Every time you run an executable command, there a file somewhere on the system that gets executed. You can use the which command to determine the location of an executable command. For example, if you want to know the location of the rm command, you can run the which rm command:
elliot@ubuntu-linux:~$ which rm
/bin/rm
So now you know that rm lives in the /bin directory. Let's see the location of the reboot command:
elliot@ubuntu-linux:~$ which reboot
/sbin/reboot
As you can see, the reboot command lives in the /sbin directory.
