dbus-daemon - Message bus daemon



   dbus-daemon [--version] [--session] [--system] [--config-file=FILE]
               [--print-address [=DESCRIPTOR]] [--print-pid [=DESCRIPTOR]]


   dbus-daemon is the D-Bus message bus daemon. See
   http://www.freedesktop.org/software/dbus/ for more information about
   the big picture. D-Bus is first a library that provides one-to-one
   communication between any two applications; dbus-daemon is an
   application that uses this library to implement a message bus daemon.
   Multiple programs connect to the message bus daemon and can exchange
   messages with one another.

   There are two standard message bus instances: the systemwide message
   bus (installed on many systems as the "messagebus" init service) and
   the per-user-login-session message bus (started each time a user logs
   in).  dbus-daemon is used for both of these instances, but with a
   different configuration file.

   The --session option is equivalent to
   "--config-file=/usr/share/dbus-1/session.conf" and the --system option
   is equivalent to "--config-file=/usr/share/dbus-1/system.conf". By
   creating additional configuration files and using the --config-file
   option, additional special-purpose message bus daemons could be

   The systemwide daemon is normally launched by an init script,
   standardly called simply "messagebus".

   The systemwide daemon is largely used for broadcasting system events,
   such as changes to the printer queue, or adding/removing devices.

   The per-session daemon is used for various interprocess communication
   among desktop applications (however, it is not tied to X or the GUI in
   any way).

   SIGHUP will cause the D-Bus daemon to PARTIALLY reload its
   configuration file and to flush its user/group information caches. Some
   configuration changes would require kicking all apps off the bus; so
   they will only take effect if you restart the daemon. Policy changes
   should take effect with SIGHUP.


   The following options are supported:

       Use the given configuration file.

       Force the message bus to fork and become a daemon, even if the
       configuration file does not specify that it should. In most
       contexts the configuration file already gets this right, though.
       This option is not supported on Windows.

       Force the message bus not to fork and become a daemon, even if the
       configuration file specifies that it should. On Windows, the
       dbus-daemon never forks, so this option is allowed but does

       Print the address of the message bus to standard output, or to the
       given file descriptor. This is used by programs that launch the
       message bus.

       Print the process ID of the message bus to standard output, or to
       the given file descriptor. This is used by programs that launch the
       message bus.

       Use the standard configuration file for the per-login-session
       message bus.

       Use the standard configuration file for the systemwide message bus.

       Print the version of the daemon.

       Print the introspection information for all D-Bus internal

       Set the address to listen on. This option overrides the address
       configured in the configuration file.

       Enable systemd-style service activation. Only useful in conjunction
       with the systemd system and session manager on Linux.

       Don't write a PID file even if one is configured in the
       configuration files.


   A message bus daemon has a configuration file that specializes it for a
   particular application. For example, one configuration file might set
   up the message bus to be a systemwide message bus, while another might
   set it up to be a per-user-login-session bus.

   The configuration file also establishes resource limits, security
   parameters, and so forth.

   The configuration file is not part of any interoperability
   specification and its backward compatibility is not guaranteed; this
   document is documentation, not specification.

   The standard systemwide and per-session message bus setups are
   configured in the files "/usr/share/dbus-1/system.conf" and
   "/usr/share/dbus-1/session.conf". These files normally <include> a
   system-local.conf or session-local.conf in /etc/dbus-1; you can put
   local overrides in those files to avoid modifying the primary
   configuration files.

   The configuration file is an XML document. It must have the following
   doctype declaration:

          <!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-Bus Bus Configuration 1.0//EN"

   The following elements may be present in the configuration file.

   *   <busconfig>

   Root element.

   *   <type>

   The well-known type of the message bus. Currently known values are
   "system" and "session"; if other values are set, they should be either
   added to the D-Bus specification, or namespaced. The last <type>
   element "wins" (previous values are ignored). This element only
   controls which message bus specific environment variables are set in
   activated clients. Most of the policy that distinguishes a session bus
   from the system bus is controlled from the other elements in the
   configuration file.

   If the well-known type of the message bus is "session", then the
   DBUS_STARTER_BUS_TYPE environment variable will be set to "session" and
   the DBUS_SESSION_BUS_ADDRESS environment variable will be set to the
   address of the session bus. Likewise, if the type of the message bus is
   "system", then the DBUS_STARTER_BUS_TYPE environment variable will be
   set to "system" and the DBUS_SESSION_BUS_ADDRESS environment variable
   will be set to the address of the system bus (which is normally well
   known anyway).

   Example: <type>session</type>

   *   <include>

   Include a file <include>filename.conf</include> at this point. If the
   filename is relative, it is located relative to the configuration file
   doing the including.

   <include> has an optional attribute "ignore_missing=(yes|no)" which
   defaults to "no" if not provided. This attribute controls whether it's
   a fatal error for the included file to be absent.

   *   <includedir>

   Include all files in <includedir>foo.d</includedir> at this point.
   Files in the directory are included in undefined order. Only files
   ending in ".conf" are included.

   This is intended to allow extension of the system bus by particular
   packages. For example, if CUPS wants to be able to send out
   notification of printer queue changes, it could install a file to
   /usr/share/dbus-1/system.d or /etc/dbus-1/system.d that allowed all
   apps to receive this message and allowed the printer daemon user to
   send it.

   *   <user>

   The user account the daemon should run as, as either a username or a
   UID. If the daemon cannot change to this UID on startup, it will exit.
   If this element is not present, the daemon will not change or care
   about its UID.

   The last <user> entry in the file "wins", the others are ignored.

   The user is changed after the bus has completed initialization. So
   sockets etc. will be created before changing user, but no data will be
   read from clients before changing user. This means that sockets and PID
   files can be created in a location that requires root privileges for

   *   <fork>

   If present, the bus daemon becomes a real daemon (forks into the
   background, etc.). This is generally used rather than the --fork
   command line option.

   *   <keep_umask>

   If present, the bus daemon keeps its original umask when forking. This
   may be useful to avoid affecting the behavior of child processes.

   *   <syslog>

   If present, the bus daemon will log to syslog.

   *   <pidfile>

   If present, the bus daemon will write its pid to the specified file.
   The --nopidfile command-line option takes precedence over this setting.

   *   <allow_anonymous>

   If present, connections that authenticated using the ANONYMOUS
   mechanism will be authorized to connect. This option has no practical
   effect unless the ANONYMOUS mechanism has also been enabled using the
   <auth> element, described below.

   *   <listen>

   Add an address that the bus should listen on. The address is in the
   standard D-Bus format that contains a transport name plus possible

   Example: <listen>unix:path=/tmp/foo</listen>

   Example: <listen>tcp:host=localhost,port=1234</listen>

   If there are multiple <listen> elements, then the bus listens on
   multiple addresses. The bus will pass its address to started services
   or other interested parties with the last address given in <listen>
   first. That is, apps will try to connect to the last <listen> address

   tcp sockets can accept IPv4 addresses, IPv6 addresses or hostnames. If
   a hostname resolves to multiple addresses, the server will bind to all
   of them. The family=ipv4 or family=ipv6 options can be used to force it
   to bind to a subset of addresses

   Example: <listen>tcp:host=localhost,port=0,family=ipv4</listen>

   A special case is using a port number of zero (or omitting the port),
   which means to choose an available port selected by the operating
   system. The port number chosen can be obtained with the --print-address
   command line parameter and will be present in other cases where the
   server reports its own address, such as when DBUS_SESSION_BUS_ADDRESS
   is set.

   Example: <listen>tcp:host=localhost,port=0</listen>

   tcp/nonce-tcp addresses also allow a bind=hostname option, used in a
   listenable address to configure the interface on which the server will
   listen: either the hostname is the IP address of one of the local
   machine's interfaces (most commonly, a DNS name that
   resolves to one of those IP addresses, '' to listen on all IPv4
   interfaces simultaneously, or '::' to listen on all IPv4 and IPv6
   interfaces simultaneously (if supported by the OS). If not specified,
   the default is the same value as "host".

   Example: <listen>tcp:host=localhost,bind=,port=0</listen>

   *   <auth>

   Lists permitted authorization mechanisms. If this element doesn't
   exist, then all known mechanisms are allowed. If there are multiple
   <auth> elements, all the listed mechanisms are allowed. The order in
   which mechanisms are listed is not meaningful.

   Example: <auth>EXTERNAL</auth>

   Example: <auth>DBUS_COOKIE_SHA1</auth>

   *   <servicedir>

   Adds a directory to scan for .service files. Directories are scanned
   starting with the first to appear in the config file (the first
   .service file found that provides a particular service will be used).

   Service files tell the bus how to automatically start a program. They
   are primarily used with the per-user-session bus, not the systemwide

   *   <standard_session_servicedirs/>

   <standard_session_servicedirs/> is equivalent to specifying a series of
   <servicedir/> elements for each of the data directories in the "XDG
   Base Directory Specification" with the subdirectory "dbus-1/services",
   so for example "/usr/share/dbus-1/services" would be among the
   directories searched.

   The "XDG Base Directory Specification" can be found at
   http://freedesktop.org/wiki/Standards/basedir-spec if it hasn't moved,
   otherwise try your favorite search engine.

   The <standard_session_servicedirs/> option is only relevant to the
   per-user-session bus daemon defined in /etc/dbus-1/session.conf.
   Putting it in any other configuration file would probably be nonsense.

   *   <standard_system_servicedirs/>

   <standard_system_servicedirs/> specifies the standard system-wide
   activation directories that should be searched for service files. This
   option defaults to /usr/share/dbus-1/system-services.

   The <standard_system_servicedirs/> option is only relevant to the
   per-system bus daemon defined in /usr/share/dbus-1/system.conf. Putting
   it in any other configuration file would probably be nonsense.

   *   <servicehelper/>

   <servicehelper/> specifies the setuid helper that is used to launch
   system daemons with an alternate user. Typically this should be the
   dbus-daemon-launch-helper executable in located in libexec.

   The <servicehelper/> option is only relevant to the per-system bus
   daemon defined in /usr/share/dbus-1/system.conf. Putting it in any
   other configuration file would probably be nonsense.

   *   <limit>

   <limit> establishes a resource limit. For example:

         <limit name="max_message_size">64</limit>
         <limit name="max_completed_connections">512</limit>

   The name attribute is mandatory. Available limit names are:

             "max_incoming_bytes"         : total size in bytes of messages
                                            incoming from a single connection
             "max_incoming_unix_fds"      : total number of unix fds of messages
                                            incoming from a single connection
             "max_outgoing_bytes"         : total size in bytes of messages
                                            queued up for a single connection
             "max_outgoing_unix_fds"      : total number of unix fds of messages
                                            queued up for a single connection
             "max_message_size"           : max size of a single message in
             "max_message_unix_fds"       : max unix fds of a single message
             "service_start_timeout"      : milliseconds (thousandths) until
                                            a started service has to connect
             "auth_timeout"               : milliseconds (thousandths) a
                                            connection is given to
             "pending_fd_timeout"         : milliseconds (thousandths) a
                                            fd is given to be transmitted to
                                            dbus-daemon before disconnecting the
             "max_completed_connections"  : max number of authenticated connections
             "max_incomplete_connections" : max number of unauthenticated
             "max_connections_per_user"   : max number of completed connections from
                                            the same user
             "max_pending_service_starts" : max number of service launches in
                                            progress at the same time
             "max_names_per_connection"   : max number of names a single
                                            connection can own
             "max_match_rules_per_connection": max number of match rules for a single
             "max_replies_per_connection" : max number of pending method
                                            replies per connection
                                            (number of calls-in-progress)
             "reply_timeout"              : milliseconds (thousandths)
                                            until a method call times out

   The max incoming/outgoing queue sizes allow a new message to be queued
   if one byte remains below the max. So you can in fact exceed the max by

   max_completed_connections divided by max_connections_per_user is the
   number of users that can work together to denial-of-service all other
   users by using up all connections on the systemwide bus.

   Limits are normally only of interest on the systemwide bus, not the
   user session buses.

   *   <policy>

   The <policy> element defines a security policy to be applied to a
   particular set of connections to the bus. A policy is made up of
   <allow> and <deny> elements. Policies are normally used with the
   systemwide bus; they are analogous to a firewall in that they allow
   expected traffic and prevent unexpected traffic.

   Currently, the system bus has a default-deny policy for sending method
   calls and owning bus names. Everything else, in particular reply
   messages, receive checks, and signals has a default allow policy.

   In general, it is best to keep system services as small, targeted
   programs which run in their own process and provide a single bus name.
   Then, all that is needed is an <allow> rule for the "own" permission to
   let the process claim the bus name, and a "send_destination" rule to
   allow traffic from some or all uids to your service.

   The <policy> element has one of four attributes:

         user="username or userid"
         group="group name or gid"

   Policies are applied to a connection as follows:

          - all context="default" policies are applied
          - all group="connection's user's group" policies are applied
            in undefined order
          - all user="connection's auth user" policies are applied
            in undefined order
          - all at_console="true" policies are applied
          - all at_console="false" policies are applied
          - all context="mandatory" policies are applied

   Policies applied later will override those applied earlier, when the
   policies overlap. Multiple policies with the same user/group/context
   are applied in the order they appear in the config file.


   A <deny> element appears below a <policy> element and prohibits some
   action. The <allow> element makes an exception to previous <deny>
   statements, and works just like <deny> but with the inverse meaning.

   The possible attributes of these elements are:

          send_type="method_call" | "method_return" | "signal" | "error"

          receive_type="method_call" | "method_return" | "signal" | "error"

          send_requested_reply="true" | "false"
          receive_requested_reply="true" | "false"

          eavesdrop="true" | "false"



          <deny send_destination="org.freedesktop.Service" send_interface="org.freedesktop.System" send_member="Reboot"/>
          <deny send_destination="org.freedesktop.System"/>
          <deny receive_sender="org.freedesktop.System"/>
          <deny user="john"/>
          <deny group="enemies"/>

   The <deny> element's attributes determine whether the deny "matches" a
   particular action. If it matches, the action is denied (unless later
   rules in the config file allow it).

   send_destination and receive_sender rules mean that messages may not be
   sent to or received from the *owner* of the given name, not that they
   may not be sent *to that name*. That is, if a connection owns services
   A, B, C, and sending to A is denied, sending to B or C will not work

   The other send_* and receive_* attributes are purely textual/by-value
   matches against the given field in the message header.

   "Eavesdropping" occurs when an application receives a message that was
   explicitly addressed to a name the application does not own, or is a
   reply to such a message. Eavesdropping thus only applies to messages
   that are addressed to services and replies to such messages (i.e. it
   does not apply to signals).

   For <allow>, eavesdrop="true" indicates that the rule matches even when
   eavesdropping. eavesdrop="false" is the default and means that the rule
   only allows messages to go to their specified recipient. For <deny>,
   eavesdrop="true" indicates that the rule matches only when
   eavesdropping. eavesdrop="false" is the default for <deny> also, but
   here it means that the rule applies always, even when not
   eavesdropping. The eavesdrop attribute can only be combined with send
   and receive rules (with send_* and receive_* attributes).

   The [send|receive]_requested_reply attribute works similarly to the
   eavesdrop attribute. It controls whether the <deny> or <allow> matches
   a reply that is expected (corresponds to a previous method call
   message). This attribute only makes sense for reply messages (errors
   and method returns), and is ignored for other message types.

   For <allow>, [send|receive]_requested_reply="true" is the default and
   indicates that only requested replies are allowed by the rule.
   [send|receive]_requested_reply="false" means that the rule allows any
   reply even if unexpected.

   For <deny>, [send|receive]_requested_reply="false" is the default but
   indicates that the rule matches only when the reply was not requested.
   [send|receive]_requested_reply="true" indicates that the rule applies
   always, regardless of pending reply state.

   user and group denials mean that the given user or group may not
   connect to the message bus.

   For "name", "username", "groupname", etc. the character "*" can be
   substituted, meaning "any." Complex globs like "foo.bar.*" aren't
   allowed for now because they'd be work to implement and maybe encourage
   sloppy security anyway.

   <allow own_prefix="a.b"/> allows you to own the name "a.b" or any name
   whose first dot-separated elements are "a.b": in particular, you can
   own "a.b.c" or "a.b.c.d", but not "a.bc" or "a.c". This is useful when
   services like Telepathy and ReserveDevice define a meaning for subtrees
   of well-known names, such as
   org.freedesktop.Telepathy.ConnectionManager.(anything) and

   It does not make sense to deny a user or group inside a <policy> for a
   user or group; user/group denials can only be inside context="default"
   or context="mandatory" policies.

   A single <deny> rule may specify combinations of attributes such as
   send_destination and send_interface and send_type. In this case, the
   denial applies only if both attributes match the message being denied.
   e.g. <deny send_interface="foo.bar" send_destination="foo.blah"/> would
   deny messages with the given interface AND the given bus name. To get
   an OR effect you specify multiple <deny> rules.

   You can't include both send_ and receive_ attributes on the same rule,
   since "whether the message can be sent" and "whether it can be
   received" are evaluated separately.

   Be careful with send_interface/receive_interface, because the interface
   field in messages is optional. In particular, do NOT specify <deny
   send_interface="org.foo.Bar"/>! This will cause no-interface messages
   to be blocked for all services, which is almost certainly not what you
   intended. Always use rules of the form: <deny
   send_interface="org.foo.Bar" send_destination="org.foo.Service"/>

   *   <selinux>

   The <selinux> element contains settings related to Security Enhanced
   Linux. More details below.

   *   <associate>

   An <associate> element appears below an <selinux> element and creates a
   mapping. Right now only one kind of association is possible:

          <associate own="org.freedesktop.Foobar" context="foo_t"/>

   This means that if a connection asks to own the name
   "org.freedesktop.Foobar" then the source context will be the context of
   the connection and the target context will be "foo_t" - see the short
   discussion of SELinux below.

   Note, the context here is the target context when requesting a name,
   NOT the context of the connection owning the name.

   There's currently no way to set a default for owning any name, if we
   add this syntax it will look like:

          <associate own="*" context="foo_t"/>

   If you find a reason this is useful, let the developers know. Right now
   the default will be the security context of the bus itself.

   If two <associate> elements specify the same name, the element
   appearing later in the configuration file will be used.

   *   <apparmor>

   The <apparmor> element is used to configure AppArmor mediation on the
   bus. It can contain one attribute that specifies the mediation mode:

          <apparmor mode="(enabled|disabled|required)"/>

   The default mode is "enabled". In "enabled" mode, AppArmor mediation
   will be performed if AppArmor support is available in the kernel. If it
   is not available, dbus-daemon will start but AppArmor mediation will
   not occur. In "disabled" mode, AppArmor mediation is disabled. In
   "required" mode, AppArmor mediation will be enabled if AppArmor support
   is available, otherwise dbus-daemon will refuse to start.

   The AppArmor mediation mode of the bus cannot be changed after the bus
   starts. Modifying the mode in the configuration file and sending a
   SIGHUP signal to the daemon has no effect on the mediation mode.


   See http://www.nsa.gov/selinux/ for full details on SELinux. Some
   useful excerpts:

   Every subject (process) and object (e.g. file, socket, IPC object, etc)
   in the system is assigned a collection of security attributes, known as
   a security context. A security context contains all of the security
   attributes associated with a particular subject or object that are
   relevant to the security policy.

   In order to better encapsulate security contexts and to provide greater
   efficiency, the policy enforcement code of SELinux typically handles
   security identifiers (SIDs) rather than security contexts. A SID is an
   integer that is mapped by the security server to a security context at

   When a security decision is required, the policy enforcement code
   passes a pair of SIDs (typically the SID of a subject and the SID of an
   object, but sometimes a pair of subject SIDs or a pair of object SIDs),
   and an object security class to the security server. The object
   security class indicates the kind of object, e.g. a process, a regular
   file, a directory, a TCP socket, etc.

   Access decisions specify whether or not a permission is granted for a
   given pair of SIDs and class. Each object class has a set of associated
   permissions defined to control operations on objects with that class.

   D-Bus performs SELinux security checks in two places.

   First, any time a message is routed from one connection to another
   connection, the bus daemon will check permissions with the security
   context of the first connection as source, security context of the
   second connection as target, object class "dbus" and requested
   permission "send_msg".

   If a security context is not available for a connection (impossible
   when using UNIX domain sockets), then the target context used is the
   context of the bus daemon itself. There is currently no way to change
   this default, because we're assuming that only UNIX domain sockets will
   be used to connect to the systemwide bus. If this changes, we'll
   probably add a way to set the default connection context.

   Second, any time a connection asks to own a name, the bus daemon will
   check permissions with the security context of the connection as
   source, the security context specified for the name in the config file
   as target, object class "dbus" and requested permission "acquire_svc".

   The security context for a bus name is specified with the <associate>
   element described earlier in this document. If a name has no security
   context associated in the configuration file, the security context of
   the bus daemon itself will be used.


   The AppArmor confinement context is stored when applications connect to
   the bus. The confinement context consists of a label and a confinement
   mode. When a security decision is required, the daemon uses the
   confinement context to query the AppArmor policy to determine if the
   action should be allowed or denied and if the action should be audited.

   The daemon performs AppArmor security checks in three places.

   First, any time a message is routed from one connection to another
   connection, the bus daemon will check permissions with the label of the
   first connection as source, label and/or connection name of the second
   connection as target, along with the bus name, the path name, the
   interface name, and the member name. Reply messages, such as
   method_return and error messages, are implicitly allowed if they are in
   response to a message that has already been allowed.

   Second, any time a connection asks to own a name, the bus daemon will
   check permissions with the label of the connection as source, the
   requested name as target, along with the bus name.

   Third, any time a connection attempts to eavesdrop, the bus daemon will
   check permissions with the label of the connection as the source, along
   with the bus name.

   AppArmor rules for bus mediation are not stored in the bus
   configuration files. They are stored in the application's AppArmor
   profile. Please see apparmor.d(5) for more details.


   If you're trying to figure out where your messages are going or why you
   aren't getting messages, there are several things you can try.

   Remember that the system bus is heavily locked down and if you haven't
   installed a security policy file to allow your message through, it
   won't work. For the session bus, this is not a concern.

   The simplest way to figure out what's happening on the bus is to run
   the dbus-monitor program, which comes with the D-Bus package. You can
   also send test messages with dbus-send. These programs have their own
   man pages.

   If you want to know what the daemon itself is doing, you might consider
   running a separate copy of the daemon to test against. This will allow
   you to put the daemon under a debugger, or run it with verbose output,
   without messing up your real session and system daemons.

   To run a separate test copy of the daemon, for example you might open a
   terminal and type:

         DBUS_VERBOSE=1 dbus-daemon --session --print-address

   The test daemon address will be printed when the daemon starts. You
   will need to copy-and-paste this address and use it as the value of the
   DBUS_SESSION_BUS_ADDRESS environment variable when you launch the
   applications you want to test. This will cause those applications to
   connect to your test bus instead of the DBUS_SESSION_BUS_ADDRESS of
   your real session bus.

   DBUS_VERBOSE=1 will have NO EFFECT unless your copy of D-Bus was
   compiled with verbose mode enabled. This is not recommended in
   production builds due to performance impact. You may need to rebuild
   D-Bus if your copy was not built with debugging in mind. (DBUS_VERBOSE
   also affects the D-Bus library and thus applications using D-Bus; it
   may be useful to see verbose output on both the client side and from
   the daemon.)

   If you want to get fancy, you can create a custom bus configuration for
   your test bus (see the session.conf and system.conf files that define
   the two default configurations for example). This would allow you to
   specify a different directory for .service files, for example.


   See http://www.freedesktop.org/software/dbus/doc/AUTHORS


   Please send bug reports to the D-Bus mailing list or bug tracker, see


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Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.