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What Exactly Are Commands?
A command can be one of four different things:
1. An executable program like all those files we saw in /usr/bin. Within this category, programs can be compiled binaries such as programs written in C and C++, or programs written in scripting languages such as the shell, perl, python, ruby, etc.
2. A command built into the shell itself. bash supports a number of commands in- ternally called shell builtins. The cd command, for example, is a shell builtin.
3. A shell function. These are miniature shell scripts incorporated into the environ- ment. We will cover configuring the environment and writing shell functions in later chapters, but for now, just be aware that they exist.
4. An alias. Commands that we can define ourselves, built from other commands.
Identifying Commands