Difference between revisions of "SysVinit"

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Revision as of 17:50, 8 August 2012

init is the first process that is executed once the Linux kernel loads. The default init program Arch uses is /sbin/init provided by sysvinit. The word init will always refer to sysvinit in this article.

inittab is the startup configuration file for init located in /etc. It contains directions for init on what programs and scripts to run when entering a specfic runlevel.

Tip: See man 5 inittab and man 8 init for a more formal and complete description.
Tip: Although Arch does use init, most of the work is delegated to the #Main Boot Scripts. This article concentrates on init and inittab; if you're looking for an overview of Arch's boot process, see Arch Boot Process.

Overview of init and inittab

init is always process 1 and, other than managing some swap space, is the parent process to all other processes. You can get an idea of where init lies in the process hierarchy of your system with pstree:

$ pstree -Ap

Besides usual initialization of system (as the name suggests), init also handles rebooting, shutdown and booting into recovery mode (single-user mode). To support these, inittab groups entries into different runlevels. The runlevels Arch uses are 0 for halt, 1 (aliased as S) for single-user mode, 3 for normal booting (multi-user mode), 5 for X and 6 for reboot. Other distros may adopt other conventions, but the meanings of 0, 1 and 6 are universal.

Upon execution, init scans inittab and carry out appropriate actions. An entry in inittab takes the form


Where id is a unique identifier for the entry (just a name, no real impact on init), and runlevels is a (not delimited) string of runlevels. If the runlevel init is entering appears in runlevels, action is carried out, executing process if appropriate. Some special actions would cause init to ignore runlevels and adopt a special matching method. More explanation follows in the next section.

Switching runlevel

Through bootloader

To change the runlevel the system boots into, simply add the desired runlevel n to the respective bootloader's configuration line. A common application of this is Start X at boot#inittab. To boot to the desired runlevel, add its number to the kernel parameters (e.g. 3 for runlevel 3).

The run-level was appended to the end so the kernel knows what run-level to start with. To use another init program (e.g. systemd), add init=/bin/systemd or similar.

Note: If using some init other than sysvinit, the runlevel parameter might be ignored.

After boot up

After the system has booted up, you may issue telinit n to inform init to change the runlevel to n. init then reads inittab and "diffs" runlevel n and current runlevel - killing processes not present in the new runlevel and carrying out actions not present in the old runlevel. Processes present in both runlevels are left untouched. The killing procedure is actually a little complex; again, technical details can be found in the init manpage.

init doesn't watch inittab. You need to call telinit explicitly to apply modifications to inittab. The command telinit q causes init to re-examine inittab but not switch runlevel.


In this section we examine common entries in inittab, in the same order as they appear in the default inittab used by Arch. After that there are a few examples to help you create your own inittab entry.

Warning: Always test a modified /etc/inittab with telinit q before you reboot, or a small syntax error can prevent your system from booting.

Default Runlevel

The default runlevel is 3. Uncomment or add this if you prefer to boot into runlevel 5 (which is used for X conventionally) by default:


Main Boot Scripts

These are the main Arch init scripts.


Single User Boot

Sometimes your kernel may fail to boot up all the way, due to a corrupted or dead hard drive or filesystem, missing key files, etc. In that case your init image may automatically enter into single-user mode which only allows root login and uses /sbin/sulogin instead of /sbin/login to control the login process. You can also boot into single-user mode by appending the letter S to your kernel command line in your GRUB, LILO, or syslinux configuration. If you would like something other than sulogin to run, specify it here.

su:S:wait:/sbin/sulogin -p

Gettys and Login

These are crucial entries that run the gettys on your terminals. Most default configurations will have several gettys running on ttys1-6 which is what pops up on your screen with the login prompt. Also see openvt, chvt, stty, and ioctl.

c1:234:respawn:/sbin/agetty 9600 tty1 xterm-color
c5:5:respawn:/sbin/agetty 57600 tty2 xterm-256color


When the special key sequence Template:Keypress is pressed, this determines what to do.

ca::ctrlaltdel:/sbin/shutdown -t3 -r now

X Programs

If you are not afraid to debug, you can figure out how to start all sorts of programs from inittab. One useful type of program is to start your login manager when the runlevel is 5, multi-user-x-mode. In the following example you can see how to start SLiM when entering runlevel 5.

x:5:respawn:/usr/bin/slim >/dev/null 2>&1
#x:5:respawn:/usr/bin/xdm -nodaemon -confi /etc/X11/xdm/archlinux/xdm-config

Power-Sensing Scripts

Init can communicate with your UPS device and execute processes based on the status of the UPS. Here are some examples:

pf::powerfail:/sbin/shutdown -f -h +2 "Power Failure; System Shutting Down"
pr:12345:powerokwait:/sbin/shutdown -c "Power Restored; Shutdown Cancelled"

Custom Keyboard Request

The following line adds a custom function for when a special key sequence is pressed. You can modify this special key sequence to be anything you like, similar to a Template:Keypress.

kb::kbrequest:/usr/bin/wall "Keyboard Request -- edit /etc/inittab to customize"

Trigger the kbrequest

You can trigger the special key sequence kbrequest by sending the WINCH signal to the init process (1) as root (via sudo). In this example, the command:

kill -WINCH 1

Causes wall to write to all ttys:

Broadcast message from root@askapachehost (console) (Wed Oct 27 14:02:26 2010):
Keyboard Request -- edit /etc/inittab to customize

See Also

External Links