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<articleinfo>
<title>Practical rc.d scripting in BSD</title>
<author>
<firstname>Yar</firstname>
<surname>Tikhiy</surname>
<affiliation>
<address><email>yar@FreeBSD.org</email></address>
</affiliation>
</author>
<copyright>
<year>2005</year>
<year>2006</year>
<year>2012</year>
<holder>The FreeBSD Project</holder>
</copyright>
<legalnotice id="trademarks" role="trademarks">
&tm-attrib.freebsd;
&tm-attrib.netbsd;
&tm-attrib.general;
</legalnotice>
<pubdate>$FreeBSD$</pubdate>
<releaseinfo>$FreeBSD$</releaseinfo>
<abstract>
<para>Beginners may find it difficult to relate the
facts from the formal documentation on the BSD
<filename>rc.d</filename> framework with the practical tasks
of <filename>rc.d</filename> scripting. In this article,
we consider a few typical cases of increasing complexity,
show <filename>rc.d</filename> 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 <filename>rc.d</filename>.</para>
</abstract>
</articleinfo>
<sect1 id="rcng-intro">
<title>Introduction</title>
<para>The historical BSD had a monolithic startup script,
<filename>/etc/rc</filename>. It was invoked by
&man.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, <filename>/etc/rc</filename> had to be modified,
and true hackers liked it.</para>
<para>The real problem with the monolithic approach was that
it provided no control over the individual components started
from <filename>/etc/rc</filename>. For instance,
<filename>/etc/rc</filename> 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
<filename>/etc/rc</filename> 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.</para>
<para>Later there was an attempt to split out some parts of
<filename>/etc/rc</filename> for the sake of starting the
most important subsystems separately. The notorious example
was <filename>/etc/netstart</filename> 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
<filename>/etc/netstart</filename> mutated into
<filename>/etc/rc.network</filename>. The latter was no
longer an ordinary script; it comprised of large, tangled
&man.sh.1; functions called from <filename>/etc/rc</filename>
at different stages of system startup. However, as the startup
tasks grew diverse and sophisticated, the
<quote>quasi-modular</quote> approach became even more of a
drag than the monolithic <filename>/etc/rc</filename> had
been.</para>
<para>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 <filename>rc</filename> system.
Thus BSD <filename>rc.d</filename> was born. Its acknowledged
fathers were Luke Mewburn and the NetBSD community. Later
it was imported into &os;. Its name refers to the location
of system scripts for individual services, which is in
<filename>/etc/rc.d</filename>. Soon we
will learn about more components of the <filename>rc.d</filename>
system and see how the individual scripts are invoked.</para>
<para>The basic ideas behind BSD <filename>rc.d</filename> are
<emphasis>fine modularity</emphasis> and <emphasis>code
reuse</emphasis>. <emphasis>Fine modularity</emphasis> means
that each basic <quote>service</quote> such as a system daemon
or primitive startup task gets its own &man.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 <filename>/etc/rc</filename> script still
drives system startup, but now it merely invokes the smaller
scripts one by one with the <option>start</option> argument.
It is easy to perform shutdown tasks as well by running the
same set of scripts with the <option>stop</option> argument,
which is done by <filename>/etc/rc.shutdown</filename>. Note
how closely this follows the Unix way of having a set of small
specialized tools, each fulfilling its task as well as possible.
<emphasis>Code reuse</emphasis> means that common operations
are implemented as &man.sh.1; functions and collected in
<filename>/etc/rc.subr</filename>. Now a typical script can
be just a few lines' worth of &man.sh.1; code. Finally, an
important part of the <filename>rc.d</filename> framework is
&man.rcorder.8;, which helps <filename>/etc/rc</filename> to
run the small scripts orderly with respect to dependencies
between them. It can help <filename>/etc/rc.shutdown</filename>,
too, because the proper order for the shutdown sequence is
opposite to that of startup.</para>
<para>The BSD <filename>rc.d</filename> design is described in
<link linkend="lukem">the original article by Luke Mewburn</link>,
and the <filename>rc.d</filename> components are documented
in great detail in <link linkend="manpages">the respective
manual pages</link>. However, it might not appear obvious
to an <filename>rc.d</filename> 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 <filename>rc.d</filename>.
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
<filename>rc.d</filename> 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.</para>
<para>There are prerequisites to understanding this article.
First of all, you should be familiar with the &man.sh.1;
scripting language in order to master <filename>rc.d</filename>.
In addition, you should know how the system performs
userland startup and shutdown tasks, which is described in
&man.rc.8;.</para>
<para>This article focuses on the &os; branch of
<filename>rc.d</filename>. Nevertheless, it may be useful
to NetBSD developers, too, because the two branches of BSD
<filename>rc.d</filename> not only share the same design
but also stay similar in their aspects visible to script
authors.</para>
</sect1>
<sect1 id="rcng-task">
<title>Outlining the task</title>
<para>A little consideration before starting
<envar>$EDITOR</envar> will not hurt. In order to write a
well-tempered <filename>rc.d</filename> script for a system
service, we should be able to answer the following questions
first:</para>
<itemizedlist>
<listitem>
<para>Is the service mandatory or optional?</para>
</listitem>
<listitem>
<para>Will the script serve a single program, e.g.,
a daemon, or perform more complex actions?</para>
</listitem>
<listitem>
<para>Which other services will our service depend on,
and vice versa?</para>
</listitem>
</itemizedlist>
<para>From the examples that follow we will see why it is
important to know the answers to these questions.</para>
</sect1>
<sect1 id="rcng-dummy">
<title>A dummy script</title>
<para>The following script just emits a message each time the
system boots up:</para>
<informalexample>
<programlisting>#!/bin/sh<co id="rcng-dummy-shebang"/>
. /etc/rc.subr<co id="rcng-dummy-include"/>
name="dummy"<co id="rcng-dummy-name"/>
start_cmd="${name}_start"<co id="rcng-dummy-startcmd"/>
stop_cmd=":"<co id="rcng-dummy-stopcmd"/>
dummy_start()<co id="rcng-dummy-startfn"/>
{
echo "Nothing started."
}
load_rc_config $name<co id="rcng-dummy-loadconfig"/>
run_rc_command "$1"<co id="rcng-dummy-runcommand"/></programlisting>
</informalexample>
<para>Things to note are:</para>
<calloutlist>
<callout arearefs="rcng-dummy-shebang">
<para>An interpreted script should begin with the magic
<quote>shebang</quote> line. That line specifies the
interpreter program for the script. Due to the shebang
line, the script can be invoked exactly like a binary
program provided that it has the execute bit set.
(See &man.chmod.1;.)
For example, a system admin can run our script manually,
from the command line:</para>
<screen>&prompt.root; <userinput>/etc/rc.d/dummy start</userinput></screen>
<note>
<para>In order to be properly managed by the
<filename>rc.d</filename> framework, its scripts need
to be written in the &man.sh.1; language. If you have
a service or port that uses a binary control utility
or a startup routine written in another language,
install that element in <filename>/usr/sbin</filename>
(for the system) or <filename>/usr/local/sbin</filename>
(for ports) and call it from a &man.sh.1; script in the
appropriate <filename>rc.d</filename> directory.</para>
</note>
<tip>
<para>If you would like to learn the details of why
<filename>rc.d</filename> scripts must be written in
the &man.sh.1; language, see how <filename>/etc/rc</filename>
invokes them by means of <function>run_rc_script</function>,
then study the implementation of
<function>run_rc_script</function> in
<filename>/etc/rc.subr</filename>.</para>
</tip>
</callout>
<callout arearefs="rcng-dummy-include">
<para>In <filename>/etc/rc.subr</filename>, a number of
&man.sh.1; functions are defined for an <filename>rc.d</filename>
script to use. The functions are documented in
&man.rc.subr.8;. While it is theoretically possible to
write an <filename>rc.d</filename> script without ever
using &man.rc.subr.8;, its functions prove extremely handy
and make the job an order of magnitude easier. So it is
no surprise that everybody resorts to &man.rc.subr.8; in
<filename>rc.d</filename> scripts. We are not going to
be an exception.</para>
<para>An <filename>rc.d</filename> script must
<quote>source</quote> <filename>/etc/rc.subr</filename>
(include it using <quote><command>.</command></quote>)
<emphasis>before</emphasis> it calls &man.rc.subr.8;
functions so that &man.sh.1; has an opportunity to learn
the functions. The preferred style is to source
<filename>/etc/rc.subr</filename> first of all.</para>
<note>
<para>Some useful functions related to networking
are provided by another include file,
<filename>/etc/network.subr</filename>.</para>
</note>
</callout>
<callout arearefs="rcng-dummy-name">
<para><anchor id="name-var"/>The mandatory variable
<envar>name</envar> specifies the name of our script. It
is required by &man.rc.subr.8;. That is, each
<filename>rc.d</filename> script <emphasis>must</emphasis>
set <envar>name</envar> before it calls &man.rc.subr.8;
functions.</para>
<para>Now it is the right time to choose a unique name for
our script once and for all. We will use it in a number
of places while developing the script. For a start, let
us give the same name to the script file, too.</para>
<note>
<para>The current style of <filename>rc.d</filename>
scripting is to enclose values assigned to variables
in double quotes. Keep in mind that it is just a style
issue that may not always be applicable. You can
safely omit quotes from around simple words without
&man.sh.1; metacharacters in them, while in certain
cases you will need single quotes to prevent any
interpretation of the value by &man.sh.1;. A programmer
should be able to tell the language syntax from style
conventions and use both of them wisely.</para>
</note>
</callout>
<callout arearefs="rcng-dummy-startcmd">
<para>The main idea behind &man.rc.subr.8; is that an
<filename>rc.d</filename> script provides handlers, or
methods, for &man.rc.subr.8; to invoke. In particular,
<option>start</option>, <option>stop</option>, and other
arguments to an <filename>rc.d</filename> script are
handled this way. A method is a &man.sh.1; expression
stored in a variable named
<envar><replaceable>argument</replaceable>_cmd</envar>,
where <replaceable>argument</replaceable> corresponds to
what can be specified on the script's command line. We
will see later how &man.rc.subr.8; provides default methods
for the standard arguments.</para>
<note>
<para>To make the code in <filename>rc.d</filename> more
uniform, it is common to use <envar>${name}</envar>
wherever appropriate. Thus a number of lines can be just
copied from one script to another.</para>
</note>
</callout>
<callout arearefs="rcng-dummy-stopcmd">
<para>We should keep in mind that &man.rc.subr.8; provides
default methods for the standard arguments. Consequently,
we must override a standard method with a no-op &man.sh.1;
expression if we want it to do nothing.</para>
</callout>
<callout arearefs="rcng-dummy-startfn">
<para>The body of a sophisticated method can be implemented
as a function. It is a good idea to make the function
name meaningful.</para>
<important>
<para>It is strongly recommended to add the prefix
<envar>${name}</envar> to the names of all functions
defined in our script so they never clash with the
functions from &man.rc.subr.8; or another common include
file.</para>
</important>
</callout>
<callout arearefs="rcng-dummy-loadconfig">
<para>This call to &man.rc.subr.8; loads &man.rc.conf.5;
variables. Our script makes no use of them yet, but it
still is recommended to load &man.rc.conf.5; because there
can be &man.rc.conf.5; variables controlling &man.rc.subr.8;
itself.</para>
</callout>
<callout arearefs="rcng-dummy-runcommand">
<para>Usually this is the last command in an
<filename>rc.d</filename> script. It invokes the
&man.rc.subr.8; machinery to perform the requested action
using the variables and methods our script has provided.</para>
</callout>
</calloutlist>
</sect1>
<sect1 id="rcng-confdummy">
<title>A configurable dummy script</title>
<para>Now let us add some controls to our dummy script. As you
may know, <filename>rc.d</filename> scripts are controlled
with &man.rc.conf.5;. Fortunately, &man.rc.subr.8; hides all
the complications from us. The following script uses
&man.rc.conf.5; via &man.rc.subr.8; to see whether it is
enabled in the first place, and to fetch a message to show
at boot time. These two tasks in fact are independent. On
the one hand, an <filename>rc.d</filename> script can just
support enabling and disabling its service. On the other
hand, a mandatory <filename>rc.d</filename> script can have
configuration variables. We will do both things in the same
script though:</para>
<informalexample>
<programlisting>#!/bin/sh
. /etc/rc.subr
name=dummy
rcvar=dummy_enable<co id="rcng-confdummy-rcvar"/>
start_cmd="${name}_start"
stop_cmd=":"
load_rc_config $name<co id="rcng-confdummy-loadconfig"/>
eval "${rcvar}=\${${rcvar}:-'NO'}"<co id="rcng-confdummy-enable"/>
dummy_msg=${dummy_msg:-"Nothing started."}<co id="rcng-confdummy-opt"/>
dummy_start()
{
echo "$dummy_msg"<co id="rcng-confdummy-msg"/>
}
run_rc_command "$1"</programlisting>
</informalexample>
<para>What changed in this example?</para>
<calloutlist>
<callout arearefs="rcng-confdummy-rcvar">
<para>The variable <envar>rcvar</envar> specifies
the name of the ON/OFF knob variable.</para>
</callout>
<callout arearefs="rcng-confdummy-loadconfig">
<para>Now <function>load_rc_config</function> is invoked
earlier in the script, before any &man.rc.conf.5; variables
are accessed.</para>
<note>
<para>While examining <filename>rc.d</filename> scripts,
keep in mind that &man.sh.1; defers the evaluation of
expressions in a function until the latter is called.
Therefore it is not an error to invoke
<function>load_rc_config</function> as late as just
before <function>run_rc_command</function> and still
access &man.rc.conf.5; variables from the method functions
exported to <function>run_rc_command</function>. This
is because the method functions are to be called by
<function>run_rc_command</function>, which is invoked
<emphasis>after</emphasis>
<function>load_rc_config</function>.</para>
</note>
</callout>
<callout arearefs="rcng-confdummy-enable">
<para>A warning will be emitted by
<function>run_rc_command</function> if <envar>rcvar</envar>
itself is set, but the indicated knob variable is unset.
If your <filename>rc.d</filename> script is for the base
system, you should add a default setting for the knob to
<filename>/etc/defaults/rc.conf</filename> and document
it in &man.rc.conf.5;. Otherwise it is your script that
should provide a default setting for the knob. A portable
approach to the latter case is shown in the example.</para>
<note>
<para>You can make &man.rc.subr.8; act as though the knob
is set to <literal>ON</literal>, irrespective of its
current setting, by prefixing the argument to the script
with <literal>one</literal> or <literal>force</literal>,
as in <option>onestart</option> or <option>forcestop</option>.
Keep in mind though that <literal>force</literal> has
other dangerous effects we will touch upon below, while
<literal>one</literal> just overrides the ON/OFF knob.
E.g., assume that <envar>dummy_enable</envar> is
<literal>OFF</literal>. The following command will run
the <option>start</option> method in spite of the
setting:</para>
<screen>&prompt.root; <userinput>/etc/rc.d/dummy onestart</userinput></screen>
</note>
</callout>
<callout arearefs="rcng-confdummy-opt">
<para>Now the message to be shown at boot time is no
longer hard-coded in the script. It is specified by an
&man.rc.conf.5; variable named <envar>dummy_msg</envar>.
This is a trivial example of how &man.rc.conf.5; variables
can control an <filename>rc.d</filename> script.</para>
<important>
<para>The names of all &man.rc.conf.5; variables used
exclusively by our script <emphasis>must</emphasis>
have the same prefix: <envar>${name}</envar>. For
example: <envar>dummy_mode</envar>,
<envar>dummy_state_file</envar>, and so on.</para>
</important>
<note>
<para>While it is possible to use a shorter name internally,
e.g., just <envar>msg</envar>, adding the unique prefix
<envar>${name}</envar> to all global names introduced by
our script will save us from possible
collisions with the &man.rc.subr.8; namespace.</para>
<para>As long as an &man.rc.conf.5; variable and its
internal equivalent are the same, we can use a more
compact expression to set the default value:</para>
<programlisting>: ${dummy_msg:="Nothing started."}</programlisting>
<para>The current style is to use the more verbose form
though.</para>
<para>As a rule, <filename>rc.d</filename> scripts of the
base system need not provide defaults for their
&man.rc.conf.5; variables because the defaults should
be set in <filename>/etc/defaults/rc.conf</filename>
instead. On the other hand, <filename>rc.d</filename>
scripts for ports should provide the defaults as shown
in the example.</para>
</note>
</callout>
<callout arearefs="rcng-confdummy-msg">
<para>Here we use <envar>dummy_msg</envar> to actually
control our script, i.e., to emit a variable message.</para>
</callout>
</calloutlist>
</sect1>
<sect1 id="rcng-daemon">
<title>Startup and shutdown of a simple daemon</title>
<para>We said earlier that &man.rc.subr.8; could provide default
methods. Obviously, such defaults cannot be too general.
They are suited for the common case of starting and shutting
down a simple daemon program. Let us assume now that we need
to write an <filename>rc.d</filename> script for such a daemon
called <command>mumbled</command>. Here it is:</para>
<informalexample>
<programlisting>#!/bin/sh
. /etc/rc.subr
name=mumbled
rcvar=mumbled_enable
command="/usr/sbin/${name}"<co id="rcng-daemon-basic-cmd"/>
load_rc_config $name
run_rc_command "$1"</programlisting>
</informalexample>
<para>Pleasingly simple, isn't it? Let us examine our little
script. The only new thing to note is as follows:</para>
<calloutlist>
<callout arearefs="rcng-daemon-basic-cmd">
<para>The <envar>command</envar> variable is meaningful to
&man.rc.subr.8;. If it is set, &man.rc.subr.8; will act
according to the scenario of serving a conventional daemon.
In particular, the default methods will be provided for
such arguments: <option>start</option>, <option>stop</option>,
<option>restart</option>, <option>poll</option>, and
<option>status</option>.</para>
<para>The daemon will be started by running
<envar>$command</envar> with command-line flags specified
by <envar>$mumbled_flags</envar>. Thus all the input
data for the default <option>start</option> method are
available in the variables set by our script. Unlike
<option>start</option>, other methods may require additional
information about the process started. For instance,
<option>stop</option> must know the PID of the process
to terminate it. In the present case, &man.rc.subr.8;
will scan through the list of all processes, looking for
a process with its name equal to <envar>$procname</envar>.
The latter is another variable of meaning to &man.rc.subr.8;,
and its value defaults to that of <envar>command</envar>.
In other words, when we set <envar>command</envar>,
<envar>procname</envar> is effectively set to the same
value. This enables our script to kill the daemon and
to check if it is running in the first place.</para>
<note>
<para>Some programs are in fact executable scripts. The
system runs such a script by starting its interpreter
and passing the name of the script to it as a command-line
argument. This is reflected in the list of processes,
which can confuse &man.rc.subr.8;. You should additionally
set <envar>command_interpreter</envar> to let &man.rc.subr.8;
know the actual name of the process if <envar>$command</envar>
is a script.</para>
<para>For each <filename>rc.d</filename> script, there
is an optional &man.rc.conf.5; variable that takes
precedence over <envar>command</envar>. Its name is
constructed as follows: <envar>${name}_program</envar>,
where <envar>name</envar> is the mandatory variable we
discussed <link linkend="name-var">earlier</link>.
E.g., in this case it will be <envar>mumbled_program</envar>.
It is &man.rc.subr.8; that arranges
<envar>${name}_program</envar> to override
<envar>command</envar>.</para>
<para>Of course, &man.sh.1; will permit you to set
<envar>${name}_program</envar> from &man.rc.conf.5; or
the script itself even if <envar>command</envar> is
unset. In that case, the special properties of
<envar>${name}_program</envar> are lost, and it becomes
an ordinary variable your script can use for its own
purposes. However, the sole use of
<envar>${name}_program</envar> is discouraged because
using it together with <envar>command</envar> became
an idiom of <filename>rc.d</filename> scripting.</para>
</note>
<para>For more detailed information on default methods,
refer to &man.rc.subr.8;.</para>
</callout>
</calloutlist>
</sect1>
<sect1 id="rcng-daemon-adv">
<title>Startup and shutdown of an advanced daemon</title>
<para>Let us add some meat onto the bones of the previous
script and make it more complex and featureful. The default
methods can do a good job for us, but we may need some of
their aspects tweaked. Now we will learn how to tune the
default methods to our needs.</para>
<informalexample>
<programlisting>#!/bin/sh
. /etc/rc.subr
name=mumbled
rcvar=mumbled_enable
command="/usr/sbin/${name}"
command_args="mock arguments > /dev/null 2>&1"<co id="rcng-daemon-adv-args"/>
pidfile="/var/run/${name}.pid"<co id="rcng-daemon-adv-pid"/>
required_files="/etc/${name}.conf /usr/share/misc/${name}.rules"<co id="rcng-daemon-adv-reqfiles"/>
sig_reload="USR1"<co id="rcng-daemon-adv-sig"/>
start_precmd="${name}_prestart"<co id="rcng-daemon-adv-precmd"/>
stop_postcmd="echo Bye-bye"<co id="rcng-daemon-adv-postcmd"/>
extra_commands="reload plugh xyzzy"<co id="rcng-daemon-adv-extra"/>
plugh_cmd="mumbled_plugh"<co id="rcng-daemon-adv-methods"/>
xyzzy_cmd="echo 'Nothing happens.'"
mumbled_prestart()
{
if checkyesno mumbled_smart; then<co id="rcng-daemon-adv-yn"/>
rc_flags="-o smart ${rc_flags}"<co id="rcng-daemon-adv-rcflags"/>
fi
case "$mumbled_mode" in
foo)
rc_flags="-frotz ${rc_flags}"
;;
bar)
rc_flags="-baz ${rc_flags}"
;;
*)
warn "Invalid value for mumbled_mode"<co id="rcng-daemon-adv-warn"/>
return 1<co id="rcng-daemon-adv-preret"/>
;;
esac
run_rc_command xyzzy<co id="rcng-daemon-adv-run"/>
return 0
}
mumbled_plugh()<co id="rcng-daemon-adv-plugh"/>
{
echo 'A hollow voice says "plugh".'
}
load_rc_config $name
run_rc_command "$1"</programlisting>
</informalexample>
<calloutlist>
<callout arearefs="rcng-daemon-adv-args">
<para>Additional arguments to <envar>$command</envar> can
be passed in <envar>command_args</envar>. They will be
added to the command line after <envar>$mumbled_flags</envar>.
Since the final command line is passed to <command>eval</command>
for its actual execution, input and output redirections
can be specified in <envar>command_args</envar>.</para>
<note>
<para><emphasis>Never</emphasis> include dashed options,
like <option>-X</option> or <option>--foo</option>, in
<envar>command_args</envar>.
The contents of <envar>command_args</envar> will
appear at the end of the final command line, hence
they are likely to follow arguments present in
<envar>${name}_flags</envar>; but most commands will
not recognize dashed options after ordinary arguments.
A better way of passing additional options
to <envar>$command</envar> is to add them
to the beginning of <envar>${name}_flags</envar>.
Another way is to modify <envar>rc_flags</envar> <link
linkend="rc-flags">as shown later</link>.</para>
</note>
</callout>
<callout arearefs="rcng-daemon-adv-pid">
<para>A good-mannered daemon should create a
<emphasis>pidfile</emphasis> so that its process can be
found more easily and reliably. The variable
<envar>pidfile</envar>, if set, tells &man.rc.subr.8;
where it can find the pidfile for its default methods to
use.</para>
<note>
<para>In fact, &man.rc.subr.8; will also use the pidfile
to see if the daemon is already running before starting
it. This check can be skipped by using the
<option>faststart</option> argument.</para>
</note>
</callout>
<callout arearefs="rcng-daemon-adv-reqfiles">
<para>If the daemon cannot run unless certain files exist,
just list them in <envar>required_files</envar>, and
&man.rc.subr.8; will check that those files do exist
before starting the daemon. There also are
<envar>required_dirs</envar> and <envar>required_vars</envar>
for directories and environment variables, respectively.
They all are described in detail in &man.rc.subr.8;.</para>
<note>
<para>The default method from &man.rc.subr.8; can be
forced to skip the prerequisite checks by using
<option>forcestart</option> as the argument to the
script.</para>
</note>
</callout>
<callout arearefs="rcng-daemon-adv-sig">
<para>We can customize signals to send to the daemon in
case they differ from the well-known ones. In particular,
<envar>sig_reload</envar> specifies the signal that makes
the daemon reload its configuration; it is
<symbol>SIGHUP</symbol> by default. Another signal is
sent to stop the daemon process; the default is
<symbol>SIGTERM</symbol>, but this can be changed by
setting <envar>sig_stop</envar> appropriately.</para>
<note>
<para>The signal names should be specified to &man.rc.subr.8;
without the <literal>SIG</literal> prefix, as it is
shown in the example. The &os; version of &man.kill.1;
can recognize the <literal>SIG</literal> prefix, but
the versions from other OS types may not.</para>
</note>
</callout>
<callout arearefs="rcng-daemon-adv-precmd rcng-daemon-adv-postcmd">
<para>Performing additional tasks before or after the default
methods is easy. For each command-argument supported by
our script, we can define
<envar><replaceable>argument</replaceable>_precmd</envar> and
<envar><replaceable>argument</replaceable>_postcmd</envar>.
These &man.sh.1; commands are invoked before and after
the respective method, as it is evident from their
names.</para>
<note>
<para>Overriding a default method with a custom
<envar><replaceable>argument</replaceable>_cmd</envar>
still does not prevent us from making use of
<envar><replaceable>argument</replaceable>_precmd</envar> or
<envar><replaceable>argument</replaceable>_postcmd</envar>
if we need to. In particular, the former is good for
checking custom, sophisticated conditions that should
be met before performing the command itself. Using
<envar><replaceable>argument</replaceable>_precmd</envar> along
with <envar><replaceable>argument</replaceable>_cmd</envar>
lets us logically separate the checks from the
action.</para>
<para>Do not forget that you can cram any valid &man.sh.1;
expressions into the methods, pre-, and post-commands
you define. Just invoking a function that makes the
real job is a good style in most cases, but never let
style limit your understanding of what is going on
behind the curtain.</para>
</note>
</callout>
<callout arearefs="rcng-daemon-adv-extra">
<para>If we would like to implement custom arguments, which
can also be thought of as <emphasis>commands</emphasis>
to our script, we need to list them in
<envar>extra_commands</envar> and provide methods to
handle them.</para>
<note>
<para>The <option>reload</option> command is special. On
the one hand, it has a preset method in &man.rc.subr.8;.
On the other hand, <option>reload</option> is not offered
by default. The reason is that not all daemons use the
same reload mechanism and some have nothing to reload
at all. So we need to ask explicitly that the builtin
functionality be provided. We can do so via
<envar>extra_commands</envar>.</para>
<para>What do we get from the default method for
<option>reload</option>? Quite often daemons reload
their configuration upon reception of a signal —
typically, <symbol>SIGHUP</symbol>. Therefore
&man.rc.subr.8; attempts to reload the daemon by sending
a signal to it. The signal is preset to
<symbol>SIGHUP</symbol> but can be customized via
<envar>sig_reload</envar> if necessary.</para>
</note>
</callout>
<callout arearefs="rcng-daemon-adv-methods rcng-daemon-adv-plugh">
<para>Our script supports two non-standard commands,
<option>plugh</option> and <option>xyzzy</option>. We
saw them listed in <envar>extra_commands</envar>, and now
it is time to provide methods for them. The method for
<option>xyzzy</option> is just inlined while that for
<option>plugh</option> is implemented as the
<function>mumbled_plugh</function> function.</para>
<para>Non-standard commands are not invoked during startup
or shutdown. Usually they are for the system admin's
convenience. They can also be used from other subsystems,
e.g., &man.devd.8; if specified in &man.devd.conf.5;.</para>
<para>The full list of available commands can be found in
the usage line printed by &man.rc.subr.8; when the script
is invoked without arguments. For example, here is the
usage line from the script under study:</para>
<screen>&prompt.root; <userinput>/etc/rc.d/mumbled</userinput>
Usage: /etc/rc.d/mumbled [fast|force|one](start|stop|restart|rcvar|reload|plugh|xyzzy|status|poll)</screen>
</callout>
<callout arearefs="rcng-daemon-adv-run">
<para>A script can invoke its own standard or non-standard
commands if needed. This may look similar to calling
functions, but we know that commands and shell functions
are not always the same thing. For instance,
<command>xyzzy</command> is not implemented as a function
here. In addition, there can be a pre-command and
post-command, which should be invoked orderly. So the
proper way for a script to run its own command is by means
of &man.rc.subr.8;, as shown in the example.</para>
</callout>
<callout arearefs="rcng-daemon-adv-yn">
<para>A handy function named <function>checkyesno</function>
is provided by &man.rc.subr.8;. It takes a variable name
as its argument and returns a zero exit code if and only
if the variable is set to <literal>YES</literal>, or
<literal>TRUE</literal>, or <literal>ON</literal>, or
<literal>1</literal>, case insensitive; a non-zero exit
code is returned otherwise. In the latter case, the
function tests the variable for being set to
<literal>NO</literal>, <literal>FALSE</literal>,
<literal>OFF</literal>, or <literal>0</literal>, case
insensitive; it prints a warning message if the variable
contains anything else, i.e., junk.</para>
<para>Keep in mind that for &man.sh.1; a zero exit code
means true and a non-zero exit code means false.</para>
<important>
<para>The <function>checkyesno</function> function takes
a <emphasis>variable name</emphasis>. Do not pass the
expanded <emphasis>value</emphasis> of a variable to
it; it will not work as expected.</para>
<para>The following is the correct usage of
<function>checkyesno</function>:</para>
<programlisting>if checkyesno mumbled_enable; then
foo
fi</programlisting>
<para>On the contrary, calling <function>checkyesno</function>
as shown below will not work — at least not as
expected:</para>
<programlisting>if checkyesno "${mumbled_enable}"; then
foo
fi</programlisting>
</important>
</callout>
<callout arearefs="rcng-daemon-adv-rcflags">
<para><anchor id="rc-flags"/>We can affect the flags to be
passed to <envar>$command</envar> by modifying
<envar>rc_flags</envar> in <envar>$start_precmd</envar>.</para>
</callout>
<callout arearefs="rcng-daemon-adv-warn">
<para>In certain cases we may need to emit an important
message that should go to <application>syslog</application>
as well. This can be done easily with the following
&man.rc.subr.8; functions: <function>debug</function>,
<function>info</function>, <function>warn</function>, and
<function>err</function>. The latter function then exits
the script with the code specified.</para>
</callout>
<callout arearefs="rcng-daemon-adv-preret">
<para>The exit codes from methods and their pre-commands
are not just ignored by default. If
<envar><replaceable>argument</replaceable>_precmd</envar> returns
a non-zero exit code, the main method will not be performed.
In turn,
<envar><replaceable>argument</replaceable>_postcmd</envar> will
not be invoked unless the main method returns a zero exit
code.</para>
<note>
<para>However, &man.rc.subr.8; can be instructed from the
command line to ignore those exit codes and invoke all
commands anyway by prefixing an argument with
<literal>force</literal>, as in
<option>forcestart</option>.</para>
</note>
</callout>
</calloutlist>
</sect1>
<sect1 id="rcng-hookup">
<title>Connecting a script to the rc.d framework</title>
<para>After a script has been written, it needs to be integrated
into <filename>rc.d</filename>. The crucial step is to install
the script in <filename>/etc/rc.d</filename> (for the base
system) or <filename>/usr/local/etc/rc.d</filename> (for
ports). Both <<filename>bsd.prog.mk</filename>> and
<<filename>bsd.port.mk</filename>> provide convenient
hooks for that, and usually you do not have to worry about
the proper ownership and mode. System scripts should be
installed from <filename>src/etc/rc.d</filename> through the
<filename>Makefile</filename> found there. Port scripts can
be installed using <makevar>USE_RC_SUBR</makevar> as described
<ulink url="&url.books.porters-handbook;/rc-scripts.html">in
the Porter's Handbook</ulink>.</para>
<para>However, we should consider beforehand the place of
our script in the system startup sequence. The service handled
by our script is likely to depend on other services. For
instance, a network daemon cannot function without the network
interfaces and routing up and running. Even if a service
seems to demand nothing, it can hardly start before the basic
filesystems have been checked and mounted.</para>
<para>We mentioned &man.rcorder.8; already. Now it is time to
have a close look at it. In a nutshell, &man.rcorder.8; takes
a set of files, examines their contents, and prints a
dependency-ordered list of files from the set to
<varname>stdout</varname>. The point is to keep dependency
information <emphasis>inside</emphasis> the files so that
each file can speak for itself only. A file can specify the
following information:</para>
<itemizedlist>
<listitem>
<para>the names of the <quote>conditions</quote> (which means
services to us) it <emphasis>provides</emphasis>;</para>
</listitem>
<listitem>
<para>the names of the <quote>conditions</quote>
it <emphasis>requires</emphasis>;</para>
</listitem>
<listitem>
<para>the names of the <quote>conditions</quote> this file
should run <emphasis>before</emphasis>;</para>
</listitem>
<listitem>
<para>additional <emphasis>keywords</emphasis> that can be
used to select a subset from the whole set of files
(&man.rcorder.8; can be instructed via options to include
or omit the files having particular keywords listed.)</para>
</listitem>
</itemizedlist>
<para>It is no surprise that &man.rcorder.8; can handle only
text files with a syntax close to that of &man.sh.1;. That
is, special lines understood by &man.rcorder.8; look like
&man.sh.1; comments. The syntax of such special lines is
rather rigid to simplify their processing. See &man.rcorder.8;
for details.</para>
<para>Besides using &man.rcorder.8; special lines, a script can
insist on its dependency upon another service by just starting
it forcibly. This can be needed when the other service is
optional and will not start by itself because the system admin
has disabled it mistakenly in &man.rc.conf.5;.</para>
<para>With this general knowledge in mind, let us consider the
simple daemon script enhanced with dependency stuff:</para>
<informalexample>
<programlisting>#!/bin/sh
# PROVIDE: mumbled oldmumble <co id="rcng-hookup-provide"/>
# REQUIRE: DAEMON cleanvar frotz<co id="rcng-hookup-require"/>
# BEFORE: LOGIN<co id="rcng-hookup-before"/>
# KEYWORD: nojail shutdown<co id="rcng-hookup-keyword"/>
. /etc/rc.subr
name=mumbled
rcvar=mumbled_enable
command="/usr/sbin/${name}"
start_precmd="${name}_prestart"
mumbled_prestart()
{
if ! checkyesno frotz_enable && \
! /etc/rc.d/frotz forcestatus 1>/dev/null 2>&1; then
force_depend frotz || return 1<co id="rcng-hookup-force"/>
fi
return 0
}
load_rc_config $name
run_rc_command "$1"</programlisting>
</informalexample>
<para>As before, detailed analysis follows:</para>
<calloutlist>
<callout arearefs="rcng-hookup-provide">
<para>That line declares the names of <quote>conditions</quote>
our script provides. Now other scripts can record a
dependency on our script by those names.</para>
<note>
<para>Usually a script specifies a single condition
provided. However, nothing prevents us from listing
several conditions there, e.g., for compatibility
reasons.</para>
<para>In any case, the name of the main, or the only,
<literal>PROVIDE:</literal> condition should be the
same as <envar>${name}</envar>.</para>
</note>
</callout>
<callout arearefs="rcng-hookup-require rcng-hookup-before">
<para>So our script indicates which <quote>conditions</quote>
provided by other scripts it depends on. According to
the lines, our script asks &man.rcorder.8; to put it after
the script(s) providing <filename>DAEMON</filename> and
<filename>cleanvar</filename>, but before that providing
<filename>LOGIN</filename>.</para>
<note>
<para>The <literal>BEFORE:</literal> line should not be
abused to work around an incomplete dependency list in
the other script. The appropriate case for using
<literal>BEFORE:</literal> is when the other script
does not care about ours, but our script can do its
task better if run before the other one. A typical
real-life example is the network interfaces vs. the
firewall: While the interfaces do not depend on the
firewall in doing their job, the system security will
benefit from the firewall being ready before there is
any network traffic.</para>
<para>Besides conditions corresponding to a single service
each, there are meta-conditions and their
<quote>placeholder</quote> scripts used to ensure that
certain groups of operations are performed before others.
These are denoted by
<filename><replaceable>UPPERCASE</replaceable></filename>
names. Their list and purposes can be found in
&man.rc.8;.</para>
<para>Keep in mind that putting a service name in the
<literal>REQUIRE:</literal> line does not guarantee
that the service will actually be running by the time
our script starts. The required service may fail to
start or just be disabled in &man.rc.conf.5;. Obviously,
&man.rcorder.8; cannot track such details, and &man.rc.8;
will not do that either. Consequently, the application
started by our script should be able to cope with any
required services being unavailable. In certain cases,
we can help it as discussed <link
linkend="forcedep">below.</link></para>
</note>
</callout>
<callout arearefs="rcng-hookup-keyword">
<para><anchor id="keywords"/>As we remember from the above text,
&man.rcorder.8; keywords can be used to select or leave
out some scripts. Namely any &man.rcorder.8; consumer
can specify through <option>-k</option> and <option>-s</option>
options which keywords are on the <quote>keep list</quote> and
<quote>skip list</quote>, respectively. From all the
files to be dependency sorted, &man.rcorder.8; will pick
only those having a keyword from the keep list (unless empty)
and not having a keyword from the skip list.</para>
<para>In &os;, &man.rcorder.8; is used by
<filename>/etc/rc</filename> and
<filename>/etc/rc.shutdown</filename>. These two scripts
define the standard list of &os; <filename>rc.d</filename>
keywords and their meanings as follows:</para>
<variablelist>
<varlistentry>
<term><literal>nojail</literal></term>
<listitem>
<para>The service is not for &man.jail.8; environment.
The automatic startup and shutdown procedures will
ignore the script if inside a jail.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>nostart</literal></term>
<listitem>
<para>The service is to be started manually or not
started at all. The automatic startup procedure
will ignore the script. In conjunction with the
<literal>shutdown</literal> keyword, this can be
used to write scripts that do something only at
system shutdown.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>shutdown</literal></term>
<listitem>
<para>This keyword is to be listed
<emphasis>explicitly</emphasis> if the service needs
to be stopped before system shutdown.</para>
<note>
<para>When the system is going to shut down,
<filename>/etc/rc.shutdown</filename> runs. It
assumes that most <filename>rc.d</filename> scripts
have nothing to do at that time. Therefore
<filename>/etc/rc.shutdown</filename>
selectively invokes <filename>rc.d</filename>
scripts with the <literal>shutdown</literal>
keyword, effectively ignoring the
rest of the scripts. For even faster shutdown,
<filename>/etc/rc.shutdown</filename> passes
the <option>faststop</option> command to the scripts
it runs so that they skip preliminary checks, e.g.,
the pidfile check. As dependent services should be
stopped before their prerequisites,
<filename>/etc/rc.shutdown</filename> runs the scripts
in reverse dependency order.</para>
<para>If writing a real
<filename>rc.d</filename> script, you should
consider whether it is relevant at system shutdown
time. E.g., if your script does its work in
response to the <option>start</option> command
only, then you need not include this keyword.
However, if your script manages a service, it is
probably a good idea to stop it before the system
proceeds to the final stage of its shutdown
sequence described in &man.halt.8;. In particular,
a service should be stopped explicitly if it needs
considerable time or special actions to shut down
cleanly. A typical example of such a service is
a database engine.</para>
</note>
</listitem>
</varlistentry>
</variablelist>
</callout>
<callout arearefs="rcng-hookup-force">
<para><anchor id="forcedep"/>To begin with,
<function>force_depend</function> should be used with
much care. It is generally better to revise the hierarchy
of configuration variables for your <filename>rc.d</filename>
scripts if they are interdependent.</para>
<para>If you still cannot do without
<function>force_depend</function>, the example offers an
idiom of how to invoke it conditionally. In the example,
our <command>mumbled</command> daemon requires that another
one, <command>frotz</command>, be started in advance.
However, <command>frotz</command> is optional, too; and
&man.rcorder.8; knows nothing about such details.
Fortunately, our script has access to all &man.rc.conf.5;
variables. If <envar>frotz_enable</envar> is true, we
hope for the best and rely on <filename>rc.d</filename>
to have started <command>frotz</command>. Otherwise we
forcibly check the status of <command>frotz</command>.
Finally, we enforce our dependency on <command>frotz</command>
if it is found to be not running. A warning message will
be emitted by <function>force_depend</function> because
it should be invoked only if a misconfiguration has been
detected.</para>
</callout>
</calloutlist>
</sect1>
<sect1 id="rcng-args">
<title>Giving more flexibility to an rc.d script</title>
<para>When invoked during startup or shutdown, an
<filename>rc.d</filename> script is supposed to act on the
entire subsystem it is responsible for. E.g.,
<filename>/etc/rc.d/netif</filename> should start or stop all
network interfaces described by &man.rc.conf.5;. Either task
can be uniquely indicated by a single command argument such
as <option>start</option> or <option>stop</option>. Between
startup and shutdown, <filename>rc.d</filename> scripts help
the admin to control the running system, and it is when the
need for more flexibility and precision arises. For instance,
the admin may want to add the settings of a new network
interface to &man.rc.conf.5; and then to start it without
interfering with the operation of the existing interfaces.
Next time the admin may need to shut down a single network
interface. In the spirit of the command line, the respective
<filename>rc.d</filename> script calls for an extra argument,
the interface name.</para>
<para>Fortunately, &man.rc.subr.8; allows for passing any number
of arguments to script's methods (within the system limits).
Due to that, the changes in the script itself can be minimal.</para>
<para>How can &man.rc.subr.8; gain
access to the extra command-line arguments. Should it just
grab them directly? Not by any means. Firstly, an &man.sh.1;
function has no access to the positional parameters of
its caller, but &man.rc.subr.8; is just a sack of such
functions. Secondly, the good manner of
<filename>rc.d</filename> dictates that it is for the
main script to decide which arguments are to be passed
to its methods.</para>
<para>So the approach adopted by &man.rc.subr.8; is as follows:
<function>run_rc_command</function> passes on all its
arguments but the first one to the respective method verbatim.
The first, omitted, argument is the name of the method itself:
<option>start</option>, <option>stop</option>, etc. It will
be shifted out by <function>run_rc_command</function>,
so what is <envar>$2</envar> in the original command line will
be presented as <envar>$1</envar> to the method, and so on.</para>
<para>To illustrate this opportunity, let us modify the primitive
dummy script so that its messages depend on the additional
arguments supplied. Here we go:</para>
<informalexample>
<programlisting>#!/bin/sh
. /etc/rc.subr
name="dummy"
start_cmd="${name}_start"
stop_cmd=":"
kiss_cmd="${name}_kiss"
extra_commands="kiss"
dummy_start()
{
if [ $# -gt 0 ]; then<co id="rcng-args-start"/>
echo "Greeting message: $*"
else
echo "Nothing started."
fi
}
dummy_kiss()
{
echo -n "A ghost gives you a kiss"
if [ $# -gt 0 ]; then<co id="rcng-args-kiss"/>
echo -n " and whispers: $*"
fi
case "$*" in
*[.!?])
echo
;;
*)
echo .
;;
esac
}
load_rc_config $name
run_rc_command "$@"<co id="rcng-args-all"/></programlisting>
</informalexample>
<para>What essential changes can we notice in the script?</para>
<calloutlist>
<callout arearefs="rcng-args-start">
<para>All arguments you type after <option>start</option>
can end up as positional parameters to the respective
method. We can use them in any way according to our
task, skills, and fancy. In the current example, we just
pass all of them to &man.echo.1; as one string in the
next line — note <envar>$*</envar> within the double
quotes. Here is how the script can be invoked now:</para>
<screen>&prompt.root; <userinput>/etc/rc.d/dummy start</userinput>
Nothing started.
&prompt.root; <userinput>/etc/rc.d/dummy start Hello world!</userinput>
Greeting message: Hello world!</screen>
</callout>
<callout arearefs="rcng-args-kiss">
<para>The same applies to any method our script provides,
not only to a standard one. We have added a custom method
named <option>kiss</option>, and it can take advantage of
the extra arguments not less than <option>start</option>
does. E.g.:</para>
<screen>&prompt.root; <userinput>/etc/rc.d/dummy kiss</userinput>
A ghost gives you a kiss.
&prompt.root; <userinput>/etc/rc.d/dummy kiss Once I was Etaoin Shrdlu...</userinput>
A ghost gives you a kiss and whispers: Once I was Etaoin Shrdlu...</screen>
</callout>
<callout arearefs="rcng-args-all">
<para>If we want just to pass all extra arguments to
any method, we can merely substitute <literal>"$@"</literal>
for <literal>"$1"</literal> in the last line of our script,
where we invoke <function>run_rc_command</function>.</para>
<important>
<para>An &man.sh.1; programmer ought to understand the
subtle difference between <envar>$*</envar> and
<envar>$@</envar> as the ways to designate all positional
parameters. For its in-depth discussion, refer to a
good handbook on &man.sh.1; scripting. <emphasis>Do
not</emphasis> use the expressions until you fully
understand them because their misuse will result in
buggy and insecure scripts.</para>
</important>
<note>
<para>Currently <function>run_rc_command</function> may
have a bug that prevents it from keeping the original
boundaries between arguments. That is, arguments with
embedded whitespace may not be processed correctly.
The bug stems from <envar>$*</envar> misuse.</para>
</note>
</callout>
</calloutlist>
</sect1>
<sect1 id="rcng-furthur">
<title>Further reading</title>
<para><anchor id="lukem"/><ulink
url="http://www.mewburn.net/luke/papers/rc.d.pdf">The original
article by Luke Mewburn</ulink> offers a general overview of
<filename>rc.d</filename> and detailed rationale for its
design decisions. It provides insight on the whole
<filename>rc.d</filename> framework and its place in a modern
BSD operating system.</para>
<para><anchor id="manpages"/>The manual pages &man.rc.8;,
&man.rc.subr.8;, and &man.rcorder.8; document the
<filename>rc.d</filename> components in great detail. You
cannot fully use the <filename>rc.d</filename> power without
studying the manual pages and referring to them while writing
your own scripts.</para>
<para>The major source of working, real-life examples is
<filename>/etc/rc.d</filename> in a live system. Its contents
are easy and pleasant to read because most rough corners are
hidden deep in &man.rc.subr.8;. Keep in mind though that the
<filename>/etc/rc.d</filename> scripts were not written by
angels, so they might suffer from bugs and suboptimal design
decisions. Now you can improve them!</para>
</sect1>
</article>