SysVinit - ArchWiki

# SysVinit

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- init

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Warning: SysVinit is not officially supported in Arch Linux [1].

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.

## Installation

Install sysvinitAUR initscripts-forkAUR from the AUR. This step will remove systemd-sysvcompat, and you will use sysvinit on reboot. To restore systemd, append `init=/usr/lib/systemd/systemd` to the kernel command line.

Additional init scripts are available at arch-rcscripts. See Init#Configuration for generic configuration steps.

## 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 `action`s 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`.

## 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

## Writing rc.d scripts

Initscripts uses rc.d scripts to used to control the starting, stopping and restarting of daemons.

### Guideline

- Source `/etc/rc.conf`, `/etc/rc.d/functions`, and optionally `/etc/conf.d/DAEMON_NAME`.
- Arguments and other daemon options should be placed in `/etc/conf.d/DAEMON_NAME`. This is done to separate configuration from logic and to keep a consistent style among daemon scripts.
- Use functions in `/etc/rc.d/functions` instead of duplicating their functionality.
- Include at least start, stop and restart as arguments to the script.

### Available functions

- There are some functions provided by `/etc/rc.d/functions`:
  - `stat_busy "message"`: set status busy for printed message (e.g. Starting daemon [BUSY])
  - `stat_done`: set status done (e.g. Starting daemon [DONE])
  - `stat_fail`: set status failed (e.g. Starting daemon [FAILED])
  - `get_pid program`: get PID of the program
  - `ck_pidfile PID-file program`: check whether PID-file is still valid for the program (e.g. ck_pidfile /var/run/daemon.pid daemon || rm -f /var/run/daemon.pid)
  - `[add|rm]_daemon program`: add/remove program to running daemons (stored in `/run/daemons/`)

Full list of functions is much longer and most possibilities (like way to control whether or not non-root users can launch daemon) are still undocumented and can be learned only from `/etc/rc.d/functions` source. See also `man rc.d`.

### Example

The following is an example for crond. Look in `/etc/rc.d` for greater variety.

The configuration file:

    /etc/conf.d/crond

    ARGS="-S -l info"

The actual script:

    /etc/rc.d/crond

    #!/bin/bash

    . /etc/rc.conf
    . /etc/rc.d/functions

    DAEMON=crond
    ARGS=

    [ -r /etc/conf.d/$DAEMON ] && . /etc/conf.d/$DAEMON

    PID=$(get_pid $DAEMON)

    case "$1" in
     start)
       stat_busy "Starting $DAEMON"
       [ -z "$PID" ] && $DAEMON $ARGS &>/dev/null
       if [ $? = 0 ]; then
         add_daemon $DAEMON
         stat_done
       else
         stat_fail
         exit 1
       fi
       ;;
     stop)
       stat_busy "Stopping $DAEMON"
       [ -n "$PID" ] && kill $PID &>/dev/null
       if [ $? = 0 ]; then
         rm_daemon $DAEMON
         stat_done
       else
         stat_fail
         exit 1
       fi
       ;;
     restart)
       $0 stop
       sleep 1
       $0 start
       ;;
     *)
       echo "usage: $0 {start|stop|restart}"
    esac

## Migration to systemd

This article or section is a candidate for merging with Init.

Notes: 2012 installation media are of no relevance, merge remainders to Init (Discuss in Talk:SysVinit#)

### Considerations before switching

- Interactive initscripts do not work with systemd.

### Supplementary information

- 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.

## See also

- Wikipedia: Init
- Linux Knowledge Base and Tutorial. Run Levels.
- Linux.com. Introduction to runlevels and inittab
- Linux.com. An introduction to services, runlevels, and rc.d scripts.

Categories:

- Boot process
- Daemons and system services

Hidden categories:

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