/bin/mail command
 

how can i change the sender address which is used for example when i do:

echo test|mail test@example.com -s "subject"

I always get root@localhost when im logged in as root.

Change your hostname and domain name for your machine. Username will depend on the user account your logged in as.

But isnt there a command line option to override this?
I run a script as a certain user, and it should mail me as a different one and i can't change to another user for this.

I don't understand what your trying to really accomplish here.. a command? Are you referring to the 'su' command?