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Latest commit 06d3161 Mar 28, 2017 @troglobit committed on GitHub Merge pull request #72 from abucodonosor/master
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Signed-off-by: Joachim Nilsson <troglobit@gmail.com>


Finit: Fast Init

Table of Contents

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Finit is an EeePC Fastinit inspired clone with process supervision similar to that of D.J. Bernstein's daemontools and Gerrit Pape's runit. It also supports free-form conditions based on events, e.g. triggering on PID file creation, Netlink events like a default route or interfaces coming up, or even custom events. Similar to systemd.

Finit focuses on small and embedded Linux systems, although fully usable on servers and desktops as well. See the contrib section for Debian and Alpine Linux examples.

Traditional SysV init style systems are scripted. For low-resource embedded systems this is quite resource intensive and often leads to long boot times. Finit reduces context switches and forking of shell scripts to provide a swift system bootstrap written entirely in C.

Configuration is read from /etc/finit.conf which details kernel modules to load, services and TTYs to start. When the initial bootstrap is done, including setting up networking, all services in /etc/finit.d/ and the familiar /etc/rc.local are started.

Example /etc/finit.conf:

    # Fallback if /etc/hostname is missing
    host wopr
    # Runlevel to start after bootstrap, runlevel 'S'
    runlevel 2
    # Services to be monitored and respawned as needed
    service [S12345] /sbin/watchdogd -L -f                       -- System watchdog daemon
    service [S12345] /sbin/syslogd -n -b 3 -D                    -- System log daemon
    service [S12345] /sbin/klogd -n                              -- Kernel log daemon
    service   [2345] /sbin/lldpd -d -c -M1 -H0 -i                -- LLDP daemon (IEEE 802.1ab)
    # For multiple instances of the same service, add :ID somewhere between
    # the service/run/task keyword and the command.
    service :1 [2345] /sbin/merecat -n -p 80   /var/www -- Web server
    service :2 [2345] /sbin/merecat -n -p 8080 /var/www -- Old web server

    # Alternative method instead of below runparts, can also use /etc/rc.local
    #task [S] /etc/init.d/keyboard-setup start -- Setting up preliminary keymap
    #task [S] /etc/init.d/acpid start          -- Starting ACPI Daemon
    #task [S] /etc/init.d/kbd start            -- Preparing console

    # Inetd services to start on demand, with alternate ports and filtering
    inetd ftp/tcp          nowait [2345] /sbin/uftpd -i -f       -- FTP daemon
    inetd tftp/udp           wait [2345] /sbin/uftpd -i -y       -- TFTP daemon
    inetd time/udp           wait [2345] internal                -- UNIX rdate service
    inetd time/tcp         nowait [2345] internal                -- UNIX rdate service
    inetd 3737/tcp         nowait [2345] internal.time           -- UNIX rdate service
    inetd telnet/tcp       nowait [2345] /sbin/telnetd -i -F     -- Telnet daemon
    inetd 2323/tcp         nowait [2345] /sbin/telnetd -i -F     -- Telnet daemon
    inetd 222/tcp@eth0     nowait [2345] /sbin/dropbear -i -R -F -- SSH service
    inetd ssh/tcp@*,!eth0  nowait [2345] /sbin/dropbear -i -R -F -- SSH service
    # Run start scripts from this directory
    # runparts /etc/start.d
    # Virtual consoles to start built-in getty on
    tty [12345] /dev/tty1    115200 linux
    tty [12345] /dev/ttyAMA0 115200 vt100

The service stanza, as well as task, run, inetd and others are described in full in doc/config.md. Here's a quick overview of some of the most common components needed to start a UNIX daemon:

    service [LVLS] <COND> /path/to/daemon ARGS -- Some text
    ^       ^      ^      ^               ^       ^
	|       |      |      |               |        `-- Optional description
	|       |      |      |                `---------- Daemon arguments
	|       |      |       `-------------------------- Path to daemon
	|       |       `--------------------------------- Optional conditions
	|        `---------------------------------------- Optional Runlevels
	 `------------------------------------------------ Monitored application

Some components are optional. It is important to note, however, is for the daemon not to fork and detach itself from the controlling TTY. This is usually an -n or -f argument to the application. If the daemon detaches itself, Finit cannot monitor it and will instead try to restart it.

For an example of a full blown embedded Linux, see TroglOS, or take a look at the contrib/ section with Alpine Linux, Debian, support and more.


Process Supervision

Start, monitor and restart services should they fail.


Finit comes with a built-in inetd server. No need to maintain a separate config file for services that you want to start on demand.

All inetd services started can be filtered per port and inbound interface, reducing the need for a full blown firewall.

Built-in optional inetd services:

  • echo RFC862
  • chargen RFC864
  • daytime RFC867
  • discard RFC863
  • time (rdate) RFC868

For more information, see doc/inetd.md.


Finit also comes with a built-in Getty for Linux console TTYs. It can parse /etc/inittab and set the speed. Then /bin/login handles all nasty bits with PAM etc.

# /etc/finit.conf
tty [12345] /dev/tty1    38400  linux
tty [12345] /dev/ttyAMA0 115200 vt100


Support for SysV init-style runlevels is available, in the same minimal style as everything else in Finit. The [2345] syntax can be applied to service, task, run, inetd, and TTY stanzas.

All services in runlevel S(1) are started first, followed by the desired run-time runlevel. Runlevel S can be started in sequence by using run [S] cmd. Changing runlevels at runtime is done like any other init, e.g. init 4, but also using the more advanced intictl tool.


Plugins can extend the functionality of Finit and hook into the different stages of the boot process and at runtime. Plugins are written in C and compiled into a dynamic library loaded automatically by finit at boot. A basic set of plugins are bundled in the plugins/ directory.


  • Hooks
    Hook into the boot at predefined points to extend Finit
  • I/O
    Listen to external events and control Finit behavior/services
  • Inetd
    Extend Finit with internal inetd services, for an example, see plugins/time.c

Extensions and functionality not purely related to what an /sbin/init needs to start a system are available as a set of plugins that either hook into the boot process or respond to various I/O.

For more information, see doc/plugins.md.

Runparts & /etc/rc.local

At the end of the boot, when networking and all services are up, Finit calls its built-in run-parts(8) on the runparts <DIR> directory, and /etc/rc.local, in that order if they exist.

    runparts /etc/rc.d/

No configuration stanza in /etc/finit.conf is required for rc.local. If it exists and is an executable shell script, finit calls it at the very end of the boot, before calling the HOOK_SYSTEM_UP. See more on hooks in doc/plugins.md, and about the system bootstrap in doc/bootstrap.md.


Basic support for runlevels is included in Finit from v1.8. By default all services, tasks, run commands and TTYs listed without a set of runlevels get a default set [234] assigned. The default runlevel after boot is 2.

Finit supports runlevels 0-9, and S, with 0 reserved for halt, 6 reboot and S for services to only run at bootstrap. Runlevel 1 is the single user level, where usually no networking is enabled. In Finit this is more of a policy for the user to define. Normally only runlevels 1-6 are used, and even more commonly, only the default runlevel is used.

To specify an allowed set of runlevels for a service, run command, task, or tty, add [NNN] to your /etc/finit.conf, like this:

    service [S12345] /sbin/syslogd -n -x     -- System log daemon
    run     [S]      /etc/init.d/acpid start -- Starting ACPI Daemon
    task    [S]      /etc/init.d/kbd start   -- Preparing console
    service [S12345] /sbin/klogd -n -x       -- Kernel log daemon
    tty     [12345]  /dev/tty1
    tty     [2]      /dev/tty2
    tty     [2]      /dev/tty3
    tty     [2]      /dev/tty4
    tty     [2]      /dev/tty5
    tty     [2]      /dev/tty6

In this example syslogd is first started, in parallel, and then acpid is called using a conventional SysV init script. It is called with the run command, meaning the following task command to start the kbd script is not called until the acpid init script has fully completed. Then the keyboard setup script is called in parallel with klogd as a monitored service.

Again, tasks and services are started in parallel, while run commands are called in the order listed and subsequent commands are not started until a run command has completed.

Switching between runlevels can be done by calling init with a single argument, e.g. init 5 switches to runlevel 5. When changing runlevels Finit also automatically reloads all .conf files in the /etc/finit.d/ directory. So if you want to set a new system config, switch to runlevel 1, change all config files in the system, and touch all .conf files in /etc/finit.d before switching back to the previous runlevel again — that way Finit can both stop old services and start any new ones for you, without rebooting the system.

Rebooting & Halting

Traditionally, rebooting and halting a UNIX system is done by changing its runlevel. Finit comes with its own tooling providing: shutdown, reboot, poweroff, and suspend, but also the traditional init and telinit, as well as a more modern initctl tool, detailed in the next section.

For compatibility reasons Finit listens to the same set of signals as BusyBox init. This is not 100% compatible with SysV init, but clearly the more common combination for Finit. For more details, see doc/signals.md.

Finit also listens to the classic SysV init FIFO, used by telinit. Support for this is implemented by the initctl.so plugin. Hence, telinit q will work as the UNIX beards intended.

~ # telinit -h
Usage: telinit [OPTIONS] [q | Q | 0-9]

  -h, --help      This help text
  -V, --version   Show Finit version

  0               Power-off the system, same as initctl poweroff
  6               Reboot the system, same as initctl reboot
  2, 3, 4, 5      Change runlevel. Starts services in new runlevel, stops any
                  services in prev. runlevel that are not allowed in new.
  q, Q            Reload *.conf in /etc/finit.d/, same as initctl reload or
                  sending SIGHUP to PID 1
  1, s, S         Enter system rescue mode, runlevel 1

Commands & Status

Finit also implements a more modern API to query status, and start/stop services, called initctl. Unlike telinit the initctl tool does not return until the given command has fully completed.

~ $ initctl -h
Usage: initctl [OPTIONS] <COMMAND>

  -d, --debug               Debug initctl (client)
  -v, --verbose             Verbose output
  -h, --help                This help text

  debug                     Toggle Finit (daemon) debug
  help                      This help text
  reload                    Reload *.conf in /etc/finit.d/ and activate changes
  runlevel [0-9]            Show or set runlevel: 0 halt, 6 reboot
  status | show             Show status of services
  cond     set   <COND>     Set (assert) condition     => +COND
  cond     clear <COND>     Clear (deassert) condition => -COND
  cond     flux  <COND>     Emulate flux condition     => ~COND
  cond     show             Show condition status
  start    <JOB|NAME>[:ID]  Start service by job# or name, with optional ID
  stop     <JOB|NAME>[:ID]  Stop/Pause a running service by job# or name
  restart  <JOB|NAME>[:ID]  Restart (stop/start) service by job# or name
  reload   <JOB|NAME>[:ID]  Reload (SIGHUP) service by job# or name
  version                   Show Finit version

For services not supporting SIGHUP the <!> notation in the .conf file must be used to tell Finit to stop and start it on reload and runlevel changes. If <> holds more conditions, these will also affect how a service is maintained.

Note: even though it is possible to start services not belonging in the current runlevel these services will not be respawned automatically by Finit if they exit (crash). Hence, if the runlevel is 2, the below Dropbear SSH service will not be restarted if it is killed or exits.

~ $ initctl status -v
1       running  476     [S12345]   /sbin/watchdog -T 16 -t 2 -F /dev/watchdog
2       running  477     [S12345]   /sbin/syslogd -n -b 3 -D
3       running  478     [S12345]   /sbin/klogd -n
4:1       inetd  0       [2345]     internal time allow *:37
4:2       inetd  0       [2345]     internal time allow *:37
4:3       inetd  0       [2345]     internal 3737 allow *:3737
5:1       inetd  0       [2345]     /sbin/telnetd allow *:23 deny eth0,eth1
5:2       inetd  0       [2345]     /sbin/telnetd allow eth0:2323,eth2:2323,eth1:2323
6:1       inetd  0       [345]      /sbin/dropbear allow eth0:222
6:2       inetd  0       [345]      /sbin/dropbear allow *:22 deny eth0


Finit is capable of running on both desktop/server systems with udev and embedded systems that usually come with BusyBox mdev. Finit probes for them at runtime and expects /dev/ to be writable, usually devtmpfs. It is also possible to run on a statically set up /dev if needed. It is however not a good idea to have both udev and mdev installed at the same time, this will lead to unpredictable results.

At boot Finit calls either mdev or udevd to populate /dev, this is done slightly differently and on systems with udev you might want to add the following one-shot task early in your /etc/finit.conf:

run [S] /sbin/udevadm settle --timeout=120 -- Waiting for udev

Finit has a built-in Getty for TTYs, but requires a working /bin/login or /bin/sh, if no TTYs are configured in /etc/finit.conf.

For a fully operational system /var, /run and /tmp must be set up properly in /etc/fstab -- which is iterated over at boot.

The built-in Inetd requires /etc/services and /etc/protocols to work with port names rather than numbers.


Having successfully built Finit it may now be time to take it for a test drive. The make install attempts to set up finit as the system system init, /sbin/init, but this is usually a symlink pointing to the current init.

So either change the symlink, or change your boot loader (GRUB, LOADLIN, LILO, U-Boot/Barebox or RedBoot) configuration to append the following to the kernel command line:


Remember to also set up an initial /etc/finit.conf before rebooting!

Finit starting TroglOS


Add finit_debug, or --debug, to the kernel command line to enable debug messages.

    append="init=/sbin/finit --debug"

To debug startup issues, in particular issues with getty/login, try configure --enable-fallback-shell. When no TTYs are detected, and Finit is configured with this option, Finit will try to start a bare /bin/sh on the boot console.

Origin & References

This project is based on the original finit by Claudio Matsuoka which was reverse engineered from syscalls of the EeePC fastinit — "gaps filled with frog DNA …"

Finit is developed and maintained by Joachim Nilsson at GitHub. Please file bug reports, clone it, or send pull requests for bug fixes and proposed extensions.