inittab − script for init
The file /etc/inittab controls process dispatching by init. The processes most typically dispatched by init are daemons.
The inittab file is composed of entries that are position dependent and have the following format:
Each entry is delimited by a newline; however, a backslash (\) preceding a newline indicates a continuation of the entry. Up to 512 characters for each 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:
One to four characters used to uniquely identify an entry. Do not use the characters "r" or "t" as the first or only character in this field. These characters are reserved for the use of rlogin(1) and telnet(1).
Define 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(s) in which it is allowed to exist. The run levels are represented by a number ranging from 0 through 6. For 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). These 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.
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:
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.
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.
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.
The entry is to be processed only at init’s boot-time read of the inittab file. init is to start the process and not wait for its termination; when it dies, it does not restart the process. In order for this instruction to be meaningful, the rstate should be the default or it must match init’s run level at boot time. This action is useful for an initialization function following a hardware reboot of the system.
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.
Execute the process associated with this entry only when init receives a power fail signal, SIGPWR (see signal(3C)).
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.
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.
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.
An entry with this action is scanned only when init is initially invoked. init uses this entry 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 will enter run level 6. This will cause the system to loop (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.
Entries of this type are
executed before init tries to access the console
(that is, before the Console Login: prompt). It is
expected that this entry will be used only to initialize
init might try to ask the run level question. These entries are executed and init waits for their completion before continuing.
Specify 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.