Copyright © 2005, 2006, 2012 The FreeBSD Project
$FreeBSD: head/en_US.ISO8859-1/articles/rc-scripting/article.xml 39880
2012-10-31 21:24:26Z eadler $
Beginners may find it difficult to relate the facts from the formal documentation on the BSD rc.d framework with the practical tasks of rc.d scripting. In this article, we consider a few typical cases of increasing complexity, show rc.d features suited for each case, and discuss how they work. Such an examination should provide reference points for further study of the design and efficient application of rc.d.
The historical BSD had a monolithic startup script, /etc/rc. It was invoked by init(8) at system boot time and performed all userland tasks required for multi-user operation: checking and mounting file systems, setting up the network, starting daemons, and so on. The precise list of tasks was not the same in every system; admins needed to customize it. With few exceptions, /etc/rc had to be modified, and true hackers liked it.
The real problem with the monolithic approach was that it provided no control over the individual components started from /etc/rc. For instance, /etc/rc could not restart a single daemon. The system admin had to find the daemon process by hand, kill it, wait until it actually exited, then browse through /etc/rc for the flags, and finally type the full command line to start the daemon again. The task would become even more difficult and prone to errors if the service to restart consisted of more than one daemon or demanded additional actions. In a few words, the single script failed to fulfil what scripts are for: to make the system admin's life easier.
Later there was an attempt to split out some parts of /etc/rc for the sake of starting the most important subsystems separately. The notorious example was /etc/netstart to bring up networking. It did allow for accessing the network from single-user mode, but it did not integrate well into the automatic startup process because parts of its code needed to interleave with actions essentially unrelated to networking. That was why /etc/netstart mutated into /etc/rc.network. The latter was no longer an ordinary script; it comprised of large, tangled sh(1) functions called from /etc/rc at different stages of system startup. However, as the startup tasks grew diverse and sophisticated, the “quasi-modular” approach became even more of a drag than the monolithic /etc/rc had been.
Without a clean and well-designed framework, the startup scripts had to bend over backwards to satisfy the needs of rapidly developing BSD-based operating systems. It became obvious at last that more steps are necessary on the way to a fine-grained and extensible rc system. Thus BSD rc.d was born. Its acknowledged fathers were Luke Mewburn and the NetBSD community. Later it was imported into FreeBSD. Its name refers to the location of system scripts for individual services, which is in /etc/rc.d. Soon we will learn about more components of the rc.d system and see how the individual scripts are invoked.
The basic ideas behind BSD rc.d are fine modularity and code reuse. Fine modularity means that each basic “service”
such as a system daemon or primitive startup task gets its own sh(1) script able to
start the service, stop it, reload it, check its status. A particular action is chosen by
the command-line argument to the script. The /etc/rc script
still drives system startup, but now it merely invokes the smaller scripts one by one
start argument. It is easy to perform shutdown tasks
as well by running the same set of scripts with the
argument, which is done by /etc/rc.shutdown. Note how closely
this follows the Unix way of having a set of small specialized tools, each fulfilling its
task as well as possible. Code
reuse means that common operations are implemented as sh(1) functions and
collected in /etc/rc.subr. Now a typical script can be just a
few lines' worth of sh(1) code. Finally,
an important part of the rc.d framework is rcorder(8), which
helps /etc/rc to run the small scripts orderly with respect to
dependencies between them. It can help /etc/rc.shutdown, too,
because the proper order for the shutdown sequence is opposite to that of startup.
The BSD rc.d design is described in the original article by Luke Mewburn, and the rc.d components are documented in great detail in the respective manual pages. However, it might not appear obvious to an rc.d newbie how to tie the numerous bits and pieces together in order to create a well-styled script for a particular task. Therefore this article will try a different approach to describe rc.d. It will show which features should be used in a number of typical cases, and why. Note that this is not a how-to document because our aim is not at giving ready-made recipes, but at showing a few easy entrances into the rc.d realm. Neither is this article a replacement for the relevant manual pages. Do not hesitate to refer to them for more formal and complete documentation while reading this article.
There are prerequisites to understanding this article. First of all, you should be familiar with the sh(1) scripting language in order to master rc.d. In addition, you should know how the system performs userland startup and shutdown tasks, which is described in rc(8).
This article focuses on the FreeBSD branch of rc.d. Nevertheless, it may be useful to NetBSD developers, too, because the two branches of BSD rc.d not only share the same design but also stay similar in their aspects visible to script authors.
|Outlining the task|