© Andrew Smallshaw
This is a continuation of Unix and Linux startup scripts, Part 1
Local start up scripts with the BSD rc.d system
The rc.d system is used on NetBSD, FreeBSD and DragonFly (and
possibly a few other systems) to launch daemon processes when the
system goes multiuser and terminate them properly at system shut
down. In the interests of brevity this article will not examine
the system comprehensively: rather, this guide is focussed solely
on adding new scripts to control additional daemons or perform
other tasks at start up and shut down. Even with this narrow focus
I will not attempt a comprehensive treatment, but this guide should
suffice for most common tasks and still remain brief enough to read
when you actually need to write such a script.
Sadly, however, start up and shut down are one of the least
standardised areas of Unix systems. This article describes recent
versions of NetBSD. Since the system has been adopted by FreeBSD
and DragonFly much here will be applicable to those too. On other
systems it is likely that little if any of this applies.
System V-inspired start up systems rely on particular naming of
start up scripts to ensure that the scripts are executed in the
proper order. In contrast, with the rc.d system each script details
internally what services that script requires to run, and in turn
what services it provides. This means that the administrator need not
determine when exactly a script should execute but at the cost of
The whole process starts with the invocation of /etc/rc which
is executed to take the system multiuser, and is ultimately
responsible for the rest of the start up procedure. The logical
complement that runs when the system shuts down is
/etc/rc.shutdown. It may be tempting to simply add whatever
we want to those scripts and be done with it. Don't. Those
are operating system files and liable to change with each upgrade.
If you really do want somewhere quick and easy to add an extra
couple of commands, put them in /etc/rc.local which exists
for precisely this purpose, and they will be executed towards the
end of the start up process. The equivalent shut down script,
/etc/rc.shutdown.local, does not exist at all as shipped,
at least with NetBSD, but the shut down scripts do test for its
existence and will run it if it is present.
In the short term this may be the simplest method of starting your
services, but in the long run it lacks flexibility. Providing
the same service on another system means isolating the relevant
commands. There is also no convenient ability to start, stop or
restart individual services either permanently or temporarily while
the system is running.
Individual service scripts
The more elegant solution is to have each service controlled by
its own script which is called by /etc/rc at the correct
point. This allows things like an "/etc/rc.d/lpd stop"
to stop the print service independently of anything else, or an
"/etc/rc.d/lpd restart" to pick up configuration changes.
A discrete file is also much easier to copy to another system if
needed. However, in order to integrate with the rest of the system
these scripts have a few special characteristics.
The first requirement for these scripts is the command line arguments
they accept. We will only consider the three most important here.
The two that really matter are "start" and "stop", which correspond
to the system starting up and shutting down. The next one is
"restart", which is equivalent to "stop" followed by "start": it
is never used by the system but is handy for the administrator when
configuration files change. Scripts may provide arbitrary additional
options for manual use based on what is useful or relevant.
(Article continues after the break)
The other special property is the presence of directives within
each script. These take the form of shell script comments. The
format is rigidly defined: each directive is on a line of its own.
The line must begin with a hash, followed by a single space, the
directive name (in uppercase), a colon, another space and finally
any parameters. The directives may be placed anywhere in the file,
but they must be in one contiguous block. That is to say, once
the first has been found, processing stops at the next line (including
blank lines) that is not a directive. Any further directives in
the script will be ignored.
Four of these directives are defined:
REQUIRE: Followed by the name of one or more
services that must be operational before we execute this script.
PROVIDE: Followed by the name of one or more
services provided by this script and which may later be required
by another script.
BEFORE: This script must be executed before
an existing one. Technically, this is redundant since
REQUIRE and PROVIDE are all that is
needed to ensure proper ordering. However, this tag may avoid the
need to modify an existing script, potentially handy if it is an
operating system or third party script.
KEYWORD: The miscellaneous catch-all directive.
Several keywords are defined, but we are only going to mention
"shutdown", which indicates the presence of a termination procedure
which needs to be called at the appropriate time.
This is already probably enough to give you an inkling of how
scripts can be written such that the correct execution order can
be determined systematically. However, manually coordinating
between large numbers of services could still prove challenging.
To combat this four placeholder "services" are provided. In and
of themselves these do nothing: their role is to allow a higher-level
structure in the dependencies between scripts. For clarity their
names are capitalised whereas the names of individual services are
by convention in lowercase. In the order they occur during start
up these placeholders are:
NETWORKING, after which basic networking
services are up and running.
SERVERS, for early start servers that general
daemons may themselves depend on.
DAEMON reflects a system where filesystems
have been mounted, basic system services are running, and the
system is in a fit state to start general processes. Most processes
you are going to want to start should require DAEMON
and will execute after this.
LOGIN is a point after which processes that
allow a user to login may be started. This does covers not only
obvious login daemons such as sshd, but also things like ftpd which
allow user access. The purpose of this phase is to fence off
processes providing user access from those providing security so
that it is impossible for a user to login to an insecure system.
It is important to note that these placeholders are points
in the start up process, not phases. For example,
DAEMON is a requirement for general background processes
to prevent them starting too early; it does not represent the period
during which they are started.
So far we have only considered the start up process. When we come
to shut down the script order is reversed. This is a matter of
common sense: if service A depends on B in order to operate, we
have to stop it first. Only then can we go about stopping B.
We now have enough details to write a suitable shell script. If
it contains the necessary tags for ordering and properly handles
the command line arguments it will work. However, there are a
number of tasks that are general to a broad range of different
applications - things like identifying the correct process to
terminate at shut down, or looking in /etc/rc.subr to see
if the service is enabled. Doing this robustly can be surprisingly
complex and we would naturally want to avoid that in each and every
To try and combat this a number of predefined routines are provided.
These replace common code with a single set of functions defined
in /etc/rc.subr. That is why if you look at many of the
existing start up scripts they consist of a number of variable
definitions and a couple of function calls. Scripts using these
functions certainly go together quickly but at the cost of immediate
clarity. This is compounded by the documentation: while it is all
there there is no user guide that distils it down to what you really
need to know.
As such, we will begin with the essentials. Firstly we need to
make these routines available which we do by sourcing the file.
/etc/rc.subr is a set of POSIX shell
functions so it follows that you must code your script in a
POSIX-compatible shell: that rules out csh
The mains guts of a script are provided by run_rc_command,
a shell function that takes the command line parameter to the script
as a whole. This determines what action is requested and executes
the required commands. This should usually be preceded by a call
to load_rc_config, which takes the name of the command
and ensures that the configuration settings for it are pulled in
from /etc/rc.conf and /etc/rc.conf.d/command.
To control run_rc_command we set few variables beforehand.
The only truly mandatory one is $name, which
should be set to the name of the script. The next in order of
usefulness is $rcvar, which gives the control variable to
enable or disable the script in /etc/rc.conf. Without this
definition the script will always be executed irrespective of the
configuration files. In general it is advisable
to set this to the same thing as $name.
For many simple tasks we only need to set one more variable:
$command. This is the full pathname of the executable we
want to invoke. With this in mind let's consider out first example.
Example 1: dictd
dictd is a server daemon for the DICT
protocol most frequently used to serve dictionary definitions. In
essence it is a simple read-only database. Being read-only simplifies
things a lot since the requirements are less onerous - there are
no issues about ensuring writes are written to disk properly, or
that the server is shut down correctly. When the time comes that
we want to terminate dictd we can simply kill it.
We will assume that dictd has been installed using default
settings using the NetBSD packages system and resides in
/usr/pkg/sbin. We will also assume that the program has
already been properly configured and will start up with a simple
"dictd" at a shell prompt. The resulting script is fairly
brief considering its capability:
# REQUIRE: DAEMON
In this script we specify execution after the DAEMON
point in the start up process. We need to avoid dictd
starting too early. It is a network service so obviously it needs
networking support. Less obvious is that pathname: there is no
guarantee that /usr is on the root filesystem. Indeed by
default the NetBSD installer arranges things so that it isn't.
Therefore we need to ensure that it only runs after /usr
is mounted. The point at which we can guarantee this is after
mountcritremote but this is an implementation detail
potentially subject to change. Since there is no advantage in having
this server start early we will make it require DAEMON
like most other additional services.
This script as written does not mark the script as providing any
additional services. That is not a problem unless we have another
service that needs dictd in order to run. In this case
it is unlikely but to resolve that we would add a line:
# PROVIDE: dictd
to the script and make the dependent script require it. Surplus
PROVIDE: tags will do no harm - there is nothing
wrong with every script tagged as providing some service. If no
other script depends on it then the statement simply has no effect.
Now to consider the operation. If the script is executable and
placed in /etc/rc.d it will be called with "/etc/rc.d/dictd
start" at system boot. load_rc_config will be called
and will look for a line dictd=YES in /etc/rc.conf
and set a variable accordingly. If present then run_rc_command,
also called with the "start" command, will start dictd.
Now login as root with dictd running and type:
and the service will stop. We can then restart it by calling the
script with the "start" command. This shows the
power of the of the standard routines: although we did not define
the shut down function one has been supplied for us. By default
this routine will look for any dictd process and
send it a SIGTERM before waiting for it to actually
exit. The "restart" command is also implemented and is equivalent
to "stop" followed by "start". There are also a number of other
options provided automatically, for example "/etc/rc.d/dictd
status" will tell us if dictd is running or not. We
can add options to dictd by defining $dictd_flags
in /etc/rc.conf : this will be added to the command used
to invoke dictd.
A few words on security are probably appropriate here, but this is
not something that we are going to consider in any great depth.
These scripts execute with root privileges and as a natural
consequence the scripts must not be publicly writable. In general
services should not run as root unless absolutely necessary. In
this instance dictd will relinquish root permissions by
reinvoking itself as user nobody, and we need not do anything
further, but the security implications should always be
considered when adding new services.
Example 2: PostgreSQL
So, our first example works fine. However, it is a very simple
example. What about a more complex situation? There are many
situations when we need to do more than simply launch an executable
at start up, and simply killing the server is not an appropriate
method of stopping many services. Many real-world, business critical
services are going to require a little more care and attention but
the routines in /etc/rc.subr cover those instances, too.
Many such services are databases so for our next example we will
consider consider PostgreSQL. As with dictd, we will assume
that the DBMS is installed from the packages collection,
has been initialised and is ready to start.
However, with this kind of package the way it is set up is bound
to vary from site to site. There is a sample script installed
as part of the package but you may not want to use it: in its
attempt to be completely general it is more complex than needed
for any given installation. Its behaviour may also be undesirable
for many users: for instance if there is no database cluster found
the script will automatically create one. That may well not be
what you want.
In this example we will consider my own installation where the database
cluster resides in /home/pgsql (DBAs can argue about that
amongst themselves), and the server process is run by user pgsql.
Unlike dictd the server does not automatically change
users so this needs doing manually. We will consider three actions
for controlling the database - to start the server up and shut it down
as normal, and also an extra "urgent" shutdown for use if, for example,
the server is running off the UPS. We can summarise the actions
Server start: /usr/pkg/bin/pg_ctl -D /home/pgsql -o -i start
Normal shut down: /usr/pkg/bin/pg_ctl -D /home/pgsql stop
Urgent shut down: /usr/pkg/bin/pg_ctl -D /home/pgsql stop -m fast
The system allows us to specify the custom actions needed for each
situation by means of more shell variables. These take the general
form action_cmd - where action is
start, stop or some other parameter we can pass
to our script. If the action is more than a one-liner then it is
advisable to define shell functions to keep these definitions brief.
We are not limited to the standard actions for our script: we can
define additional actions based on what is our appropriate for our
script, so in this case we will add an "urgentstop" option for our
expedited shut down. If we do this we need to notify the subroutines
of this via the $extra_commands variable.
Finally we call run_rc_command as before which will notice
our custom action scripts and invoke them as and when appropriate.
The net result of this is something along the lines of the following
# REQUIRE: DAEMON
# PROVIDE: pgsql
# KEYWORD: shutdown
start_cmd='su pgsql -c "/usr/pkg/bin/pg_ctl -D /home/pgsql -o -i start"'
stop_cmd='su pgsql -c "/usr/pkg/bin/pg_ctl -D /home/pgsql stop"'
urgentstop_cmd='su pgsql -c "/usr/pkg/bin/pg_ctl -D /home/pgsql stop -m fast"'
Once we install this script and enable it in /etc/rc.conf
we can see that everything works as intended.
It is an example such as this that begins to show both the power and
weaknesses inherent in the rc.d system. The script is fairly brief
and comparatively simple considering what it does and its flexibility.
However, there is no denying there is a great deal of red tape
involved. This is a problem that only gets worse if the actions
needed are more than simple one-liners. In that case we need to
define separate shell functions for each action. Not a big deal, but
it is more bloat not directly solving the task in hand.
As such I think a certain amount of judgment is warranted. The
system of directives controlling dependencies and execution ordering
is undeniably elegant, but the advantage of the other infrastructure
is less clear cut for more complex examples. The use of appropriate
directives is mandatory for the system to work properly, but use of
the supplied subroutines to control the actions taken is not. A
complex set of actions may well be clearer if coded as a conventional
freestanding script even if such a script is slightly longer than one
using the infrastructure to its fullest extent.
Ask yourself which is clearer and easier to write: a script like that
above, full of opaque variable definitions, or a case statement that
looked at $1 and took the appropriate action? Fortunately
a mixed approach poses few problems in practice so there is no reason
to come to a rigid conclusion regardless of the particular
and Implementation of the NetBSD rc.d system,
by Luke Mewburn. This paper is a fairly comprehensive overview of
the entire system. There is a lot of background here and much on
the rationale for the overall design of the system. However, much
of it is not relevant when simply creating a new start up script.
NetBSD Guide, Chapter 7. Background to the system
from an administrative perspective. Details the starting and
stopping of individual services but has very little on actually creating
new scripts for additional services.
rcorder(8) man page. rcorder is what resolves
dependencies between the start up scripts in /etc/rc.d
and determines the order in which to execute them.
rc.subr(8) man page. Describes the /etc/rc.subr
routines in greater depth - we have only really scratched the surface here
in terms of the capabilities on offer.
Finally, of course, there are the /etc/rc* scripts
themselves. These are all shell scripts so they are easy
enough to open up and examine even if you do not have the sources
installed on your system.
© Andrew Smallshaw
7 December 2009.
Unix and Linux startup scripts, Part 3 explores init replacements such as Launchd and Upstart.
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