How init Controls the System State

How init Controls the System State

The init process is the first general process created by the system at startup. It reads the file /etc/inittab, which defines exactly which processes exist for each run level.

In the multiuser state (run level 2), init scans the file for entries that have a tag (also 2) for the run level and executes everything after the third colon on the line containing the tag. For complete information, see the inittab(4) reference page.

The system /etc/inittab looks something like this:

is:2:initdefault: 
fs::sysinit:/etc/bcheckrc </dev/console >/dev/console 2>&1 
mt::sysinit:/etc/brc </dev/console >/dev/console 2>&1 
s0:06s:wait:/etc/rc0 >/dev/console 2>&1 </dev/console
s1:1:wait:/etc/shutdown -y -iS -g0 >/dev/console 2>&1 </dev/console 
s2:23:wait:/etc/rc2 >/dev/console 2>&1 </dev/console 
s3:3:wait:/etc/rc3 >/dev/console 2>&1 </dev/console 
or:06:wait:/etc/umount -ak -b /proc,/debug > /dev/console 2>&1
of:0:wait:/etc/uadmin 2 0 >/dev/console 2>&1 </dev/console 
RB:6:wait:echo "The system is being restarted." >/dev/console 
rb:6:wait:/etc/uadmin 2 1 >/dev/console 2>&1 </dev/console 
#

This display has been edited for brevity; the actual /etc/inittab file is lengthy. If /etc/inittab is removed by mistake and is missing during shutdown, init enters the single-user state (init s). While entering single-user state, /usr remains mounted, and processes not spawned by init continue to run. Immediately replace /etc/inittab before changing states again. The format of each line in inittab is:

id:level:action:process

where:

id is one or two characters that uniquely identify an entry.
level is zero or more numbers and letters (0 through 6, s, a, b, and c) that determine in what level(s) action is to take place. If level is null, the action is valid in all levels.
action can be one of the following:
- sysinit
  Run process before init sends anything to the system console (Console Login).
- bootwait
  Start process the first time init goes from single-user to multiuser state after the system is started. (If initdefault is set to 2, the process runs right after the startup.) init starts the process, waits for its termination and, when it dies, does not restart the process.
- wait
  When going to level, start process and wait until it has finished.
- initdefault
  When init starts, it enters level; the process field for this action has no meaning.
- once
  Run process once. If it finishes, don't start it again.
- powerfail
  Run process whenever a direct powerdown of the computer is requested.
- respawn
  If process does not exist, start it, wait for it to finish, and then start another.
- ondemand
  Synonymous with respawn, but used only with level a, b, or c.
- off
  When in level, kill process or ignore it.
process is any executable program, including shell procedures.
# can be used to add a comment to the end of a line. init ignores all lines beginning with the # character.

When changing levels, init kills all processes not specified for that level.