Iterating through a series of commands is a common programming practice. Often, you need to repeat a set of commands until a specific condition has been met, such as processing all the files in a directory, all the users on a system, or all the lines...

Often, you’ll find yourself trying to evaluate a variable’s value, looking for a specific value within a set of possible values. In this scenario, you end up having to write a lengthy if-then-else statement, like this:

Two additions to the bash shell provide advanced features that you can use in if-then statements: Double parentheses for mathematical expressions Double square brackets for advanced string handling functions The following sections describe each of these features in more detail.

The if-then statement allows you to use Boolean logic to combine tests. You can use these two Boolean operators: [ condition1 ] && [ condition2 ] [ condition1 ] || [ condition2 ] The first Boolean operation uses the AND Boolean operator to combine two conditions....

So far, all you’ve seen in the if statement line are normal shell commands. You might be wondering if the bash if-then statement has the ability to evaluate any condition other than a command’s exit status code. The answer is no, it can’t. However, there’s a...

Sometimes, you must check for several situations in your script code. For these situations, you can nest the if-then statements: To check if a logon name is not in the /etc/passwd file and yet a directory for that user still exists, use a nested if-then statement....