Where are the commands?

A 'command' is usually just a program on the filesystem somewhere. A few commands, however are built-in to the command line.

Finding commands

To find out where a program is, you can use the which command.

For example, let's find out where ls is:

% which ls
/bin/ls

This tells us that ls is a program in /bin. So typing ls is just the same as typing /bin/ls.

Question

Where do some other programs, like echo, cp, or hostname, live? Are they all in /bin?

Knowing where to look

So how does the command line know which directories to look in? The answer is the $PATH environment variable. To see what's in this variable right now, you can use echo to print it:

% echo $PATH

(Remember that echo just prints out its arguments to stdout.)

Note

As you can see, the directories in $PATH are separated by colons. Can you use tr in a pipeline with echo as above, to print them out seperated by newlines instead?

Hint: '\n' is the way to write a newline character so tr can understand it (but make sure and put the '\n' in single quotes).

You will probably see that the command line is looking in multiple directories for programs to run - it simply starts at the top and works down the list until it finds a program with the given name. If it can't find one, it prints a 'command not found' error instead.

So if you want to break your terminal, try resetting $PATH:

% PATH=""
% ls
command not found: ls

Uh-oh! You'll have to start a new terminal now, or re-set $PATH to the right values, to get it back.