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REBOOT(8) System Manager's Manual REBOOT(8)

halt, reboot — stopping and restarting the system

halt [-lNnqu]

reboot [-lNnqu]

The halt and reboot utilities flush the file system cache to disk, send all running processes a SIGTERM (and subsequently a SIGKILL) and, respectively, halt or restart the system. The action is logged, including entering a shutdown record into the user accounting database.

When the system is halted with the halt command, the system is powered off.

The options are as follows:

The halt or reboot is logged to the system log. This option is intended for applications such as shutdown(8), that call halt or reboot and log this themselves.
The file system cache is not flushed during the initial process clean-up, however the kernel level reboot(2) is still processed with a sync. This option can be useful for performing a “best-effort” reboot when devices might be unavailable. This can happen when devices have been disconnected.
The file system cache is not flushed. This option should probably not be used.
The system is halted or restarted quickly and ungracefully, and only the flushing of the file system cache is performed (if the -n option is not specified). This option should probably not be used.
The system is halted up until the point of removing system power, but waits before removing power for 5 minutes so that an external UPS (uninterruptible power supply) can forcibly remove power. This simulates a dirty shutdown to permit a later automatic power on. macOS uses this mode automatically with supported UPSs in emergency shutdowns.

Normally, the shutdown(8) utility is used when the system needs to be halted or restarted, giving users advance warning of their impending doom and cleanly terminating specific programs.

The SIGKILL will follow the SIGTERM by an intentionally indeterminate period of time. Programs are expected to take only enough time to flush all dirty data and exit. Developers are encouraged to file a bug with the OS vendor, should they encounter an issue with this functionality.

getutxent(3), wtmp(5), shutdown(8), sync(8)

A reboot utility appeared in 4.0BSD.

December 20, 2017 macOS