Where

where is a very useful system command used to find out how many different versions of the same command or file you have strewn throughout your path and in which order they will be found if you don't specify a path when asking for one. For example:

ph34r# where time
time: shell built-in
/usr/bin/time
ph34r#

See also which for just finding out where the first returned program name from your path directory is without bothering to list all the others, or whereis for finding man pages and sources as well as the program itself.