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systemd.exec


SYNOPSIS

       service.service, socket.socket, mount.mount, swap.swap


DESCRIPTION

       Unit configuration files for services, sockets, mount points, and swap
       devices share a subset of configuration options which define the
       execution environment of spawned processes.

       This man page lists the configuration options shared by these four unit
       types. See systemd.unit(5) for the common options of all unit
       configuration files, and systemd.service(5), systemd.socket(5),
       systemd.swap(5), and systemd.mount(5) for more information on the
       specific unit configuration files. The execution specific configuration
       options are configured in the [Service], [Socket], [Mount], or [Swap]
       sections, depending on the unit type.


OPTIONS

       WorkingDirectory=
           Takes an absolute directory path. Sets the working directory for
           executed processes. If not set, defaults to the root directory when
           systemd is running as a system instance and the respective user's
           home directory if run as user.

       RootDirectory=
           Takes an absolute directory path. Sets the root directory for
           executed processes, with the chroot(2) system call. If this is
           used, it must be ensured that the process and all its auxiliary
           files are available in the chroot() jail.

       User=, Group=
           Sets the Unix user or group that the processes are executed as,
           respectively. Takes a single user or group name or ID as argument.
           If no group is set, the default group of the user is chosen.

       SupplementaryGroups=
           Sets the supplementary Unix groups the processes are executed as.
           This takes a space-separated list of group names or IDs. This
           option may be specified more than once in which case all listed
           groups are set as supplementary groups. When the empty string is
           assigned the list of supplementary groups is reset, and all
           assignments prior to this one will have no effect. In any way, this
           option does not override, but extends the list of supplementary
           groups configured in the system group database for the user.

       Nice=
           Sets the default nice level (scheduling priority) for executed
           processes. Takes an integer between -20 (highest priority) and 19
           (lowest priority). See setpriority(2) for details.

       OOMScoreAdjust=
           Sets the adjustment level for the Out-Of-Memory killer for executed
           processes. Takes an integer between -1000 (to disable OOM killing

       CPUSchedulingPolicy=
           Sets the CPU scheduling policy for executed processes. Takes one of
           other, batch, idle, fifo or rr. See sched_setscheduler(2) for
           details.

       CPUSchedulingPriority=
           Sets the CPU scheduling priority for executed processes. The
           available priority range depends on the selected CPU scheduling
           policy (see above). For real-time scheduling policies an integer
           between 1 (lowest priority) and 99 (highest priority) can be used.
           See sched_setscheduler(2) for details.

       CPUSchedulingResetOnFork=
           Takes a boolean argument. If true, elevated CPU scheduling
           priorities and policies will be reset when the executed processes
           fork, and can hence not leak into child processes. See
           sched_setscheduler(2) for details. Defaults to false.

       CPUAffinity=
           Controls the CPU affinity of the executed processes. Takes a
           space-separated list of CPU indices. This option may be specified
           more than once in which case the specified CPU affinity masks are
           merged. If the empty string is assigned, the mask is reset, all
           assignments prior to this will have no effect. See
           sched_setaffinity(2) for details.

       UMask=
           Controls the file mode creation mask. Takes an access mode in octal
           notation. See umask(2) for details. Defaults to 0022.

       Environment=
           Sets environment variables for executed processes. Takes a
           space-separated list of variable assignments. This option may be
           specified more than once in which case all listed variables will be
           set. If the same variable is set twice, the later setting will
           override the earlier setting. If the empty string is assigned to
           this option, the list of environment variables is reset, all prior
           assignments have no effect. Variable expansion is not performed
           inside the strings, however, specifier expansion is possible. The $
           character has no special meaning. If you need to assign a value
           containing spaces to a variable, use double quotes (") for the
           assignment.

           Example:

               Environment="VAR1=word1 word2" VAR2=word3 "VAR3=$word 5 6"

           gives three variables "VAR1", "VAR2", "VAR3" with the values "word1
           word2", "word3", "$word 5 6".

           See environ(7) for details about environment variables.

           once in which case all specified files are read. If the empty
           string is assigned to this option, the list of file to read is
           reset, all prior assignments have no effect.

           The files listed with this directive will be read shortly before
           the process is executed (more specifically, after all processes
           from a previous unit state terminated. This means you can generate
           these files in one unit state, and read it with this option in the
           next).

           Settings from these files override settings made with Environment=.
           If the same variable is set twice from these files, the files will
           be read in the order they are specified and the later setting will
           override the earlier setting.

       PassEnvironment=
           Pass environment variables from the systemd system manager to
           executed processes. Takes a space-separated list of variable names.
           This option may be specified more than once, in which case all
           listed variables will be set. If the empty string is assigned to
           this option, the list of environment variables is reset, all prior
           assignments have no effect. Variables that are not set in the
           system manager will not be passed and will be silently ignored.

           Variables passed from this setting are overridden by those passed
           from Environment= or EnvironmentFile=.

           Example:

               PassEnvironment=VAR1 VAR2 VAR3

           passes three variables "VAR1", "VAR2", "VAR3" with the values set
           for those variables in PID1.

           See environ(7) for details about environment variables.

       StandardInput=
           Controls where file descriptor 0 (STDIN) of the executed processes
           is connected to. Takes one of null, tty, tty-force, tty-fail or
           socket.

           If null is selected, standard input will be connected to /dev/null,
           i.e. all read attempts by the process will result in immediate EOF.

           If tty is selected, standard input is connected to a TTY (as
           configured by TTYPath=, see below) and the executed process becomes
           the controlling process of the terminal. If the terminal is already
           being controlled by another process, the executed process waits
           until the current controlling process releases the terminal.

           tty-force is similar to tty, but the executed process is forcefully
           and immediately made the controlling process of the terminal,

       StandardOutput=
           Controls where file descriptor 1 (STDOUT) of the executed processes
           is connected to. Takes one of inherit, null, tty, journal, syslog,
           kmsg, journal+console, syslog+console, kmsg+console or socket.

           inherit duplicates the file descriptor of standard input for
           standard output.

           null connects standard output to /dev/null, i.e. everything written
           to it will be lost.

           tty connects standard output to a tty (as configured via TTYPath=,
           see below). If the TTY is used for output only, the executed
           process will not become the controlling process of the terminal,
           and will not fail or wait for other processes to release the
           terminal.

           journal connects standard output with the journal which is
           accessible via journalctl(1). Note that everything that is written
           to syslog or kmsg (see below) is implicitly stored in the journal
           as well, the specific two options listed below are hence supersets
           of this one.

           syslog connects standard output to the syslog(3) system syslog
           service, in addition to the journal. Note that the journal daemon
           is usually configured to forward everything it receives to syslog
           anyway, in which case this option is no different from journal.

           kmsg connects standard output with the kernel log buffer which is
           accessible via dmesg(1), in addition to the journal. The journal
           daemon might be configured to send all logs to kmsg anyway, in
           which case this option is no different from journal.

           journal+console, syslog+console and kmsg+console work in a similar
           way as the three options above but copy the output to the system
           console as well.

           socket connects standard output to a socket acquired via socket
           activation. The semantics are similar to the same option of
           StandardInput=.

           This setting defaults to the value set with DefaultStandardOutput=
           in systemd-system.conf(5), which defaults to journal.

       StandardError=
           Controls where file descriptor 2 (STDERR) of the executed processes
           is connected to. The available options are identical to those of
           StandardOutput=, with one exception: if set to inherit the file
           descriptor used for standard output is duplicated for standard
           error. This setting defaults to the value set with
           DefaultStandardError= in systemd-system.conf(5), which defaults to

       TTYVTDisallocate=
           If the terminal device specified with TTYPath= is a virtual console
           terminal, try to deallocate the TTY before and after execution.
           This ensures that the screen and scrollback buffer is cleared.
           Defaults to "no".

       SyslogIdentifier=
           Sets the process name to prefix log lines sent to the logging
           system or the kernel log buffer with. If not set, defaults to the
           process name of the executed process. This option is only useful
           when StandardOutput= or StandardError= are set to syslog, journal
           or kmsg (or to the same settings in combination with +console).

       SyslogFacility=
           Sets the syslog facility to use when logging to syslog. One of
           kern, user, mail, daemon, auth, syslog, lpr, news, uucp, cron,
           authpriv, ftp, local0, local1, local2, local3, local4, local5,
           local6 or local7. See syslog(3) for details. This option is only
           useful when StandardOutput= or StandardError= are set to syslog.
           Defaults to daemon.

       SyslogLevel=
           Default syslog level to use when logging to syslog or the kernel
           log buffer. One of emerg, alert, crit, err, warning, notice, info,
           debug. See syslog(3) for details. This option is only useful when
           StandardOutput= or StandardError= are set to syslog or kmsg. Note
           that individual lines output by the daemon might be prefixed with a
           different log level which can be used to override the default log
           level specified here. The interpretation of these prefixes may be
           disabled with SyslogLevelPrefix=, see below. For details see sd-
           daemon(3). Defaults to info.

       SyslogLevelPrefix=
           Takes a boolean argument. If true and StandardOutput= or
           StandardError= are set to syslog, kmsg or journal, log lines
           written by the executed process that are prefixed with a log level
           will be passed on to syslog with this log level set but the prefix
           removed. If set to false, the interpretation of these prefixes is
           disabled and the logged lines are passed on as-is. For details
           about this prefixing see sd-daemon(3). Defaults to true.

       TimerSlackNSec=
           Sets the timer slack in nanoseconds for the executed processes. The
           timer slack controls the accuracy of wake-ups triggered by timers.
           See prctl(2) for more information. Note that in contrast to most
           other time span definitions this parameter takes an integer value
           in nano-seconds if no unit is specified. The usual time units are
           understood too.

       LimitCPU=, LimitFSIZE=, LimitDATA=, LimitSTACK=, LimitCORE=, LimitRSS=,
       LimitNOFILE=, LimitAS=, LimitNPROC=, LimitMEMLOCK=, LimitLOCKS=,
           of microseconds is implied. Also, note that the effective
           granularity of the limits might influence their enforcement. For
           example, time limits specified for LimitCPU= will be rounded up
           implicitly to multiples of 1s. For LimitNICE= the value may be
           specified in two syntaxes: if prefixed with "+" or "-", the value
           is understood as regular Linux nice value in the range -20..19. If
           not prefixed like this the value is understood as raw resource
           limit parameter in the range 0..40 (with 0 being equivalent to 1).

           Note that most process resource limits configured with these
           options are per-process, and processes may fork in order to acquire
           a new set of resources that are accounted independently of the
           original process, and may thus escape limits set. Also note that
           LimitRSS= is not implemented on Linux, and setting it has no
           effect. Often it is advisable to prefer the resource controls
           listed in systemd.resource-control(5) over these per-process
           limits, as they apply to services as a whole, may be altered
           dynamically at runtime, and are generally more expressive. For
           example, MemoryLimit= is a more powerful (and working) replacement
           for LimitRSS=.

           Table 1. Limit directives and their equivalent with ulimit
           +-----------------+-------------------+---------------------+
           |Directive        | ulimit equivalent | Unit                |
           +-----------------+-------------------+---------------------+
           |LimitCPU=        | ulimit -t         | Seconds             |
           +-----------------+-------------------+---------------------+
           |LimitFSIZE=      | ulimit -f         | Bytes               |
           +-----------------+-------------------+---------------------+
           |LimitDATA=       | ulimit -d         | Bytes               |
           +-----------------+-------------------+---------------------+
           |LimitSTACK=      | ulimit -s         | Bytes               |
           +-----------------+-------------------+---------------------+
           |LimitCORE=       | ulimit -c         | Bytes               |
           +-----------------+-------------------+---------------------+
           |LimitRSS=        | ulimit -m         | Bytes               |
           +-----------------+-------------------+---------------------+
           |LimitNOFILE=     | ulimit -n         | Number of File      |
           |                 |                   | Descriptors         |
           +-----------------+-------------------+---------------------+
           |LimitAS=         | ulimit -v         | Bytes               |
           +-----------------+-------------------+---------------------+
           |LimitNPROC=      | ulimit -u         | Number of Processes |
           +-----------------+-------------------+---------------------+
           |LimitMEMLOCK=    | ulimit -l         | Bytes               |
           +-----------------+-------------------+---------------------+
           |LimitLOCKS=      | ulimit -x         | Number of Locks     |
           +-----------------+-------------------+---------------------+
           |LimitSIGPENDING= | ulimit -i         | Number of Queued    |
           |                 |                   | Signals             |
           +-----------------+-------------------+---------------------+
           |LimitMSGQUEUE=   | ulimit -q         | Bytes               |

       CapabilityBoundingSet=
           Controls which capabilities to include in the capability bounding
           set for the executed process. See capabilities(7) for details.
           Takes a whitespace-separated list of capability names as read by
           cap_from_name(3), e.g.  CAP_SYS_ADMIN, CAP_DAC_OVERRIDE,
           CAP_SYS_PTRACE. Capabilities listed will be included in the
           bounding set, all others are removed. If the list of capabilities
           is prefixed with "~", all but the listed capabilities will be
           included, the effect of the assignment inverted. Note that this
           option also affects the respective capabilities in the effective,
           permitted and inheritable capability sets, on top of what
           Capabilities= does. If this option is not used, the capability
           bounding set is not modified on process execution, hence no limits
           on the capabilities of the process are enforced. This option may
           appear more than once in which case the bounding sets are merged.
           If the empty string is assigned to this option, the bounding set is
           reset to the empty capability set, and all prior settings have no
           effect. If set to "~" (without any further argument), the bounding
           set is reset to the full set of available capabilities, also
           undoing any previous settings.

       AmbientCapabilities=
           Controls which capabilities to include in the ambient capability
           set for the executed process. Takes a whitespace-separated list of
           capability names as read by cap_from_name(3), e.g.  CAP_SYS_ADMIN,
           CAP_DAC_OVERRIDE, CAP_SYS_PTRACE. This option may appear more than
           once in which case the ambient capability sets are merged. If the
           list of capabilities is prefixed with "~", all but the listed
           capabilities will be included, the effect of the assignment
           inverted. If the empty string is assigned to this option, the
           ambient capability set is reset to the empty capability set, and
           all prior settings have no effect. If set to "~" (without any
           further argument), the ambient capability set is reset to the full
           set of available capabilities, also undoing any previous settings.
           Note that adding capabilities to ambient capability set adds them
           to the process's inherited capability set.

           Ambient capability sets are useful if you want to execute a process
           as a non-privileged user but still want to give it some
           capabilities. Note that in this case option keep-caps is
           automatically added to SecureBits= to retain the capabilities over
           the user change.

       SecureBits=
           Controls the secure bits set for the executed process. Takes a
           space-separated combination of options from the following list:
           keep-caps, keep-caps-locked, no-setuid-fixup,
           no-setuid-fixup-locked, noroot, and noroot-locked. This option may
           appear more than once in which case the secure bits are ORed. If
           the empty string is assigned to this option, the bits are reset to
           0. See capabilities(7) for details.
           ReadWriteDirectories= are accessible from within the namespace with
           the same access rights as from outside. Directories listed in
           ReadOnlyDirectories= are accessible for reading only, writing will
           be refused even if the usual file access controls would permit
           this. Directories listed in InaccessibleDirectories= will be made
           inaccessible for processes inside the namespace. Note that
           restricting access with these options does not extend to submounts
           of a directory that are created later on. These options may be
           specified more than once in which case all directories listed will
           have limited access from within the namespace. If the empty string
           is assigned to this option, the specific list is reset, and all
           prior assignments have no effect.

           Paths in ReadOnlyDirectories= and InaccessibleDirectories= may be
           prefixed with "-", in which case they will be ignored when they do
           not exist. Note that using this setting will disconnect propagation
           of mounts from the service to the host (propagation in the opposite
           direction continues to work). This means that this setting may not
           be used for services which shall be able to install mount points in
           the main mount namespace.

       PrivateTmp=
           Takes a boolean argument. If true, sets up a new file system
           namespace for the executed processes and mounts private /tmp and
           /var/tmp directories inside it that is not shared by processes
           outside of the namespace. This is useful to secure access to
           temporary files of the process, but makes sharing between processes
           via /tmp or /var/tmp impossible. If this is enabled, all temporary
           files created by a service in these directories will be removed
           after the service is stopped. Defaults to false. It is possible to
           run two or more units within the same private /tmp and /var/tmp
           namespace by using the JoinsNamespaceOf= directive, see
           systemd.unit(5) for details. Note that using this setting will
           disconnect propagation of mounts from the service to the host
           (propagation in the opposite direction continues to work). This
           means that this setting may not be used for services which shall be
           able to install mount points in the main mount namespace.

       PrivateDevices=
           Takes a boolean argument. If true, sets up a new /dev namespace for
           the executed processes and only adds API pseudo devices such as
           /dev/null, /dev/zero or /dev/random (as well as the pseudo TTY
           subsystem) to it, but no physical devices such as /dev/sda. This is
           useful to securely turn off physical device access by the executed
           process. Defaults to false. Enabling this option will also remove
           CAP_MKNOD from the capability bounding set for the unit (see
           above), and set DevicePolicy=closed (see systemd.resource-
           control(5) for details). Note that using this setting will
           disconnect propagation of mounts from the service to the host
           (propagation in the opposite direction continues to work). This
           means that this setting may not be used for services which shall be
           able to install mount points in the main mount namespace.
           accessible).

       ProtectSystem=
           Takes a boolean argument or "full". If true, mounts the /usr and
           /boot directories read-only for processes invoked by this unit. If
           set to "full", the /etc directory is mounted read-only, too. This
           setting ensures that any modification of the vendor supplied
           operating system (and optionally its configuration) is prohibited
           for the service. It is recommended to enable this setting for all
           long-running services, unless they are involved with system updates
           or need to modify the operating system in other ways. Note however
           that processes retaining the CAP_SYS_ADMIN capability can undo the
           effect of this setting. This setting is hence particularly useful
           for daemons which have this capability removed, for example with
           CapabilityBoundingSet=. Defaults to off.

       ProtectHome=
           Takes a boolean argument or "read-only". If true, the directories
           /home, /root and /run/user are made inaccessible and empty for
           processes invoked by this unit. If set to "read-only", the three
           directories are made read-only instead. It is recommended to enable
           this setting for all long-running services (in particular
           network-facing ones), to ensure they cannot get access to private
           user data, unless the services actually require access to the
           user's private data. Note however that processes retaining the
           CAP_SYS_ADMIN capability can undo the effect of this setting. This
           setting is hence particularly useful for daemons which have this
           capability removed, for example with CapabilityBoundingSet=.
           Defaults to off.

       MountFlags=
           Takes a mount propagation flag: shared, slave or private, which
           control whether mounts in the file system namespace set up for this
           unit's processes will receive or propagate mounts or unmounts. See
           mount(2) for details. Defaults to shared. Use shared to ensure that
           mounts and unmounts are propagated from the host to the container
           and vice versa. Use slave to run processes so that none of their
           mounts and unmounts will propagate to the host. Use private to also
           ensure that no mounts and unmounts from the host will propagate
           into the unit processes' namespace. Note that slave means that file
           systems mounted on the host might stay mounted continuously in the
           unit's namespace, and thus keep the device busy. Note that the file
           system namespace related options (PrivateTmp=, PrivateDevices=,
           ProtectSystem=, ProtectHome=, ReadOnlyDirectories=,
           InaccessibleDirectories= and ReadWriteDirectories=) require that
           mount and unmount propagation from the unit's file system namespace
           is disabled, and hence downgrade shared to slave.

       UtmpIdentifier=
           Takes a four character identifier string for an utmp/wtmp entry for
           this service. This should only be set for services such as getty
           implementations where utmp/wtmp entries must be created and cleared
           Takes a profile name as argument. The process executed by the unit
           will switch to this profile when started. Profiles must already be
           loaded in the kernel, or the unit will fail. This result in a non
           operation if AppArmor is not enabled. If prefixed by "-", all
           errors will be ignored.

       SmackProcessLabel=
           Takes a SMACK64 security label as argument. The process executed by
           the unit will be started under this label and SMACK will decide
           whether the processes is allowed to run or not based on it. The
           process will continue to run under the label specified here unless
           the executable has its own SMACK64EXEC label, in which case the
           process will transition to run under that label. When not
           specified, the label that systemd is running under is used. This
           directive is ignored if SMACK is disabled.

           The value may be prefixed by "-", in which case all errors will be
           ignored. An empty value may be specified to unset previous
           assignments.

       IgnoreSIGPIPE=
           Takes a boolean argument. If true, causes SIGPIPE to be ignored in
           the executed process. Defaults to true because SIGPIPE generally is
           useful only in shell pipelines.

       NoNewPrivileges=
           Takes a boolean argument. If true, ensures that the service process
           and all its children can never gain new privileges. This option is
           more powerful than the respective secure bits flags (see above), as
           it also prohibits UID changes of any kind. This is the simplest,
           most effective way to ensure that a process and its children can
           never elevate privileges again.

       SystemCallFilter=
           Takes a space-separated list of system call names. If this setting
           is used, all system calls executed by the unit processes except for
           the listed ones will result in immediate process termination with
           the SIGSYS signal (whitelisting). If the first character of the
           list is "~", the effect is inverted: only the listed system calls
           will result in immediate process termination (blacklisting). If
           running in user mode and this option is used, NoNewPrivileges=yes
           is implied. This feature makes use of the Secure Computing Mode 2
           interfaces of the kernel ('seccomp filtering') and is useful for
           enforcing a minimal sandboxing environment. Note that the execve,
           rt_sigreturn, sigreturn, exit_group, exit system calls are
           implicitly whitelisted and do not need to be listed explicitly.
           This option may be specified more than once in which case the
           filter masks are merged. If the empty string is assigned, the
           filter is reset, all prior assignments will have no effect.

           If you specify both types of this option (i.e. whitelisting and
           blacklisting), the first encountered will take precedence and will
           immediately when the filter is triggered.

       SystemCallArchitectures=
           Takes a space separated list of architecture identifiers to include
           in the system call filter. The known architecture identifiers are
           x86, x86-64, x32, arm as well as the special identifier native.
           Only system calls of the specified architectures will be permitted
           to processes of this unit. This is an effective way to disable
           compatibility with non-native architectures for processes, for
           example to prohibit execution of 32-bit x86 binaries on 64-bit
           x86-64 systems. The special native identifier implicitly maps to
           the native architecture of the system (or more strictly: to the
           architecture the system manager is compiled for). If running in
           user mode and this option is used, NoNewPrivileges=yes is implied.
           Note that setting this option to a non-empty list implies that
           native is included too. By default, this option is set to the empty
           list, i.e. no architecture system call filtering is applied.

       RestrictAddressFamilies=
           Restricts the set of socket address families accessible to the
           processes of this unit. Takes a space-separated list of address
           family names to whitelist, such as AF_UNIX, AF_INET or AF_INET6.
           When prefixed with ~ the listed address families will be applied as
           blacklist, otherwise as whitelist. Note that this restricts access
           to the socket(2) system call only. Sockets passed into the process
           by other means (for example, by using socket activation with socket
           units, see systemd.socket(5)) are unaffected. Also, sockets created
           with socketpair() (which creates connected AF_UNIX sockets only)
           are unaffected. Note that this option has no effect on 32-bit x86
           and is ignored (but works correctly on x86-64). If running in user
           mode and this option is used, NoNewPrivileges=yes is implied. By
           default, no restriction applies, all address families are
           accessible to processes. If assigned the empty string, any previous
           list changes are undone.

           Use this option to limit exposure of processes to remote systems,
           in particular via exotic network protocols. Note that in most
           cases, the local AF_UNIX address family should be included in the
           configured whitelist as it is frequently used for local
           communication, including for syslog(2) logging.

       Personality=
           Controls which kernel architecture uname(2) shall report, when
           invoked by unit processes. Takes one of x86 and x86-64. This is
           useful when running 32-bit services on a 64-bit host system. If not
           specified, the personality is left unmodified and thus reflects the
           personality of the host system's kernel.

       RuntimeDirectory=, RuntimeDirectoryMode=
           Takes a list of directory names. If set, one or more directories by
           the specified names will be created below /run (for system
           services) or below $XDG_RUNTIME_DIR (for user services) when the


ENVIRONMENT VARIABLES IN SPAWNED PROCESSES

       Processes started by the system are executed in a clean environment in
       which select variables listed below are set. System processes started
       by systemd do not inherit variables from PID 1.

       $PATH
           Colon-separated list of directories to use when launching
           executables. Systemd uses a fixed value of
           /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin.

       $LANG
           Locale. Can be set in locale.conf(5) or on the kernel command line
           (see systemd(1) and kernel-command-line(7)).

       $USER, $LOGNAME, $HOME, $SHELL
           User name (twice), home directory, and the login shell. The
           variables are set for the units that have User= set. See passwd(5).

       $XDG_RUNTIME_DIR
           The directory for volatile state. Set in user sessions. See
           pam_systemd(8).

       $XDG_SESSION_ID, $XDG_SEAT, $XDG_VTNR
           The identifier of the session, the seat name, and virtual terminal
           of the session. Set by pam_systemd(8) for login sessions.
           $XDG_SEAT and $XDG_VTNR will only be set when attached to a seat
           and a tty.

       $MAINPID
           The PID of the units main process if it is known. This is only set
           for control processes as invoked by ExecReload= and similar.

       $LISTEN_FDS, $LISTEN_PID
           Information about file descriptors passed to a service for socket
           activation. See sd_listen_fds(3).

       $TERM
           Terminal type, set only for units connected to a terminal
           (StandardInput=tty, StandardOutput=tty, or StandardError=tty). See
           termcap(5).

       Additional variables may be configured by the following means: for
       processes spawned in specific units, use the Environment= and
       EnvironmentFile= options above; to specify variables globally, use
       DefaultEnvironment= (see systemd-system.conf(5)) or the kernel option
       systemd.setenv= (see systemd(1)). Additional variables may also be set
       through PAM, cf. pam_env(8).


SEE ALSO

       systemd(1), systemctl(1), journalctl(8), systemd.unit(5),
       systemd.service(5), systemd.socket(5), systemd.swap(5),
       systemd.mount(5), systemd.kill(5), systemd.resource-control(5),
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