which (locate a command)
This command shows you where in your executable path a program resides. For example:
$ which touch
Don't type in the dollar sign; I used it to indicate the command prompt. The output shows that /usr/bin/touch is executed whenever the command →touch is invoked. Now try which with the option -a (“all”). This will show all places in the path where an executable file is found:
$ which -a touch
You can see that my system also has a file called /bin/touch, but /usr/bin/touch has priority over it, that's why it's listed first. In this case, there is no real difference between the two “competing” commands because /usr/bin/touch is a symbolic link to /bin/touch (see →ln). But it may happen that you're not sure which program is executed when you call a particular command, and that's when which comes in handy.
It's important to note that which can only identify executable files. Shell builtins such as →cd or →alias do not have their own executable file, so which will not give you any information about them. Another common stumbling block for newcomers is that root has its own executable path. It follows that you won't get any output when you call which shutdown, for example, because →shutdown is in root's path. You have to run sudo which shutdown in this case, which will output the following:
The directory /sbin is not part of a normal user's executable path because it contains programs for administrative work.