shutdown provides an automated procedure to notify users when the system is to be shut down. time specifies when shutdown will bring the system
down; it may be the word now (indicating an immediate shutdown), or it may specify a future time in one of two formats: +number and hour:min. The first form brings the system down in number minutes, and the second brings the system down at the time
of day indicated in 24-hour notation.
At intervals that get closer as the apocalypse approaches, warning messages are displayed at terminals of all logged-in users, and of users who have remote mounts on that machine.
At shutdown time a message is written to the system log daemon, syslogd(1M), containing the time of shutdown, the instigator of the shutdown, and the reason. Then a terminate signal is sent to init, which brings the system down to single-user
mode.
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