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## Auto-converted by kwiki2moinmoin v2005-10-07
Pronunciation:{{{
 day'mn or dee'mn
#language en
~-[[DebianWiki/EditorGuide#translation|Translation(s)]]: English - [[fr/Daemon|Français]] - [[it/Daemon|Italiano]]-~
----
A [[WikiPedia:Daemon_(computing)|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:

 * sshd - listens for and manages incoming [[ssh|SSH]] connections
 * [[acpid]] - listens for power management events and executes scripts based on them
 * [[Apache|apache]] - provides a local HTTP web server

<<TableOfContents>>


== 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:
 * '''start''': starts a service immediately
 * '''stop''': stops a service immediately
 * '''restart''': restarts a service
 * '''reload''': asks a service to reload its configuration
 * '''status''': shows the current status of a service

For instance, the sshd daemon is restarted by
{{{
# systemctl restart ssh
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From the mythological meaning, later rationalized as the acronym `Disk And Execution ["MONitor"]'. === 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:
 * '''enable''': enable service to be started during boot sequence
 * '''disable''': do not start service during boot sequence
 * '''is-enabled''': check if a service is already enabled
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A program that is not invoked explicitly, but lies dormant waiting for some condition(s) to occur. The idea is that the perpetrator of the condition need not be aware that a daemon is lurking (though often a program will commit an action only because it knows that it will implicitly invoke a daemon). For example, under ITS writing a file on the LPT spooler's directory would invoke the spooling daemon, which would then print the file. The advantage is that programs wanting (in this example) files printed need neither compete for access to nor understand any idiosyncrasies of the LPT. They simply enter their implicit requests and let the daemon decide what to do with them. Daemons are usually spawned automatically by the system, and may either live forever or be regenerated at intervals. Daemon and demon are often used interchangeably, but seem to have distinct connotations. The term `daemon' was introduced to computing by CTSS people (who pronounced it ''dee'mon'') and used it to refer to what ITS called a dragon; the prototype was a program called DAEMON that automatically made tape backups of the file system. Although the meaning and the pronunciation have drifted, we think this glossary reflects current (2000) usage. From Jargon Dictionary http://www.tldp.org/LDP/Linux-Dictionary/html/index.html

A background process of the operating system that usually has root security level permission. A daemon usually lurks in the background until something triggers it into activity, such as a specific time or date, time interval, receipt of e-mail, etc. From I-gloss http://www.tldp.org/LDP/Linux-Dictionary/html/index.html

A process lurking in the background, usually unnoticed, until something triggers it into action. For example, the \cmd{update} daemon wakes up every thirty seconds or so to flush the buffer cache, and the \cmd{sendmail} daemon awakes whenever someone sends mail. http://www.tldp.org/LDP/Linux-Dictionary/html/index.html

A program that runs continuously in the background, until activated by a particular event. A daemon can constantly query for requests or await direct action from a user or other process. From Redhat-9-Glossary http://www.tldp.org/LDP/Linux-Dictionary/html/index.html
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.
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A program which runs for an extended period (usually "forever") to handle requests for service as needed. From Linux Guide @FirstLinux http://www.tldp.org/LDP/Linux-Dictionary/html/index.html == Daemon management with sysvinit ==
Before [[DebianJessie]], [[WikiPedia:Init#SysV-style|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]].
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A program, usually on a computer running UNIX, that serves some obscure function (such as routing electronic mail to its recipients) and usually has a very limited user interface. There's some debate about the origins of the word, but most say it derives from the devilish spirits of Greek mythology. From QUECID http://www.tldp.org/LDP/Linux-Dictionary/html/index.html The [[http://www.debian.org/doc/debian-policy/|Debian Policy Manual]] (sections [[http://www.debian.org/doc/debian-policy/ch-opersys.html#s-sysvinit|9.3]] and [[http://www.debian.org/doc/debian-policy/ch-opersys.html#s9.4|9.4]]) is an excellent resource for better understanding daemon init scripts in Debian.
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Disk And Execution ["MONitor"] (Unix) From VERA http://www.tldp.org/LDP/Linux-Dictionary/html/index.html === 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.
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Daemon (service)
On ["UNIX"], a daemon is a program running in the background, usually providing some sort of service. Typical daemons are those that provide e-mail, printing, telnet, FTP, and web access. From Hacking-Lexicon http://www.tldp.org/LDP/Linux-Dictionary/html/index.html
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:
 * '''start''': start a service
 * '''stop''': stop a service
 * '''restart''': restart a service without reloading its job config file
 * '''reload''': send a SIGHUP signal to running process
 * '''status''': return the status of a service

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 [[DebianMan:8/update-rc.d|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: DebianPkg:rcconf and DebianPkg:sysv-rc-conf.

== See also ==
 * [[Init]]
 * [[RunLevel]]
 * manpages: [[DebianMan:5/inittab|inittab]], [[DebianMan:8/init|init]], [[DebianMan:5/rcS|rcS]], [[DebianMan:8/update-rc.d|update-rc.d]], [[DebianMan:8/runlevel|runlevel]], [[DebianMan:8/insserv|insserv]]

----
CategoryBootProcess

Translation(s): English - Français - Italiano


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:

  • sshd - listens for and manages incoming ?SSH connections

  • acpid - listens for power management events and executes scripts based on them

  • apache - provides a local HTTP web server

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:

  • start: starts a service immediately

  • stop: stops a service immediately

  • restart: restarts a service

  • reload: asks a service to reload its configuration

  • status: shows the current status of a service

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:

  • enable: enable service to be started during boot sequence

  • disable: do not start service during boot sequence

  • is-enabled: check if a service is already enabled

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:

  • start: start a service

  • stop: stop a service

  • restart: restart a service without reloading its job config file

  • reload: send a SIGHUP signal to running process

  • status: return the status of a service

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