/usr/bin/who

Print information about users who are currently logged in.

Usage: who [OPTION]... [ FILE | ARG1 ARG2 ]

Arguments:
  [FILE]...

Options:
  -a, --all       same as -b -d --login -p -r -t -T -u
  -b, --boot      time of last system boot
  -d, --dead      print dead processes
  -H, --heading   print line of column headings
  -l, --login     print system login processes
      --lookup    attempt to canonicalize hostnames via DNS
  -m              only hostname and user associated with stdin
  -p, --process   print active processes spawned by init
  -q, --count     all login names and number of users logged on
  -r, --runlevel  print current runlevel
  -s, --short     print only name, line, and time (default)
  -t, --time      print last system clock change
  -u, --users     list users logged in
  -T, --mesg      add user's message status as +, - or ? [aliases: -w, --message, --writable]
  -h, --help      Print help
  -V, --version   Print version

If FILE is not specified, use /var/run/utmp. /var/log/wtmp as FILE is common.
If ARG1 ARG2 given, -m presumed: 'am i' or 'mom likes' are usual.