Each application/program over time, as it evolves (hopefully for the better!), is released in several versions. Ever wondered which version is being used or where/what all the versions installed are? Its quite easy with just using simple english in unix!
Lets start with a quick snippet on what an executable is with ‘whatis’. It searches for whole words within the whatis database that contains short descriptions of system commands.
$ whatis ruby irb(1), erb(1), ri(1), rdoc(1), testrb(1) - Ruby helper programs ruby(1) - Interpreted object-oriented scripting language
Now ‘whereis’ does the job of indicating where the specified executable is.