who - print list of currently logged in users
who # Print user names, terminals and times
Who prints a list of currently logged in users. For each one, the user
name, terminal, and login time is printed. This program gets its
information from the file /etc/utmp, which is updated by init and login.
If the file does not exist, neither of these will create it, and who will
not work. Note that if you decide to create an empty /usr/adm/wtmp to
enable the login accounting, it will grow forever and eventually fill up
your disk unless you manually truncate it from time to time. If an
optional file name is provided, the logins in that file will be printed.