Книга: Embedded Linux Primer: A Practical, Real-World Approach

6.3.1. inittab

6.3.1. inittab

When init is started, it reads the system configuration file /etc/inittab. This file contains directives for each runlevel, as well as directives that apply to all run-levels. This file and init 's behavior are well documented in man pages on most Linux workstations, as well as by several books covering system administration. We do not attempt to duplicate those works; we focus on how a developer might configure inittab for an embedded system. For a detailed explanation of how inittab and init work together, view the man page on most Linux workstations by typing man init and man inittab.

Let's take a look at a typical inittab for a simple embedded system. Listing 6-6 contains a simple inittab example for a system that supports a single runlevel as well as shutdown and reboot.

Listing 6-6. Simple Example inittab 

# /etc/inittab
# The default runlevel (2 in this example)
id:2:initdefault:
# This is the first process (actually a script) to be run.
si::sysinit:/etc/rc.sysinit
# Execute our shutdown script on entry to runlevel 0
l0:0:wait:/etc/init.d/sys.shutdown
# Execute our normal startup script on entering runlevel 2
l2:2:wait:/etc/init.d/runlvl2.startup
# This line executes a reboot script (runlevel 6)
l6:6:wait:/etc/init.d/sys.reboot
# This entry spawns a login shell on the console
# Respawn means it will be restarted each time it is killed
con:2:respawn:/bin/sh

This very simple[52] inittab script describes three individual runlevels. Each run-level is associated with a script, which must be created by the developer for the desired actions in each runlevel. When this file is read by init, the first script to be executed is /etc/rc.sysinit. This is denoted by the sysinit tag. Then init enters runlevel 2, and executes the script defined for runlevel 2. From this example, this would be /etc/init.d/runlvl2.startup. As you might guess from the :wait: tag in Listing 6-6, init waits until the script completes before continuing. When the runlevel 2 script completes, init spawns a shell on the console (through the /bin/sh symbolic link), as shown in the last line of Listing 6-6. The respawn keyword instructs init to restart the shell each time it detects that it has exited. Listing 6-7 shows what it looks like during boot.

Listing 6-7. Example Startup Messages

...
VFS: Mounted root (nfs filesystem).
Freeing init memory: 304K
INIT: version 2.78 booting
This is rc.sysinit
INIT: Entering runlevel: 2
This is runlvl2.startup

The startup scripts in this example do nothing except announce themselves for illustrative purposes. Of course, in an actual system, these scripts enable features and services that do useful work! Given the simple configuration in this example, you would enable the services and applications for your particular widget in the /etc/init.d/runlvl2.startup script and do the reversedisable your applications, services, and devicesin your shutdown and/or reboot scripts. In the next section, we look at some typical system configurations and the required entries in the startup scripts to enable these configurations.

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