Software & Apps Linux inittab—Linux / Unix Command The 'inittab' file specifies startup and normal-ops procedures for Linux ops by Juergen Haas Writer Former Lifewire writer Juergen Haas is a software developer, data scientist, and a fan of the Linux operating system. our editorial process Juergen Haas Updated on July 29, 2020 Linux Switching from Windows Tweet Share Email The inittab file describes which processes are started at bootup and during normal operation. Init distinguishes different 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 on-demand entries. An entry in the inittab file has the following format: id:runlevels:action:process Lines beginning with `#' are ignored. In the file, id is a unique sequence of 1-4 characters that identifies an entry in inittab. Getty Images / PeopleImages.com 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. Runlevels, Action, and Processes The iniitab file contains information including: 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 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. Action Standards 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.ondemand: A process marked with an ondemand runlevel will be executed whenever the specifiedondemand 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 orbootwait 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 informormed 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. 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 Example 'inittab' Files This is an example of a inittab which resembles the old Linux inittab: # inittab for linuxid:1:initdefault:rc::bootwait:/etc/rc1:1:respawn:/etc/getty 9600 tty12:1:respawn:/etc/getty 9600 tty23:1:respawn:/etc/getty 9600 tty34:1: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: # Level to run inid: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.haltl1:1:wait:/etc/rc.d/rc.singlel2:2345:wait:/etc/rc.d/rc.multil6:6: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 linux2:23:respawn:/sbin/getty tty2 VC linux3:23:respawn:/sbin/getty tty3 VC linux4:23:respawn:/sbin/getty tty4 VC linuxS2:3:respawn:/sbin/uugetty ttyS2 M19200 Use the man command (% man) to see how a command is used on your particular computer.