When you’re trying to find your way around the Linux file system and want some information on specific commands, the whereis, whatis, and which commands can help. Each provides a different view of the command you’re asking about. In this post, I’ll compare these commands and explain what they tell us and what they don’t tell us.
The which command is the simplest of the three. When you use it to ask about a Linux command, it will run down your search path looking for executable files by the name you specify. These can be commands that are available on your system as well as scripts. As long as the files provide you with execute privilege, they fit the bill. Here are some examples:
Source:: Network World – Linux