what is su?

su stands for superuser

actually its stands for substitute user according to the man pages.. but its mainly used to su to root, so superuser it can be called as well.

Like trickykid said,

it stands for substitute user, and from the name you can figure out what it does.

if you do a:

su

it will default to switching to root, but you can also use it this way:

su <user name>

for example I usually do a su (to become root) to edit some configuration files, then use su <my user name> to go back to my own username, since being root always is not a very good idea.

consult the man pages for more details.

You can also simply type exit to go back to user.

Anne

su will allow you to switch between users without having to logoff/logon again as that user. One more thing not mentioned above is that if you want to inheret the permissions of the user you are switching to, you need to use the - switch like so:

su - username

If you don't do that, you often times won't be able to run commands or have access to things that you normally would as that user.

Also, in the example above where the person su'd to root, then su'd back...you are still leaving a running root session open which can be a security problem and it takes up inodes, etc. You are better off typing exit when you are done.

Ryan