inittab - format of the inittab file used by the sysv-compatible init process
The
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:
id:
runlevels:
action:
process
Lines beginning with `#' are ignored.
- id
- is a unique sequence of 1-4 characters which identifies an
entry in inittab (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.
- runlevels
- lists the runlevels for which the specified action should
be taken.
- action
- describes which action should be taken.
- process
- 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
length of this field is limited to 127 characters.
Please note that including certain characters in the process field will
result in init attempting to launch a shell to interpret the
command contained in the process field. The characters which will trigger
a shell are: ~`!$^&*()=|{}[];
On systems which do not have a shell to be launched or which do not wish to
use a shell to interpret the process field, the process field can be
prefixed with the @ symbol. The @ will be ignored and everything followed
will be treated as a command to be launched, literally, by the init
service.
In cases where both a + and @ sign are to be used (to turn off logging and
shell interpretation), place the + sign before the @ symbol. Both flags
will be handled and then everything following the @ will be handled by
init
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:
- respawn
- The process will be restarted whenever it terminates (e.g.
getty).
- wait
- The process will be started once when the specified
runlevel is entered and init will wait for its termination.
- once
- The process will be executed once when the specified
runlevel is entered.
- boot
- The process will be executed during system boot. The
runlevels field is ignored.
- bootwait
- The process will be executed during system boot, while
init waits for its termination (e.g. /etc/rc). The runlevels
field is ignored.
- off
- This does nothing.
- ondemand
- 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').
- initdefault
- 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.
- sysinit
- The process will be executed during system boot. It will be
executed before any boot or bootwait entries. The
runlevels field is ignored.
- powerwait
- 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.
- powerfail
- As for powerwait, except that init does not
wait for the process's completion.
- powerokwait
- This process will be executed as soon as init is
informed that the power has been restored.
- powerfailnow
- 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).
- ctrlaltdel
- 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.
- kbrequest
- The process will be executed when init receives a
signal from the keyboard handler that a special key combination was
pressed on the console keyboard.
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
This 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
This inittab file executes
/etc/rc during boot and starts gettys on
tty1-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
/etc/inittab
init was written by
Miquel van
Smoorenburg This manual page was written by
Sebastian
Lederer and modified by
Michael
Haardt
init(8),
telinit(8)