Difference between revisions of "Daemons"

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Revision as of 16:49, 2 July 2011

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A daemon is a program that runs in the background, waiting for events to occur and offering services. A good example is a webserver that waits for a request to deliver a page or a ssh server waiting for someone trying to log in. While these are full featured applications, there are daemons whose work is not that visible. Daemons are for tasks like writing messages into a log file (e.g. syslog, metalog) or lowering your CPU frequency when the system is idle (e.g. cpufreq).

Starting on Boot

A default install of Arch Linux will leave you with very few services (or daemons) enabled during boot. You can add or remove services by editing the DAEMONS array in your rc.conf file. It will initially look something like this:

DAEMONS=(syslog-ng network netfs crond)

They will start in the order you have them listed. You can disable one and keep it in the array by prefixing it with an exclamation mark (!). You can also have them start in the background by adding an at (@) symbol in front of it.

Daemon scripts are stored in Template:Filename. You can print the list of all the available daemons on your system, along with their current status, with:

$ rc.d list

Performing daemon actions manually

Every daemon has a series of actions that can be called with specific commands: usually there are at least start, stop, and restart. You can issue each with:

# /etc/rc.d/daemon-name {start|stop|restart|...}

For example:

# /etc/rc.d/vboxdrv setup

A completely equivalent way is:

# rc.d {start|stop|restart|...} daemon-name-1 daemon-name-2 daemon-name-3 ...

which, as it is clear from the example, works also with a list of daemons, calling for each the given action. For example:

# rc.d setup vboxdrv

For a list of all the available commands for a specific daemon, check its documentation, or just open the script in a text viewer.

Essentials

You do not have to add any more services, if you do not feel the need. However, a typical desktop user will add at least CUPS and dbus. As you install new services, you will have to manually add them to the DAEMONS array in /etc/rc.conf. (The DAEMONS array is at the end of the default rc.conf file.)

Note: Some services will start other services. For example, HAL will automatically start D-Bus and Acpid. But keep in mind, as it has been mentioned elsewhere, that HAL will sometimes fail to automatically start D-Bus, without the user's awareness. It is considered good practice, therefore, to add D-Bus explicitly before HAL and not to "background" it. This will let the user know during bootup if it fails to start, before other services dependent on D-Bus break. Don't forget that dbus will still be useful even after programs stop using hal.

Starting Daemons in Background

This is helpful for starting a service and letting the next service start before the previous one has finished. Which services to start background depends on your needs. Do not background anything you need immediately. Here is an example:

DAEMONS=(syslog-ng gensplash dbus hal network netfs @avahi-daemon @samba @crond @openntpd @cups @mpd)

Starting openntpd in the background could lead to synchronization errors between the actual time and the time stored on your computer. If you recognize an increasing time difference between your desktop clock and the actual time, try to start the openntpd daemon normally and not in the background.

Rc.conf GUI Frontends

Rc.conf GUI Frontends allow you to easily change settings in /etc/rc.conf using graphical aplication.

List of Daemons

(Here is a list of daemons. Any package can use a daemon if it needs to, so this list will never be complete. Please feel free to add any missing daemons here, in alphabetical order.)

Daemon Description
acpid Delivers ACPI events.
alsa Advanced Linux Sound Architecture; provides device drivers for sound cards.
atd run jobs queued for later execution.
avahi-daemon Allows programs to automatically find local network services.
avahi-dnsconfd
crond Daemon to schedule and time events.
cups Common UNIX Printing System daemon.
dbus Message bus system for software communication.
fam File Alteration Monitor.
fbsplash Graphical boot splash screen for the user.
gensplash (see fbsplash)
hal Hardware Abstraction Layer.
mdadm MD Administration (Linux Software RAID).
mpd Music Player Daemon.
mysqld MySQL database server.
netfs Mounts network file systems.
network To bring up the network connections.
networkmanager Combine with HAL to replace network, and provide configuration and detection for automatic network connections.
nsyslogd
ntpd Network Time Protocol daemon (client and server).
openntpd alternate Network Time Protocol daemon (client and server).
pure-ftpd FTP server.
rsyslogd The latest version of a system logger.
slim Simple Login Manager
samba File and print services for Microsoft Windows clients.
saned To share the scanner system over network.
sensors Hardware (temperature, fans etc) monitoring.
smartd Self-Monitoring, Analysis, and Reporting Technology (S.M.A.R.T) Hard Disk Monitoring
sshd OpenSSH (secure shell) daemon.
stbd This daemon was previously necessary for gnome-system-tools. However, as of gnome-tools 2.28, it is no longer needed.
syslogd This was the older and basic system logger.
syslog-ng System logger next generation.
timidity++ Software synthesizer for MIDI.
vsftpd FTP server.
wicd Combine with dbus to replace network, a lightweight alternative to networkmanager.

See also

Examples for writing rc.d scripts