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/bin/init and its descendants

init

systemd

upstart

Upstart is a project that aims to replace to old init system by providing one standard way of starting and stopping daemons with the correct environment. A second goal is to speed up a computer's boot time. It achieves this by removing the slow init shell scripts, and also by parallelizing as much of the startup as possible. Where old init daemons start daemons in successive order, upstart issues "events" on which "jobs" can listen.

Such an event can be e.g.: filesystem - indicates that the system has mounted all its filesystems and we can proceed to start any jobs that would depend on a filesystem. Each job then becomes an event of its own, upon which others can depend. These events can be broken up into stages: starting(7), and started(7); and stopping(7), and stopped(7) respectively.

A good starting point for learning how different jobs on a system are interconnected is initctl(8)'s show-config command:

igalic@tynix ~ % initctl show-config
avahi-daemon
  start on (filesystem and started dbus)
  stop on stopping dbus
cgroup-lite
  start on mounted MOUNTPOINT=/sys
elasticsearch
  start on (filesystem or runlevel [2345])
  stop on runlevel [!2345]
mountall-net
  start on net-device-up
...

This snippet reveals that upstart will stop the avahi-daemon at the same time as dbus. Unlike many other daemons that depend on the whole filesystem, upstart will start cgroup-lite as soon as the /sys filesystem is mounted.

Upstart is also able to "supervise" programs: that is, to restart a program after it crashed, or was killed. To achieve this, upstart needs to "follow" a programs progression. It uses the ptrace(2) system call to do so. However, following a daemons forks is complex, because not all daemons are written alike. The upstart documentation recommends to avoid this whenever possible and force a to remain in the foreground. That makes upstart's job a lot easier.

Finally, upstart can also switch to a predefined user (and group) before starting the program. Unlike systemd and SMF, however, it cannot drop to a limited set of capabilities(7) before doing so.

Putting it all together in an example:

# httpd-examplecom - Apache HTTP Server for example.com
#

# description     "Apache HTTP Server for example.com"

start on filesystems
stop on runlevel [06]

respawn
respawn limit 5 30

setuid examplecom
setgid examplecom
console log

script
    exec /opt/httpd/bin/httpd -f /etc/httpds/example.com/httpd.conf -DNO_DETACH -k start
end script

In this example we define an upstart job for serving example.com from the Apache HTTP Server. We switch to the user/group examplecom and start httpd in the foreground, by passing the option -DNO_DETACH.

To activate this job, we simply place it in a file in /etc/init/, e.g. /etc/init/httpd-examplecom.conf. We can then start/stop the job by issuing:

% sudo start httpd-examplecom

Note that this job definition alone already will guarantee that the system will start the job on reboot. If this is not what we want, we can add the stanza manual to the job definition.

SMF

daemontools