The Linux / Unix operating systems come with many commands that the user can enter into the computer from the keyboard and use to interact with the computer. There are two kinds of commands that come with a Linux / Unix operating system: Shell Commands and Linux/Unix Commands. Here is a comparison of the two:

Built-in Shell Commands:

What are they?

They are part of a shell. In other words, each shell (e.g., C Shell, Bourne Shell and Korn Shell) has a set of commands built into its program.

Are the shell commands always the same on every computer?

More Here.