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<p><FONT class=normalfont><B><font color=blue>linux Boot Scripts</font></B></FONT><BR><FONT class=smallfont color=#ff9900>2004-04-23 15:18 pm</FONT><BR><FONT class=normalfont>作者：Richard Gooch<br>来自：Linux知识宝库<br>联系方式：无名<br><br>21-NOV-2002<br>
Introduction<br>
This document describes the new linux boot scripts that I'm developing. These boot scripts are a result of frustration with existing boot scripts and intensive dicussions with Patrick Jordan. We (Patrick and I) have also had email discussions with Larry McVoy and David Parsons, and their input is appreciated (even if they don't totally agree with us:-).<br>
<br>
The new boot scripts require a modified version of simpleinit(8), which is distributed with the util-linux package. See http://www.kernel.org/ for a mirror site near you. I'm the new maintainer of simpleinit(8), and the changes I've made are available in the latest util-linux package.<br>
<br>
OK, with all that out of the way, let me tell you why we think a new boot script scheme is needed. The two main existing schemes are the so-called "BSD" and "SysV" styles. Each have their respective disadvantages:<br>
BSD-style problems<br>
In this scheme, booting is controlled by one of a very few number of boot scripts. Often, there is a master boot script (typically /etc/rc) which orchestrates the whole boot procedure. This scheme is fairly easy to understand, as it has only a small number of scripts to read and the order in which things are started up is quite clear from the master boot script. It is fast, simple and efficient.<br>
<br>
Where this scheme fails is in its scalability. If a 3rd-party package needs to have an initialisation script run during the boot procedure, it needs to edit one of the existing boot scripts. Such editing is dangerous, as boot scripts are fragile at the best of times. A simple mistake by the installer can lead to an unbootable system.<br>
SysV-style problems<br>
These boot scripts, in the tradition of SysV, can do anything. They are flexible. They are scalable. They can run industrial-strength systems. Unfortunately, they're bloated and ugly. OK, maybe they're not ugly to everyone (not that I've ever heard anyone admit they're elegant, only "it works"), but they sure are bloated. They are complex and hard to navigate. To see this, read about how they work:<br>
<br>
This scheme places a number of mini-scripts in a master directory (typically /etc/rc.d/init.d/) which collectively can boot most of the system. Each of these mini-scripts starts and stops one "service". This is quite neat and modular.<br>
A master boot script is used to orchestrate the boot process, which does some "special" setup (i.e. anything which was too hard to put into a mini-script), and then proceeds to run each of the mini-scripts in another directory. The order is based on shell wildcard expansion rules.<br>
<br>
The "other directory" is populated with symbolic links back into /etc/rc.d/init.d/ (where the scripts are kept). Each script usually has two links to it. One starts with 'S' and the other with 'K'. The "S*" scripts are called when booting up the system, the "K*" scripts are called when shutting down the system. The desired ordering is achieved by using numbers after the 'S' and 'K' in the link names.<br>
<br>
So a link with name "S10" will run before "S15", which in turn runs before "S20". It is the responsibility of the system integrator to name these links such that services are started and stopped in the correct order. A 3rd-party software installer can "simply" place their startup script in /etc/rc.d/init.d/ and then create a symbolic link to the script in the "other directory". The installer has to pick a name that is not already taken, and has to determine the number to use (which depends on how far into the boot procedure the script must be run).<br>
The author of the system boot scripts must therefore allocate numbers with sufficient gaps between them to allow for later insertions. Typically, the numbers 10, 20, 30, 40, 50, 60, 70 and so on are chosen. This reminds me of when I was a youngster programming BASIC on my Apple ][. Every line had to be given a number, and you had to be careful to leave "space" for later insertions. The SysV numbering isn't quite so restrictive, as it is possible to append an arbitrary string to the number, which effectively increases the number space. Typically, the name of the script is appended, such as 10inetd and 10named. Thus, it is possible to "group" scripts so that the order between groups is well-defined, while ordering within a group is unknown (or knowable but not important).<br>
<br>
The SysV booting scheme also supports the concept of "runlevels". What this means is that the system may be booted "all the way" (by convention, this is runlevel 5) by default, but may also be booted only part of the way. The most common purpose is to allow the system to be booted "single-user" (i.e. maintenance/repair mode), where only a handful of services are started. The runlevel scheme is supported by splitting the symlinks in the "other directory" into a number of directories, each directory corresponding to a runlevel. These directories are typically named: /etc/rc.d/rc0.d/ /etc/rc.d/rc1.d/ /etc/rc.d/rc2.d/ /etc/rc.d/rc3.d/ /etc/rc.d/rc4.d/ /etc/rc.d/rc5.d/ /etc/rc.d/rc6.d/.<br>
<br>
The master boot script will start all scripts in the runlevel directory corresponding to the desired runlevel. Thus, the system can be booted to runlevel 1 by running the scripts in /etc/rc.d/rc1.d/ (this is often "single-user" mode). Then, perhaps after some maintenance work the system can be booted all the way by switching to runlevel 5 by stopping services for runlevel 1 and starting the scripts in /etc/rc.d/rc5.d/. Similarly, the system can be taken from a higher runlevel to a lower one by stopping services.<br>
<br>
The problem with all this, if it isn't obvious already, is that it's complex. The directories of symlinks make it difficult to see what is being run and how all the pieces fit together. If that doesn't convince you, consider the number of words required to describe this scheme. Note how the BSD-style scheme is much easier to describe (and by extension, understand).<br>
<br>
All that is going for the SysV-style scheme is that it works. We believe that it is not elegant, and is confusing to experienced system administrators (when first exposed to the scheme), let alone novice administrators. And there remains a problem for 3rd-party boot scripts: which symlink name should be chosen? Usually the script is started in runlevel 6, because by that time "most" services are available. The simplest solution is to pick a random high number, which "should" work. Also, the use of numerical runlevels is far from intuitive. While old-guard SysV administrators may feel the runlevel definitions are simple to learn, the reality is the numbers convey no meaning. Certainly novice system administrators (the bulk of the linux population now) will just scratch their collective heads and say "ah well, I guess that's just Unix".<br>
The New Scheme<br>
OK, so you've gotten past the ranting section. Thanks for your patience.<br>
<br>
The solution we came up with is simple yet powerful. There is no master script which orchestrates everything. It's all done by init(8) and need(8) which provide the mechanism. The master script (which my old boot scripts have, and which SysV also has, despite their attempts to modularise), is broken into a bunch of smaller mini scripts.<br>
<br>
The mini scripts are kept in /sbin/init.d and init(8) runs all of them, in random order. Ordering of the mini scripts is controlled by the scripts themselves. Each script runs any other scripts it depends on, using the need(8) programme which ensures that a script is only run once. It doesn't matter which order init(8) starts running the scripts, it all magically sorts itself out.<br>
<br>
3rd-party scripts need only use need(8) to ensure services they require are running. That's all there is to the scheme. It's worth noting that Mastodon linux (by David Parsons) has a similar dependency-based scheme (although it doesn't go quite as far). Thanks to Larry McVoy for pointing this out. NetBSD subsequently implemented a similar system although less flexible scheme, described here.<br>
Implementation details<br>
By default, simpleinit(8) will run /etc/rc as its startup script. The modified version allows you (either at the boot prompt or in /etc/inittab) to specify an alternative script to run. If the script specified is in fact a directory, all the scripts in that directory are run, in random order.<br>
<br>
In the new scheme, init(8) is configured to run all mini startup scripts in /sbin/init.d/. Each script starts/stops one service (i.e. printing, filesystem checks, NFS mounting and so on). Ordering requirements are solved by each script pulling in other scripts it depends on using the need(8) programme. This basically means "run with dependency check". So the NFS export script would be:<br>
<br>
#! /bin/sh<br>
# /sbin/init.d/nfs-export<br>
<br>
case "$1" in<br>
  start)<br>
    need portmap || exit 1<br>
    rpc.mountd<br>
    rpd.nfsd<br>
    ;;<br>
  stop)<br>
    # Stop it<br>
    ;;<br>
esac<br>
# End<br>
<br>
or something like that. The need(8) programme is used to run a script with. If the programme has not been run before, need(8) will run it. If it has already run, need(8) does nothing.<br>
Single-user and runlevels<br>