daemon(7) — Linux manual page


DAEMON(7)                          daemon                          DAEMON(7)

NAME         top

       daemon - Writing and packaging system daemons

DESCRIPTION         top

       A daemon is a service process that runs in the background and
       supervises the system or provides functionality to other processes.
       Traditionally, daemons are implemented following a scheme originating
       in SysV Unix. Modern daemons should follow a simpler yet more
       powerful scheme (here called "new-style" daemons), as implemented by
       systemd(1). This manual page covers both schemes, and in particular
       includes recommendations for daemons that shall be included in the
       systemd init system.

   SysV Daemons
       When a traditional SysV daemon starts, it should execute the
       following steps as part of the initialization. Note that these steps
       are unnecessary for new-style daemons (see below), and should only be
       implemented if compatibility with SysV is essential.

        1. Close all open file descriptors except standard input, output,
           and error (i.e. the first three file descriptors 0, 1, 2). This
           ensures that no accidentally passed file descriptor stays around
           in the daemon process. On Linux, this is best implemented by
           iterating through /proc/self/fd, with a fallback of iterating
           from file descriptor 3 to the value returned by getrlimit() for

        2. Reset all signal handlers to their default. This is best done by
           iterating through the available signals up to the limit of _NSIG
           and resetting them to SIG_DFL.

        3. Reset the signal mask using sigprocmask().

        4. Sanitize the environment block, removing or resetting environment
           variables that might negatively impact daemon runtime.

        5. Call fork(), to create a background process.

        6. In the child, call setsid() to detach from any terminal and
           create an independent session.

        7. In the child, call fork() again, to ensure that the daemon can
           never re-acquire a terminal again. (This relevant if the program
           — and all its dependencies — does not carefully specify
           `O_NOCTTY` on each and every single `open()` call that might
           potentially open a TTY device node.)

        8. Call exit() in the first child, so that only the second child
           (the actual daemon process) stays around. This ensures that the
           daemon process is re-parented to init/PID 1, as all daemons
           should be.

        9. In the daemon process, connect /dev/null to standard input,
           output, and error.

       10. In the daemon process, reset the umask to 0, so that the file
           modes passed to open(), mkdir() and suchlike directly control the
           access mode of the created files and directories.

       11. In the daemon process, change the current directory to the root
           directory (/), in order to avoid that the daemon involuntarily
           blocks mount points from being unmounted.

       12. In the daemon process, write the daemon PID (as returned by
           getpid()) to a PID file, for example /run/foobar.pid (for a
           hypothetical daemon "foobar") to ensure that the daemon cannot be
           started more than once. This must be implemented in race-free
           fashion so that the PID file is only updated when it is verified
           at the same time that the PID previously stored in the PID file
           no longer exists or belongs to a foreign process.

       13. In the daemon process, drop privileges, if possible and

       14. From the daemon process, notify the original process started that
           initialization is complete. This can be implemented via an
           unnamed pipe or similar communication channel that is created
           before the first fork() and hence available in both the original
           and the daemon process.

       15. Call exit() in the original process. The process that invoked the
           daemon must be able to rely on that this exit() happens after
           initialization is complete and all external communication
           channels are established and accessible.

       The BSD daemon() function should not be used, as it implements only a
       subset of these steps.

       A daemon that needs to provide compatibility with SysV systems should
       implement the scheme pointed out above. However, it is recommended to
       make this behavior optional and configurable via a command line
       argument to ease debugging as well as to simplify integration into
       systems using systemd.

   New-Style Daemons
       Modern services for Linux should be implemented as new-style daemons.
       This makes it easier to supervise and control them at runtime and
       simplifies their implementation.

       For developing a new-style daemon, none of the initialization steps
       recommended for SysV daemons need to be implemented. New-style init
       systems such as systemd make all of them redundant. Moreover, since
       some of these steps interfere with process monitoring, file
       descriptor passing and other functionality of the init system, it is
       recommended not to execute them when run as new-style service.

       Note that new-style init systems guarantee execution of daemon
       processes in a clean process context: it is guaranteed that the
       environment block is sanitized, that the signal handlers and mask is
       reset and that no left-over file descriptors are passed. Daemons will
       be executed in their own session, with standard input connected to
       /dev/null and standard output/error connected to the
       systemd-journald.service(8) logging service, unless otherwise
       configured. The umask is reset.

       It is recommended for new-style daemons to implement the following:

        1. If SIGTERM is received, shut down the daemon and exit cleanly.

        2. If SIGHUP is received, reload the configuration files, if this

        3. Provide a correct exit code from the main daemon process, as this
           is used by the init system to detect service errors and problems.
           It is recommended to follow the exit code scheme as defined in
           the LSB recommendations for SysV init scripts[1].

        4. If possible and applicable, expose the daemon's control interface
           via the D-Bus IPC system and grab a bus name as last step of

        5. For integration in systemd, provide a .service unit file that
           carries information about starting, stopping and otherwise
           maintaining the daemon. See systemd.service(5) for details.

        6. As much as possible, rely on the init system's functionality to
           limit the access of the daemon to files, services and other
           resources, i.e. in the case of systemd, rely on systemd's
           resource limit control instead of implementing your own, rely on
           systemd's privilege dropping code instead of implementing it in
           the daemon, and similar. See systemd.exec(5) for the available

        7. If D-Bus is used, make your daemon bus-activatable by supplying a
           D-Bus service activation configuration file. This has multiple
           advantages: your daemon may be started lazily on-demand; it may
           be started in parallel to other daemons requiring it — which
           maximizes parallelization and boot-up speed; your daemon can be
           restarted on failure without losing any bus requests, as the bus
           queues requests for activatable services. See below for details.

        8. If your daemon provides services to other local processes or
           remote clients via a socket, it should be made socket-activatable
           following the scheme pointed out below. Like D-Bus activation,
           this enables on-demand starting of services as well as it allows
           improved parallelization of service start-up. Also, for
           state-less protocols (such as syslog, DNS), a daemon implementing
           socket-based activation can be restarted without losing a single
           request. See below for details.

        9. If applicable, a daemon should notify the init system about
           startup completion or status updates via the sd_notify(3)

       10. Instead of using the syslog() call to log directly to the system
           syslog service, a new-style daemon may choose to simply log to
           standard error via fprintf(), which is then forwarded to syslog
           by the init system. If log levels are necessary, these can be
           encoded by prefixing individual log lines with strings like "<4>"
           (for log level 4 "WARNING" in the syslog priority scheme),
           following a similar style as the Linux kernel's printk() level
           system. For details, see sd-daemon(3) and systemd.exec(5).

       11. As new-style daemons are invoked without a controlling TTY (but
           as their own session leaders) care should be taken to always
           specify `O_NOCTTY` on `open()` calls that possibly reference a
           TTY device node, so that no controlling TTY is accidentally

       These recommendations are similar but not identical to the Apple
       MacOS X Daemon Requirements[2].

ACTIVATION         top

       New-style init systems provide multiple additional mechanisms to
       activate services, as detailed below. It is common that services are
       configured to be activated via more than one mechanism at the same
       time. An example for systemd: bluetoothd.service might get activated
       either when Bluetooth hardware is plugged in, or when an application
       accesses its programming interfaces via D-Bus. Or, a print server
       daemon might get activated when traffic arrives at an IPP port, or
       when a printer is plugged in, or when a file is queued in the printer
       spool directory. Even for services that are intended to be started on
       system bootup unconditionally, it is a good idea to implement some of
       the various activation schemes outlined below, in order to maximize
       parallelization. If a daemon implements a D-Bus service or listening
       socket, implementing the full bus and socket activation scheme allows
       starting of the daemon with its clients in parallel (which speeds up
       boot-up), since all its communication channels are established
       already, and no request is lost because client requests will be
       queued by the bus system (in case of D-Bus) or the kernel (in case of
       sockets) until the activation is completed.

   Activation on Boot
       Old-style daemons are usually activated exclusively on boot (and
       manually by the administrator) via SysV init scripts, as detailed in
       the LSB Linux Standard Base Core Specification[1]. This method of
       activation is supported ubiquitously on Linux init systems, both
       old-style and new-style systems. Among other issues, SysV init
       scripts have the disadvantage of involving shell scripts in the boot
       process. New-style init systems generally employ updated versions of
       activation, both during boot-up and during runtime and using more
       minimal service description files.

       In systemd, if the developer or administrator wants to make sure that
       a service or other unit is activated automatically on boot, it is
       recommended to place a symlink to the unit file in the .wants/
       directory of either multi-user.target or graphical.target, which are
       normally used as boot targets at system startup. See systemd.unit(5)
       for details about the .wants/ directories, and systemd.special(7) for
       details about the two boot targets.

   Socket-Based Activation
       In order to maximize the possible parallelization and robustness and
       simplify configuration and development, it is recommended for all
       new-style daemons that communicate via listening sockets to employ
       socket-based activation. In a socket-based activation scheme, the
       creation and binding of the listening socket as primary communication
       channel of daemons to local (and sometimes remote) clients is moved
       out of the daemon code and into the init system. Based on per-daemon
       configuration, the init system installs the sockets and then hands
       them off to the spawned process as soon as the respective daemon is
       to be started. Optionally, activation of the service can be delayed
       until the first inbound traffic arrives at the socket to implement
       on-demand activation of daemons. However, the primary advantage of
       this scheme is that all providers and all consumers of the sockets
       can be started in parallel as soon as all sockets are established. In
       addition to that, daemons can be restarted with losing only a minimal
       number of client transactions, or even any client request at all (the
       latter is particularly true for state-less protocols, such as DNS or
       syslog), because the socket stays bound and accessible during the
       restart, and all requests are queued while the daemon cannot process

       New-style daemons which support socket activation must be able to
       receive their sockets from the init system instead of creating and
       binding them themselves. For details about the programming interfaces
       for this scheme provided by systemd, see sd_listen_fds(3) and
       sd-daemon(3). For details about porting existing daemons to
       socket-based activation, see below. With minimal effort, it is
       possible to implement socket-based activation in addition to
       traditional internal socket creation in the same codebase in order to
       support both new-style and old-style init systems from the same
       daemon binary.

       systemd implements socket-based activation via .socket units, which
       are described in systemd.socket(5). When configuring socket units for
       socket-based activation, it is essential that all listening sockets
       are pulled in by the special target unit sockets.target. It is
       recommended to place a WantedBy=sockets.target directive in the
       [Install] section to automatically add such a dependency on
       installation of a socket unit. Unless DefaultDependencies=no is set,
       the necessary ordering dependencies are implicitly created for all
       socket units. For more information about sockets.target, see
       systemd.special(7). It is not necessary or recommended to place any
       additional dependencies on socket units (for example from
       multi-user.target or suchlike) when one is installed in

   Bus-Based Activation
       When the D-Bus IPC system is used for communication with clients,
       new-style daemons should employ bus activation so that they are
       automatically activated when a client application accesses their IPC
       interfaces. This is configured in D-Bus service files (not to be
       confused with systemd service unit files!). To ensure that D-Bus uses
       systemd to start-up and maintain the daemon, use the SystemdService=
       directive in these service files to configure the matching systemd
       service for a D-Bus service. e.g.: For a D-Bus service whose D-Bus
       activation file is named org.freedesktop.RealtimeKit.service, make
       sure to set SystemdService=rtkit-daemon.service in that file to bind
       it to the systemd service rtkit-daemon.service. This is needed to
       make sure that the daemon is started in a race-free fashion when
       activated via multiple mechanisms simultaneously.

   Device-Based Activation
       Often, daemons that manage a particular type of hardware should be
       activated only when the hardware of the respective kind is plugged in
       or otherwise becomes available. In a new-style init system, it is
       possible to bind activation to hardware plug/unplug events. In
       systemd, kernel devices appearing in the sysfs/udev device tree can
       be exposed as units if they are tagged with the string "systemd".
       Like any other kind of unit, they may then pull in other units when
       activated (i.e. plugged in) and thus implement device-based
       activation. systemd dependencies may be encoded in the udev database
       via the SYSTEMD_WANTS= property. See systemd.device(5) for details.
       Often, it is nicer to pull in services from devices only indirectly
       via dedicated targets. Example: Instead of pulling in
       bluetoothd.service from all the various bluetooth dongles and other
       hardware available, pull in bluetooth.target from them and
       bluetoothd.service from that target. This provides for nicer
       abstraction and gives administrators the option to enable
       bluetoothd.service via controlling a bluetooth.target.wants/ symlink
       uniformly with a command like enable of systemctl(1) instead of
       manipulating the udev ruleset.

   Path-Based Activation
       Often, runtime of daemons processing spool files or directories (such
       as a printing system) can be delayed until these file system objects
       change state, or become non-empty. New-style init systems provide a
       way to bind service activation to file system changes. systemd
       implements this scheme via path-based activation configured in .path
       units, as outlined in systemd.path(5).

   Timer-Based Activation
       Some daemons that implement clean-up jobs that are intended to be
       executed in regular intervals benefit from timer-based activation. In
       systemd, this is implemented via .timer units, as described in

   Other Forms of Activation
       Other forms of activation have been suggested and implemented in some
       systems. However, there are often simpler or better alternatives, or
       they can be put together of combinations of the schemes above.
       Example: Sometimes, it appears useful to start daemons or .socket
       units when a specific IP address is configured on a network
       interface, because network sockets shall be bound to the address.
       However, an alternative to implement this is by utilizing the Linux
       IP_FREEBIND socket option, as accessible via FreeBind=yes in systemd
       socket files (see systemd.socket(5) for details). This option, when
       enabled, allows sockets to be bound to a non-local, not configured IP
       address, and hence allows bindings to a particular IP address before
       it actually becomes available, making such an explicit dependency to
       the configured address redundant. Another often suggested trigger for
       service activation is low system load. However, here too, a more
       convincing approach might be to make proper use of features of the
       operating system, in particular, the CPU or I/O scheduler of Linux.
       Instead of scheduling jobs from userspace based on monitoring the OS
       scheduler, it is advisable to leave the scheduling of processes to
       the OS scheduler itself. systemd provides fine-grained access to the
       CPU and I/O schedulers. If a process executed by the init system
       shall not negatively impact the amount of CPU or I/O bandwidth
       available to other processes, it should be configured with
       CPUSchedulingPolicy=idle and/or IOSchedulingClass=idle. Optionally,
       this may be combined with timer-based activation to schedule
       background jobs during runtime and with minimal impact on the system,
       and remove it from the boot phase itself.


   Writing systemd Unit Files
       When writing systemd unit files, it is recommended to consider the
       following suggestions:

        1. If possible, do not use the Type=forking setting in service
           files. But if you do, make sure to set the PID file path using
           PIDFile=. See systemd.service(5) for details.

        2. If your daemon registers a D-Bus name on the bus, make sure to
           use Type=dbus in the service file if possible.

        3. Make sure to set a good human-readable description string with

        4. Do not disable DefaultDependencies=, unless you really know what
           you do and your unit is involved in early boot or late system

        5. Normally, little if any dependencies should need to be defined
           explicitly. However, if you do configure explicit dependencies,
           only refer to unit names listed on systemd.special(7) or names
           introduced by your own package to keep the unit file operating

        6. Make sure to include an [Install] section including installation
           information for the unit file. See systemd.unit(5) for details.
           To activate your service on boot, make sure to add a
           WantedBy=multi-user.target or WantedBy=graphical.target
           directive. To activate your socket on boot, make sure to add
           WantedBy=sockets.target. Usually, you also want to make sure that
           when your service is installed, your socket is installed too,
           hence add Also=foo.socket in your service file foo.service, for a
           hypothetical program foo.

   Installing systemd Service Files
       At the build installation time (e.g.  make install during package
       build), packages are recommended to install their systemd unit files
       in the directory returned by pkg-config systemd
       --variable=systemdsystemunitdir (for system services) or pkg-config
       systemd --variable=systemduserunitdir (for user services). This will
       make the services available in the system on explicit request but not
       activate them automatically during boot. Optionally, during package
       installation (e.g.  rpm -i by the administrator), symlinks should be
       created in the systemd configuration directories via the enable
       command of the systemctl(1) tool to activate them automatically on

       Packages using autoconf(1) are recommended to use a configure script
       excerpt like the following to determine the unit installation path
       during source configuration:

                [AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files])],,
           AS_IF([test "x$with_systemdsystemunitdir" = "xyes" -o "x$with_systemdsystemunitdir" = "xauto"], [
                def_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)

                AS_IF([test "x$def_systemdsystemunitdir" = "x"],
              [AS_IF([test "x$with_systemdsystemunitdir" = "xyes"],
               [AC_MSG_ERROR([systemd support requested but pkg-config unable to query systemd package])])
           AS_IF([test "x$with_systemdsystemunitdir" != "xno"],
                 [AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])])
           AM_CONDITIONAL([HAVE_SYSTEMD], [test "x$with_systemdsystemunitdir" != "xno"])

       This snippet allows automatic installation of the unit files on
       systemd machines, and optionally allows their installation even on
       machines lacking systemd. (Modification of this snippet for the user
       unit directory is left as an exercise for the reader.)

       Additionally, to ensure that make distcheck continues to work, it is
       recommended to add the following to the top-level Makefile.am file in
       automake(1)-based projects:


       Finally, unit files should be installed in the system with an
       automake excerpt like the following:

           if HAVE_SYSTEMD
           systemdsystemunit_DATA = \
             foobar.socket \

       In the rpm(8) .spec file, use snippets like the following to
       enable/disable the service during installation/deinstallation. This
       makes use of the RPM macros shipped along systemd. Consult the
       packaging guidelines of your distribution for details and the
       equivalent for other package managers.

       At the top of the file:

           BuildRequires: systemd

       And as scriptlets, further down:

           %systemd_post foobar.service foobar.socket

           %systemd_preun foobar.service foobar.socket


       If the service shall be restarted during upgrades, replace the
       "%postun" scriptlet above with the following:

           %systemd_postun_with_restart foobar.service

       Note that "%systemd_post" and "%systemd_preun" expect the names of
       all units that are installed/removed as arguments, separated by
       spaces.  "%systemd_postun" expects no arguments.
       "%systemd_postun_with_restart" expects the units to restart as

       To facilitate upgrades from a package version that shipped only SysV
       init scripts to a package version that ships both a SysV init script
       and a native systemd service file, use a fragment like the following:

           %triggerun -- foobar < 0.47.11-1
           if /sbin/chkconfig --level 5 foobar ; then
             /bin/systemctl --no-reload enable foobar.service foobar.socket >/dev/null 2>&1 || :

       Where 0.47.11-1 is the first package version that includes the native
       unit file. This fragment will ensure that the first time the unit
       file is installed, it will be enabled if and only if the SysV init
       script is enabled, thus making sure that the enable status is not
       changed. Note that chkconfig is a command specific to Fedora which
       can be used to check whether a SysV init script is enabled. Other
       operating systems will have to use different commands here.


       Since new-style init systems such as systemd are compatible with
       traditional SysV init systems, it is not strictly necessary to port
       existing daemons to the new style. However, doing so offers
       additional functionality to the daemons as well as simplifying
       integration into new-style init systems.

       To port an existing SysV compatible daemon, the following steps are

        1. If not already implemented, add an optional command line switch
           to the daemon to disable daemonization. This is useful not only
           for using the daemon in new-style init systems, but also to ease

        2. If the daemon offers interfaces to other software running on the
           local system via local AF_UNIX sockets, consider implementing
           socket-based activation (see above). Usually, a minimal patch is
           sufficient to implement this: Extend the socket creation in the
           daemon code so that sd_listen_fds(3) is checked for already
           passed sockets first. If sockets are passed (i.e. when
           sd_listen_fds() returns a positive value), skip the socket
           creation step and use the passed sockets. Secondly, ensure that
           the file system socket nodes for local AF_UNIX sockets used in
           the socket-based activation are not removed when the daemon shuts
           down, if sockets have been passed. Third, if the daemon normally
           closes all remaining open file descriptors as part of its
           initialization, the sockets passed from the init system must be
           spared. Since new-style init systems guarantee that no left-over
           file descriptors are passed to executed processes, it might be a
           good choice to simply skip the closing of all remaining open file
           descriptors if sockets are passed.

        3. Write and install a systemd unit file for the service (and the
           sockets if socket-based activation is used, as well as a path
           unit file, if the daemon processes a spool directory), see above
           for details.

        4. If the daemon exposes interfaces via D-Bus, write and install a
           D-Bus activation file for the service, see above for details.


       It is recommended to follow the general guidelines for placing
       package files, as discussed in file-hierarchy(7).

SEE ALSO         top

       systemd(1), sd-daemon(3), sd_listen_fds(3), sd_notify(3), daemon(3),
       systemd.service(5), file-hierarchy(7)

NOTES         top

        1. LSB recommendations for SysV init scripts

        2. Apple MacOS X Daemon Requirements

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a bug
       report for this manual page, see
       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨https://github.com/systemd/systemd.git⟩ on 2020-07-14.  (At that
       time, the date of the most recent commit that was found in the repos‐
       itory was 2020-07-14.)  If you discover any rendering problems in
       this HTML version of the page, or you believe there is a better or
       more up-to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not part
       of the original manual page), send a mail to man-pages@man7.org

systemd 246                                                        DAEMON(7)

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