which

shows the full path of (shell) commands

References

Options

-a, --all Print all matching executables in PATH, not just the first
-i, --read-alias Read aliases (default)
--skip-alias Ignore option --read-alias, if any.
--read-functions Read shell function definitions from stdin, reporting matching ones on stdout
--skip-functions Ignore option --read-functions, if any.
……

Print the first matching executable

which <command_name>

# e.g.
$ which bash
/bin/bash

Print all matching executables

$ which -a bash
/bin/bash
/usr/bin/bash

On BSD, builtin where == which -a

$ where php
/usr/local/bin/php
/usr/bin/php

$ which php
/usr/local/bin/php
/usr/bin/php