Which

This is a very useful system command used to find out exactly what is going to get executed when you ask for a command without specifying a path. For instance, the csh shell has a built-in function called time, but there is also a version at /bin/time. So which one gets executed if you simply type "time"?

ph34r# which time
time: shell built-in command.
ph34r#

See also where for figuring out how many different versions of the same command or file you have strewn throughout your path.