The which command locates an executable file in your shell’s search path. If you’ve been invoking a program by typing its name:

$ who

the which command tells you where this command is located:

$ which who
/usr/bin/who

You can even find the which program itself:

$ which which
/usr/bin/which

If several programs in your search path have the same name (for example, /usr/bin/who and /usr/local/bin/who), which reports only the first.