How to check if a string contains a substring in Bash?

To check if a string contains a substring in Bash, you can use the following methods:

1. Using [[ ]] with Wildcard Matching

string="Hello World"
substring="World"

if [[ "$string" == *"$substring"* ]]; then
    echo "Substring found!"
else
    echo "Substring not found."
fi

Explanation:

• *"$substring"* acts as a wildcard pattern to match the substring anywhere in the string.
• Quotes around variables prevent globbing or splitting issues.
• Best for simple substring checks.

2. Using =~ (Regex Operator)

string="Hello World"
substring="Wor"

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

Notes:

• The =~ operator treats the substring as a regular expression.
• Escape regex special characters (e.g., ., *, +) if you want literal matches:

  substring="a\.c"  # Matches "a.c" literally (not "abc")

3. Using case Statements

string="Hello World"
substring="lo W"

case "$string" in
    *"$substring"*) echo "Found" ;;
    *) echo "Not found" ;;
esac

Use Case:

• Useful in POSIX-compliant scripts (works in sh and bash).

4. Using grep (External Command)

string="Hello World"
substring="World"

if echo "$string" | grep -q "$substring"; then
    echo "Substring found!"
fi

Notes:

• -q suppresses output (quiet mode).
• Avoid this method in performance-critical scripts (spawns a subshell).

Key Differences

Method	Pros	Cons
Wildcard ([[ ]])	Fast, no subshell, simple	Bash-specific syntax
Regex (=~)	Flexible (regex support)	Requires escaping special characters
case Statement	POSIX-compliant	Verbose for simple checks
grep	Works in all shells	Overhead of external process

Examples

Check for Multiple Substrings

string="Hello World"
if [[ "$string" == *"Hello"* && "$string" == *"World"* ]]; then
    echo "Both substrings found!"
fi

Case-Insensitive Check

shopt -s nocasematch  # Enable case-insensitive matching
string="Hello World"
substring="world"

if [[ "$string" =~ "$substring" ]]; then
    echo "Found (case-insensitive)!"
fi
shopt -u nocasematch  # Disable afterward

Summary

• Preferred Method: Use [[ "$str" == *"$substr"* ]] for simplicity and efficiency.
• Regex Use: Opt for =~ if you need pattern matching (e.g., partial words, wildcards).
• Avoid grep unless necessary for compatibility.