The Design and Implementation of the NetBSD rc.d system

Luke Mewburn
The NetBSD Foundation

Copyright © 2001, 2003 Luke Mewburn. All rights reserved.


In this paper I cover the design and implementation of the rc.d system start-up mechanism that has been a part of NetBSD since NetBSD 1.5, which replaced the monolithic /etc/rc start-up file inherited from 4.4BSD. Topics covered include a history of various UNIX start-up mechanisms (including NetBSD prior to 1.5), design considerations that evolved over six years of discussions, implementation details, an examination of the human issues that occurred during the design and implementation, and enhancements made since the initial integration.

1. Introduction

Three years ago NetBSD converted from the traditional 4.4BSD monolithic /etc/rc start-up script to an /etc/rc.d mechanism, where there is a separate script to manage each service or daemon, and these scripts are executed in a specific order at system boot.

This paper covers the motivation, design, and implementation of the rc.d system; from the history of what NetBSD had before to the system that NetBSD 1.5 shipped with in December 2000, and the enhancements made since then in NetBSD.

The changes were contentious and generated some of the liveliest discussions about any feature change ever made in NetBSD. Parts of those discussions will be covered to provide insight into some of the design and implementation decisions.

2. History

There is great diversity in the system start-up mechanisms used by various UNIX variants. A few of the more pertinent schemes are detailed below. As NetBSD is derived from 4.4BSD, it follows that a description of the latter's method is relevant. Solaris' start-up method is also detailed, as it is the most common System V UNIX variant.

2.1. 4.4BSD

4.4BSD has a rather simple start-up sequence.

When booting multi-user, the kernel runs init (located in /sbin/init), which spawns a shell (/bin/sh) to run /etc/rc, which contains commands to check the consistency of the file-systems, mount the disks, start up system processes, etc. /etc/rc invokes /etc/netstart to configure the network and any associated services, and /etc/rc.local (if it exists) for locally added services. After /etc/rc has successfully completed, init forks a copy of itself for each terminal in /etc/ttys, usually running /usr/libexec/getty on them. [1].

Administrative configuration of system services is controlled by editing the scripts (/etc/rc, /etc/rc.local, /etc/netstart). In some instances, only shell variables need to be changed, in others commands are added, changed, or removed. [2].

4.4BSD has no specific shut down procedure. After init receives a SIGTERM signal it sends a SIGHUP signal to each process with a controlling terminal, which the process was expected to catch and handle appropriately. Ten seconds later, this is repeated with SIGTERM instead of the SIGHUP, and another ten seconds after that SIGKILL is sent. After all processes have exited or when thirty seconds had elapsed, init then drops to single user mode, reboots, or shuts down, as appropriate.

2.2. Solaris 9

Solaris is the most common System V variant, and serves as a good reference implementation of the System V init.d mechanism, as implemented by System V Release 4 (SVR4).

When running, the system can be in one of eight distinct run levels [3], which are distinct states in which selected groups of processes may run. The run level may be changed at any time by a privileged user running the init with the run level as the argument, and the current run level may be determined at any time with the "who -r" command.

When the system is booted, the kernel runs init (located in /sbin/init), whose purpose is to spawn processes defined in /etc/inittab [4]. For each configuration line in /etc/inittab that has a run level field (`rlevel') which matches the current run level, init starts the process defined on that line as per the given `action' field. The different run levels are:

0 Shut down the operating system so that it's safe to turn off the power.
s or S Single user mode, with some file systems mounted.
1 Single user mode, with all file systems mounted. User logins are disabled.
2 Multi user mode, with all services running except NFS server daemons.
3 Multi-user mode with all services running. This is usually the default.
4 Currently unavailable.
5 Shut down the system and attempt to turn off the power.
6 Shut down the system to level 0, and reboot.

For a given run level X, an shell script /sbin/rcX exists to control the run level change, and /etc/rcX.d contains scripts to be executed at the change. /sbin/rcX stops the services in the files matching /etc/rcX.d/K* in lexicographical order, and then starts the services matching /etc/rcX.d/S* in order.

To add a new service foo requires adding /etc/rcX.d/S*foo in the appropriate run level to start the service, and then /etc/rcY.d/K*foo in all the other run levels Y where the service is not to be run. Usually these files are actually links to the appropriate script in /etc/init.d which implements the start up and shut down procedures for a given service.

To disable or remove a service foo, any files matching /etc/rc?.d/[KS]*foo need to be removed.

2.3. NetBSD prior to 1.5

Prior to the release of NetBSD 1.3, NetBSD's start-up mechanism was similar to 4.4BSD's, with relatively minor changes, as described below.

2.3.1. NetBSD 1.3

In NetBSD 1.3 (released in January 1998), two major user-visible additions were made to the start-up system; /etc/rc.conf and /etc/rc.lkm.

/etc/rc.conf contains variables to control which services are started by /etc/rc and /etc/netstart. For each service foo, two variables may be provided:

foo Can be "yes" or "no" (or various other boolean equivalents). If set to "yes", the service or action relating to foo is started.
foo_flags Optional flags to invoke foo with.

The aim of /etc/rc.conf was to separate the scripts that start services from the configuration information about the services. This allows updating of the start-up scripts in an operating system upgrade with less chance of losing site-specific configuration.

Similar /etc/rc.conf functionality has been implemented in commercial UNIX and freely-available BSD derived systems, including current systems such as FreeBSD. By the time this change was considered for NetBSD, it had a reasonable number of users of the prior art to help justify its implementation.

/etc/rc.lkm was added to provide control over how load-able kernel modules (LKMs) are loaded at boot time. /etc/rc.lkm is invoked at three separate stages during the boot process; before networking is started, before non-critical file systems (i.e., file systems other than /, /usr, /var) are mounted, and after all file-systems are mounted. This complexity is required because an LKM may be located on a local or remote file system. The configuration file /etc/lkm.conf controls behavior of /etc/rc.lkm.

2.3.2. NetBSD 1.4

In NetBSD 1.4 (released in May 1999), two more additions were made; /etc/rc.shutdown and /etc/rc.wscons.

/etc/rc.shutdown is run at shut down time by shutdown. This occurs before the global SIGHUP is sent (as described in section 2.1. This is useful because there are some services that should be shut down in order (e.g., database-using applications before their databases) and some services that require more than SIGHUP for a clean shutdown.

/etc/rc.wscons was added to control how the wscons console driver was configured at boot time, and to allow manual reconfiguration. /etc/wscons.conf controls this behavior.

2.3.3. Summary prior to NetBSD 1.5

At multiuser boot, init calls /etc/rc to initialize the system. /etc/rc calls /etc/netstart to setup network services, /etc/rc.local for local services, /etc/rc.lkm to initialize load-able kernel modules, and /etc/rc.wscons to configure the wscons console driver. The start-up of services is controlled by variables in /etc/rc.conf.

At system shutdown time, shutdown calls /etc/rc.shutdown to shut down specific services which have to be shut down before the global SIGHUP that init sends.

3. Design considerations

Over a six year period, various ideas on how to enhance the start-up system were floated on the public NetBSD mailing lists `current-users' and `tech-userlevel', as well as on the NetBSD developer-only mailing list.

There was no consensus on `One True Design'; there was too much contention for that. What is described below is an amalgamation of what a few developers felt was a reasonable analysis of the problems and feedback as well as the most reasonable solution to support the widest variety of circumstances.

3.1. Problems with the old system

The old system was perceived to suffer from the following problems:

3.2. Requirements of the new system

Given the problems in the old system, and observations of what other systems have done, including those described in section 2, the following design considerations were defined.

Some of these considerations were not determined during discussion prior to implementation, but were identified once users were actively using the implementation.

3.2.1. Dependency ordering

Dependency ordering is a strong requirement.

The following dependency ordering requirements were determined:

After various discussions and implementation tests, it was decided that a dedicated dynamic ordering tool, rcorder (see section 4.2.6), was the most appropriate mechanism; using make or tsort and awk would require moving those programs to /bin (`bloating' the root file-system for machines with limited resources), and a dedicated tool could provide better feedback in certain error situations.

3.2.2. Manipulation of individual services

Most people seem to agree that the ability to manipulate an individual service (via a script) is one of the benefits of the System V init.d start-up mechanism. Having a script that allows direct starting, stopping, and restarting of a service, as well as other per-service options like `reloading configuration files', significantly reduces system administration overheads.

Having the same script be used by the start-up sequence is also highly desirable, as opposed to using a monolithic /etc/rc for booting and separate /etc/rc.d scripts for manual control (which had been suggested).

It is interesting to note that some System V init.d implementations often start multiple services in the one file, which defeats the purpose of providing per-service control files. An example is Solaris' /etc/init.d/inetsvc, which configures network interfaces, starts named and starts inetd.

3.2.3. Support third-party scripts

An important requirement is the ability to support third-party scripts, especially by allowing them to be inserted at any place in the boot sequence order.

The current system does support third-party scripts if they are installed into /etc/rc.d. There has been discussion about allowing for different directories to be used for local and third-party scripts, in order to provide a separate `name-space' to prevent possible conflicts with a local script and a future base system script, but so far none of the suggestions has been considered sufficiently complete to provide in the default system. This, however, does not prevent a site from implementing their own method.

3.2.4. Maintain /etc/rc.conf

/etc/rc.conf was introduced in NetBSD 1.3, and most users seem fairly happy with the concept.

One of the concerns about a traditional System V init.d style mechanism is that the control of service start-up is managed by the existing of a link (or symbolic link) from /etc/rc2.d/S69inet to /etc/init.d/inetinit, which is difficult to manage in a traditional configuration change management environment (such as RCS). Similar concerns exist regarding the suggestion of using mode bits on files in /etc/rc.d to control start-up.

/etc/rc.conf was further enhanced as described in section 3.3.

3.2.5. Promote code re-use

Traditional System V init.d implementations do not appear to re-use any code between scripts. From experience, maintaining local scripts in a traditional init.d environment is a maintenance nightmare. We achieved code re-use with common functions in /etc/rc.subr which results in the average /etc/rc.d script being a small (5-10 line) file.

3.2.6. Service shut down

The ability to shut down certain services at system shutdown time with /etc/rc.shutdown was a useful feature of the previous system and of other systems, and it makes sense to retain this feature.

In the initial implementation, we reverse the dependency order, and shut down any services which are tagged with a "shutdown" keyword (see section 4.2.6) within the script. We may modify or enhance this behavior if observation of in-field use reveals a more complicated scheme is required.

3.2.7. Avoid mandatory run levels

We avoided the use of System V run levels (also known as run states or init states) and /etc/inittab. This was the result of many discussions about the design, which can be summarized to:

3.2.8. Other issues

After various discussions, we settled on the name /etc/rc.d instead of /etc/init.d, because the implementation was different enough from the System V init.d mechanism that we decided not to confuse people expecting the exact System V semantics. Many system administrators may be used to referring directly to /etc/init.d/foo or /sbin/init.d/bar when manipulating a service; a symbolic link from /etc/init.d or /sbin/init.d to /etc/rc.d on their systems could help retain their sanity.

The first implementation of /etc/rc.d that I released for evaluation supported all three start-up schemes; the original monolithic /etc/rc, a System V init.d (without run-levels), and the current /etc/rc.d. These were all built from the same sources, and a command was provided to generate the style that an administrator preferred. After feedback and discussion, this functionality was abandoned, because:

As architects of the NetBSD operating system, we have the responsibility to provide useful solutions to problems. In general, those solutions should be as flexible as possible, without introducing unnecessary flexibility, which will only cause confusion. Therefore, the alternative mechanisms were dropped.

That said, the current system is flexible enough that if a site decided to use a System V init.d approach, it is fairly trivial to populate /etc/rcN.d with a symbolic link farm to files in /etc/rc.d (using rcorder to build the dependency list), and modify /etc/rc to run the scripts in /etc/rcN.d/ in lexicographical order, or to even implement a System V /etc/inittab and run states.

Unfortunately, there is no easy solution for people who want to retain /etc/rc. However, as NetBSD is an Open Source project and allows for public access to the CVS source code repository (via anonymous CVS as well as via a WWW front-end [6]), nothing prevents users from reverting to the old style /etc/rc.

3.3. Configuration improvements

The /etc/rc.conf mechanism was enhanced in two ways:

  1. The default configuration settings were moved from /etc/rc.conf to /etc/defaults/rc.conf, and /etc/rc.conf sources the former. Site specific configuration overrides are placed in /etc/rc.conf. This enables easier upgrades (both manual and automatic) of the default settings in /etc/defaults/rc.conf for new or changed services.

    There was debate about this change, but a significant majority of users agreed with the change. Also, FreeBSD had made a similar change some time before, with a similar debate and outcome, and subsequent upgrade benefits observed which helped the case supporting the change.

  2. An optional per-service configuration file in /etc/rc.conf.d/SERVICE was added. This configuration file (if it exists) is read after /etc/rc.conf, to allow per-service overrides. This optional functionality was added to allow automated third-party installation mechanisms to easily add configuration data.

    /etc/rc.conf.d/SERVICE may also contain variable assignments to override the variables defined in calling script (usually /etc/rc.d/SERVICE), to provide an easy mechanism for a system administrator to override the behaviour of a given rc.d script without requiring the editing of the script. This feature has been used for purposes such as using /etc/rc.d/postfix to start /usr/pkg/sbin/postfix instead of /usr/sbin/postfix, without editing the rc.d script.

    Migrating entirely away from /etc/rc.conf to a multitude of /etc/rc.conf.d/SERVICE files was considered, but no consensus was reached, and after a local trial, we decided that providing for the latter but retaining the former satisfies proponents of either side.

Thus, the order that configuration information for a given service foo is read in is as follows:

  1. foo sources /etc/rc.conf.

  2. /etc/rc.conf sources in /etc/defaults/rc.conf (if it exists), and machine specific overrides of the defaults are added at the end of /etc/rc.conf.

  3. A per-service configuration file in /etc/rc.conf.d/foo (if it exists) will be loaded. This allows for automated maintenance of /etc/rc.conf.d configuration files, whilst retaining the popular /etc/rc.conf semantics.

4. Implementation & aftermath

The system was implemented as described above in the design section, although the design was slightly fluid and did change as feedback was incorporated.

There are two elements to the post-implementation analysis; the human issues, and the technical details.

4.1. The human issues

There was a lot of feedback, debate, angst, flames, and hate-mail. The change has been one of the most contentious in the history of the project.

The first commits to the source code repository were made with the intention of providing a mostly complete implementation which was to be incrementally improved over a few months before the release of NetBSD 1.5.

Unfortunately, we made one of our largest implementation mistakes at this point; we didn't warn the user-base that this was our intention, and the commits were seen as a `stealth attack'. This was partly because we felt that there had been enough debate and announcing our intentions would have delayed the project another few months for a rehash of the same debate (which had been going on for five years at that point).

After the initial implementation, various technical and `religious' complaints were raised about the system. A summary of these is:

Because some of the detractors were quite vocal in the complaints, there was a perception for a time that the work was against a majority decision. This was far from the truth; many users and developers had become jaded with the discussion over the years and did not bother to argue in support of the change, since they agreed with it in principle, if not in implementation particulars. This was borne out by the level of support for the change in the time since implementation.

4.2. The technical details

The rc.d system comprises of the following components:

/etc/rc System start-up script.
/etc/rc.shutdown System shutdown script.
/etc/rc.d/* Individual start-up scripts.
/etc/rc.subr Common shell code used by various scripts.
/etc/defaults/rc.conf Default system configuration.
/etc/rc.conf System configuration file.
/etc/rc.conf.d/* Per service configuration file.

4.2.1. /etc/rc

On system start-up, /etc/rc is executed by init. If init is starting an automatic boot into multi-user mode, /etc/rc is invoked with an argument of "autoboot".

/etc/rc then calls rcorder to order the scripts in /etc/rc.d that do not have a "nostart" rcorder keyword to obtain a dependency list of script names. /etc/rc then invokes each script in turn with the argument of "start" to start the service.

The purpose of the "nostart" support is to allow (primarily third-party) scripts which are only to be manipulated manually (and not started automatically) to be installed into /etc/rc.d. No scripts in the standard NetBSD distribution use this feature as yet.

4.2.2. /etc/rc.shutdown

At system shutdown, /etc/rc.shutdown is executed by shutdown. halt, reboot, and poweroff do not call this script.

/etc/rc.shutdown then calls rcorder to order the scripts in /etc/rc.d that have a "shutdown" rcorder keyword to obtain a dependency list of script names. This dependency list is then reversed, and /etc/rc.shutdown then invokes each script in turn with the argument of "stop" to stop the service.

The rationale for this is that only a few services (such as databases) actually require a shutdown mechanism more complicated than the SIGHUP sent by init at shutdown time. Also, having every script perform "stop" slows down system shutdown as well as causing problems in other areas (such as cleanly un-mounting a `busy' NFS mount once the networking services have been stopped).

4.2.3. /etc/rc.d/* scripts

The scripts in /etc/rc.d are invoked by /etc/rc (with an argument of "start") and /etc/rc.shutdown (with an argument of "stop") in the order specified by rcorder to start and stop (respectively) a given service.

The following file naming conventions are used in /etc/rc.d/:

ALLUPPERCASE Scripts that are `placeholders' or `barriers', to ensure that certain operations are performed before others. In order of startup, these are:

NETWORKING Ensure basic network services are running, including general network configuration (network), and dhclient.
SERVERS Ensure basic services (such as NETWORKING, ppp, syslogd, and kdc) exist for services that start early (such as named), because they're required by DAEMON below.
DAEMON Before all general purpose daemons such as dhcpd, lpd, and ntpd.
LOGIN Before user login services (inetd, telnetd, rshd, sshd, and xdm), as well as before services which might run commands as users (cron, postfix, and sendmail). Scripts that are to be sourced into the current shell rather than a subshell have a `.sh' suffix. Extreme care must be taken in using this, as the startup sequence will terminate if the script does.
bar Scripts that are to be sourced in a sub shell.

Each script should support the following (mutually exclusive) arguments:

start Start the service. This should check that the service is to be started as controlled by /etc/rc.conf. Also checks if the service is already running and refuses to start if it is. This latter check is not performed by standard NetBSD scripts if the system is starting directly to multi-user mode, to speed up the boot process. If `forcestart' is given, ignore the /etc/rc.conf check (but still determine if the service is already running).
stop Stop the service if /etc/rc.conf specifies that it should have been started. This should check that the service is running and complain if it is not. If `forcestop' is given, ignore the /etc/rc.conf check and attempt to stop.
restart Perform stop then start.
status If the script starts a process (rather than performing a one-off operation), show the status of the process. Otherwise, it's not necessary to support this argument. Defaults to displaying the process ID of the service (if running).
rcvar Display which /etc/rc.conf variables are used to control the start-up of the service (if any).

Each script should contain rcorder keywords, especially an appropriate "PROVIDE" entry.

Other arguments for manual use by a system administrator (such as reload, etc) can be added on a per service basis. For example, /etc/rc.d/named supports reload to reload named's configuration files without interrupting service.

4.2.4. /etc/defaults/rc.conf, /etc/rc.conf, /etc/rc.conf.d/*

/etc/defaults/rc.conf contains the default settings for the standard system services, and is provided to facilitate easier system upgrades. End users should not edit this file.

/etc/rc.conf is the primary system start-up configuration file. It reads in /etc/defaults/rc.conf (if it exists), and the end-user puts site-local overrides of these settings at the end of the /etc/rc.conf. This makes it more obvious to differentiate between what is a system default and what is a site-local change, and provides similar functionality to FreeBSD's /etc/defaults mechanism.

For a given service foo, it is possible to have a per-service configuration file in /etc/rc.conf.d/foo, which is read after /etc/rc.conf. This was provided to allow third-party installation tools to install a default configuration without requiring them to in-line edit /etc/rc.conf.

Example /etc/rc.conf entries for dhclient are:

    dhclient_flags="-q tlp0"

To ensure that the system doesn't start into multi-user mode without the system administrator actually checking the configuration of the system, the variable rc_configured is set to "no" by default, and must be set to "yes" by the system administrator. If this is not set, the system will not boot into multi-user mode, and instead remain in single-user mode. The system installation tool sysinst makes this change for you when configuring a newly-installed system, but users performing manual installations or upgrades need to be aware of this.

As /etc/rc.conf is a sh script, it is possible to put various shell commands into the script to conditionally set flags if necessary. Be aware, however, that if the script exits then any script that sources /etc/rc.conf (such as the system boot scripts) will exit too. As /etc/rc.conf may be loaded early in the boot sequence (possibly before /usr is mounted), not all commands may not be available for use.

4.2.5. /etc/rc.subr

/etc/rc.subr is a shell script that's sourced by the /etc/rc.d scripts. It contains `helper' shell functions for commonly used operations:

In traditional System V init.d systems (e.g., Solaris), each script contains the code to determine if a script should be started or shut down, and often re-implemented the checks for a running process, etc. These scripts become difficult to maintain, and are often 1-2 pages long.

By using the functions in /etc/rc.subr, the standard NetBSD rc.d scripts are quite small in comparison.

For example, the /etc/rc.d/dhclient script (sans comments which aren't used by rcorder) is:

    # PROVIDE: dhclient
    # REQUIRE: network mountcritlocal

    . /etc/rc.subr


    load_rc_config $name
    run_rc_command "$1"

It is not mandatory for scripts to use these functions. An ordinary shell script (with the appropriate rcorder control comment lines) which supports the arguments start and stop should work at system start-up and shutdown without modification. In order to be consistent with the existing rc.d scripts, in may help to also support restart, status, rcvar (if appropriate), as well as the `force' prefix.

4.2.6 rcorder

The ordering of the scripts in /etc/rc.d is performed by rcorder (located in /sbin/rcorder), which prints a dependency ordering of a set of interdependent scripts. rcorder reads each script for special comment lines which describe how the script is dependent upon other services, and what services this script provides.

Example rcorder comment lines for /etc/rc.d/dhclient follow:

    # PROVIDE: dhclient
    # REQUIRE: network mountcritlocal

In this case, dhclient requires the services `network' (to configure basic network services) and `mountcritlocal' (to mount critical file-systems that are required early in the boot sequence, usually /var), provides the service `dhclient', and must run before the barrier script /etc/rc.d/NETWORKING.

It is possible to tag a script with a keyword which can be used to conditionally include or exclude the script from being returned by rcorder in the result. /etc/rc uses this to exclude scripts that have a "nostart" keyword, and /etc/rc.shutdown uses this to only include scripts that have a "shutdown" keyword. For example, as xdm needs to be shut down cleanly on some platforms, /etc/rc.d/xdm contains:

    # KEYWORD: shutdown

The rcorder dependency mechanism enables third-party scripts to be installed into /etc/rc.d and therefore added into the dependency tree at the appropriate start-up point without difficulty.

5. Enhancements since NetBSD 1.5

rc.d has been improved since its first release in NetBSD 1.5:

6. Future Work

I'd like to implement `runrcto' to allow you to start up (or shut down) services from service A to service B. This would allow you to start in single user mode, and then start up enough to get the network running, or start all services until just before `multi user login', or just those between `network running' to `database start', etc. This could be a fairly simple system, and would provide most of the functionality that most people seem to want run states for.

We also need a functional chkconfig command (similar to the equivalent command in IRIX), to manage /etc/rc.conf.d by displaying a setting or changing its value. An incomplete implementation exists at this time.

I encourage other systems that are still using a monolithic /etc/rc and who would like to resolve some of the similar issues NetBSD had, to consider this work. I would like to liaise with the maintainers of those systems to ensure as much code re-use as possible.

7. Conclusion

NetBSD 1.5 has a start-up system which implements useful functionality such the ability to control the dependency ordering of services at system boot and manipulate individual services, as well as retaining useful features of previous releases such as /etc/rc.conf.

This work was extremely contentious and difficult to implement because of this contentious nature. The implementation phase did provide valuable insight into some of the difficulties involved in the design and development of large open source projects.

In the long run I believe that this work will have benefits for a majority of users, both in day-to-day operation of the system as well as during future upgrades from NetBSD 1.5 and later releases.


This work first appeared in NetBSD 1.5, which was released in December, 2000 [7]. It was enhanced for NetBSD 1.6 [8], and continues to evolve based on feedback from users. It is used by NetBSD pkgsrc [9] on various platforms. It has since appeared in FreeBSD 5.0 [10], albeit with minor changes that were required to suit their needs.

The CVSweb interface [6] can be used to browse the work and its CVS history.


Many people contributed to the discussions and design of the current system.

However, some people in particular provided important elements: Matthew Green for finishing rcorder and providing the initial attempt at splitting /etc/rc into /etc/rc.d, and Perry Metzger for the idea of providing dependencies using a "PROVIDE" and "REQUIRE" mechanism, and for the initial rcorder implementation.


[1] M. K. McKusick, K. Bostic, M. J. Karels, & J. S. Quarterman, The Design and Implementation of the 4.4BSD Operating System, Addison-Wesley, Reading, MA, 1996.
[2] M. K. McKusick, K. Bostic, M. J. Karels, & S. J. Leffler, "Installing and Operating 4.4BSD UNIX", in 4.4BSD System Managers Manual, pp. 1:44-53, O'Reilly & Associates, Sebastopol, CA, 1994.
[3] Sun Microsystems, System Administration Guide: Basic Administration, Sun Microsystems, Santa Clara, CA, 2002.
[4] Sun Microsystems, "init(1M)", in man pages section 1M: System Administration Commands, Sun Microsystems, Santa Clara, CA, 2002.
[5] Robert Elz, in email to
[6] The NetBSD Project, "CVS Repository",
[7] The NetBSD Project, "Information about NetBSD 1.5.x",
[8] The NetBSD Project, "Information about NetBSD 1.6.x",
[9] The NetBSD Project, "pkgsrc: the NetBSD Packages Collection",
[10] The FreeBSD Project, "FreeBSD/i386 5.0-RELEASE Release Notes",