W

The w command (literally a lower-case 'W') shows the uptime of the system as well as the users who are logged on and certain details about them.

mail# w
 4:00PM  up 95 days, 16:48, 2 users, load averages: 0.00, 0.00, 0.00
USER             TTY      FROM              LOGIN@  IDLE WHAT
glyn             p0       213.133.196.148   Fri10AM     - w
glyn             p1       192.168.100.4     Sat09AM 15:59 _su (csh)

This shows the uptime of the system in the same format as the uptime command (see that article for details). The lines following it show:

• USER - who is logged onto the system;
• TTY - terminal session;
• FROM - remote host name or IP address of the user;
• LOGIN@ - The time of logging on;
• IDLE - time since the user typed anything;
• WHAT - current command being run and its parameters (if any).

See also uptime command.