Which

which 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 /usr/bin/time. So which one gets executed if you simply type "time"?

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

This is particularly useful to know in the case of the time command above, because csh's built-in "time" does not have the same featureset described in the man page for time, which is based on the version to be found at /usr/bin/time.

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