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.

   In addition, options which control resources through Linux Control
   Groups (cgroups) are listed in systemd.resource-control(5). Those
   options complement options listed here.

AUTOMATIC DEPENDENCIES

   A few execution parameters result in additional, automatic dependencies
   to be added.

   Units with WorkingDirectory= or RootDirectory= set automatically gain
   dependencies of type Requires= and After= on all mount units required
   to access the specified paths. This is equivalent to having them listed
   explicitly in RequiresMountsFor=.

   Similar, units with PrivateTmp= enabled automatically get mount unit
   dependencies for all mounts required to access /tmp and /var/tmp.

   Units whose standard output or error output is connected to journal,
   syslog or kmsg (or their combinations with console output, see below)
   automatically acquire dependencies of type After= on
   systemd-journald.socket.

OPTIONS

   WorkingDirectory=
       Takes a directory path relative to the service's root directory
       specified by RootDirectory=, or the special value "~". Sets the
       working directory for executed processes. If set to "~", the home
       directory of the user specified in User= is used. 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.
       If the setting is prefixed with the "-" character, a missing
       working directory is not considered fatal. If RootDirectory= is not
       set, then WorkingDirectory= is relative to the root of the system
       running the service manager. Note that setting this parameter might
       result in additional dependencies to be added to the unit (see
       above).

   RootDirectory=
       Takes a directory path relative to the host's root directory (i.e.
       the root of the system running the service manager). Sets the root
       directory for executed processes, with the chroot(2) system call.
       If this is used, it must be ensured that the process binary and all
       its auxiliary files are available in the chroot() jail. Note that
       setting this parameter might result in additional dependencies to
       be added to the unit (see above).

       The PrivateUsers= setting is particularly useful in conjunction
       with RootDirectory=. For details, see below.

   User=, Group=
       Set the UNIX user or group that the processes are executed as,
       respectively. Takes a single user or group name, or numeric ID as
       argument. For system services (services run by the system service
       manager, i.e. managed by PID 1) and for user services of the root
       user (services managed by root's instance of systemd --user), the
       default is "root", but User= may be used to specify a different
       user. For user services of any other user, switching user identity
       is not permitted, hence the only valid setting is the same user the
       user's service manager is running as. If no group is set, the
       default group of the user is used. This setting does not affect
       commands whose command line is prefixed with "+".

   DynamicUser=
       Takes a boolean parameter. If set, a UNIX user and group pair is
       allocated dynamically when the unit is started, and released as
       soon as it is stopped. The user and group will not be added to
       /etc/passwd or /etc/group, but are managed transiently during
       runtime. The nss-systemd(8) glibc NSS module provides integration
       of these dynamic users/groups into the system's user and group
       databases. The user and group name to use may be configured via
       User= and Group= (see above). If these options are not used and
       dynamic user/group allocation is enabled for a unit, the name of
       the dynamic user/group is implicitly derived from the unit name. If
       the unit name without the type suffix qualifies as valid user name
       it is used directly, otherwise a name incorporating a hash of it is
       used. If a statically allocated user or group of the configured
       name already exists, it is used and no dynamic user/group is
       allocated. Dynamic users/groups are allocated from the UID/GID
       range 61184...65519. It is recommended to avoid this range for
       regular system or login users. At any point in time each UID/GID
       from this range is only assigned to zero or one dynamically
       allocated users/groups in use. However, UID/GIDs are recycled after
       a unit is terminated. Care should be taken that any processes
       running as part of a unit for which dynamic users/groups are
       enabled do not leave files or directories owned by these
       users/groups around, as a different unit might get the same UID/GID
       assigned later on, and thus gain access to these files or
       directories. If DynamicUser= is enabled, RemoveIPC=, PrivateTmp=
       are implied. This ensures that the lifetime of IPC objects and
       temporary files created by the executed processes is bound to the
       runtime of the service, and hence the lifetime of the dynamic
       user/group. Since /tmp and /var/tmp are usually the only
       world-writable directories on a system this ensures that a unit
       making use of dynamic user/group allocation cannot leave files
       around after unit termination. Moreover ProtectSystem=strict and
       ProtectHome=read-only are implied, thus prohibiting the service to
       write to arbitrary file system locations. In order to allow the
       service to write to certain directories, they have to be
       whitelisted using ReadWritePaths=, but care must be taken so that
       UID/GID recycling doesn't create security issues involving files
       created by the service. Use RuntimeDirectory= (see below) in order
       to assign a writable runtime directory to a service, owned by the
       dynamic user/group and removed automatically when the unit is
       terminated. Defaults to off.

   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. This
       does not affect commands prefixed with "+".

   RemoveIPC=
       Takes a boolean parameter. If set, all System V and POSIX IPC
       objects owned by the user and group the processes of this unit are
       run as are removed when the unit is stopped. This setting only has
       an effect if at least one of User=, Group= and DynamicUser= are
       used. It has no effect on IPC objects owned by the root user.
       Specifically, this removes System V semaphores, as well as System V
       and POSIX shared memory segments and message queues. If multiple
       units use the same user or group the IPC objects are removed when
       the last of these units is stopped. This setting is implied if
       DynamicUser= is set.

   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 I/O 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 I/O scheduling priority for executed processes. Takes an
       integer between 0 (highest priority) and 7 (lowest priority). The
       available priorities depend on the selected I/O 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 list
       of CPU indices or ranges separated by either whitespace or commas.
       CPU ranges are specified by the lower and upper CPU indices
       separated by a dash. 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.

   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, lines without an "=" separator, or 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, 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,
       socket or fd.

       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.

       The fd option connects the input stream to a single file descriptor
       provided by a socket unit. A custom named file descriptor can be
       specified as part of this option, after a ":" (e.g.  "fd:foobar").
       If no name is specified, "stdin" is assumed (i.e.  "fd" is
       equivalent to "fd:stdin"). At least one socket unit defining such
       name must be explicitly provided via the Sockets= option, and file
       descriptor name may differ from the name of its containing socket
       unit. If multiple matches are found, the first one will be used.
       See FileDescriptorName= in systemd.socket(5) for more details about
       named descriptors and ordering.

       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, journal, syslog,
       kmsg, journal+console, syslog+console, kmsg+console, socket or fd.

       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=.

       The fd option connects the output stream to a single file
       descriptor provided by a socket unit. A custom named file
       descriptor can be specified as part of this option, after a ":"
       (e.g.  "fd:foobar"). If no name is specified, "stdout" is assumed
       (i.e.  "fd" is equivalent to "fd:stdout"). At least one socket unit
       defining such name must be explicitly provided via the Sockets=
       option, and file descriptor name may differ from the name of its
       containing socket unit. If multiple matches are found, the first
       one will be used. See FileDescriptorName= in systemd.socket(5) for
       more details about named descriptors and ordering.

       If the standard output (or error output, see below) of a unit is
       connected to the journal, syslog or the kernel log buffer, the unit
       will implicitly gain a dependency of type After= on
       systemd-journald.socket (also see the automatic dependencies
       section above).

       This setting defaults to the value set with DefaultStandardOutput=
       in systemd-system.conf(5), which defaults to journal. Note that
       setting this parameter might result in additional dependencies to
       be added to the unit (see above).

   StandardError=
       Controls where file descriptor 2 (STDERR) of the executed processes
       is connected to. The available options are identical to those of
       StandardOutput=, with some exceptions: if set to inherit the file
       descriptor used for standard output is duplicated for standard
       error, while fd operates on the error stream and will look by
       default for a descriptor named "stderr".

       This setting defaults to the value set with DefaultStandardError=
       in systemd-system.conf(5), which defaults to inherit. Note that
       setting this parameter might result in additional dependencies to
       be added to the unit (see above).

   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 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=
       The 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=
       Set soft and hard limits on various resources for executed
       processes. See setrlimit(2) for details on the resource limit
       concept. Resource limits may be specified in two formats: either as
       single value to set a specific soft and hard limit to the same
       value, or as colon-separated pair soft:hard to set both limits
       individually (e.g.  "LimitAS=4G:16G"). Use the string infinity to
       configure no limit on a specific resource. The multiplicative
       suffixes K, M, G, T, P and E (to the base 1024) may be used for
       resource limits measured in bytes (e.g. LimitAS=16G). For the
       limits referring to time values, the usual time units ms, s, min, h
       and so on may be used (see systemd.time(7) for details). Note that
       if no time unit is specified for LimitCPU= the default unit of
       seconds is implied, while for LimitRTTIME= the default unit 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=.

       For system units these resource limits may be chosen freely. For
       user units however (i.e. units run by a per-user instance of
       systemd(1)), these limits are bound by (possibly more restrictive)
       per-user limits enforced by the OS.

       Resource limits not configured explicitly for a unit default to the
       value configured in the various DefaultLimitCPU=,
       DefaultLimitFSIZE=, ... options available in systemd-
       system.conf(5), and -- if not configured there -- the kernel or
       per-user defaults, as defined by the OS (the latter only for user
       services, see above).

       Table 1. Resource limit directives, their equivalent ulimit shell
       commands and the unit used
       
       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               
       
       LimitNICE=        ulimit -e          Nice Level          
       
       LimitRTPRIO=      ulimit -r          Realtime Priority   
       
       LimitRTTIME=      No equivalent      Microseconds        
       

   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, 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. 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. This does not affect commands
       prefixed with "+".

   AmbientCapabilities=
       Controls which capabilities to include in the ambient capability
       set for the executed process. Takes a whitespace-separated list of
       capability names, 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.  AmbientCapabilities= does not affect commands
       prefixed with "+".

   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. This does not affect commands prefixed with "+". See
       capabilities(7) for details.

   ReadWritePaths=, ReadOnlyPaths=, InaccessiblePaths=
       Sets up a new file system namespace for executed processes. These
       options may be used to limit access a process might have to the
       file system hierarchy. Each setting takes a space-separated list of
       paths relative to the host's root directory (i.e. the system
       running the service manager). Note that if paths contain symlinks,
       they are resolved relative to the root directory set with
       RootDirectory=.

       Paths listed in ReadWritePaths= are accessible from within the
       namespace with the same access modes as from outside of it. Paths
       listed in ReadOnlyPaths= are accessible for reading only, writing
       will be refused even if the usual file access controls would permit
       this. Nest ReadWritePaths= inside of ReadOnlyPaths= in order to
       provide writable subdirectories within read-only directories. Use
       ReadWritePaths= in order to whitelist specific paths for write
       access if ProtectSystem=strict is used. Paths listed in
       InaccessiblePaths= will be made inaccessible for processes inside
       the namespace (along with everything below them in the file system
       hierarchy).

       Note that restricting access with these options does not extend to
       submounts of a directory that are created later on. Non-directory
       paths may be specified as well. These options may be specified more
       than once, in which case all paths 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 ReadWritePaths=, ReadOnlyPaths= and InaccessiblePaths= 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. Note that the effect of
       these settings may be undone by privileged processes. In order to
       set up an effective sandboxed environment for a unit it is thus
       recommended to combine these settings with either
       CapabilityBoundingSet=~CAP_SYS_ADMIN or SystemCallFilter=~@mount.

   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. This setting is implied if
       DynamicUser= is set. For this setting the same restrictions
       regarding mount propagation and privileges apply as for
       ReadOnlyPaths= and related calls, see above.

   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, system
       memory /dev/mem, system ports /dev/port and others. This is useful
       to securely turn off physical device access by the executed
       process. Defaults to false. Enabling this option will install a
       system call filter to block low-level I/O system calls that are
       grouped in the @raw-io set, will also remove CAP_MKNOD and
       CAP_SYS_RAWIO 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. The /dev
       namespace will be mounted read-only and 'noexec'. The latter may
       break old programs which try to set up executable memory by using
       mmap(2) of /dev/zero instead of using MAP_ANON. This setting is
       implied if DynamicUser= is set. For this setting the same
       restrictions regarding mount propagation and privileges apply as
       for ReadOnlyPaths= and related calls, see above.

   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).

   PrivateUsers=
       Takes a boolean argument. If true, sets up a new user namespace for
       the executed processes and configures a minimal user and group
       mapping, that maps the "root" user and group as well as the unit's
       own user and group to themselves and everything else to the
       "nobody" user and group. This is useful to securely detach the user
       and group databases used by the unit from the rest of the system,
       and thus to create an effective sandbox environment. All files,
       directories, processes, IPC objects and other resources owned by
       users/groups not equaling "root" or the unit's own will stay
       visible from within the unit but appear owned by the "nobody" user
       and group. If this mode is enabled, all unit processes are run
       without privileges in the host user namespace (regardless if the
       unit's own user/group is "root" or not). Specifically this means
       that the process will have zero process capabilities on the host's
       user namespace, but full capabilities within the service's user
       namespace. Settings such as CapabilityBoundingSet= will affect only
       the latter, and there's no way to acquire additional capabilities
       in the host's user namespace. Defaults to off.

       This setting is particularly useful in conjunction with
       RootDirectory=, as the need to synchronize the user and group
       databases in the root directory and on the host is reduced, as the
       only users and groups who need to be matched are "root", "nobody"
       and the unit's own user and group.

   ProtectSystem=
       Takes a boolean argument or the special values "full" or "strict".
       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. If set to "strict" the entire
       file system hierarchy is mounted read-only, except for the API file
       system subtrees /dev, /proc and /sys (protect these directories
       using PrivateDevices=, ProtectKernelTunables=,
       ProtectControlGroups=). This setting ensures that any modification
       of the vendor-supplied operating system (and optionally its
       configuration, and local mounts) 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. If this option is used,
       ReadWritePaths= may be used to exclude specific directories from
       being made read-only. This setting is implied if DynamicUser= is
       set. For this setting the same restrictions regarding mount
       propagation and privileges apply as for ReadOnlyPaths= and related
       calls, see above. 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. This setting is implied if DynamicUser= is
       set. For this setting the same restrictions regarding mount
       propagation and privileges apply as for ReadOnlyPaths= and related
       calls, see above.

   ProtectKernelTunables=
       Takes a boolean argument. If true, kernel variables accessible
       through /proc/sys, /sys, /proc/sysrq-trigger, /proc/latency_stats,
       /proc/acpi, /proc/timer_stats, /proc/fs and /proc/irq will be made
       read-only to all processes of the unit. Usually, tunable kernel
       variables should only be written at boot-time, with the sysctl.d(5)
       mechanism. Almost no services need to write to these at runtime; it
       is hence recommended to turn this on for most services. For this
       setting the same restrictions regarding mount propagation and
       privileges apply as for ReadOnlyPaths= and related calls, see
       above. Defaults to off. Note that this option does not prevent
       kernel tuning through IPC interfaces and external programs. However
       InaccessiblePaths= can be used to make some IPC file system objects
       inaccessible.

   ProtectControlGroups=
       Takes a boolean argument. If true, the Linux Control Groups
       (cgroups(7)) hierarchies accessible through /sys/fs/cgroup will be
       made read-only to all processes of the unit. Except for container
       managers no services should require write access to the control
       groups hierarchies; it is hence recommended to turn this on for
       most services. For this setting the same restrictions regarding
       mount propagation and privileges apply as for ReadOnlyPaths= and
       related calls, see above. 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=, ProtectKernelTunables=,
       ProtectControlGroups=, ReadOnlyPaths=, InaccessiblePaths=,
       ReadWritePaths=) 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(5) and wtmp
       entry for this service. This should only be set for services such
       as getty implementations (such as agetty(8)) where utmp/wtmp
       entries must be created and cleared before and after execution, or
       for services that shall be executed as if they were run by a getty
       process (see below). 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.

   UtmpMode=
       Takes one of "init", "login" or "user". If UtmpIdentifier= is set,
       controls which type of utmp(5)/wtmp entries for this service are
       generated. This setting has no effect unless UtmpIdentifier= is set
       too. If "init" is set, only an INIT_PROCESS entry is generated and
       the invoked process must implement a getty-compatible utmp/wtmp
       logic. If "login" is set, first an INIT_PROCESS entry, followed by
       a LOGIN_PROCESS entry is generated. In this case, the invoked
       process must implement a login(1)-compatible utmp/wtmp logic. If
       "user" is set, first an INIT_PROCESS entry, then a LOGIN_PROCESS
       entry and finally a USER_PROCESS entry is generated. In this case,
       the invoked process may be any process that is suitable to be run
       as session leader. Defaults to "init".

   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 authorize the transition. This directive is
       ignored if SELinux is disabled. If prefixed by "-", all errors will
       be ignored. This does not affect commands prefixed with "+". See
       setexeccon(3) for details.

   AppArmorProfile=
       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. This does not affect commands prefixed with
       "+".

   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 process 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. This does not affect commands prefixed with "+".

   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 and
       most effective way to ensure that a process and its children can
       never elevate privileges again. Defaults to false, but in the user
       manager instance certain settings force NoNewPrivileges=yes,
       ignoring the value of this setting. Those is the case when
       SystemCallFilter=, SystemCallArchitectures=,
       RestrictAddressFamilies=, PrivateDevices=, ProtectKernelTunables=,
       ProtectKernelModules=, MemoryDenyWriteExecute=, or
       RestrictRealtime= are specified.

   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, or in system mode, but without the
       CAP_SYS_ADMIN capability (e.g. setting User=nobody),
       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, exit, exit_group, getrlimit,
       rt_sigreturn, sigreturn system calls and the system calls for
       querying time and sleeping 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. This does not affect commands prefixed with "+".

       Note that strict system call filters may impact execution and error
       handling code paths of the service invocation. Specifically, access
       to the execve system call is required for the execution of the
       service binary --- if it is blocked service invocation will
       necessarily fail. Also, if execution of the service binary fails
       for some reason (for example: missing service executable), the
       error handling logic might require access to an additional set of
       system calls in order to process and log this failure correctly. It
       might be necessary to temporarily disable system call filters in
       order to simplify debugging of such failures.

       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.)

       As the number of possible system calls is large, predefined sets of
       system calls are provided. A set starts with "@" character,
       followed by name of the set.

       Table 2. Currently predefined system call sets
       
       Set             Description                
       
       @basic-io       System calls for basic     
                       I/O: reading, writing,     
                       seeking, file descriptor   
                       duplication and closing    
                       (read(2), write(2), and    
                       related calls)             
       
       @clock          System calls for changing  
                       the system clock           
                       (adjtimex(2),              
                       settimeofday(2), and       
                       related calls)             
       
       @cpu-emulation  System calls for CPU       
                       emulation functionality    
                       (vm86(2) and related       
                       calls)                     
       
       @debug          Debugging, performance     
                       monitoring and tracing     
                       functionality (ptrace(2),  
                       perf_event_open(2) and     
                       related calls)             
       
       @io-event       Event loop system calls    
                       (poll(2), select(2),       
                       epoll(7), eventfd(2) and   
                       related calls)             
       
       @ipc            Pipes, SysV IPC, POSIX     
                       Message Queues and other   
                       IPC (mq_overview(7),       
                       svipc(7))                  
       
       @keyring        Kernel keyring access      
                       (keyctl(2) and related     
                       calls)                     
       
       @module         Kernel module control      
                       (init_module(2),           
                       delete_module(2) and       
                       related calls)             
       
       @mount          File system mounting and   
                       unmounting (mount(2),      
                       chroot(2), and related     
                       calls)                     
       
       @network-io     Socket I/O (including      
                       local AF_UNIX): socket(7), 
                       unix(7)                    
       
       @obsolete       Unusual, obsolete or       
                       unimplemented              
                       (create_module(2),         
                       gtty(2), ...)              
       
       @privileged     All system calls which     
                       need super-user            
                       capabilities               
                       (capabilities(7))          
       
       @process        Process control,           
                       execution, namespaces      
                       (clone(2), kill(2),        
                       namespaces(7), ...         
       
       @raw-io         Raw I/O port access        
                       (ioperm(2), iopl(2),       
                       pciconfig_read(), ...)     
       
       @resources      System calls for changing  
                       resource limits, memory    
                       and scheduling parameters  
                       (setrlimit(2),             
                       setpriority(2), ...)       
       
       Note that as new system calls are added to the kernel, additional
       system calls might be added to the groups above, so the contents of
       the sets may change between systemd versions.

       It is recommended to combine the file system namespacing related
       options with SystemCallFilter=~@mount, in order to prohibit the
       unit's processes to undo the mappings. Specifically these are the
       options PrivateTmp=, PrivateDevices=, ProtectSystem=, ProtectHome=,
       ProtectKernelTunables=, ProtectControlGroups=, ReadOnlyPaths=,
       InaccessiblePaths= and ReadWritePaths=.

   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
       the same as for ConditionArchitecture= described in
       systemd.unit(5), as well as x32, mips64-n32, mips64-le-n32, and 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, or in system mode, but
       without the CAP_SYS_ADMIN capability (e.g. setting User=nobody),
       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, or in system mode, but without the CAP_SYS_ADMIN capability
       (e.g. setting User=nobody), 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. This does not
       affect commands prefixed with "+".

   ProtectKernelModules=
       Takes a boolean argument. If true, explicit module loading will be
       denied. This allows to turn off module load and unload operations
       on modular kernels. It is recommended to turn this on for most
       services that do not need special file systems or extra kernel
       modules to work. Default to off. Enabling this option removes
       CAP_SYS_MODULE from the capability bounding set for the unit, and
       installs a system call filter to block module system calls, also
       /usr/lib/modules is made inaccessible. For this setting the same
       restrictions regarding mount propagation and privileges apply as
       for ReadOnlyPaths= and related calls, see above. Note that limited
       automatic module loading due to user configuration or kernel
       mapping tables might still happen as side effect of requested user
       operations, both privileged and unprivileged. To disable module
       auto-load feature please see sysctl.d(5) kernel.modules_disabled
       mechanism and /proc/sys/kernel/modules_disabled documentation.

   Personality=
       Controls which kernel architecture uname(2) shall report, when
       invoked by unit processes. Takes one of the architecture
       identifiers x86, x86-64, ppc, ppc-le, ppc64, ppc64-le, s390 or
       s390x. Which personality architectures are supported depends on the
       system architecture. Usually the 64bit versions of the various
       system architectures support their immediate 32bit personality
       architecture counterpart, but no others. For example, x86-64
       systems support the x86-64 and x86 personalities but no others. The
       personality feature 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
       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 unprivileged
       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).

   MemoryDenyWriteExecute=
       Takes a boolean argument. If set, attempts to create memory
       mappings that are writable and executable at the same time, or to
       change existing memory mappings to become executable, or mapping
       shared memory segments as executable are prohibited. Specifically,
       a system call filter is added that rejects mmap(2) system calls
       with both PROT_EXEC and PROT_WRITE set, mprotect(2) system calls
       with PROT_EXEC set and shmat(2) system calls with SHM_EXEC set.
       Note that this option is incompatible with programs that generate
       program code dynamically at runtime, such as JIT execution engines,
       or programs compiled making use of the code "trampoline" feature of
       various C compilers. This option improves service security, as it
       makes harder for software exploits to change running code
       dynamically.

   RestrictRealtime=
       Takes a boolean argument. If set, any attempts to enable realtime
       scheduling in a process of the unit are refused. This restricts
       access to realtime task scheduling policies such as SCHED_FIFO,
       SCHED_RR or SCHED_DEADLINE. See sched(7) for details about these
       scheduling policies. Realtime scheduling policies may be used to
       monopolize CPU time for longer periods of time, and may hence be
       used to lock up or otherwise trigger Denial-of-Service situations
       on the system. It is hence recommended to restrict access to
       realtime scheduling to the few programs that actually require them.
       Defaults to off.

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 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, which includes
       user systemd instances. See passwd(5).

   $INVOCATION_ID
       Contains a randomized, unique 128bit ID identifying each runtime
       cycle of the unit, formatted as 32 character hexadecimal string. A
       new ID is assigned each time the unit changes from an inactive
       state into an activating or active state, and may be used to
       identify this specific runtime cycle, in particular in data stored
       offline, such as the journal. The same ID is passed to all
       processes run as part of the unit.

   $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 unit's 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, $LISTEN_FDNAMES
       Information about file descriptors passed to a service for socket
       activation. See sd_listen_fds(3).

   $NOTIFY_SOCKET
       The socket sd_notify() talks to. See sd_notify(3).

   $WATCHDOG_PID, $WATCHDOG_USEC
       Information about watchdog keep-alive notifications. See
       sd_watchdog_enabled(3).

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

   $JOURNAL_STREAM
       If the standard output or standard error output of the executed
       processes are connected to the journal (for example, by setting
       StandardError=journal) $JOURNAL_STREAM contains the device and
       inode numbers of the connection file descriptor, formatted in
       decimal, separated by a colon (":"). This permits invoked processes
       to safely detect whether their standard output or standard error
       output are connected to the journal. The device and inode numbers
       of the file descriptors should be compared with the values set in
       the environment variable to determine whether the process output is
       still connected to the journal. Note that it is generally not
       sufficient to only check whether $JOURNAL_STREAM is set at all as
       services might invoke external processes replacing their standard
       output or standard error output, without unsetting the environment
       variable.

       This environment variable is primarily useful to allow services to
       optionally upgrade their used log protocol to the native journal
       protocol (using sd_journal_print(3) and other functions) if their
       standard output or standard error output is connected to the
       journal anyway, thus enabling delivery of structured metadata along
       with logged messages.

   $SERVICE_RESULT
       Only defined for the service unit type, this environment variable
       is passed to all ExecStop= and ExecStopPost= processes, and encodes
       the service "result". Currently, the following values are defined:
       "timeout" (in case of an operation timeout), "exit-code" (if a
       service process exited with a non-zero exit code; see $EXIT_CODE
       below for the actual exit code returned), "signal" (if a service
       process was terminated abnormally by a signal; see $EXIT_CODE below
       for the actual signal used for the termination), "core-dump" (if a
       service process terminated abnormally and dumped core), "watchdog"
       (if the watchdog keep-alive ping was enabled for the service but it
       missed the deadline), or "resources" (a catch-all condition in case
       a system operation failed).

       This environment variable is useful to monitor failure or
       successful termination of a service. Even though this variable is
       available in both ExecStop= and ExecStopPost=, it is usually a
       better choice to place monitoring tools in the latter, as the
       former is only invoked for services that managed to start up
       correctly, and the latter covers both services that failed during
       their start-up and those which failed during their runtime.

   $EXIT_CODE, $EXIT_STATUS
       Only defined for the service unit type, these environment variables
       are passed to all ExecStop=, ExecStopPost= processes and contain
       exit status/code information of the main process of the service.
       For the precise definition of the exit code and status, see
       wait(2).  $EXIT_CODE is one of "exited", "killed", "dumped".
       $EXIT_STATUS contains the numeric exit code formatted as string if
       $EXIT_CODE is "exited", and the signal name in all other cases.
       Note that these environment variables are only set if the service
       manager succeeded to start and identify the main process of the
       service.

       Table 3. Summary of possible service result variable values
       
       $SERVICE_RESULT  $EXIT_STATUS       $EXIT_CODE          
       
       "timeout"        "killed"           "TERM", "KILL"      
                        
                        "exited"           "0", "1", "2", "3", 
                                           ..., "255"          
       
       "exit-code"      "exited"           "0", "1", "2", "3", 
                                           ..., "255"          
       
       "signal"         "killed"           "HUP", "INT",       
                                           "KILL", ...         
       
       "core-dump"      "dumped"           "ABRT", "SEGV",     
                                           "QUIT", ...         
       
       "watchdog"       "dumped"           "ABRT"              
                        
                        "killed"           "TERM", "KILL"      
                        
                        "exited"           "0", "1", "2", "3", 
                                           ..., "255"          
       
       "resources"      any of the above   any of the above    
       
       Note: the process may be also terminated by a signal not  
       sent by systemd. In particular the process may send an    
       arbitrary signal to itself in a handler for any of the    
       non-maskable signals. Nevertheless, in the "timeout" and  
       "watchdog" rows above only the signals that systemd sends 
       have been included.                                       
       

   Additional variables may be configured by the following means: for
   processes spawned in specific units, use the Environment=,
   EnvironmentFile= and PassEnvironment= 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),
   systemd.time(7), systemd.directives(7), tmpfiles.d(5), exec(3)

NOTES

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





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