You can use finger to find out someone’s login name.

What if you don’t know someone’s login name? finger is sometimes smart enough to find users from only their first name or a portion of their name. For example, if someone were looking for me, only knowing that my name was Linda Mui and I worked at http://ora.com, he or she might type:

% finger mui@ora.com

and see:

[ora.com]
Login name: lmui In real life: Linda Mui
Directory: /home/lmui Shell: /bin/tcsh
On since Oct 20 09:43:14 on ttyr5 from opal:0.0
7 minutes 14 seconds Idle Time
Mail last read Thu Oct 20 15:06:24 1994
No Plan.

Login name: peter In real life: Peter Mui
Directory: /home/peter Shell: /bin/tcsh
Last login Thu Oct 20 08:33 on ttyqa from ora-east-nb.ora.
New mail received Thu Oct 20 15:51:38 1994;
  unread since Thu Oct 20 08:33:25 1994
No Plan.

From here, you can tell that there are two Muis with accounts at http://ora.com, me and my brother Peter. ...

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