Checking if a String Contains a Substring in Bash

In Bash, checking if a string contains a substring is a common task that can be accomplished using various methods. In this tutorial, we will explore the most efficient and idiomatic ways to perform this check.

Using Double Brackets

One way to check if a string contains a substring is by using double brackets [[ ]] with the == operator and wildcard patterns. The syntax is as follows:

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

In this example, $string is the variable containing the string to be searched, and $substring is the variable containing the substring to search for. The * wildcards match any characters before and after the substring.

Using Regex

Another way to check if a string contains a substring is by using regular expressions with the =~ operator. The syntax is as follows:

if [[ $string =~ "$substring" ]]; then
  echo "It's there!"
fi

In this example, $string and $substring are used in the same way as before.

Using Case Statements

A more efficient way to check if a string contains a substring is by using a case statement. The syntax is as follows:

case $string in
  *"$substring"*)
    echo "It's there!"
    ;;
esac

This method is more efficient because it avoids the overhead of spawning a new process or using regex engines.

Using Grep

You can also use grep to check if a string contains a substring. The syntax is as follows:

if grep -q "$substring" <<< "$string"; then
  echo "It's there!"
fi

In this example, grep searches for the substring in the string and returns an exit status indicating success or failure.

Case-Insensitive Matching

To perform case-insensitive matching, you can use parameter expansion to convert both the string and the substring to lowercase or uppercase. The syntax is as follows:

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

Alternatively, you can modify the case statement to perform case-insensitive matching by converting the string and substring to lowercase or uppercase.

Conclusion

In conclusion, checking if a string contains a substring in Bash can be accomplished using various methods, including double brackets, regex, case statements, and grep. The most efficient method is often the case statement, which avoids overhead and is easy to read. By choosing the right method for your use case, you can write more efficient and effective Bash scripts.