what is an environment variable?

[Thufir Hawat]

I've looked at https://help.ubuntu.com/community/EnvironmentVariables and 
some other resources, but it's still unclear to me *what* $PATH is.  
There's no corresponding file, just various files which add to the this 
variable.  (It's a shell variable?)

How do you see all the files which effect $PATH?  You just have to go 
through ~/.bash* and ~/.profile* and such, as well as /etc/evironment?

Once something is added to $PATH, how do you remove it?

Specifically:

thufir at caladan:~$ 
thufir at caladan:~$ nl /etc/environment 
     1	PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/
bin:/usr/games"
thufir at caladan:~$ 
thufir at caladan:~$ echo $PATH
/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/
bin:/sbin:/bin:/usr/games:/home/thufir/.rvm/bin
thufir at caladan:~$ 


I cannot find any existing file which would add .rvm to $PATH, so why 
does that "stick" there?  I logged out, just to be sure.  



-Thufir