On Unix-like operating systems, the shutdown command shuts down or reboots the system.

Description

The shutdown command brings the system down in a secure way. All logged-in users are notified that the system is going down, and login operations are blocked. It is possible to shut the system down immediately, or after a specified delay.

  • Description
  • Syntax
  • Details
  • Access control
  • Halting vs. powering off
  • Files
  • Examples
  • Related commands
  • Linux commands help

All processes are first notified that the system is going down by the signal SIGTERM. This gives programs like vi time to save the file being edited, mail and news processing programs a chance to exit cleanly, etc.

shutdown does its job by signalling the init process, asking it to change the runlevel. Runlevel 0 is used to halt the system, runlevel 6 is used to reboot the system, and runlevel 1 is used to put the system into a state where administrative tasks can be performed (single-user mode). Runlevel 1 is the default, unless the -h or -r options are specified.

Your system may have a different set of options for the shutdown command; check your documentation for details.

Syntax

shutdown [-akrhPHfFnc] [-t sec] time [message]

Options

Details

If a shutdown is scheduled for the future, it creates the advisory file /etc/nologin which causes programs such as login not to allow new user logins. This file is created five minutes before the shutdown sequence starts. shutdown removes this file if it’s stopped before it can signal init (i.e., it is canceled or something goes wrong). It also removes it before calling init to change the runlevel.

The -f flag means “reboot fast”. This only creates an advisory file /fastboot which can be tested by the system when it comes up again. The system boot rc file (“rc” stands for “runcom”, which is short for “run commands”) can test if this file is present, and decide not to run fsck since the system was shut down in the proper way. After that, the boot process should remove /fastboot.

The -F flag means “force fsck”. This only creates an advisory file /forcefsck which can be tested by the system when it comes up again. The boot rc file can test if this file is present, and decide to run fsck with a special “force” flag so that even properly unmounted file systems get checked. After that, the boot process should remove /forcefsck.

The -n flag causes shutdown not to call init, but to kill all running processes itself. Shutdown then turns off quota, accounting, and swapping and unmount all file systems.

Access control

shutdown can be called from init when the Ctrl+Alt+Del key combination is pressed, if there exists an appropriate entry in /etc/inittab. This would mean that everyone who has physical access to the console keyboard can shut the system down, however.

To prevent this, shutdown can check to see if an authorized user is logged in. If shutdown is called with the -a argument (add this to the invocation of shutdown in /etc/inittab), it checks to see if the file /etc/shutdown.allow is present. It then compares the login names in that file with the list of people that are logged in on a virtual console (from /var/run/utmp). Only if one of those authorized users or root is logged in, it proceeds. Otherwise, it writes the message

shutdown: no authorized users logged in

…to the (physical) system console. The format of /etc/shutdown.allow is one username per line. Empty lines and comment lines (prefixed by a #) are allowed. Currently there is a limit of 32 users in this file.

Note that if /etc/shutdown.allow is not present, the -a argument is ignored.

Halting vs. powering off

The -H option sets the init environment variable INIT_HALT to HALT, and the -P option sets that variable to POWEROFF. The shutdown script that calls halt as the last thing in the shutdown sequence should check these environment variables and call halt with the right options for these options to actually have any effect.

Files

/fastboot /etc/inittab /etc/init.d/halt /etc/init.d/reboot /etc/shutdown.allow

Examples

shutdown 8:00

Schedule the system to shut down at 8 A.M.

shutdown 20:00

Schedule the system to shut down at 8 P.M.

shutdown +15 “Upgrading hardware, downtime should be minimal”

Schedule the system to shut down in fifteen minutes. Along with the normal message alerting users that the system is shutting down, they are given the descriptive message about a hardware upgrade.

shutdown now

Bring down the system immediately.

shutdown -r now

Bring down the system immediately, and automatically reboot it.

shutdown -P now

Bring down the system immediately, and automatically power off the system.

halt — Stop the computer.poweroff — Stop the computer.reboot — Stop the computer.wall — Send a message to all logged-in users.