Difference between revisions of "SysVinit"

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{{Warning|SysVinit is obsoleted in Arch Linux [https://www.archlinux.org/news/end-of-initscripts-support/] and has been dropped from the [[official repositories]]. You need to upgrade to [[systemd]].}}
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{{Warning|SysVinit is obsoleted in Arch Linux [https://www.archlinux.org/news/end-of-initscripts-support/] and has been dropped from the [[official repositories]]. You need to [[#Migration_to_systemd|migrate to systemd]].}}
  
 
On systems based on SysVinit, '''init''' is the first process that is executed once the Linux kernel loads. The default init program used by the kernel is {{ic|/sbin/init}} provided by {{Pkg|systemd-sysvcompat}} (by default on new installs, see [[systemd]]) or {{Aur|sysvinit}}. The word '''init''' will always refer to sysvinit in this article.
 
On systems based on SysVinit, '''init''' is the first process that is executed once the Linux kernel loads. The default init program used by the kernel is {{ic|/sbin/init}} provided by {{Pkg|systemd-sysvcompat}} (by default on new installs, see [[systemd]]) or {{Aur|sysvinit}}. The word '''init''' will always refer to sysvinit in this article.

Revision as of 22:31, 27 January 2014

Warning: SysVinit is obsoleted in Arch Linux [1] and has been dropped from the official repositories. You need to migrate to systemd.

On systems based on SysVinit, init is the first process that is executed once the Linux kernel loads. The default init program used by the kernel is /sbin/init provided by systemd-sysvcompat (by default on new installs, see systemd) or sysvinitAUR. 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 specific runlevel.

Although a SysVinit-based Arch system does use init, most of the work is delegated to the #Main Boot Scripts. This article concentrates on init and inittab.

Migration to systemd

Note:

Considerations before switching

  • Do some reading about systemd.
  • Note the fact that systemd has a journal system that replaces syslog, although the two can co-exist. See #Journal.
  • While systemd can replace some of the functionality of cron, acpid, or xinetd, there is no need to switch away from using the traditional daemons unless you want to.
  • Interactive initscripts are not working with systemd. In particular, netcfg-menu cannot be used at system start-up (FS#31377).

Installation procedure

  1. Install systemd from the official repositories.
  2. Append the following to your kernel parameters: init=/usr/lib/systemd/systemd.
  3. Once completed you may enable any desired services via the use of systemctl enable service_name (this roughly equates to what you included in the DAEMONS array. New names can be found in Daemons List).
  4. Reboot your system and verify that systemd is currently active by issuing the following command: cat /proc/1/comm. This should return the string systemd.
  5. Make sure your hostname is set correctly under systemd: hostnamectl set-hostname myhostname or /etc/hostname.
  6. Proceed to remove initscripts and sysvinit from your system and install systemd-sysvcompat.
  7. Optionally, remove the init=/usr/lib/systemd/systemd parameter. It is no longer needed because systemd-sysvcompat provides a symlink to systemd's init where sysvinit used to be.

Supplementary information

  • If you have quiet in your kernel parameters, you might want to remove it for your first couple of systemd boots, to assist with identifying any issues during boot.
  • It is not necessary to add your user to groups (sys, disk, lp, network, video, audio, optical, storage, scanner, power, etc.) for most use cases with systemd. The groups can even cause some functionality to break. For example, the audio group will break fast user switching and allows applications to block software mixing. Every PAM login provides a logind session, which for a local session will give you permissions via POSIX ACLs on audio/video devices, and allow certain operations like mounting removable storage via udisks.

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
init(1)-+-acpid(3432)
        |-crond(3423)
        |-dbus-daemon(3469)
        |-gpm(3485)
        |-mylogin(3536)
        |-ngetty(3535)---login(3954)---zsh(4043)---pstree(4326)
        |-polkitd(4033)---{polkitd}(4035)
        |-syslog-ng(3413)---syslog-ng(3414)
        `-udevd(643)-+-udevd(3194)
                     `-udevd(3218)

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

id:runlevels:action:process

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.

See also man 5 inittab and man 8 init.

Installation

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: This section should explain how to replace systemd with sysvinitAUR. (Discuss in Talk:SysVinit#)

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 Login#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.

inittab

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:

id:5:initdefault:

Main Boot Scripts

These are the main Arch init scripts.

rc::sysinit:/etc/rc.sysinit
rs:S1:wait:/etc/rc.single
rm:2345:wait:/etc/rc.multi
rh:06:wait:/etc/rc.shutdown

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

Ctrl+Alt+Del

When the special key sequence Ctrl+Alt+Del 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 Ctrl+Alt+Del.

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