Various Linux Daemons and Libraries

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    In Linux, there are many daemons, libraries, and processes that administrators should be familiar with. Not knowing the purpose of a process may make it more difficult to find the cause and solution to a problem. This article will discuss some of the most important daemons, libraries, and processes.

    DBus is a bus messenger system that allows applications to communicate. DBus has two parts. One, called the session bus, allows applications to communicate within the desktop environment. The second part, the system bus, allows communication with the system daemons. DBus mimics IP to some extent. Applications that are going to use DBus are given a random DBus name which is like the IP addresses used in networks. Some applications have a set DBus name. For instance, the Evince document viewer has a DBus name of "org.gnome.evince.Daemon". Most (if not all) processes that make the desktop environment produce an error notification bubble like "notify-send" are using DBus to transfer such data.

    Compiz is a window manager. It places and manages the windows. If this process is killed, then the graphical applications will not work. For example, if there is no program to hold other programs in a window/box for the GUI, then no programs will be seen. However, sometimes the border disappears but not the contents (bug maybe?). Compiz also provides special effects for windows. For example, moving windows can make them wobble like gelatin (if that feature is enabled).

    OpenGL is an application programming interface (API) for graphics.

    Mesa is a library for graphics. This library uses OpenGL to render graphics.

    Pango is a text renderer used in many applications (like Linux versions of Firefox) and toolkits (like GTK+).

    GStreamer is a multimedia framework that is made of codecs, input/output drivers, etc. Gstreamer is used by many media players and off support for many media formats. There are three sets of GStreamer plugin packages - good, bad, and ugly. The "good" package contains high-quality tested plugins while the "bad" package contains plugins that may be low quality, untested, etc. The "ugly" set has the worst quality and may be unstable.

    GTK+ (Gimp ToolKit) is a toolkit for creating graphical user interfaces for programs.

    PulseAudio is a sound system that passes sounds from applications to the hardware. PulseAudio is like a sound proxy server. PulseAudio can also send sound to other computers over the network.

    OSS is an alternative to ALSA and PulseAudio. The Open Sound System (OSS) manages the sound system.

    acpid is the ACPI manager Daemon. ACPI controls power management events. For example, when the laptop lid closes, acpid will turn off the screen. Otherwise, power is being wasted if the screen is still on.

    Autofs is the daemon that mounts removable storage when it is plugged in. Anytime a USB flash drive or a FireWire storage device is plugged in, Autofs will auto-mount the storage. If not, then the user would need to use a command-line to mount their removable storage every time it was plugged in. Autofs also auto-mounts network shares.

    Chrony is a daemon for light-weight NTP servers and clients.

    clamd is a common anti-virus software daemon for Unix systems.

    crond is a very commonly known daemon in Linux. Cron manages scheduled tasks.

    cupsd is the printing service daemon. If this daemon is missing, then the system will not be able to submit print jobs to real or virtual printers.

    For those of you using Dropbox, you should see a dropboxd running.

    NOTE: To see the currently running processes/daemon, type "top" in the command-line. Alternately, use your preferred task/process manager GUI to view the currently running processes.

    GDM is the Gnome Display manager; GDM is the login screen. The display manager for KDE is called KDM. Again, LXDE uses lxdm as its display manager for login screens. xdm is the X Display Manager and the daemon with the Arch theme is called xdm-archlinux.

    ftpd is the FTP daemon for FTP clients/servers.

    httpd is the HTTP daemon for HTTP clients/servers.

    NOTE: As you may have noticed, server systems have a daemon named just like the server type but with a "d" on the end. For example, MySQL servers run the "mysql" + "d" daemon.

    The music player daemon is called mpd. This daemon is commonly used for music servers.

    The mythbackend is the daemon that runs in the background to transcode media files and detect commercial breaks among many other tasks. This daemon is commonly seen in Mythubuntu.

    The Point-to-Point Protocol (PPP) requires pppd to be an active daemon.

    Onion routing requires the tor daemon. Tor is used for Internet privacy and popular among Anonymous.

    VirtualBox runs the vboxservice daemon.

    The spamd daemon is an email filtering service known as spamassassin.

    The syslogd daemon is used for logging. This is a very common and active daemon.

    There are many other daemons, but these are the most important to know.

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