who —
display who is logged in
  
    | who | [ -abdHlmqrsTtuv]
      [file] | 
The who utility displays a list of all users currently
  logged on, showing for each user the login name, tty name, the date and time
  of login, and hostname if not local.
Available options:
  - -a
- Same as -bdlprTtuv.
- -b
- Time of last system boot.
- -d
- Print dead processes.
- -H
- Write column headings above the regular output.
- -l
- Print system login processes.
- -m
- Only print information about the current terminal. This is the POSIX way
      of saying whoam i.
- -p
- Print active processes spawned by
      init(8).
- -q
- “Quick mode”: List only the names and the number of users
      currently logged on. When this option is used, all other options are
      ignored.
- -r
- Print the current runlevel. Supported runlevels are:
    
      - d(- DEATH)
- The system has halted.
- s(- SINGLE_USER)
- The system is running in single user mode.
- r(- RUNCOM)
- The system is executing /etc/rc.
- t(- READ_TTYS)
- The system is processing /etc/ttys.
- m(- MULTI_USER)
- The system is running in multi-user mode.
- T(- CLEAN_TTYS)
- The system is in the process of stopping processes associated with
          terminal devices.
- c(- CATATONIA)
- The system is in the process of shutting down and will not create new
          processes.
 
- -s
- List only the name, line and time fields. This is the default.
- -T
- Print a character after the user name indicating the state of the terminal
      line: ‘+’ if the terminal is writable; ‘-’ if
      it is not; and ‘?’ if a bad line is encountered.
- -t
- Print last system clock change.
- -u
- Print the idle time for each user, and the associated process ID.
- -v
- When printing of more information is requested with
      -u, this switch can be used to also printed
      process termination signals, process exit status, session id for windowing
      and the type of the entry, see documentation of ut_type in
      getutxent(3).
- am I
- Returns the invoker's real user name.
- file
- By default, whogathers information from the file
      /var/run/utmpx. An alternative
      file may be specified which is usually
      /var/log/wtmpx (or
      /var/log/wtmp, or
      /var/log/wtmpx.[0-6] or
      /var/log/wtmp.[0-6] depending on site policy as
      wtmpx can grow quite large and daily versions may
      or may not be kept around after compression by
      ac(8)). The
      wtmpx and wtmp file
      contains a record of every login, logout, crash, shutdown and date change
      since wtmpx and wtmp were
      last truncated or created.
If /var/log/wtmpx or
    /var/log/wtmp are being used as the file, the user
    name may be empty or one of the special characters '|', '}' and '~'. Logouts
    produce an output line without any user name. For more information on the
    special characters, see
  utmp(5).
  - /var/run/utmp
-  
- /var/run/utmpx
-  
- /var/log/wtmp
-  
- /var/log/wtmp.[0-6]
-  
- /var/log/wtmpx
-  
- /var/log/wtmpx.[0-6]
-  
Thewho utility is expected to conform to
  IEEE Std 1003.2-1992 (“POSIX.2”).