The program /sbin/init (hereafter called just init) is the parent of all user processes. Init's job is to create other user processes by following the instructions found in the file /etc/inittab. Technically, the kernel itself has completely booted before init runs--- it has to, since init is a user process. Despite this, most consider init to be "part of the boot process".
The inittab script usually has entries to tell init to run programs like mingetty that provide login prompts, and to run scripts like those found in /etc/rc.d/rc3.d that in turn start still more processes and services like xinetd, NFS, and crond. As a result, a typical Linux workstation environment may have as many as 50 different processes running before the user even logs in for the first time.
Workstations usually modify system behavior by modifying the contents of inittab, or the contents of the subdirectories under /etc/rc.d. This capability makes it easy to make large-scale changes to a system's runtime behavior without needing to recompile (or in some cases, even reboot) the system. The conventions followed by inittab and /etc/rc.d scripts are well documented and pervasive (they predate Linux by a number of years), and lend themselves to automated modification during installation of user software.
To change the final stages of an embedded Linux startup process, you can either provide a modified inittab and run init, or you can replace init entirely, with an application of your own design--- perhaps your embedded application itself. You can even experiment a bit, by providing the names of programs like /bin/sh in the kernel's init= command line parameter of a Linux workstation. The kernel will simply run the specified program at the end of the boot process, instead of init.
Figure 11 shows an excerpt from a typical inittab file, that runs the scripts in /etc/rc.d/rc.sysinit and /etc/rc.d/rc3.d, and launches a few mingetty's to provide login prompts.
id:3:initdefault: # System initialization. si::sysinit:/etc/rc.d/rc.sysinit l0:0:wait:/etc/rc.d/rc 0 l1:1:wait:/etc/rc.d/rc 1 l2:2:wait:/etc/rc.d/rc 2 l3:3:wait:/etc/rc.d/rc 3 l4:4:wait:/etc/rc.d/rc 4 l5:5:wait:/etc/rc.d/rc 5 l6:6:wait:/etc/rc.d/rc 6 # Things to run in every runlevel. ud::once:/sbin/update # Trap CTRL-ALT-DELETE ca::ctrlaltdel:/sbin/shutdown -t3 -r now # Run gettys in standard runlevels 1:2345:respawn:/sbin/mingetty tty1 2:2345:respawn:/sbin/mingetty tty2 3:2345:respawn:/sbin/mingetty tty3 4:2345:respawn:/sbin/mingetty tty4 5:2345:respawn:/sbin/mingetty tty5 6:2345:respawn:/sbin/mingetty tty6 |
Figure 11. Excerpt from a typical inittab.
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