who # logged-in users, terminal, login time
who -H # with column headers
whoami # your effective username (useful after su/sudo -u)
id # your uid/gid and group memberships (more detail than whoami)
w # who is logged in, what they're running, idle time, and load average (combines who + ps)
w's header line is the same as uptime (see uptime): current time, uptime, user count, and load averages. The per-user rows show idle time and CPU used by their current process, useful for spotting a runaway process on a shared server or confirming nobody else is using a box before rebooting it. last (also util-linux) shows historical logins from /var/log/wtmp.