The four categories of linux commands

All Linux commands must fall into one of these following four categories:

1. An executable program: Which is usually written in the C programming language. The cp command is an example of an executable command.
2. An alias: Which is basically another name for a command (or a group of commands).
3. A shell builtin: The shell supports internal commands as well. The exit and cd commands are two examples of a shell builtin command.

1. A shell function: These are functions that help us achieve a specific task and are essential in writing shell scripts.  We will cover this in more detail later, for now, just know they exist.