Halt, power -off or reboot the machine
shutdown [OPTIONS...] [TIME] [WALL...]
shutdown may be used to halt, power-off or reboot the machine.
The first argument may be a time string (which is usually "now"). Optionally, this may be followed by a wall message to be sent to all logged-in users before going down.
The time string may either be in the format "hh:mm" for hour/minutes specifying the time to execute the shutdown at, specified in 24h clock format. Alternatively it may be in the syntax "+m" referring to the specified number of minutes m from now. "now" is an alias for "+0", i.e. for triggering an immediate shutdown. If no time argument is specified, "+1" is implied.
Note that to specify a wall message you must specify a time argument, too.
If the time argument is used, 5 minutes before the system goes down the /run/nologin file is created to ensure that further logins shall not be allowed.
The following options are understood:
Print a short help text and exit.
Halt the machine.
Power-off the machine (the default).
Reboot the machine.
Equivalent to --poweroff, unless --halt is specified.
Do not halt, power-off, reboot, just write wall message.
Do not send wall message before halt, power-off, reboot.
Cancel a pending shutdown. This may be used to cancel the effect of an invocation of shutdown with a time argument that is not "+0" or "now".
On success, 0 is returned, a non-zero failure code otherwise.