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 ondemandentries. An entry in the inittab file has the following format:
Lines beginning with `#’ are ignored.
- is a unique sequence of 1-4 characters which identifies an entry in inittab (for versions of sysvinit compiled with libraries < 5.2.18 or a.out libraries the limit is 2 characters).Note: For gettys or other login processes, the id field should be the tty suffix of the corresponding tty, e.g. 1 for tty1. Otherwise, the login accounting might not work correctly.
- 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, initwill 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, 123specifies that the process should be started in runlevels 1, 2, and 3. The runlevels forondemand entries may contain an A, B, or C. The runlevels field of sysinit, boot, and bootwaitentries 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 specifiedondemand runlevel is called. However, no runlevel change will occur (ondemandrunlevels 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 orbootwait 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 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: # System initialization before anything else. si::sysinit:/etc/rc.d/bcheckrc # Runlevel 0,6 is halt and reboot, 1 is maintenance mode. l0:0:wait:/etc/rc.d/rc.halt l1:1:wait:/etc/rc.d/rc.single l2:2345:wait:/etc/rc.d/rc.multi l6:6:wait:/etc/rc.d/rc.reboot # What to do at the "3 finger salute". ca::ctrlaltdel:/sbin/shutdown -t5 -rf now # Runlevel 2&3: getty on console, level 3 also getty on modem port. 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 S2:3:respawn:/sbin/uugetty ttyS2 M19200