Print all usernames currently logged on, showing for each user the login name, tty name, the date and time of login, and hostname if not local.
Syntax who [-mTuH] [file] who am i Options -m Only print information about the current terminal. This is the POSIX way of saying who am i. -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. -u Print the idle time for each user. -H Write column headings above the regular output. am I Returns the invoker's real user name. file By default, who gathers information from the file /var/run/utmp. An alternative file may be specified which is usually /var/log/wtmp (or /var/log/wtmp.[0-6] depending on site policy as wtmp can grow quite large and daily versions may or may not be kept around after compression by ac(8)). The wtmp file contains a record of every login, logout, crash, shutdown and date change since wtmp was last truncated or created.
If /var/log/wtmp is being used as the file, the user name may be empty or one of the special characters '|', '}' and '~'. Logouts produce an out- put line without any user name. For more information on the special characters, see utmp(5).
“The more the merrier” ~ John Heywood
who man page - Apple.com
last - indicate last logins of users and ttys.
mesg - display (or do not display) messages from other users.
users - Print login names of users currently logged in.
w - Show who is logged on and what they are doing.