How to Check if a String Contains a Substring in Bash

When it comes to string manipulation in Bash, checking if a string contains a specific substring is a common requirement. In this post, we’ll explore various methods to achieve this, ranging from simple conditional checks to more advanced techniques.

Using Conditional Expressions

The simplest way to check if a string contains a substring is by using conditional expressions. Here’s how you can do it:

string="My string"
substring="foo"

if [[ $string == *"$substring"* ]]; then
    echo "It's there!"
else
    echo "Not found."
fi

Explanation

• The double square brackets [[ ... ]] allow for advanced string comparison in Bash.
• The * wildcards are used to match any characters before and after the substring.

Case Sensitivity

By default, the check is case-sensitive. If you want to perform a case-insensitive check, you can modify the string before comparing:

if [[ "${string,,}" == *"${substring,,}"* ]]; then
    echo "It's there!"
fi

In this example, ${string,,} converts the string to lowercase before the comparison.

Using grep

Another method involves using grep, which is especially useful when you’re dealing with larger text or need regex capabilities:

if echo "$string" | grep -q "$substring"; then
    echo "It's there!"
else
    echo "Not found."
fi

Explanation

• The -q option tells grep to be quiet, meaning it won’t output anything but will set the exit status based on the match.
• The command substitution echo "$string" pipes the string to grep.

Performance Consideration

Using grep introduces additional processes, which can be less efficient for simple checks compared to direct string comparison.

Using a case Statement

For a more POSIX-compliant approach, you can use a case statement, which is often cleaner and avoids some of the pitfalls of string comparisons:

case "$string" in
    *"$substring"*)
        echo "It's there!"
        ;;
    *)
        echo "Not found."
        ;;
esac

Why Use case?

• The case statement can handle multiple patterns and is generally faster because it avoids the overhead of external commands.
• It’s portable across different shells, making it a good choice for scripts intended to run in varied environments.

Creating a Function for Reusability

If you find yourself needing to check for substrings frequently, consider wrapping the logic in a function:

string_contains() {
    local string="$1"
    local substring="$2"

    if [[ $string == *"$substring"* ]]; then
        return 0  # Found
    else
        return 1  # Not found
    fi
}

# Usage
if string_contains "$string" "My"; then
    echo "Substring found!"
else
    echo "Substring not found!"
fi

In Bash, there are several effective ways to check if a string contains a substring. Depending on your needs—whether it’s performance, portability, or additional features—you can choose the method that best suits your situation. Whether using conditional expressions, grep, case statements, or custom functions, mastering string manipulation will enhance your scripting skills and efficiency.