4 Types Of Commands

1) An executable program
    - these are compiled binaries written in Java, or scripting  languages like Perl or Python.
    - Examples in /usr/bin


2) Commands built into the shell
    - Internal commands are called shell builtins contained in bash
    - Example is cd


3) Shell functions
    - Miniature shell scripts incorporated in the the environment.


4) Alias
    - Custom commands, built from other commands




Identifying commands with 'type'


type command


Example
$ type cd
cd is a shell builtin




Identifying the executable's location with 'which'


which command


Example
$ which ls
/usr/bin/ls