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In this article, learn to shut down or reboot your Linux system, warn users that the system is going down, and switch to single-user mode or a more or less restrictive runlevel. Learn to:
- Set the default runlevel
- Change between runlevels
- Change to single-user mode
- Shut down or reboot the system from the command line
- Alert users about major system events, including switching to another runlevel
- Terminate processes properly
Unless otherwise noted, the examples in this article use a Fedora 8 system with a 2.6.26 kernel. The upstart examples use Fedora 13 with a 2.6.34 kernel, or Ubuntu 10.10 with a 2.6.35 kernel. Your results on other systems may differ.
This article helps you prepare for Objective 101.3 in Topic 101 of the Linux Professional Institute's Junior Level Administration (LPIC-1) exam 101. The objective has a weight of 3.
To get the most from the articles in this series, you should have a basic knowledge of Linux and a working Linux system on which to practice the commands covered in this article. Sometimes different versions of a program will format output differently, so your results may not always look exactly like the listings and figures shown here. In particular, the newer upstart system is changing many things that might be familiar to users of the traditional System V init process (see Init vs. Upstart for details). This article is directed specifically at the traditional System V init process, with only a very cursory overview of how upstart differs.
Runlevels define what tasks can be accomplished in the current state (or runlevel) of a Linux system. Every Linux system supports three basic runlevels, plus one or more runlevels for normal operation. The basic runlevels are shown in Table 1.
Table 1. Linux basic runlevels
| Level | Purpose |
|---|---|
| 0 | Shut down (or halt) the system |
| 1 | Single-user mode; usually aliased as s or S |
| 6 | Reboot the system |
Beyond the basics, runlevel usage differs among distributions. One common usage set is shown in Table 2.
Table 2. Other common Linux runlevels
| Level | Purpose |
|---|---|
| 2 | Multiuser mode without networking |
| 3 | Multiuser mode with networking |
| 5 | Multiuser mode with networking and the X Window System |
The Slackware distribution uses runlevel 4 instead of 5 for a full system running the X Window system. Debian and derivatives, such as Ubuntu, use a single runlevel for any multiuser mode, typically runlevel 2. Be sure to consult the documentation for your distribution.
When a Linux system starts, the default runlevel is determined from the
id: entry in /etc/inittab. Listing 1 illustrates a typical entry
for a system such as Fedora 8 or openSUSE 11.2, both of which use runlevel
5 for the X Window System.
Listing 1. Default runlevel in /etc/inittab
[root@pinguino ~]# grep "^id:" /etc/inittab
id:5:initdefault:
|
Edit this value if you want your system to start in a different runlevel, say runlevel 3.
There are several ways to change runlevels. To make a permanent change, you can edit /etc/inittab and change the default level as you just saw above.
If you only need to bring the system up in a different runlevel for one boot, you can do this. For example, suppose you just installed a new kernel and need to build some kernel modules after the system booted with the new kernel, but before you start the X Window System. You might want to bring up the system in runlevel 3 to accomplish this. You do this at boot time by editing the kernel line (GRUB) or adding a parameter after the selected system name (LILO). Use a single digit to specify the desired runlevel (3, in this case). We'll illustrate the process with a GRUB example. Suppose your /boot/grub/menu.lst file contains the stanza shown in Listing 2.
Listing 2. Typical GRUB stanza to boot Fedora 8
title Fedora (2.6.26.8-57.fc8)
root (hd0,5)
kernel /boot/vmlinuz-2.6.26.8-57.fc8 ro root=LABEL=FEDORA8 rhgb quiet
initrd /boot/initrd-2.6.26.8-57.fc8.img
|
To bring this system up in runlevel 3, wait till the boot entries are displayed, select this entry and enter 'e' to edit the entry. Depending on your GRUB options, you may need to press a key to display the boot entries and also enter 'p' and a password to unlock editing. The GRUB screen on our Fedora 8 system looks like Figure 1.
Figure 1. Selecting a boot choice in GRUB
In this example, you should now see the root, kernel, and initrd lines displayed. Move the cursor to the line starting with "kernel" and press 'e' to edit the line. The GRUB screen on our Fedora 8 system now looks like Figure 2.
Figure 2. Selecting the kernel entry for editing
Finally, move the cursor to the end of the line, and add a space and the digit '3'. You may remove 'quiet' if you wish, or modify any other parameters as needed. The GRUB screen on our Fedora 8 system now looks like Figure 3.
Figure 3. Setting the starting runlevel to 3
Finally, press Enter to save the changes, then type 'b' to boot the system.
Note: The steps for doing this using LILO or GRUB2 differ from those for GRUB, but the basic principle of editing the way the kernel is started remains. Even GRUB screens on other systems or other distributions may look quite different to those shown here. Prompts will usually be available to help you.
Once you have finished your setup work in runlevel 3, you will probably
want to switch to runlevel 5. Fortunately, you do not need to reboot the
system. You can use the telinit command to
switch to another runlevel. Use the runlevel
command to show both the previous runlevel and the current one. If the
first output character is 'N', the runlevel has not been changed since the
system was booted. Listing 3
illustrates verifying and changing the runlevel.
Listing 3. Verifying and changing the runlevel
[root@pinguino ~]# runlevel
N 3
[root@pinguino ~]# telinit 5
|
After you enter telinit 5 you will see
several messages flash by and your display will switch to the configured
graphical login screen. Open a terminal window and verify that the
runlevel has been changed as shown in Listing 4.
Listing 4. Confirming the new runlevel
[root@pinguino ~]# runlevel
3 5
|
If you use the ls command to display a long
listing of the telinit command, you will see
that it really is a symbolic link to the init
command. We illustrate this in Listing 5
Listing 5. telinit is really a symbolic link to init
[root@pinguino ~]# ls -l $(which telinit)
lrwxrwxrwx 1 root root 4 2008-04-01 07:50 /sbin/telinit -> init
|
The init executable knows whether it was called
as init or telinit
and behaves accordingly. Since init runs as PID
1 at boot time, it is also smart enough to know when you subsequently
invoke it using init rather than
telinit. If you do, it will assume you want it
to behave as if you had called telinit instead.
For example, you may use init 5 instead
of telinit 5 to switch to runlevel 5.
In contrast to personal computer operating systems such as DOS or Windows, Linux is inherently a multiuser system. However, there are times when that can be a problem, such as when you need to recover a major filesystem or database, or install and test some new hardware. Runlevel 1, or single-user mode, is your answer for these situations. The actual implementation varies by distribution, but you will usually start in a shell with only a minimal system. Usually there will be no networking and no (or very few) daemons running. On some systems, you must authenticate by logging in, but on others you go straight into a shell prompt as root. Single-user mode can be a lifesaver, but you can also destroy your system, so always be careful whenever you are running with root authority. Reboot to normal multiuser mode as soon as you are done.
As with switching to regular multiuser runlevels, you can also switch to
single-user mode using telinit 1. As
noted in Table 1, 's' and 'S' are
aliases for runlevel 1, so you could, for example, use
telinit s instead.
While you can use telinit or
init to stop multiuser activity and switch to
single-user mode, this can be rather abrupt and cause users to lose work
and processes to terminate abnormally.
The preferred method to shut down or reboot the system is to use the
shutdown command, which first sends a warning
message to all logged-in users and blocks any further logins. It then
signals init to switch runlevels. The
init process then sends all running processes a
SIGTERM signal, giving them a chance to save data or otherwise properly
terminate. After 5 seconds, or another delay if specified,
init sends a SIGKILL signal to forcibly end
each remaining process.
By default, shutdown switches to runlevel 1
(single-user mode). You may specify the -h
option to halt the system, or the -r option to
reboot. A standard message is issued in addition to any message you
specify. The time may be specified as an absolute time in hh:mm
format, or as a relative time, n, where n is the number of
minutes until shutdown. For immediate shutdown, use now, which is
equivalent to +0.
If you have issued a delayed shutdown and the time has not yet expired, you
may cancel the shutdown by pressing Ctrl-c if the command
is running in the foreground, or by issuing
shutdown with the -c
option to cancel a pending shutdown. Listing 6 shows several examples of the use of
shutdown, along with ways to cancel the
command.
Listing 6. Shutdown examples
[root@pinguino ~]# shutdown 5 File system recovery needed
Broadcast message from root (pts/1) (Tue Jan 4 08:05:24 2011):
File system recovery needed
The system is going DOWN to maintenance mode in 5 minutes!
^C
Shutdown cancelled.
[root@pinguino ~]# shutdown -r 10 Reloading updated kernel&
[1] 18784
[root@pinguino ~]#
Broadcast message from root (pts/1) (Tue Jan 4 08:05:53 2011):
Reloading updated kernel
The system is going DOWN for reboot in 10 minutes!
[root@pinguino ~]# fg
shutdown -r 10 Reloading updated kernel
^C
Shutdown cancelled.
[root@pinguino ~]# shutdown -h 23:59&
[1] 18788
[root@pinguino ~]# shutdown -c
Shutdown cancelled.
[1]+ Done shutdown -h 23:59
|
You may have noticed that our last example did not cause a warning message
to be sent. If the time till shutdown exceeds 15 minutes, then the message
is not sent until 15 minutes before the event as shown in Listing 7. Listing 7 also shows the use of the
-t option to increase the default delay between
SIGTERM and SIGKILL signals from 5 seconds to 60 seconds.
Listing 7. Another shutdown example
[root@pinguino ~]# date;shutdown -t60 17 Time to do backups&
Tue Jan 4 08:12:55 EST 2011
[1] 18825
[root@pinguino ~]# date
Tue Jan 4 08:14:13 EST 2011
[root@pinguino ~]#
Broadcast message from root (pts/1) (Tue Jan 4 08:14:55 2011):
Time to do backups
The system is going DOWN to maintenance mode in 15 minutes! |
If you do cancel a shutdown, you should use the
wall command to send a warning to all users
alerting them to the fact that the system is not going
down.
As we said earlier, it is also possible to use
telinit (or init) to
shut down or reboot the system. As with other uses of
telinit, no warning is sent to users, and the
command takes effect immediately, although there is still a delay between
SIGTERM and SIGKILL signals. For additional options of
telinit, init, and
shutdown, consult the appropriate man pages.
You should know about a few more commands related to shutdown and reboot.
- The
haltcommand halts the system. - The
poweroffcommand is a symbolic link to thehaltcommand, which halts the system and then attempts to power it off. - The
rebootcommand is another symbolic link to thehaltcommand, which halts the system and then reboots it.
If any of these are called when the system is not in runlevel 0 or 6, then
the corresponding shutdown command will be
invoked instead.
For additional options that you may use with these commands, as well as more detailed information on their operation, consult the man page.
By now, you may be wondering why pressing Ctrl-Alt-Delete
on some systems causes a reboot, or how all this runlevel stuff is
configured. Remember the id field in
/etc/inittab? Well, there are several other fields in /etc/inittab, along
with a set of init scripts in directories such as rc1.d or rc5.d, where
the digit identifies the runlevel to which the scripts in that directory
apply. Listing 8 shows the full
inittab from our Fedora 8 system.
Listing 8. Full inittab from Fedora 8
# # inittab This file describes how the INIT process should set up # the system in a certain run-level. # # Author: Miquel van Smoorenburg, <miquels@drinkel.nl.mugnet.org> # Modified for RHS Linux by Marc Ewing and Donnie Barnes # # Default runlevel. The runlevels used by RHS are: # 0 - halt (Do NOT set initdefault to this) # 1 - Single user mode # 2 - Multiuser, without NFS (The same as 3, if you do not have networking) # 3 - Full multiuser mode # 4 - unused # 5 - X11 # 6 - reboot (Do NOT set initdefault to this) # id:5:initdefault: # System initialization. si::sysinit:/etc/rc.d/rc.sysinit l0:0:wait:/etc/rc.d/rc 0 l1:1:wait:/etc/rc.d/rc 1 l2:2:wait:/etc/rc.d/rc 2 l3:3:wait:/etc/rc.d/rc 3 l4:4:wait:/etc/rc.d/rc 4 l5:5:wait:/etc/rc.d/rc 5 l6:6:wait:/etc/rc.d/rc 6 # Trap CTRL-ALT-DELETE ca::ctrlaltdel:/sbin/shutdown -t3 -r now # When our UPS tells us power has failed, assume we have a few minutes # of power left. Schedule a shutdown for 2 minutes from now. # This does, of course, assume you have powerd installed and your # UPS connected and working correctly. pf::powerfail:/sbin/shutdown -f -h +2 "Power Failure; System Shutting Down" # If power was restored before the shutdown kicked in, cancel it. pr:12345:powerokwait:/sbin/shutdown -c "Power Restored; Shutdown Cancelled" # Run gettys in standard runlevels 1:2345:respawn:/sbin/mingetty tty1 2:2345:respawn:/sbin/mingetty tty2 3:2345:respawn:/sbin/mingetty tty3 4:2345:respawn:/sbin/mingetty tty4 5:2345:respawn:/sbin/mingetty tty5 6:2345:respawn:/sbin/mingetty tty6 # Run xdm in runlevel 5 x:5:respawn:/etc/X11/prefdm -nodaemon |
As usual, lines starting with # are comments. Other lines have several
fields with the following format:
id:runlevels:action:process
- id
- is a unique identifier of one to four characters. Older versions limited this to two characters, so you will often see only two characters used.
- runlevels
- lists the runlevels for which the action for this id should be taken. If no runlevels are listed, do the action for all runlevels.
- action
- describes which of several possible actions should be taken
- process
- tells which process, if any, should be run when the action on this line is performed.
Some of the common actions that may be specified in /etc/inittab are shown in Table 3. See the man pages for inittab for other possibilities.
Table 3. Some common inittab actions
| Action | Purpose |
|---|---|
| respawn | Restart the process whenever it terminates. Usually used for getty processes, which monitor for logins. |
| wait | Start the process once when the specified runlevel is entered and wait for its termination before init proceeds. |
| once | Start the process once when the specified runlevel is entered. |
| initdefault | Specifies the runlevel to enter after system boot. |
| ctrlaltdel | Execute the associated process when init receives the SIGINT signal, for example, when someone on the system console presses CTRL-ALT-DEL. |
Listing 9 shows just the entry for Ctrl-Alt-Delete from Listing 8. So now you see why pressing Ctrl-Alt-Delete causes the system to be rebooted.
Listing 9. Trapping Ctrl-Alt-Delete
# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -r now
|
You may have noticed several lines in Listing 8, such as
l5:5:wait:/etc/rc.d/rc 5 |
In this example, init will run the
/etc/rc.d/rc script (or command) with the
parameter of 5 whenever runlevel 5 is entered.
init will wait until this command completes
before doing anything else.
These scripts used by init when starting the
system, changing runlevels, or shutting down are typically stored in the
/etc/init.d or /etc/rc.d directory. A series of symbolic links in the
rcn.d directories, one directory for each runlevel
n, control whether a script is started when entering a runlevel
or stopped when leaving it. These links start with either a K or an S,
followed by a two-digit number and then the name of the service, as shown
in Listing 10.
Listing 10. Init scripts
[root@pinguino ~]# find /etc -path "*rc[0-9]*.d/???au*"
/etc/rc.d/rc2.d/S27auditd
/etc/rc.d/rc2.d/K72autofs
/etc/rc.d/rc4.d/S27auditd
/etc/rc.d/rc4.d/S28autofs
/etc/rc.d/rc5.d/S27auditd
/etc/rc.d/rc5.d/S28autofs
/etc/rc.d/rc0.d/K72autofs
/etc/rc.d/rc0.d/K73auditd
/etc/rc.d/rc6.d/K72autofs
/etc/rc.d/rc6.d/K73auditd
/etc/rc.d/rc1.d/K72autofs
/etc/rc.d/rc1.d/K73auditd
/etc/rc.d/rc3.d/S27auditd
/etc/rc.d/rc3.d/S28autofs
[root@pinguino ~]# cd /etc/rc.d/rc5.d
[root@pinguino rc5.d]# ls -l ???a*
lrwxrwxrwx 1 root root 16 2008-04-07 11:29 S27auditd -> ../init.d/auditd
lrwxrwxrwx 1 root root 16 2008-04-01 07:51 S28autofs -> ../init.d/autofs
lrwxrwxrwx 1 root root 15 2008-04-01 14:03 S44acpid -> ../init.d/acpid
lrwxrwxrwx 1 root root 13 2008-04-01 07:50 S95atd -> ../init.d/atd
lrwxrwxrwx 1 root root 22 2008-04-01 07:54 S96avahi-daemon -> ../init.d/avahi-daemon
lrwxrwxrwx 1 root root 17 2008-11-17 13:40 S99anacron -> ../init.d/anacron |
Here you see that the audit and
autofs services have Knn entries in
all runlevels and Snn entries for both in runlevels 3 and 5. The
S indicates that the service is started when that runlevel is entered,
while the K entry indicates that it should be stopped. The nn
component of the link name indicates the priority order in which the
service should be started or stopped. In this example,
audit is started before
autofs, and it is stopped later.
Consult the man pages for init and
inittab for more information.
As we have seen here, the traditional method of booting a Linux system is
based on the UNIX System V init process. It involves loading an initial
RAM disk (initrd) and then passing control to a program called
init, a program that is usually installed as
part of the sysvinit package. The init program
then runs a series of scripts in a predefined order to bring up the
system. If something that is expected is not available, the init process
typically waits until it is. While this worked adequately for systems
where everything is known and connected when the system starts, modern
systems with hot-pluggable devices, network file systems, and even network
interfaces that may not be available at start time present new challenges.
Certainly, waiting for hardware that may not come available for a long
time, or even just a relatively long time, is not desirable.
An alternative initialization process called upstart was first introduced in Ubuntu 6.10 ("Edgy Eft") in 2006. This has now supplanted the init process in Ubuntu and Fedora, among others, although vestiges of init remain and the full power of upstart may not be realized for some time yet.
In contrast to the static set of init scripts used in earlier systems, the upstart system is driven by events. Events may be triggered by hardware changes, starting or stopping or tasks, or by any other process on the system. Events are used to trigger tasks or services, collectively known as jobs. So, for example, connecting a USB drive might cause the udev service to send a block-device-added event, which would cause a defined task to check /etc/fstab and mount the drive if appropriate. As another example, an Apache web server may be started only when both a network and required filesystem resources are available.
The upstart initialization program replaces /sbin/init. Upstart jobs are defined in the /etc/init directory and its subdirectories. The upstart system will currently process /etc/inittab and System V init scripts. On systems such as recent Fedora releases, /etc/inittab is likely to contain only the id entry for the initdefault action. Recent Ubuntu systems do not have /etc/inittab by default, although you can create one if you want to specify a default runlevel.
Upstart also has the initctl command to allow interaction with the upstart
init daemon. This allows you to start or stop jobs, list jobs, get status
of jobs, emit events, restart the init process, and so on. Listing 11 shows how to use
initctl to obtain a list of upstart jobs on a
Fedora 13 system.
Listing 11. Interacting with upstart init daemon using initctl
[ian@echidna ~]$ initctl list rc stop/waiting tty (/dev/tty3) start/running, process 1486 tty (/dev/tty2) start/running, process 1484 tty (/dev/tty6) start/running, process 1492 tty (/dev/tty5) start/running, process 1490 tty (/dev/tty4) start/running, process 1488 plymouth-shutdown stop/waiting control-alt-delete stop/waiting system-setup-keyboard start/running, process 1000 readahead-collector stop/waiting vpnc-cleanup stop/waiting quit-plymouth stop/waiting rcS stop/waiting prefdm start/running, process 1479 init-system-dbus stop/waiting ck-log-system-restart stop/waiting readahead stop/waiting ck-log-system-start stop/waiting start-ttys stop/waiting readahead-disable-services stop/waiting ck-log-system-stop stop/waiting rcS-sulogin stop/waiting serial stop/waiting |
To learn more about upstart, see Resources.
This completes your introduction to Linux runlevels, shutdown, and reboot.
Learn
- Use the developerWorks roadmap for
LPIC-1 to find the developerWorks articles to help you study for
LPIC-1 certification based on the April 2009 objectives.
- At the LPIC
Program site, find detailed objectives, task lists, and sample
questions for the three levels of the Linux Professional Institute's Linux
system administration certification. In particular, see their April 2009
objectives for LPI exam 101 and LPI exam 102. Always refer to the LPIC Program site for the
latest objectives.
- Review the entire LPI exam prep series on
developerWorks to learn Linux fundamentals and prepare for system
administrator certification based on earlier LPI exam objectives prior to
April 2009.
-
The Linux
BootPrompt-HowTo helps you understand boot parameters.
- See the Upstart overview for more
information on upstart.
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Ian Shields works on a multitude of Linux projects for the developerWorks Linux zone. He is a Senior Programmer at IBM at the Research Triangle Park, NC. He joined IBM in Canberra, Australia, as a Systems Engineer in 1973, and has since worked on communications systems and pervasive computing in Montreal, Canada, and RTP, NC. He has several patents. His undergraduate degree is in pure mathematics and philosophy from the Australian National University. He has an M.S. and Ph.D. in computer science from North Carolina State University. You can contact Ian at ishields@us.ibm.com.
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