built-in Bash)
Parameter expansion: Fast (no external command)
grep: Slower (external process)
Regex: Medium speed (pattern matching)

Use [[ $text == *"pattern"* ]] for best performance.