how to login to another user with its enviroment on ubuntu
su is short for switch user.
With su jenkins you become user jenkins, but don't execute his profile scripts.
Calling su - jenkins or su -l jenkins does the same, but also executes his profile scripts.
See also the su manpage.
In case you don't have the password for user jenkins (or don't want to login with password), but are allowed to call "sudo" you can also add "sudo" in front of the su (i.e. sudo su - jenkins )