Just for fun, change your shell prompt. $ should now change; see Figure
6.2 for details.

export is not necessary, because you're changing the shell's own behavior.
There's no reason to export the variable into the environment for other
programs to see. Technically, PS1 is a shell variable rather than an
environment variable.

If you wanted to, you could export the shell variable, transforming it
into an environment variable. If you do this, programs you run from the
shell can see it.

Where Commands Reside: The PATH Variable

When you type a command into the shell, it has to find the program on your
hard disk before executing it. If the shell had to look all over the disk,
it would be very slow; instead, it looks in a list of directories
contained in the PATH environment variable. This list of directories makes
up the shell's search path; when you enter a command, it goes through each
one in turn looking for the program you asked to run.

You may need to change the PATH variable if you install programs yourself
in a non-standard location. The value of PATH is a colon-separated list of
directories. The default value on Debian systems is as follows:

/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games

This value is defined in the file /etc/profile and applies to all users.
You can easily change the value, just as you can change any environment