Bash Shell Scripting/A Few Notes On Terminology

The language of Bash commands is a Turing-complete programming language, meaning that if it is even theoretically possible for a given computation to be performed, then it is theoretically possible to perform it in Bash. Furthermore, from a practical standpoint, it is possible to do some very sophisticated things in Bash, and people often do. Nonetheless, it is usually described as a "shell-scripting language" (or similar) rather than a "programming language", and the programs written in it are usually referred to as "Bash shell scripts" (or similar) rather than "Bash programs". We will follow that convention here, preferring script for a Bash script and program only when referring to external programs that might be invoked from a Bash script.

We will also often use the term utility. This term is somewhat subjective, and usage varies, but it typically describes an external program that requires no human interaction and is intended to work well with other programs. All utilities are programs, so we will use the terms somewhat interchangeably. The extreme opposite of a "utility" is an application; that term is also subjective, with variable usage, but it typically describes an external program that has a relatively complex graphical user interface (GUI) and is not intended to work well with other programs. Since applications, by their nature, are not generally called from Bash scripts, we will have less occasion to use that term. Many programs are neither "utilities" nor "applications", and some programs have elements both of utilities and of applications.

Of the various elements of a Bash script, some are typically called commands, while others are typically called statements. We will observe this distinction here, but you need not overly concern yourself with it. Normally "command" is used in reference to invocations of external programs, of Bash built-ins such as echo that resemble external programs, of shell functions (which we will see below), and so on. Normally "statement" is used in reference to instances of Bash programming constructs such as if … then … else … fi or variable assignment (which we will see shortly). Perhaps confusingly, command is also used in reference to any Bash built-in, even if a use of that built-in would result in a "statement" rather than a "command"; for example, "an if statement" is a statement that uses "the if command". If this distinction seems too fine, do not worry; in context, it should always be sufficiently clear what is meant by one term or the other.