Add a basic OpenRC users guide
X-Gentoo-Bug: 513024 X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=513024
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Patrick Lauer
William Hubbs
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guide.md
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guide.md
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# Purpose and description
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OpenRC is an init system for Unixoid operating systems. It takes care of
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startup and shutdown of the whole system, including services.
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It evolved out of the Gentoo "Baselayout" package which was a custom pure-shell
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startup solution. (This was both hard to maintain and debug, and not very
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performant)
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Most of the core parts are written in C99 for performance and flexibility
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reasons, while everything else is posix sh.
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The License is 2-clause BSD
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Current size is about 10k LoC C, and about 4k LoC shell.
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OpenRC is known to work on Linux, many BSDs (FreeBSD, OpenBSD, DragonFlyBSD at
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least) and HURD.
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Services are stateful (i.e. start; start will lead to "it's already started")
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# Startup
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Usually PID1 (aka. init) calls the OpenRC binary ("/sbin/openrc" by default).
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(The default setup assumes sysvinit for this)
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openrc scans the runlevels (default: "/etc/runlevels") and builds a dependency
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graph, then starts the needed service scripts, either serialized (default) or in
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parallel.
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When all the init scripts are started openrc terminates. There is no persistent
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daemon. (Integration with tools like monit, runit or s6 can be done)
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# Shutdown
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On change to runlevel 0/6 or running "reboot", "halt" etc., openrc stops all
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services that are started and runs the services in the "shutdown" runlevel.
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# Modifying Service Scripts
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Any service can, at any time, be started/stopped/restarted by executing
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"rc-service someservice start", "rc-service someservice stop", etc.
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Another, less preferred method, is to run the service script directly,
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e.g. "/etc/init.d/service start", "/etc/init.d/service stop", etc.
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OpenRC will take care of dependencies, e.g starting apache will start network
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first, and stopping network will stop apache first.
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There is a special command "zap" that makes OpenRC 'forget' that a service is
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started; this is mostly useful to reset a crashed service to stopped state
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without invoking the (possibly broken) stop function of the service script.
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Calling "openrc" without any arguments will try to reset all services so
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that the current runlevel is satisfied; if you manually started apache it will be
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stopped, and if squid died but is in the current runlevel it'll be restarted.
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There is a "service" helper that emulates the syntax seen on e.g. older Redhat
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and Ubuntu ("service nginx start" etc.)
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# Runlevels
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OpenRC has a concept of runlevels, similar to what sysvinit historically
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offered. A runlevel is basically a collection of services that needs to be
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started. Instead of random numbers they are named, and users can create their
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own if needed. This allows, for example, to have a default runlevel with
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"everything" enabled, and a "powersaving" runlevel where some services are
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disabled.
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The "rc-status" helper will print all currently active runlevels and the state
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of init scripts in them:
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# rc-status
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* Caching service dependencies ... [ ok ]
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Runlevel: default
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modules [ started ]
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lvm [ started ]
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All runlevels are represented as folders in /etc/runlevels/ with symlinks to
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the actual init scripts.
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Calling openrc with an argument ("openrc default") will switch to that
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runlevel; this will start and stop services as needed.
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Managing runlevels is usually done through the "rc-update" helper, but could of
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course be done by hand if desired.
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e.g. "rc-update add nginx default" - add nginx to the default runlevel
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Note: This will not auto-start nginx! You'd still have to trigger "rc" or run
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the initscript by hand.
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FIXME: Document stacked runlevels
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The default startup uses the runlevels "boot", "sysinit" and "default", in that
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order. Shutdown uses the "shutdown" runlevel.
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# Syntax of Service Scripts
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Service scripts are shell scripts. OpenRC aims at using only the standardized
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POSIX sh subset for portability reasons. The default interpreter (build-time
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toggle) is /bin/sh, so using for example mksh is not a problem.
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OpenRC has been tested with busybox sh, ash, dash, bash, mksh, zsh and possibly
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others. Using busybox sh has been difficult as it replaces commands with
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builtins that don't offer the expected features.
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The interpreter for initscripts is #!/sbin/openrc-run
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Not using this interpreter will break the use of dependencies and is not
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supported. (iow: if you insist on using #!/bin/sh you're on your own)
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A "depend" function declares the dependencies of this service script.
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All scripts must have start/stop/status functions, but defaults are provided.
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Extra functions can be added easily:
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extra_commands="checkconfig"
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checkconfig() {
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doSomething
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}
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This exports the checkconfig function so that "/etc/init.d/someservice
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checkconfig" will be available, and it "just" runs this function.
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While commands defined in extra_commands are always available, commands
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defined in extra_started_commands will only work when the service is started
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and those defined in extra_stopped_commands will only work when the service is
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stopped. This can be used for implementing graceful reload and similar
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behaviour.
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Adding a restart function will not work, this is a design decision within
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OpenRC. Since there may be dependencies involved (e.g. network -> apache) a
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restart function is in general not going to work.
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restart is internally mapped to stop() + start() (plus handling dependencies).
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If a service needs to behave differently when it is being restarted vs
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started or stopped, it should test the $RC_CMD variable, for example:
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[ "$RC_CMD" = restart ] && do_something
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# The Depend Function
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This function declares the dependencies for a service script. This
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determines the order the service scripts start.
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depend() {
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need net
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use dns logger netmount
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want coolservice
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}
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"need" declares a hard dependency - net always needs to be started before this
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service does
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"use" is a soft dependency - if dns, logger or netmount is in this runlevel
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start it before, but we don't care if it's not in this runlevel.
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"want" is between need and use - try to start coolservice if it is
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installed on the system, regardless of whether it is in the
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runlevel, but we don't care if it starts.
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"before" declares that we need to be started before another service
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"after" declares that we need to be started after another service, without
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creating a dependency (so on calling stop the two are independent)
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"provide" allows multiple implementations to provide one service type, e.g.:
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'provide cron' is set in all cron-daemons, so any one of them started
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satisfies a cron dependency
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"keyword" allows platform-specific overrides, e.g. "keyword -lxc" makes this
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service script a noop in lxc containers. Useful for things like keymaps,
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module loading etc. that are either platform-specific or not available
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in containers/virtualization/...
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FIXME: Anything missing in this list?
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# The Default Functions
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All service scripts are assumed to have the following functions:
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start()
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stop()
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status()
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There are default implementations in rc/sh/runscript.sh - this allows very
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compact service scripts. These functions can be overridden per service script as
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needed.
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The default functions assume the following variables to be set in the service
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script:
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command=
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command_args=
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pidfile=
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Thus the 'smallest' service scripts can be half a dozen lines long
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# The Magic of Conf.d
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Most service scripts need default values. It would be fragile to
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explicitly source some arbitrary files. By convention openrc-run will source
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the matching file in /etc/conf.d/ for any script in /etc/init.d/
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This allows you to set random startup-related things easily. Example:
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conf.d/foo:
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START_OPTS="--extraparameter sausage"
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init.d/foo:
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start() {
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/usr/sbin/foo-daemon ${STARTOPTS}
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}
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The big advantage of this split is that most of the time editing of the init
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script can be avoided.
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# Start-Stop-Daemon
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OpenRC has its own modified version of s-s-d, which is historically related and
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mostly syntax-compatible to Debian's s-s-d, but has been rewritten from scratch.
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It helps with starting daemons, backgrounding, creating PID files and many
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other convenience functions related to managing daemons.
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# /etc/rc.conf
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This file manages the default configuration for OpenRC, and it has examples of
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per-service-script variables.
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Among these are rc_parallel (for parallelized startup), rc_log (logs all boot
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messages to a file), and a few others.
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# ulimit and CGroups
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Setting ulimit and nice values per service can be done through the rc_ulimit
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variable.
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Under Linux, OpenRC can optionally use CGroups for process management.
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By default each service script's processes are migrated to their own CGroup.
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By changing certain values in the conf.d file limits can be enforced per
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service. It is easy to find orphan processes of a service that persist after
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stop(), but by default these will NOT be terminated.
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To change this add rc_cgroup_cleanup="yes" in the conf.d files for services
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where you desire this functionality.
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# Caching
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For performance reasons OpenRC keeps a cache of pre-parsed initscript metadata
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(e.g. depend). The default location for this is /${RC_SVCDIR}/cache.
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The cache uses mtime to check for file staleness. Should any service script
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change it'll re-source the relevant files and update the cache
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# Convenience functions
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OpenRC has wrappers for many common output tasks in libeinfo.
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This allows to print colour-coded status notices and other things.
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To make the output consistent the bundled initscripts all use ebegin/eend to
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print nice messages.
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