The PATH Environment Variable in UNIX

The PATH environment variable has a special format. Let's see what it looks like:

dbettis@rhino[~]$ echo $PATH
/usr/local/bin:/bin:/usr/bin:/sbin:/usr/sbin:.

It's essentially a :-separated list of directories. When you execute a command, the shell searches through each of these directories, one by one, until it finds a directory where the executable exists. Remember that we found ls in /bin, right? /bin is the second item in the PATH variable. So let's remove /bin from PATH. We can do this by using the export command:

dbettis@rhino[~]$ export PATH=/usr/local/bin:/usr/bin:/sbin:/usr/sbin:.

Make sure that the variable is set correctly:

dbettis@rhino[~]$ echo $PATH
/usr/local/bin:/usr/bin:/sbin:/usr/sbin:.

Now, if we try to run ls, the shell no longer knows to look in /bin!

dbettis@rhino[~]$ ls
-bash: ls: command not found

As expected, ls can no longer be found. Let's add /bin back to PATH, as ls is a very useful thing to have.

dbettis@rhino[~]$ export PATH=/usr/local/bin:/bin:/usr/bin:/sbin:/usr/sbin:.

Adding to PATH
There are many times where you'll want to append an item to PATH. First, let's see what the current PATH is:

dbettis@rhino[~]$ echo $PATH
/usr/local/bin:/bin:/usr/bin:/sbin:/usr/sbin:.

The way to add a directory is as follows:

dbettis@rhino[~]$ export PATH=$PATH:/new/path

This command adds /new/path to PATH. Let's see if it got updated:

dbettis@rhino[~]$ echo $PATH
/usr/local/bin:/bin:/usr/bin:/sbin:/usr/sbin:.:/new/path

Making this happen every time you login

There's a special file in your home directory called .bashrc In UNIX, a convention is that files beginning with . are configuration files, and thus should be hidden from view. ls will only list files beginning with a . if passed the -a flag. e.g.

dbettis@rhino[~]$ ls -a

At any rate, this file (.bashrc), simply contains a list of commands. Each one of these commands gets executed every time you create a new shell.

dbettis@rhino[~]$ cat .bashrc
export PATH="$PATH:/p/firefox/bin"
..

Every time a shell is started, /p/firefox/bin is added to PATH. If you wish to have certain directories automatically added to PATH, simply place those commands at the end of this file. Log out and log back in to view the changes. Alternatively, you can load the contents of that file in the current session:

dbettis@rhino[~]$ . .bashrc