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