SysVinit
Related articles
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.
Contents
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.
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.
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
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, theaudio
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.