Knowing the right shell commands may be all the artificial intelligence you need to make your computer work for you.
If each computer program could only perform one, unchangeable sequence of actions, software and computers would be almost useless compared to what they can do today. For this reason, every programming language since the invention of vacuum tubes has keywords and syntax structures that allow the programmer to implement flow control.
In a nutshell, flow control is the capability of a program to autonomously understands which actions to perform, or repeat, according to the current values of variables.
Of course, the kind of autonomous decision making you can implement with shell flow control is no artificial intelligence. However, if shell flow control is inadequate for what you need to do, that is a sign that a shell script might not be the right solution for your problem.
To the extent of the coverage in these Bash tutorials [1] [2], flow control has two forms, and each form has a simple and a more complex variant. The simpler variant consists of explaining to a script how to decide which action or sequence of actions to execute among a set of two or more possible choices. The more complex, but flexible variant is about iterations: You use keywords to make a script repeat some sequence of actions, possibly over all the elements of some set, one at a time, for a fixed or variable number of times.
In practice, both forms of flow control can be, and frequently are, nested in all ways imaginable. Unsurprisingly, it is also possible for a script to either alter – or just interrupt – the default sequence of actions started by any flow control statement. I will show how to do all this in this installment. Please note that, for space reasons, I only focus here on relatively high-level issues (i.e., when and how to use and mix the several flow control constructs). For the actual Bash test operators that can trigger any of these constructs, see the Advanced Bash-Scripting guide [3] for a list with plenty of examples.
Almost all of the content covered in this tutorial series is valid for all versions of the Bash shell currently installed with Linux. The main, if not only, exception is a few Bash array features described in the previous installment of this tutorial series [1, 2], which are supported only on Bash v4 or later.
In the interest of complete, up-to_date information, the Bash landscape became just a little bit more complicated on January 7th, 2019, with the release of Bash 5.0. Besides fixing assorted bugs, version 5.0 introduces several new features. The most relevant changes deal with Bash special variables. The $@ and $* variables, which I discussed in the previous installment [2], are expanded in different ways. In addition, there are now new variables called BASH_ARGV0 , EPOCHSECONDS , and EPOCHREALTIME , plus an option to expand associative arrays.
The Bash if / then / elif / else construct (Listing 1) shown in Figure 1 (where elif is simply a shortcut for "else if") does just what its name implies. That chunk of code in Listing 1 tells Bash the following:
