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SYSTEMD.EXEC(5)			 systemd.exec		       SYSTEMD.EXEC(5)

NAME
       systemd.exec - Execution environment configuration

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
	   for this process) and 1000 (to make killing of this process under
	   memory pressure very likely). See proc.txt[1] for details.

       IOSchedulingClass=
	   Sets the IO scheduling class for executed processes. Takes an
	   integer between 0 and 3 or one of the strings none, realtime,
	   best-effort or idle. See ioprio_set(2) for details.

       IOSchedulingPriority=
	   Sets the IO scheduling priority for executed processes. Takes an
	   integer between 0 (highest priority) and 7 (lowest priority). The
	   available priorities depend on the selected IO scheduling class
	   (see above). See ioprio_set(2) for details.

       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 specificed 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.

       EnvironmentFile=
	   Similar to Environment= but reads the environment variables from a
	   text file. The text file should contain new-line-separated variable
	   assignments. Empty lines and lines starting with ; or # will be
	   ignored, which may be used for commenting. A line ending with a
	   backslash will be concatenated with the following one, allowing
	   multiline variable definitions. The parser strips leading and
	   trailing whitespace from the values of assignments, unless you use
	   double quotes (").

	   The argument passed should be an absolute filename or wildcard
	   expression, optionally prefixed with "-", which indicates that if
	   the file does not exist, it will not be read and no error or
	   warning message is logged. This option may be specified more than
	   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, this means 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.

       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,
	   potentially removing previous controlling processes from the
	   terminal.  tty-fail is similar to tty but if the terminal already
	   has a controlling process start-up of the executed process fails.
	   The socket option is only valid in socket-activated services, and
	   only when the socket configuration file (see systemd.socket(5) for
	   details) specifies a single socket only. If this option is set,
	   standard input will be connected to the socket the service was
	   activated from, which is primarily useful for compatibility with
	   daemons designed for use with the traditional inetd(8) daemon. This
	   setting defaults to null.

       StandardOutput=
	   Controls where file descriptor 1 (STDOUT) of the executed processes
	   is connected to. Takes one of inherit, null, tty, syslog, kmsg,
	   journal, syslog+console, kmsg+console, journal+console or socket.
	   If set to inherit, the file descriptor of standard input is
	   duplicated for standard output. If set to null, standard output
	   will be connected to /dev/null, i.e. everything written to it will
	   be lost. If set to tty, standard output will be connected 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.  syslog connects standard output
	   to the syslog(3) system syslog service.  kmsg connects it with the
	   kernel log buffer which is accessible via dmesg(1).	journal
	   connects it with the journal which is accessible via journalctl(1)
	   (Note that everything that is written to syslog or kmsg is
	   implicitly stored in the journal as well, those options are hence
	   supersets of this one).  syslog+console, journal+console and
	   kmsg+console work similarly but copy the output to the system
	   console as well.  socket connects standard output to a socket from
	   socket activation, semantics are similar to the respective 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
	   inherit.

       TTYPath=
	   Sets the terminal device node to use if standard input, output, or
	   error are connected to a TTY (see above). Defaults to /dev/console.

       TTYReset=
	   Reset the terminal device specified with TTYPath= before and after
	   execution. Defaults to "no".

       TTYVHangup=
	   Disconnect all clients which have opened the terminal device
	   specified with TTYPath= before and after execution. Defaults to
	   "no".

       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 syslog 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 or kmsg.

       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=,
       LimitSIGPENDING=, LimitMSGQUEUE=, LimitNICE=, LimitRTPRIO=,
       LimitRTTIME=
	   These settings control various resource limits for executed
	   processes. See setrlimit(2) for details. Use the string infinity to
	   configure no limit on a specific resource.

       PAMName=
	   Sets the PAM service name to set up a session as. If set, the
	   executed process will be registered as a PAM session under the
	   specified service name. This is only useful in conjunction with the
	   User= setting. If not set, no PAM session will be opened for the
	   executed processes. See pam(8) for details.

       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.

       SecureBits=
	   Controls the secure bits set for the executed process. See
	   capabilities(7) for details. Takes a list of strings: keep-caps,
	   keep-caps-locked, no-setuid-fixup, no-setuid-fixup-locked, noroot
	   and/or 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.

       Capabilities=
	   Controls the capabilities(7) set for the executed process. Take a
	   capability string describing the effective, permitted and inherited
	   capability sets as documented in cap_from_text(3). Note that these
	   capability sets are usually influenced by the capabilities attached
	   to the executed file. Due to that CapabilityBoundingSet= is
	   probably the much more useful setting.

       ReadWriteDirectories=, ReadOnlyDirectories=, InaccessibleDirectories=
	   Sets up a new file system namespace for executed processes. These
	   options may be used to limit access a process might have to the
	   main file system hierarchy. Each setting takes a space-separated
	   list of absolute directory paths. Directories listed in
	   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. You must list submounts separately in these
	   settings to ensure the same limited access. 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.

       PrivateNetwork=
	   Takes a boolean argument. If true, sets up a new network namespace
	   for the executed processes and configures only the loopback network
	   device "lo" inside it. No other network devices will be available
	   to the executed process. This is useful to securely turn off
	   network access by the executed process. Defaults to false. It is
	   possible to run two or more units within the same private network
	   namespace by using the JoinsNamespaceOf= directive, see
	   systemd.unit(5) for details. Note that this option will disconnect
	   all socket families from the host, this includes AF_NETLINK and
	   AF_UNIX. The latter has the effect that AF_UNIX sockets in the
	   abstract socket namespace will become unavailable to the processes
	   (however, those located in the file system will continue to be
	   accessible).

       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 continously in the
	   unit's namespace, and thus keep the device busy. Note that the file
	   system namespace related options (PrivateTmp=, PrivateDevices=,
	   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
	   before and after execution. If the configured string is longer than
	   four characters, it is truncated and the terminal four characters
	   are used. This setting interprets %I style string replacements.
	   This setting is unset by default, i.e. no utmp/wtmp entries are
	   created or cleaned up for this service.

       SELinuxContext=
	   Set the SELinux security context of the executed process. If set,
	   this will override the automated domain transition. However, the
	   policy still needs to autorize the transition. This directive is
	   ignored if SELinux is disabled. If prefixed by "-", all errors will
	   be ignored. See setexeccon(3) for details.

       AppArmorProfile=
	   Take 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.

       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
	   dictate the default action (termination or approval of a system
	   call). Then the next occurrences of this option will add or delete
	   the listed system calls from the set of the filtered system calls,
	   depending of its type and the default action. (For example, if you
	   have started with a whitelisting of read and write, and right after
	   it add a blacklisting of write, then write will be removed from the
	   set.)

       SystemCallErrorNumber=
	   Takes an "errno" error number name to return when the system call
	   filter configured with SystemCallFilter= is triggered, instead of
	   terminating the process immediately. Takes an error name such as
	   EPERM, EACCES or EUCLEAN. When this setting is not used, or when
	   the empty string is assigned, the process will be terminated
	   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 32bit 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 32bit services on a 64bit 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
	   unit is started and removed when the unit is stopped. The
	   directories will have the access mode specified in
	   RuntimeDirectoryMode=, and will be owned by the user and group
	   specified in User= and Group=. Use this to manage one or more
	   runtime directories of the unit and bind their lifetime to the
	   daemon runtime. The specified directory names must be relative, and
	   may not include a "/", i.e. must refer to simple directories to
	   create or remove. This is particularly useful for unpriviliges
	   daemons that cannot create runtime directories in /run due to lack
	   of privileges, and to make sure the runtime directory is cleaned up
	   automatically after use. For runtime directories that require more
	   complex or different configuration or lifetime guarantees, please
	   consider using tmpfiles.d(5).

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, but processes started
       by user systemd instances inherit all environment variables from the
       user systemd instance.

       $PATH
	   Colon-separated list of directiories 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, which includes
	   user systemd instances. See passwd(5).

       $XDG_RUNTIME_DIR
	   The directory for volatile state. Set for the user systemd
	   instance, and also 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.

       $MANAGERPID
	   The PID of the user systemd instance, set for processes spawned by
	   it.

       $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, c.f. pam_env(8).

SEE ALSO
       systemd(1), systemctl(8), journalctl(8), systemd.unit(5),
       systemd.service(5), systemd.socket(5), systemd.swap(5),
       systemd.mount(5), systemd.kill(5), systemd.resource-control(5),
       systemd.directives(7), tmpfiles.d(5), exec(3)

NOTES
	1. proc.txt
	   https://www.kernel.org/doc/Documentation/filesystems/proc.txt

systemd 212						       SYSTEMD.EXEC(5)
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