9.2. Configuring init to start
getty: the
/etc/inittab file
When it starts up, init reads the
/etc/inittab
configuration file. While the system is running, it will
re-read it, if sent the HUP signal (kill -HUP 1);
this feature makes it unnecessary to boot the system to make
changes to the init configuration take
effect.
The /etc/inittab file is
a bit complicated. We'll start with the simple case
of configuring getty lines. Lines in
/etc/inittab consist of four colon-delimited
fields:
id:runlevels:action:process |
The fields are described below. In addition,
/etc/inittab can contain empty lines, and
lines that begin with a number sign (`
#');
these are both ignored.
- id
This identifies the line in the file. For
getty lines, it specifies the terminal
it runs on (the characters after /dev/tty
in the device file name). For other lines,
it doesn't matter (except for length restrictions),
but it should be unique.
- runlevels
The run levels the line should be considered
for. The run levels are given as single digits,
without delimiters. (Run levels are described
in the next section.)
- action
What action should be taken by the line, e.g.,
respawn to run the command in the
next field again, when it exits, or once
to run it just once.
- process
The command to run.
To start a
getty on the first virtual terminal
(
/dev/tty1), in all the normal multi-user
run levels (2-5), one would write the following line:
1:2345:respawn:/sbin/getty 9600 tty1 |
The first field says that this is the line for
/dev/tty1.
The second field says that it applies to run levels 2, 3, 4,
and 5. The third field means that the command should be run
again, after it exits (so that one can log in, log out, and
then log in again). The last field is the command that runs
getty on the first virtual terminal.
Different versions of getty are run
differently. Consult your manual page, and make sure it is
the correct manual page.
If you wanted to add terminals or dial-in modem lines to a
system, you'd add more lines to /etc/inittab,
one for each terminal or dial-in line. For more details, see the
manual pages init, inittab,
and getty.
If a command fails when it starts,
and init is configured to
restart it, it will use a lot of
system resources: init starts it,
it fails, init starts it, it fails,
init starts it, it fails, and so on, ad
infinitum. To prevent this, init will keep
track of how often it restarts a command, and if the frequency
grows to high, it will delay for five minutes before restarting
again.
