A substring is a adjacent sequence of characters within a string. In Bash, you can check if a string contains a substring using grep command and comparison methods.

if [[ $string == *substring* ]]; then

    # do something if substring is found

fi

Here, the * characters are wildcards, which match any sequence of characters. If the substring appears anywhere in the string, the comparison will evaluate to true, and the code inside the if block will be executed.

Alternatively, you can use the grep command to search for the substring in the string. Here's an example:
if echo "$string" | grep -q "substring"; then

    # do something if substring is found

fi

In this case, the -q option tells grep to operate in quiet mode, which means it won't output anything to the console. Instead, it will return a zero exit code if the substring is found, and a non-zero exit code otherwise. The if statement checks the exit code and executes the code inside the block if the substring is found.