Linux System Initialization
Now that I've explained a significant part of how Linux system initialization works, I'll tell you how Linux compares to some of the systems I've worked with.
For BSD-style systems, the first time I saw Slackware, I was amazed at its similarity in boot-up to Ultrix which I was using on some DEC-5000s—it has the same structure with the rc scripts in /etc/rc.d and the same names. If Slackware used any system as a pattern, Ultrix could have been one of them. I haven't used any newer BSD-style systems, so I cannot comment further.
For System V, I can compare the various Linux distributions to several others. The one with the most resemblance seems to be Sun Solaris, which uses the same structure as Debian, but uses runlevel 3 as its default and implements XDM startup as Debian. Also, runlevel 5 is used for system shutdown, and the rc scripts are moved to /sbin. HP-UX 10.20 is also similar, but HP puts the init.d, rc.d and other runlevel directories under /sbin. IBM's AIX uses System V style initialization, but with most of the individual scripts for subprocesses called directly from its inittab. Finally, SCO OpenServer uses a system similar to Debian for its boot-time initialization, but does not use symbolic links to init.d. Instead, all start-kill scripts are located in rc2.d.
The latest Filesystem Hierarchy Standard (FHS) v2.0 for Linux dated 26 October 1997 states either BSD or System V style initialization is acceptable. It stopped short, however, of outlining exactly where the rc scripts would go, except to say they would be below /etc, and future revisions to the standard may provide further guidance. I find that unlikely, since Red Hat and Debian, both very popular distributions, do it a little differently. I have no particular preference, and in fact my system has symbolic links which make each look like the other in case an install process makes an invalid assumption about how my systems are configured. I will tell you that as lazy as I am, less typing to start and stop daemons is more to my liking, so /etc/init.d/ gets my vote.
While this article hasn't been all-encompassing by any means, hopefully you've gained some knowledge of how your Linux system initializes during boot-up. All these tables and scripts are simple ASCII text files easily modified with vi or any text editor of your choice. Just read them and follow their logic. I've shown you how to read and interpret /etc/inittab and provided you with basic information regarding how init works.
I've also shown you how to recover in case you've managed to create a script that hangs the boot process or prevents init from starting. Take a look at your inittab and the scripts it runs to better understand your system and optimize it for your own use.
All listings referred to in this article are available by anonymous download in the file ftp.linuxjournal.com/pub/lj/listings/issue56/3016.tgz.
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