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A daemon, or system service, is a background process usually started during the initial boot sequence by Init. Daemons typically run independent of users, waiting for system events to occur and providing services in response. Some common daemons include:

ToDo: needs to be rewritten for systemd.

Daemons in Debian

Debian makes use of System V-style init scripts for daemon management. This allows daemons to operate conditionally, based on the current RunLevel of the computer. For example, a daemon can be configured to run only when the computer is in single-user mode (runlevel 1) or, more commonly, when in multi-user mode (runlevels 2-5). For more information, see Init and RunLevel.

The Debian Policy Manual (sections 9.3 and 9.4) is an excellent resource for better understanding daemon init scripts in Debian.

Daemon management

A brief introduction to Debian init scripts

Daemon init scripts are stored in /etc/init.d/ along with the system's other boot-time init scripts.

When a daemon is enabled or disabled, symbolic links targeting the respective init script are created or removed under the various /etc/rc*.d/ directories, corresponding to the RunLevel(s) in which the daemon is to run.

Daemon init scripts are treated as configuration files by dpkg. This means they remain on the system after a package is uninstalled, unless the 'purge' option is used.

Common daemon controls

Most daemon scripts accept a common set of options, which can be passed directly to the script at the command line, for example:

# service ssh restart
Restarting OpenBSD Secure Shell server: sshd.

A brief description of the most common options follows:

Starting and stopping daemons in this manner is temporary and will not survive a reboot. Refer to the next section to enable/disable daemons on a permanent basis.

Enable/disable daemons

Note: When in doubt, refer to the documentation of the daemon itself.

To (re)enable/disable a daemon (using the default settings & runlevels) run the following command, where <daemon> corresponds to the name of the init script as listed in /etc/init.d/:

# update-rc.d <daemon> enable|disable

For more detail on what these commands do, refer to the update-rc.d manpage.

For more information on Debian's way of managing and writing init scripts see LSBInitScripts/DependencyBasedBoot and LSBInitScripts.

GUI utilities for daemon management

Several GUI tools exist to make daemon management even simpler; some popular examples available for Debian include: rcconf and sysv-rc-conf.

See also