With aliases and search paths, it can be easy to lose track of what is executed when you enter a command. The which command can tell you which file is executed if you enter a command line that begins with a specific command. The where command can tell you where versions of the command are located. For example:

which kill

tells you:

kill: shell built-in command.

and the command:

where kill

tells you:

kill is a shell built-in
/bin/kill