How do you check if a string contains a substring in shell scripting?

In shell scripting, checking whether a string contains another substring can be achieved in several ways. I will focus on two common methods: using the grep command and leveraging Shell's built-in features.

Method One: Using the grep Command

grep is a powerful text search tool that can be used to check if a string contains a specific substring. Here is an example using grep:

bash
#!/bin/bash

# Define the main string and the substring to check
mainString="Hello, welcome to the world of Linux"
subString="world"

# Use grep to check for the substring
if echo "$mainString" | grep -q "$subString"; then
    echo "The substring exists in the main string."
else
    echo "The substring does not exist in the main string."
fi

In this script, we use the -q option of grep for quiet mode searching, so grep does not print matching lines to standard output; instead, it relies on the exit status code to indicate whether a match was found (with an exit status code of 0 when a match is present).

Method Two: Using Shell's Built-in Features (e.g., bash's Conditional Expressions)

In bash shell, we can directly use built-in string manipulation features to check if a string contains another string without invoking external commands like grep. This method is typically more efficient as it avoids the overhead of starting new processes. Here is an example:

bash
#!/bin/bash

# Define the main string and the substring to check
mainString="Hello, welcome to the world of Linux"
subString="world"

# Use bash's conditional expression to detect the substring
if [[ "$mainString" == *"$subString"* ]]; then
    echo "The substring exists in the main string."
else
    echo "The substring does not exist in the main string."
fi

Here, we use bash's [[ ... ]] conditional expression and employ the wildcard * to match any number of characters. If subString is part of mainString, the conditional expression evaluates to true.

Summary

These two methods have their pros and cons: the grep method is more general and can be used across various Shell environments; while the method using bash's built-in features is more efficient but depends on bash-specific features and may not be available in all Shells. In practical applications, you can choose the appropriate method based on your specific requirements and environment.