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

Daemon management with systemd

Since DebianJessie, systemd is used to manage daemons.

Common daemon control

Daemons are controlled with the systemctl command:

# systemctl <COMMAND> <NAME>

where NAME is the name of the service or daemon. The most common commands for controlling daemons are:

For instance, the sshd daemon is restarted by

# systemctl restart ssh

Enable/disable daemons

The systemctl command is also used to enable or disable the start of a daemon during system boot:

# systemctl <COMMAND> <NAME>

where NAME is the name of the service or daemon. The most important commands to enable/disable daemons are:

Note that the enable/disable commands only affect the system at the next boot. To change the system immediately, use the --now option. For instance:

# systemctl --now disable ssh

will prevent starting sshd at the next boot and stop the daemon immediately.

Daemon management with sysvinit

Before DebianJessie, System V-style init scripts were used for daemon management by default. 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.

A brief introduction to Debian sysvinit 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


CategoryBootProcess