Determining a command's type

You can use the type command to determine the type (category) of a command. For example, if you want to know the type of the pwd command you can simply run the type pwd command:

elliot@ubuntu-linux:~$ type pwd 
pwd is a shell builtin

So now you know that the pwd command is a shell builtin command. Now let's figure out the type of the ls command:

elliot@ubuntu-linux:~$ type ls
ls is aliased to `ls --color=auto'

As you can see, the ls command is aliased to ls --color=auto. Now you know why you see a colorful output every time you run the ls command. Let's see the type of the date command:

elliot@ubuntu-linux:~$ type date 
date is /bin/date

Any command that lives in /bin or /sbin is an executable program. Therefore, we can conclude that the date command is an executable program as it resides in /bin.

Finally, let's determine the type of the type command itself:

elliot@ubuntu-linux:~$ type type 
type is a shell builtin

It turns out the type command is a shell builtin command.