How do services in Debian work, and how can I manage them?

There are currently 3 main init systems used by linux. A few years ago, there was just one, SysVinit. But SysVinit was seriously lacking in capabilities such as service dependency graphing, so it's been deprecated in most distros by now. Currently most distros are switching to systemd. Though there is also upstart.

But here's the answer to your question for each of the 3 init systems:

 

SysVinit

SysVinit currently used by Debian and RedHat. Though the next version of RedHat (7) will be using systemd.

The univeral way of enabling SysVinit services on boot is to symlink them in /etc/rc3.d (or /etc/rc2.d). All services can be found in /etc/init.d. Note however that distros will often have their own tool for managing these files, and that tool should be used instead. (Fedora/RedHat has service and chkconfig, ubuntu has update-rc.d)

List services:

ls /etc/init.d/

Start service:

/etc/init.d/{SERVICENAME} start

or

service {SERVICENAME} start

Stop service:

/etc/init.d/{SERVICENAME} stop

or

service {SERVICENAME} stop

Enable service:

cd /etc/rc3.d
ln -s ../init.d/{SERVICENAME} S95{SERVICENAME}

(the S95 is used to specify the order. S01 will start before S02, etc)

Disable service:

rm /etc/rc3.d/*{SERVICENAME}

 

Systemd

The most notable distribution using systemd is Fedora. Though it is used by many others. Additionally, with Debian having chosen to go with systemd over upstart, it will become the defacto upstart system for most distributions (ubuntu has already announced they will be dropping upstart for systemd).

List services:

systemctl list-unit-files

Start service:

systemctl start {SERVICENAME}

Stop service:

systemctl stop {SERVICENAME}

Enable service:

systemctl enable {SERVICENAME}

Disable service:

systemctl disable {SERVICENAME}

 

Upstart

Upstart was developed by the Ubuntu folks. But after debian decided to go with systemd, Ubuntu announced they would drop upstart.

Upstart was also briefly used by RedHat, as it is present in RHEL-6, but it is not commonly used.

List services:

initctl list

Start service:

initctl start {SERVICENAME}

Stop service:

initctl stop {SERVICENAME}

Enable service:

2 ways unfortunately:

1. There will be a file /etc/default/{SERVICENAME} which contains a line ENABLED=.... Change this line to ENABLED=1.

2. There will be a file /etc/init/{SERVICENAME}.override. Make sure it contains start (or is absent entirely), not manual.

Disable service:

echo manual > /etc/init/{SERVICENAME}.override

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Note: There is also the 'OpenRC' init system which is used by Gentoo. Currently Gentoo is the only distro which uses it, and it is not being considered for use, nor supported by any other distro. So I am not covering it's usage (though if opinion is that I do, I can add it).

Different distributions use different mechanisms to manage services. The software to manage services is called init, after the traditional name for the very first process (with process ID 1) which is in charge of starting the others.

Debian uses the traditional SysVinit variant of init. Under this system, there is a collection of scripts in the directory /etc/init (this and other location may vary slightly between distributions that use SysVinit). These scripts are not invoked directly, but through symbolic links in directories /etc/rc?.d. It is the presence and the name of these symbolic links that determine when services are started. For more details, read the chapter on init in the Debian Reference.

Have a look in /etc/rc?.d to see what services are already present. The letter or digit before the dot is the runlevel; entries whose name starts with S are executed with the argument start when entering the runlevel, and entries whose name starts with K are executed when leaving the runlevel. The normal runlevel sequence is: S during boot (so /etc/rcS.d/S* are executed), then 2 (so /etc/rc2.d/S* are executed). At shutdown time, /etc/rc2.d/K* are executed, then the runlevel switches to 0 (or 6 for a reboot).

In a nutshell, if you want to create a startup script for a new service:

• Write a shell script in /etc/init.d. This script must accept one argument which may be start, stop, force-reload, restart, or (optional) reload or status. The difference between reload and restart is that restart is equivalent to stop followed by start while reload reloads the configuration without stopping anything (if the service supports it); force-reload does reload if available and restart otherwise. See the existing files and Making scripts run at boot time with Debian for examples.
• Run update-rc.d to create symbolic links to start and stop your service. Most services run in runlevels 2, 3, 4 and 5.

Note that to provide svn access, it may be easier to set up Apache and use the HTTP or HTTPS protocol. This has the side benefit of allowing quick repository browsing through a web browser.

From A traditional unix background, there is nothing special about services. Services are just process, but with two exceptions: they don't need a terminal and they get started at boot. how they get started at boot depends on init (which could be sysv init, bsd init, upstart, systemd or something else; check your man page for init) and whether your are using a wrapper for the task or for the init configuration. There is nothing stopping you from running a service from a terminal, in fact it is common for testing purposes.