inittab(4) UNIX System V inittab(4)
NAME
inittab - script for init
DESCRIPTION
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:
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 a one to four character tag 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.
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inittab(4) UNIX System V inittab(4)
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 only at init's
boot-time read of the inittab file. init is to
start the process, not wait for its termination;
and when it dies, 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.
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.
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inittab(4) UNIX System V inittab(4)
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 of this type are executed before init
tries to access the console (i.e., before the
Console Login: prompt). It is expected that this
entry will be only 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.
SEE ALSO
init(1M), ttymon(1M), exec(2), open(2), signal(2)
sh(1), who(1) in the User's Reference Manual
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