Working with Commands

commands can be one of four different things:
- an executable program.
- a command built into the shell itself.
- a shell function.
- an alias.

type

display a command’s type,command is the name that you want to examine.

type command

which

dispaly an executable’s location.Only works for executable program,not builtins and aliases

which ls#will show /bin/ls

which cd#cd is a shell builtin command,can't use which

what is

display the name and one line description of a man page matching a specified keyword.

whatis ls#will show  "ls         (1) - list directory contents"

alias

Using the alias command,we can create our own command.

alias foo='cd /usr;ls -alh;cd -'

then you can use the foo command,it’s function is to change the directory to /usr,then ls and return to the previous directory.
Also,you can remove the alias command using unalias command:

unalias foo