How to Check if a String Contains a Substring in Bash

One of the most common operations when working with strings in Bash is to determine whether or not a string contains another string.

In this article, we will show you several ways to check if a string contains a substring.

Using Wildcards #

The easiest approach is to surround the substring with asterisk wildcard symbols (asterisk) * and compare it with the string. Wildcard is a symbol used to represent zero, one or more characters.

If the test returns true, the substring is contained in the string.

In the example below we are using the if statement and the equality operator (==) to check whether the substring SUB is found within the string STR:

#!/bin/bash

STR='GNU/Linux is an operating system'
SUB='Linux'
if [[ "$STR" == *"$SUB"* ]]; then
  echo "It's there."
fi

When executed the script will output:

It's there.

Using the case operator #

Instead of using the if statement you can also use the case statement to check whether or not a string includes another string.

#!/bin/bash

STR='GNU/Linux is an operating system'
SUB='Linux'

case $STR in

  *"$SUB"*)
    echo -n "It's there."
    ;;
esac

Using Regex Operator #

Another option to determine whether a specified substring occurs within a string is to use the regex operator =~. When this operator is used, the right string is considered as a regular expression.

The period followed by an asterisk .* matches zero or more occurrences any character except a newline character.

#!/bin/bash

STR='GNU/Linux is an operating system'
SUB='Linux'

if [[ "$STR" =~ .*"$SUB".* ]]; then
  echo "It's there."
fi

The script will echo the following:

It's there.

Using Grep #

The grep command can also be used to find strings in another string.

In the following example, we are passing the string $STR as an input to grep and checking if the string $SUB is found within the input string. The command will return true or false as appropriate.

#!/bin/bash

STR='GNU/Linux is an operating system'
SUB='Linux'

if grep -q "$SUB" <<< "$STR"; then
  echo "It's there"
fi

The -q option tells grep to be quiet, to omit the output.

Conclusion #

Checking if a string contains a substring is one of the most basic and frequently used operations in Bash scripting.

After reading this tutorial, you should have a good understanding of how to test whether a string includes another string. You can also use other commands like awk or sed for testing.

If you have any questions or feedback, feel free to leave a comment.