Linux and Unix halt, poweroff and reboot commands

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About halt, poweroff and reboot
Syntax
Examples
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About halt, poweroff and reboot

Stop, power down, or reboot the system.

Overview

halt, poweroff, and reboot are commands you can run as root to stop the system hardware.

  • halt instructs the hardware to stop all CPU functions.
  • poweroff sends an ACPI signal which instructs the system to power down.
  • reboot instructs the system to reboot.

These commands require superuser privileges. If you are not logged in as root, you will need to prefix the command with sudo or the signal will not be sent.

Command Syntax

reboot [OPTION]... [REBOOTCOMMAND]
halt [OPTION]...
poweroff [OPTION]... 

Technical Description

These programs allow a system administrator to reboot, halt or poweroff the system.

When called with --force or when in runlevel 0 or 6, this tool invokes the reboot system call itself (with REBOOTCOMMAND argument passed) and directly reboots the system. Otherwise this simply invokes the shutdown tool with the appropriate arguments without passing REBOOTCOMMAND argument.

Before invoking reboot, a shutdown time record is first written to /var/log/wtmp

Options

-f, --force Does not invoke shutdown and instead performs the actual action you would expect from the name.
-p, --poweroff Instructs the halt command to instead behave as poweroff.
-w, --wtmp-only Does not call shutdown or the reboot system call and instead only writes the shutdown record to /var/log/wtmp.
--verbose Outputs slightly more verbose messages when rebooting, which can be useful for debugging problems with shutdown.

Environment

RUNLEVEL reboot will read the current runlevel from this environment variable if set in preference to reading from /var/run/utmp.

Files

/var/run/utmp File where the current runlevel will be read from; this file will also be updated with the runlevel record being replaced by a shutdown time record.
/var/log/wtmp A new runlevel record for the shutdown time will be appended to this file.

Examples

halt

If you are logged in as root, issuing the halt command will cease all CPU function on the system. On most systems, this will drop you into single-user mode and then power off the machine.

sudo halt

If you are not logged in as root, you will need to prefix the halt command with sudo to run the command as the superuser.

poweroff

If you are logged in as root, issuing the poweroff command will send an ACPI hardware signal which will instruct the system to commence with a complete and immediate shutdown. This is roughly equivalent to pressing the power button on a typical desktop computer.

sudo poweroff

Execute the poweroff command as root.

reboot

If you are logged in as root, issuing the reboot command will immediately initiate a reboot sequence. The system will shut down and then commence a warm boot.

sudo reboot

Execute the reboot command as root.

Related commands

shutdown — Schedule a time for the system to be powered down.