INITTABSection: Linux System Administrator's Manual (5)
Updated: Dec 4, 2001
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NAMEinittab - format of the inittab file used by the sysv-compatible init process
DESCRIPTIONThe inittab file describes which processes are started at bootup and during normal operation (e.g. /etc/init.d/boot, /etc/init.d/rc, gettys...). Init(8) distinguishes multiple runlevels, each of which can have its own set of processes that are started. Valid runlevels are 0-6 plus A, B, and C for ondemand entries. An entry in the inittab file has the following format:
is a unique sequence of 1-4 characters which identifies an entry in
(for versions of sysvinit compiled with the old libc5 (< 5.2.18) or
a.out libraries the limit is 2 characters).
Note: traditionally, for getty and other login processes, the value of the id field is kept the same as the suffix of the corresponding tty, e.g. 1 for tty1. Some ancient login accounting programs might expect this, though I can't think of any.
- lists the runlevels for which the specified action should be taken.
- describes which action should be taken.
- specifies the process to be executed. If the process field starts with a `+' character, init will not do utmp and wtmp accounting for that process. This is needed for gettys that insist on doing their own utmp/wtmp housekeeping. This is also a historic bug.
The runlevels field may contain multiple characters for different runlevels. For example, 123 specifies that the process should be started in runlevels 1, 2, and 3. The runlevels for ondemand entries may contain an A, B, or C. The runlevels field of sysinit, boot, and bootwait entries are ignored.
When the system runlevel is changed, any running processes that are not specified for the new runlevel are killed, first with SIGTERM, then with SIGKILL.
Valid actions for the action field are:
- The process will be restarted whenever it terminates (e.g. getty).
- The process will be started once when the specified runlevel is entered and init will wait for its termination.
- The process will be executed once when the specified runlevel is entered.
- The process will be executed during system boot. The runlevels field is ignored.
- The process will be executed during system boot, while init waits for its termination (e.g. /etc/rc). The runlevels field is ignored.
- This does nothing.
- A process marked with an ondemand runlevel will be executed whenever the specified ondemand runlevel is called. However, no runlevel change will occur (ondemand runlevels are `a', `b', and `c').
- An initdefault entry specifies the runlevel which should be entered after system boot. If none exists, init will ask for a runlevel on the console. The process field is ignored.
- The process will be executed during system boot. It will be executed before any boot or bootwait entries. The runlevels field is ignored.
- The process will be executed when the power goes down. Init is usually informed about this by a process talking to a UPS connected to the computer. Init will wait for the process to finish before continuing.
- As for powerwait, except that init does not wait for the process's completion.
- This process will be executed as soon as init is informormed that the power has been restored.
- This process will be executed when init is told that the battery of the external UPS is almost empty and the power is failing (provided that the external UPS and the monitoring process are able to detect this condition).
- The process will be executed when init receives the SIGINT signal. This means that someone on the system console has pressed the CTRL-ALT-DEL key combination. Typically one wants to execute some sort of shutdown either to get into single-user level or to reboot the machine.
The process will be executed when init receives a signal from the
keyboard handler that a special key combination was pressed on the
The documentation for this function is not complete yet; more documentation can be found in the kbd-x.xx packages (most recent was kbd-0.94 at the time of this writing). Basically you want to map some keyboard combination to the "KeyboardSignal" action. For example, to map Alt-Uparrow for this purpose use the following in your keymaps file:
alt keycode 103 = KeyboardSignal
EXAMPLESThis is an example of a inittab which resembles the old Linux inittab:
# inittab for linux id:1:initdefault: rc::bootwait:/etc/rc 1:1:respawn:/etc/getty 9600 tty1 2:1:respawn:/etc/getty 9600 tty2 3:1:respawn:/etc/getty 9600 tty3 4:1:respawn:/etc/getty 9600 tty4
A more elaborate inittab with different runlevels (see the comments inside):
# Level to run in id:2:initdefault: # Boot-time system configuration/initialization script. si::sysinit:/etc/init.d/rcS # What to do in single-user mode. ~:S:wait:/sbin/sulogin # /etc/init.d executes the S and K scripts upon change # of runlevel. # # Runlevel 0 is halt. # Runlevel 1 is single-user. # Runlevels 2-5 are multi-user. # Runlevel 6 is reboot. l0:0:wait:/etc/init.d/rc 0 l1:1:wait:/etc/init.d/rc 1 l2:2:wait:/etc/init.d/rc 2 l3:3:wait:/etc/init.d/rc 3 l4:4:wait:/etc/init.d/rc 4 l5:5:wait:/etc/init.d/rc 5 l6:6:wait:/etc/init.d/rc 6 # What to do at the "3 finger salute". ca::ctrlaltdel:/sbin/shutdown -t1 -h now # Runlevel 2,3: getty on virtual consoles # Runlevel 3: getty on terminal (ttyS0) and modem (ttyS1) 1:23:respawn:/sbin/getty tty1 VC linux 2:23:respawn:/sbin/getty tty2 VC linux 3:23:respawn:/sbin/getty tty3 VC linux 4:23:respawn:/sbin/getty tty4 VC linux S0:3:respawn:/sbin/getty -L 9600 ttyS0 vt320 S1:3:respawn:/sbin/mgetty -x0 -D ttyS1
AUTHORInit was written by Miquel van Smoorenburg (firstname.lastname@example.org). This manual page was written by Sebastian Lederer (email@example.com) and modified by Michael Haardt (firstname.lastname@example.org).
SEE ALSOinit(8), telinit(8)
examples of using inittab
Run Levels in a Nutshell