9.1. init comes first
init is one of those programs that
are absolutely essential to the operation of a Linux system,
but that you still can mostly ignore. A good Linux distribution
will come with a configuration for init
that will work for most systems, and on these systems there is
nothing you need to do about init. Usually,
you only need to worry about init if you hook
up serial terminals, dial-in (not dial-out) modems, or if you
want to change the default run level.
When the kernel has started itself (has been loaded
into memory, has started running, and has initialized all
device drivers and data structures and such), it finishes its
own part of the boot process by starting a user level program,
init. Thus, init is always
the first process (its process number is always 1).
The kernel looks for init
in a few locations that have been historically used
for it, but the proper location for it (on a Linux
system) is /sbin/init. If the
kernel can't find init, it tries to run
/bin/sh, and if that also fails, the startup
of the system fails.
When init starts, it finishes the
boot process by doing a number of administrative tasks, such
as checking filesystems, cleaning up /tmp,
starting various services, and starting a getty
for each terminal and virtual console where users should be able
to log in (see Chapter 10).
After the system is properly up, init
restarts getty for each terminal
after a user has logged out (so that the next user can log
in). init also adopts orphan processes: when
a process starts a child process and dies before its child, the
child immediately becomes a child of init.
This is important for various technical reasons, but it is good
to know it, since it makes it easier to understand process lists
and process tree graphs.
There are a few variants of init
available. Most Linux distributions
use sysvinit (written by Miquel
van Smoorenburg), which is based on the System V
init design. The BSD versions of Unix have
a different init. The primary difference
is run levels: System V has them, BSD does not (at least
traditionally). This difference is not essential. We'll look
at sysvinit only.