inittab(4)

Description

The inittab file is composed of entries that are position dependent and have the following format:

   id:rstate:action:process

Each entry is delimited by a newline, however, a backslash (\) preceding a newline indicates a continuation of the entry. Up to 512 characters per entry are permitted. Comments may be inserted in the process field using the convention for comments described in sh(1). There are no limits (other than maximum entry size) imposed on the number of entries in the inittab file. The entry fields are:

id

This is one to four characters used to uniquely identify an entry.

rstate

This defines the run level in which this entry is to be processed. Run-levels effectively correspond to a configuration of processes in the system. That is, each process spawned by init is assigned a run level or run levels in which it is allowed to exist. The run levels are represented by a number ranging from 0 through 6. As an example, if the system is in run level 1, only those entries having a 1 in the rstate field are processed. When init is requested to change run levels, all processes that do not have an entry in the rstate field for the target run level are sent the warning signal SIGTERM and allowed a 5-second grace period before being forcibly terminated by the kill signal SIGKILL. The rstate field can define multiple run levels for a process by selecting more than one run level in any combination from 0 through 6. If no run level is specified, then the process is assumed to be valid at all run levels 0 through 6. There are three other values, a, b and c, which can appear in the rstate field, even though they are not true run levels. Entries which have these characters in the rstate field are processed only when an init or telinit process requests them to be run (regardless of the current run level of the system). See init(1M). They differ from run levels in that init can never enter run level a, b or c. Also, a request for the execution of any of these processes does not change the current run level. Furthermore, a process started by an a, b or c command is not killed when init changes levels. They are killed only if their line in inittab is marked off in the action field, their line is deleted entirely from inittab, or init goes into single-user state.

action

Key words in this field tell init how to treat the process specified in the process field. The actions recognized by init are as follows:

respawn

If the process does not exist, then start the process; do not wait for its termination (continue scanning the inittab file), and when the process dies, restart the process. If the process currently exists, do nothing and continue scanning the inittab file.

wait

When init enters the run level that matches the entry's rstate, start the process and wait for its termination. All subsequent reads of the inittab file while init is in the same run level cause init to ignore this entry.

once

When init enters a run level that matches the entry's rstate, start the process, do not wait for its termination. When it dies, do not restart the process. If init enters a new run level and the process is still running from a previous run level change, the program is not restarted.

boot

The entry is to be processed the first time init goes from single-user to multi-user state after the system is booted. (If initdefault is set to 2, the process runs right after the boot.) init starts the process, does not wait for its termination and, when it dies, does not restart the process.

bootwait

The entry is to be processed the first time init goes from single-user to multi-user state after the system is booted. (If initdefault is set to 2, the process runs right after the boot.) init starts the process, waits for its termination and, when it dies, does not restart the process.

powerfail

Execute the process associated with this entry only when init receives a power fail signal, SIGPWR [see signal(2)].

powerwait

Execute the process associated with this entry only when init receives a power fail signal, SIGPWR, and wait until it terminates before continuing any processing of inittab.

off

If the process associated with this entry is currently running, send the warning signal SIGTERM and wait 5 seconds before forcibly terminating the process with the kill signal SIGKILL. If the process is nonexistent, ignore the entry.

ondemand

This instruction is really a synonym for the respawn action. It is functionally identical to respawn but is given a different keyword in order to divorce its association with run levels. This instruction is used only with the a, b or c values described in the rstate field.

initdefault

An entry with this action is scanned only when init is initially invoked. init uses this entry, if it exists, to determine which run level to enter initially. It does this by taking the highest run level specified in the rstate field and using that as its initial state. If the rstate field is empty, this is interpreted as 0123456 and init therefore enters run level 6. This will cause the system to loop, that is, it will go to firmware and reboot continuously. Additionally, if init does not find an initdefault entry in inittab, it requests an initial run level from the user at reboot time.

sysinit

Entries with this action are scanned only when init is initially invoked. Among other things, sysinit entries may be used to initialize devices on which init might try to ask the run level question. These entries are executed and waited for before continuing.

process

This is a command to be executed. The entire process field is prefixed with exec and passed to a forked sh as sh -c 'exec command'. For this reason, any legal sh syntax can appear in the process field.

Notices

Application code should not attempt to modify the /etc/inittab file during a run-level change, since the etc/init program ignores inittab changes then. In particular, modifying the /etc/inittab file while the system is shutting down will result in minor root file system damage.

Files

References