OpenRC is an init system for Unixoid operating systems. It takes care of startup and shutdown of the whole system, including services.
It evolved out of the Gentoo "Baselayout" package which was a custom pure-shell startup solution. (This was both hard to maintain and debug, and not very performant)
Most of the core parts are written in C99 for performance and flexibility reasons, while everything else is posix sh. The License is 2-clause BSD
Current size is about 10k LoC C, and about 4k LoC shell.
OpenRC is known to work on Linux, many BSDs (FreeBSD, OpenBSD, DragonFlyBSD at least) and HURD.
Services are stateful (i.e. start; start will lead to "it's already started")
Usually PID1 (aka. init) calls the OpenRC binary (/sbin/openrc by default).
(The default setup assumes sysvinit for this)
openrc scans the runlevels (default: /etc/runlevels) and builds a dependency
graph, then starts the needed service scripts, either serialized (default) or in
parallel.
When all the init scripts are started openrc terminates. There is no persistent daemon. (Integration with tools like monit, runit or s6 can be done)
On change to runlevel 0/6 or running reboot, halt etc., openrc stops all
services that are started and runs the services in the shutdown runlevel.
Any service can, at any time, be started/stopped/restarted by executing
rc-service someservice start, rc-service someservice stop, etc.
Another, less preferred method, is to run the service script directly,
e.g. /etc/init.d/service start, /etc/init.d/service stop, etc.
OpenRC will take care of dependencies, e.g starting apache will start network first, and stopping network will stop apache first.
There is a special command zap that makes OpenRC 'forget' that a service is
started; this is mostly useful to reset a crashed service to stopped state
without invoking the (possibly broken) stop function of the service script.
Calling openrc without any arguments will try to reset all services so
that the current runlevel is satisfied; if you manually started apache it will be
stopped, and if squid died but is in the current runlevel it'll be restarted.
OpenRC has a concept of runlevels, similar to what sysvinit historically offered. A runlevel is basically a collection of services that needs to be started. Instead of random numbers they are named, and users can create their own if needed. This allows, for example, to have a default runlevel with "everything" enabled, and a "powersaving" runlevel where some services are disabled.
The rc-status helper will print all currently active runlevels and the state
of init scripts in them:
# rc-status
* Caching service dependencies ... [ ok ]
Runlevel: default
modules [ started ]
lvm [ started ]
All runlevels are represented as folders in /etc/runlevels/ with symlinks to
the actual init scripts.
Calling openrc with an argument (openrc default) will switch to that
runlevel; this will start and stop services as needed.
Managing runlevels is usually done through the rc-update helper, but could of
course be done by hand if desired.
e.g. rc-update add nginx default - add nginx to the default runlevel
Note: This will not auto-start nginx! You'd still have to trigger rc or run
the initscript by hand.
FIXME: Document stacked runlevels
The default startup uses the runlevels boot, sysinit and default, in that
order. Shutdown uses the shutdown runlevel.
Service scripts are shell scripts. OpenRC aims at using only the standardized
POSIX sh subset for portability reasons. The default interpreter (build-time
toggle) is /bin/sh, so using for example mksh is not a problem.
OpenRC has been tested with busybox sh, ash, dash, bash, mksh, zsh and possibly others. Using busybox sh has been difficult as it replaces commands with builtins that don't offer the expected features.
The interpreter for initscripts is #!/sbin/openrc-run.
Not using this interpreter will break the use of dependencies and is not
supported. (iow: if you insist on using #!/bin/sh you're on your own)
A depend function declares the dependencies of this service script.
All scripts must have start/stop/status functions, but defaults are provided.
Extra functions can be added easily:
extra_commands="checkconfig"
checkconfig() {
doSomething
}
This exports the checkconfig function so that /etc/init.d/someservice checkconfig will be available, and it "just" runs this function.
While commands defined in extra_commands are always available, commands
defined in extra_started_commands will only work when the service is started
and those defined in extra_stopped_commands will only work when the service is
stopped. This can be used for implementing graceful reload and similar
behaviour.
Adding a restart function will not work, this is a design decision within
OpenRC. Since there may be dependencies involved (e.g. network -> apache) a
restart function is in general not going to work.
restart is internally mapped to stop() + start() (plus handling dependencies).
If a service needs to behave differently when it is being restarted vs
started or stopped, it should test the $RC_CMD variable, for example:
[ "$RC_CMD" = restart ] && do_something
This function declares the dependencies for a service script. This determines the order the service scripts start.
depend() {
need net
use dns logger netmount
want coolservice
}
need declares a hard dependency - net always needs to be started before this
service does
use is a soft dependency - if dns, logger or netmount is in this runlevel
start it before, but we don't care if it's not in this runlevel.
want is between need and use - try to start coolservice if it is
installed on the system, regardless of whether it is in the
runlevel, but we don't care if it starts.
before declares that we need to be started before another service
after declares that we need to be started after another service, without
creating a dependency (so on calling stop the two are independent)
provide allows multiple implementations to provide one service type, e.g.:
provide cron is set in all cron-daemons, so any one of them started
satisfies a cron dependency
keyword allows platform-specific overrides, e.g. keyword -lxc makes this
service script a noop in lxc containers. Useful for things like keymaps,
module loading etc. that are either platform-specific or not available
in containers/virtualization/...
FIXME: Anything missing in this list?
All service scripts are assumed to have the following functions:
start()
stop()
status()
There are default implementations in lib/rc/sh/openrc-run.sh - this allows very
compact service scripts. These functions can be overridden per service script as
needed.
The default functions assume the following variables to be set in the service script:
command=
command_args=
pidfile=
Thus the 'smallest' service scripts can be half a dozen lines long
Most service scripts need default values. It would be fragile to
explicitly source some arbitrary files. By convention openrc-run will source
the matching file in /etc/conf.d/ for any script in /etc/init.d/
This allows you to set random startup-related things easily. Example:
conf.d/foo:
START_OPTS="--extraparameter sausage"
init.d/foo:
start() {
/usr/sbin/foo-daemon ${STARTOPTS}
}
The big advantage of this split is that most of the time editing of the init script can be avoided.
OpenRC has its own modified version of s-s-d, which is historically related and mostly syntax-compatible to Debian's s-s-d, but has been rewritten from scratch.
It helps with starting daemons, backgrounding, creating PID files and many other convenience functions related to managing daemons.
This file manages the default configuration for OpenRC, and it has examples of per-service-script variables.
Among these are rc_parallel (for parallelized startup), rc_log (logs all boot
messages to a file), and a few others.
Setting ulimit and nice values per service can be done through the
rc_ulimit variable.
Under Linux, OpenRC can use cgroups for process management as well. Once
the kernel is configured appropriately, the rc_cgroup_mode setting in
/etc/rc.conf should be used to control whether cgroups version one,,
two, or both are used. The default is to use both if they are available.
By changing certain settings in the service's conf.d file limits can be
enforced per service. These settings are documented in detail in the
default /etc/rc.conf under LINUX CGROUPS RESOURCE MANAGEMENT.
It is possible to get into a state where there are orphaned processes running which were part of a service. For example, if you are monitoring a service with supervise-daemon and supervise-daemon dies for an unknown reason. The way to deal with this will be different for each system.
On Linux systems with cgroups enabled, the cgroup_cleanup command is added to all services. You can run it manually, when the service is stopped, by using:
# rc-service someservice cgroup_cleanup
The rc_cgroup_cleanup setting can be changed to yes to make this
happen automatically when the service is stopped.
For performance reasons OpenRC keeps a cache of pre-parsed initscript metadata
(e.g. depend). The default location for this is /${RC_SVCDIR}/cache.
The cache uses mtime to check for file staleness. Should any service script
change it'll re-source the relevant files and update the cache
OpenRC has wrappers for many common output tasks in libeinfo. This allows to print colour-coded status notices and other things. To make the output consistent the bundled initscripts all use ebegin/eend to print nice messages.