The [[ Operator

When to use the [[ operator

We’ve gotten acquainted with if statements. They call a Bash built-in command or a utility in a specific condition.

Let’s consider the options we have to make an if condition. Let’s suppose we want to check if a text file contains a phrase. When the phrase is present, we print a message in the log file.

We can combine the if statement and the grep utility to solve the task. Let’s put the grep call in the if condition. If the utility succeeds, it returns zero exit status. In this case, the if condition equals true and Bash prints the message.

The grep utility prints its result to the output stream. We do not need it when calling grep in the if condition. We can get rid of the utility’s output using the -q option. Then, we get the following if statement:

if grep -q -R "General Public License" /usr/share/doc/bash
then
  echo "Bash has the GPL license"
fi

Let’s click on the “Run” button and then run the script using ./script.sh.