How to Check if a String Contains a Substring in Bash

Introduction

Before we begin talking about how to check if a string contains a substring in Bash, let's briefly understand – What is String ?

String is a data type used to represent and manipulate text. It is a sequence of characters enclosed in quotation marks. Understanding what a string is and its importance in programming is crucial.

Strings can contain letters, numbers, symbols, and spaces, enabling programmers to work with and combine textual information efficiently. String manipulation involves tasks like concatenation (joining strings together), extracting substrings, and searching for patterns within strings.

In this tutorial, you will check if a string contains a substring in Bash. We will also address a few FAQs on how to check if a string contains a substring in Bash.

Advantages of String

Versatility: Strings can store and manipulate alphanumeric characters, making them adaptable for various tasks.
Ease of Use: Manipulating strings is straightforward due to built-in methods for concatenation, searching, and extraction.
Text Processing: Strings enable efficient handling of textual data, enabling tasks like parsing, formatting, and validation.
Interoperability: Strings facilitate seamless data exchange between different systems and programming languages.
User Interaction: Strings play a crucial role in developing user interfaces, allowing for input validation, error messages, and display of dynamic content. (89 words)

Using Wildcards

The simplest method is to use the asterisk * as a wildcard and compare the substring to the original text. Wildcards may stand in for a single character or a range of them.

If the test results in true, then the substring is found inside the string.

To determine whether the substring SUB is present in the string STR, we may use the if statement in conjunction with the equality operator (==), as seen in the following example:

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

Whenever this script is run, it will provide the following results:

Output

It's there.

Using the case operator

In addition to the if statement, the case statement may be used to determine whether a string contains a certain string.


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

case $STR in

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

Using Regex Operator

It is also possible to use the regex operator =~ to check for the presence of a substring inside a string. Using this operator will treat the correct string as a regular expression.

A period followed by an asterisk (.*) matches zero or more instances of any character other than a newline.


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

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

The script will echo the following:

Output

It's there.

Using Grep

If you need to locate text inside another string, the grep command can do that, too.

For this example, we'll use grep to determine whether the substring $SUB appears anywhere in the string $STR. Depending on the circumstances, the command may return true or be false.

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

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

With the -q option, grep is instructed to suppress its output.

FAQs to Check if a String Contains a Substring in Bash

Can I use regular expressions to check for substrings?

Yes, by using the =~ operator, you can match substrings using regular expressions in Bash.

How do I check if a substring exists at the beginning or end of a string?

With the ^ symbol, you can check for a substring at the beginning, while the $ symbol checks for a substring at the end of a string.

Is there a way to count the number of occurrences of a substring in a string?

Yes, by using the wc command with the -l option along with grep, you can count the number of occurrences of a substring.

Are substring searches case-sensitive in Bash?

Yes, by default, substring searches are case-sensitive in Bash. However, you can use the nocasematch option to perform case-insensitive searches.

What if I want to perform a substring search ignoring the leading or trailing spaces?

By using the parameter expansion feature, you can trim leading and trailing white spaces before performing the substring search.

How can I check if a string contains a substring using a case-insensitive search?

Set the nocasematch option using shopt to perform a case-insensitive substring search in Bash.

Can I use variables to search for substrings dynamically?

Absolutely, you can use variables to store the substring you are searching for, allowing dynamic substring searches in Bash.

Conclusion

In Bash programming, one of the most fundamental and often used operations is testing whether a string includes a substring.

In this tutorial, you will learn to determine whether a string contains another string by comparing them. In addition to the sed and awk tests, you may also use additional commands.

If you have any queries, please leave a comment below and we’ll be happy to respond to them.