GNU/Linux is full of all sort of files. Files are only which user deals with, after cluttering of various types and amount of files used need tool to make searching simpler.
`which` is a tool used to locate the command. It gives the executable address of command or tool. It has maily two addresses possible
1. /bin/command -- it is place where all the command which are already in linux by default.
2. /usr/bin/command -- it is place where user installed programs command resides.
which [option] [command]
[ -a ] :It gives output information.