openrc/guide.md

Purpose and description

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")

Startup

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)

Shutdown

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.

Modifying Service Scripts

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.

There is a service helper that emulates the syntax seen on e.g. older Redhat and Ubuntu (service nginx start etc.)

Runlevels

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.

Syntax of Service Scripts

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

The Depend Function

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?

The Default Functions

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

The Magic of conf.d

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.

Start-Stop-Daemon

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.

/etc/rc.conf

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.

ulimit and CGroups

Setting ulimit and nice values per service can be done through the rc_ulimit variable.

Under Linux, OpenRC can optionally use CGroups for process management. By default each service script's processes are migrated to their own CGroup.

By changing certain values in the conf.d file limits can be enforced per service. It is easy to find orphan processes of a service that persist after stop(), but by default these will NOT be terminated. To change this add rc_cgroup_cleanup="yes" in the conf.d files for services where you desire this functionality.

Caching

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

Convenience functions

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.