man: fix link to systemd-networkd-wait-online.service in systemd-networkd.service(8)

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<refentry id="systemd.exec">
        <refentryinfo>
                <title>systemd.exec</title>
                <productname>systemd</productname>

                <authorgroup>
                        <author>
                                <contrib>Developer</contrib>
                                <firstname>Lennart</firstname>
                                <surname>Poettering</surname>
                                <email>lennart@poettering.net</email>
                        </author>
                </authorgroup>
        </refentryinfo>

        <refmeta>
                <refentrytitle>systemd.exec</refentrytitle>
                <manvolnum>5</manvolnum>
        </refmeta>

        <refnamediv>
                <refname>systemd.exec</refname>
                <refpurpose>Execution environment configuration</refpurpose>
        </refnamediv>

        <refsynopsisdiv>
                <para><filename><replaceable>service</replaceable>.service</filename>,
                <filename><replaceable>socket</replaceable>.socket</filename>,
                <filename><replaceable>mount</replaceable>.mount</filename>,
                <filename><replaceable>swap</replaceable>.swap</filename></para>
        </refsynopsisdiv>

        <refsect1>
                <title>Description</title>

                <para>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.</para>

                <para>This man page lists the configuration options
                shared by these four unit types. See
                <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
                for the common options of all unit configuration
                files, and
                <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                and
                <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
                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.</para>
        </refsect1>

        <refsect1>
                <title>Options</title>

                <variablelist class='unit-directives'>

                        <varlistentry>
                                <term><varname>WorkingDirectory=</varname></term>

                                <listitem><para>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.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>RootDirectory=</varname></term>

                                <listitem><para>Takes an absolute
                                directory path. Sets the root
                                directory for executed processes, with
                                the
                                <citerefentry project='man-pages'><refentrytitle>chroot</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                system call. If this is used, it must
                                be ensured that the process and all
                                its auxiliary files are available in
                                the <function>chroot()</function>
                                jail.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>User=</varname></term>
                                <term><varname>Group=</varname></term>

                                <listitem><para>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.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>SupplementaryGroups=</varname></term>

                                <listitem><para>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.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>Nice=</varname></term>

                                <listitem><para>Sets the default nice
                                level (scheduling priority) for
                                executed processes. Takes an integer
                                between -20 (highest priority) and 19
                                (lowest priority). See
                                <citerefentry><refentrytitle>setpriority</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                for details.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>OOMScoreAdjust=</varname></term>

                                <listitem><para>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 <ulink
                                url="https://www.kernel.org/doc/Documentation/filesystems/proc.txt">proc.txt</ulink>
                                for details.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>IOSchedulingClass=</varname></term>

                                <listitem><para>Sets the IO scheduling
                                class for executed processes. Takes an
                                integer between 0 and 3 or one of the
                                strings <option>none</option>,
                                <option>realtime</option>,
                                <option>best-effort</option> or
                                <option>idle</option>. See
                                <citerefentry><refentrytitle>ioprio_set</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                for details.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>IOSchedulingPriority=</varname></term>

                                <listitem><para>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
                                <citerefentry><refentrytitle>ioprio_set</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                for details.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>CPUSchedulingPolicy=</varname></term>

                                <listitem><para>Sets the CPU
                                scheduling policy for executed
                                processes. Takes one of
                                <option>other</option>,
                                <option>batch</option>,
                                <option>idle</option>,
                                <option>fifo</option> or
                                <option>rr</option>. See
                                <citerefentry><refentrytitle>sched_setscheduler</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                for details.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>CPUSchedulingPriority=</varname></term>

                                <listitem><para>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 <citerefentry><refentrytitle>sched_setscheduler</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                for details.
                                </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>CPUSchedulingResetOnFork=</varname></term>

                                <listitem><para>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
                                <citerefentry><refentrytitle>sched_setscheduler</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                for details. Defaults to false.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>CPUAffinity=</varname></term>

                                <listitem><para>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
                                <citerefentry><refentrytitle>sched_setaffinity</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                for details.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>UMask=</varname></term>

                                <listitem><para>Controls the file mode
                                creation mask. Takes an access mode in
                                octal notation. See
                                <citerefentry><refentrytitle>umask</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                for details. Defaults to
                                0022.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>Environment=</varname></term>

                                <listitem><para>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.</para>

                                <para>Example:
                                <programlisting>Environment="VAR1=word1 word2" VAR2=word3 "VAR3=$word 5 6"</programlisting>
                                gives three variables <literal>VAR1</literal>,
                                <literal>VAR2</literal>, <literal>VAR3</literal>
                                with the values <literal>word1 word2</literal>,
                                <literal>word3</literal>, <literal>$word 5 6</literal>.
                                </para>

                                <para>
                                See
                                <citerefentry project='man-pages'><refentrytitle>environ</refentrytitle><manvolnum>7</manvolnum></citerefentry>
                                for details about environment variables.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><varname>EnvironmentFile=</varname></term>
                                <listitem><para>Similar to
                                <varname>Environment=</varname> 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 (").</para>

                                <para>The argument passed should be an
                                absolute filename or wildcard
                                expression, optionally prefixed with
                                <literal>-</literal>, 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.</para>

                                <para>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
                                <varname>Environment=</varname>. 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.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>StandardInput=</varname></term>
                                <listitem><para>Controls where file
                                descriptor 0 (STDIN) of the executed
                                processes is connected to. Takes one
                                of <option>null</option>,
                                <option>tty</option>,
                                <option>tty-force</option>,
                                <option>tty-fail</option> or
                                <option>socket</option>.</para>

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

                                <para>If <option>tty</option> is
                                selected, standard input is connected
                                to a TTY (as configured by
                                <varname>TTYPath=</varname>, 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.</para>

                                <para><option>tty-force</option> is similar
                                to <option>tty</option>, but the
                                executed process is forcefully and
                                immediately made the controlling
                                process of the terminal, potentially
                                removing previous controlling
                                processes from the
                                terminal.</para>

                                <para><option>tty-fail</option> is
                                similar to <option>tty</option> but if
                                the terminal already has a controlling
                                process start-up of the executed
                                process fails.</para>

                                <para>The <option>socket</option>
                                option is only valid in
                                socket-activated services, and only
                                when the socket configuration file
                                (see
                                <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
                                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
                                <citerefentry><refentrytitle>inetd</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                                daemon.</para>

                                <para>This setting defaults to
                                <option>null</option>.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><varname>StandardOutput=</varname></term>
                                <listitem><para>Controls where file
                                descriptor 1 (STDOUT) of the executed
                                processes is connected to. Takes one
                                of <option>inherit</option>,
                                <option>null</option>,
                                <option>tty</option>,
                                <option>journal</option>,
                                <option>syslog</option>,
                                <option>kmsg</option>,
                                <option>journal+console</option>,
                                <option>syslog+console</option>,
                                <option>kmsg+console</option> or
                                <option>socket</option>.</para>

                                <para><option>inherit</option>
                                duplicates the file descriptor of
                                standard input for standard
                                output.</para>

                                <para><option>null</option> connects
                                standard output to
                                <filename>/dev/null</filename>,
                                i.e. everything written to it will be
                                lost.</para>

                                <para><option>tty</option> connects
                                standard output to a tty (as
                                configured via
                                <varname>TTYPath=</varname>, 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.</para>

                                <para><option>journal</option>
                                connects standard output with the
                                journal which is accessible via
                                <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
                                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.</para>

                                <para><option>syslog</option> connects
                                standard output to the <citerefentry
                                project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                                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
                                <option>journal</option>.</para>

                                <para><option>kmsg</option> connects
                                standard output with the kernel log
                                buffer which is accessible via
                                <citerefentry
                                project='man-pages'><refentrytitle>dmesg</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                                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
                                <option>journal</option>.</para>

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

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

                                <para>This setting defaults to the
                                value set with
                                <option>DefaultStandardOutput=</option>
                                in
                                <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                                which defaults to
                                <option>journal</option>.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><varname>StandardError=</varname></term>
                                <listitem><para>Controls where file
                                descriptor 2 (STDERR) of the
                                executed processes is connected to.
                                The available options are identical to
                                those of
                                <varname>StandardOutput=</varname>,
                                with one exception: if set to
                                <option>inherit</option> the file
                                descriptor used for standard output is
                                duplicated for standard error. This
                                setting defaults to the value set with
                                <option>DefaultStandardError=</option>
                                in
                                <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                                which defaults to
                                <option>inherit</option>.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><varname>TTYPath=</varname></term>
                                <listitem><para>Sets the terminal
                                device node to use if standard input, output,
                                or error are connected to a
                                TTY (see above). Defaults to
                                <filename>/dev/console</filename>.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><varname>TTYReset=</varname></term>
                                <listitem><para>Reset the terminal
                                device specified with
                                <varname>TTYPath=</varname> before and
                                after execution. Defaults to
                                <literal>no</literal>.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><varname>TTYVHangup=</varname></term>
                                <listitem><para>Disconnect all clients
                                which have opened the terminal device
                                specified with
                                <varname>TTYPath=</varname>
                                before and after execution. Defaults
                                to
                                <literal>no</literal>.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><varname>TTYVTDisallocate=</varname></term>
                                <listitem><para>If the terminal
                                device specified with
                                <varname>TTYPath=</varname> 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
                                <literal>no</literal>.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><varname>SyslogIdentifier=</varname></term>
                                <listitem><para>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
                                <varname>StandardOutput=</varname> or
                                <varname>StandardError=</varname> are
                                set to <option>syslog</option>,
                                <option>journal</option> or
                                <option>kmsg</option> (or to the same
                                settings in combination with
                                <option>+console</option>).</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><varname>SyslogFacility=</varname></term>
                                <listitem><para>Sets the syslog
                                facility to use when logging to
                                syslog. One of <option>kern</option>,
                                <option>user</option>,
                                <option>mail</option>,
                                <option>daemon</option>,
                                <option>auth</option>,
                                <option>syslog</option>,
                                <option>lpr</option>,
                                <option>news</option>,
                                <option>uucp</option>,
                                <option>cron</option>,
                                <option>authpriv</option>,
                                <option>ftp</option>,
                                <option>local0</option>,
                                <option>local1</option>,
                                <option>local2</option>,
                                <option>local3</option>,
                                <option>local4</option>,
                                <option>local5</option>,
                                <option>local6</option> or
                                <option>local7</option>. See
                                <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                                for details. This option is only
                                useful when
                                <varname>StandardOutput=</varname> or
                                <varname>StandardError=</varname> are
                                set to <option>syslog</option>.
                                Defaults to
                                <option>daemon</option>.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><varname>SyslogLevel=</varname></term>
                                <listitem><para>Default syslog level
                                to use when logging to syslog or the
                                kernel log buffer. One of
                                <option>emerg</option>,
                                <option>alert</option>,
                                <option>crit</option>,
                                <option>err</option>,
                                <option>warning</option>,
                                <option>notice</option>,
                                <option>info</option>,
                                <option>debug</option>. See
                                <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                                for details. This option is only
                                useful when
                                <varname>StandardOutput=</varname> or
                                <varname>StandardError=</varname> are
                                set to <option>syslog</option> or
                                <option>kmsg</option>. 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
                                <varname>SyslogLevelPrefix=</varname>,
                                see below. For details see
                                <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>.

                                Defaults to
                                <option>info</option>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>SyslogLevelPrefix=</varname></term>
                                <listitem><para>Takes a boolean
                                argument. If true and
                                <varname>StandardOutput=</varname> or
                                <varname>StandardError=</varname> are
                                set to <option>syslog</option>,
                                <option>kmsg</option> or
                                <option>journal</option>, 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
                                <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
                                Defaults to true.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>TimerSlackNSec=</varname></term>
                                <listitem><para>Sets the timer slack
                                in nanoseconds for the executed
                                processes. The timer slack controls
                                the accuracy of wake-ups triggered by
                                timers. See
                                <citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                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.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>LimitCPU=</varname></term>
                                <term><varname>LimitFSIZE=</varname></term>
                                <term><varname>LimitDATA=</varname></term>
                                <term><varname>LimitSTACK=</varname></term>
                                <term><varname>LimitCORE=</varname></term>
                                <term><varname>LimitRSS=</varname></term>
                                <term><varname>LimitNOFILE=</varname></term>
                                <term><varname>LimitAS=</varname></term>
                                <term><varname>LimitNPROC=</varname></term>
                                <term><varname>LimitMEMLOCK=</varname></term>
                                <term><varname>LimitLOCKS=</varname></term>
                                <term><varname>LimitSIGPENDING=</varname></term>
                                <term><varname>LimitMSGQUEUE=</varname></term>
                                <term><varname>LimitNICE=</varname></term>
                                <term><varname>LimitRTPRIO=</varname></term>
                                <term><varname>LimitRTTIME=</varname></term>
                                <listitem><para>These settings control
                                various resource limits for executed
                                processes. See
                                <citerefentry><refentrytitle>setrlimit</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                for details. Use the string
                                <varname>infinity</varname> to
                                configure no limit on a specific
                                resource.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>PAMName=</varname></term>
                                <listitem><para>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
                                <varname>User=</varname> setting. If
                                not set, no PAM session will be opened
                                for the executed processes. See
                                <citerefentry project='man-pages'><refentrytitle>pam</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                                for details.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>CapabilityBoundingSet=</varname></term>

                                <listitem><para>Controls which
                                capabilities to include in the
                                capability bounding set for the
                                executed process. See
                                <citerefentry project='man-pages'><refentrytitle>capabilities</refentrytitle><manvolnum>7</manvolnum></citerefentry>
                                for details. Takes a whitespace-separated
                                list of capability names as read by
                                <citerefentry><refentrytitle>cap_from_name</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
                                e.g. <constant>CAP_SYS_ADMIN</constant>,
                                <constant>CAP_DAC_OVERRIDE</constant>,
                                <constant>CAP_SYS_PTRACE</constant>.
                                Capabilities listed will be included
                                in the bounding set, all others are
                                removed. If the list of capabilities
                                is prefixed with <literal>~</literal>,
                                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
                                <varname>Capabilities=</varname>
                                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 <literal>~</literal> (without any
                                further argument), the bounding set is
                                reset to the full set of available
                                capabilities, also undoing any
                                previous settings.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>SecureBits=</varname></term>
                                <listitem><para>Controls the secure
                                bits set for the executed process.
                                Takes a space-separated combination of
                                options from the following list:
                                <option>keep-caps</option>,
                                <option>keep-caps-locked</option>,
                                <option>no-setuid-fixup</option>,
                                <option>no-setuid-fixup-locked</option>,
                                <option>noroot</option>, and
                                <option>noroot-locked</option>. This
                                option may appear more than once in
                                which case the secure bits are ORed.
                                If the empty string is assigned to
                                this option, the bits are reset to 0.
                                See <citerefentry
                                project='man-pages'><refentrytitle>capabilities</refentrytitle><manvolnum>7</manvolnum></citerefentry>
                                for details.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>Capabilities=</varname></term>
                                <listitem><para>Controls the
                                <citerefentry project='man-pages'><refentrytitle>capabilities</refentrytitle><manvolnum>7</manvolnum></citerefentry>
                                set for the executed process. Take a
                                capability string describing the
                                effective, permitted and inherited
                                capability sets as documented in
                                <citerefentry><refentrytitle>cap_from_text</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
                                Note that these capability sets are
                                usually influenced (and filtered) by the capabilities
                                attached to the executed file. Due to
                                that
                                <varname>CapabilityBoundingSet=</varname>
                                is probably a much more useful
                                setting.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>ReadWriteDirectories=</varname></term>
                                <term><varname>ReadOnlyDirectories=</varname></term>
                                <term><varname>InaccessibleDirectories=</varname></term>

                                <listitem><para>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
                                <varname>ReadWriteDirectories=</varname>
                                are accessible from within the
                                namespace with the same access rights
                                as from outside. Directories listed in
                                <varname>ReadOnlyDirectories=</varname>
                                are accessible for reading only,
                                writing will be refused even if the
                                usual file access controls would
                                permit this. Directories listed in
                                <varname>InaccessibleDirectories=</varname>
                                will be made inaccessible for
                                processes inside the namespace. Note
                                that restricting access with these
                                options does not extend to submounts
                                of a directory that are created later
                                on. These options may be specified
                                more than once in which case all
                                directories listed will have limited
                                access from within the namespace. If
                                the empty string is assigned to this
                                option, the specific list is reset,
                                and all prior assignments have no
                                effect.</para>
                                <para>Paths in
                                <varname>ReadOnlyDirectories=</varname>
                                and
                                <varname>InaccessibleDirectories=</varname>
                                may be prefixed with
                                <literal>-</literal>, 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.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>PrivateTmp=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If true, sets up a new file
                                system namespace for the executed
                                processes and mounts private
                                <filename>/tmp</filename> and
                                <filename>/var/tmp</filename>
                                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
                                <filename>/tmp</filename> or
                                <filename>/var/tmp</filename>
                                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
                                <filename>/tmp</filename> and
                                <filename>/var/tmp</filename>
                                namespace by using the
                                <varname>JoinsNamespaceOf=</varname>
                                directive, see
                                <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
                                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.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>PrivateDevices=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If true, sets up a new /dev
                                namespace for the executed processes
                                and only adds API pseudo devices such
                                as <filename>/dev/null</filename>,
                                <filename>/dev/zero</filename> or
                                <filename>/dev/random</filename> (as
                                well as the pseudo TTY subsystem) to
                                it, but no physical devices such as
                                <filename>/dev/sda</filename>. This is
                                useful to securely turn off physical
                                device access by the executed
                                process. Defaults to false. Enabling
                                this option will also remove
                                <constant>CAP_MKNOD</constant> from
                                the capability bounding set for the
                                unit (see above), and set
                                <varname>DevicePolicy=closed</varname>
                                (see
                                <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>
                                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.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>PrivateNetwork=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If true, sets up a new
                                network namespace for the executed
                                processes and configures only the
                                loopback network device
                                <literal>lo</literal> 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
                                <varname>JoinsNamespaceOf=</varname>
                                directive, see
                                <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
                                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).</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>ProtectSystem=</varname></term>

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

                        <varlistentry>
                                <term><varname>ProtectHome=</varname></term>

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

                        <varlistentry>
                                <term><varname>MountFlags=</varname></term>

                                <listitem><para>Takes a mount
                                propagation flag:
                                <option>shared</option>,
                                <option>slave</option> or
                                <option>private</option>, which
                                control whether mounts in the file
                                system namespace set up for this
                                unit's processes will receive or
                                propagate mounts or unmounts. See
                                <citerefentry><refentrytitle>mount</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                for details. Defaults to
                                <option>shared</option>. Use
                                <option>shared</option> to ensure that
                                mounts and unmounts are propagated
                                from the host to the container and
                                vice versa. Use <option>slave</option>
                                to run processes so that none of their
                                mounts and unmounts will propagate to
                                the host. Use <option>private</option>
                                to also ensure that no mounts and
                                unmounts from the host will propagate
                                into the unit processes'
                                namespace. Note that
                                <option>slave</option> 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
                                (<varname>PrivateTmp=</varname>,
                                <varname>PrivateDevices=</varname>,
                                <varname>ProtectSystem=</varname>,
                                <varname>ProtectHome=</varname>,
                                <varname>ReadOnlyDirectories=</varname>,
                                <varname>InaccessibleDirectories=</varname>
                                and
                                <varname>ReadWriteDirectories=</varname>)
                                require that mount and unmount
                                propagation from the unit's file
                                system namespace is disabled, and
                                hence downgrade
                                <option>shared</option> to
                                <option>slave</option>.
                                </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>UtmpIdentifier=</varname></term>

                                <listitem><para>Takes a four
                                character identifier string for an
                                utmp/wtmp entry for this service. This
                                should only be set for services such
                                as <command>getty</command>
                                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.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>SELinuxContext=</varname></term>

                                <listitem><para>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
                                <literal>-</literal>, all errors will
                                be ignored. See
                                <citerefentry><refentrytitle>setexeccon</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                                for details.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>AppArmorProfile=</varname></term>

                                <listitem><para>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 <literal>-</literal>, all errors
                                will be ignored.
                                </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>SmackProcessLabel=</varname></term>

                                <listitem><para>Takes a
                                <option>SMACK64</option> security
                                label as argument. The process
                                executed by the unit will be started
                                under this label and SMACK will decide
                                whether the processes is allowed to
                                run or not based on it. The process
                                will continue to run under the label
                                specified here unless the executable
                                has its own
                                <option>SMACK64EXEC</option> 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.</para>

                                <para>The value may be prefixed by
                                <literal>-</literal>, in which case
                                all errors will be ignored. An empty
                                value may be specified to unset
                                previous assignments.</para>
                                </listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>IgnoreSIGPIPE=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If true, causes <constant>SIGPIPE</constant> to be
                                ignored in the executed
                                process. Defaults to true because
                                <constant>SIGPIPE</constant> generally is useful only in
                                shell pipelines.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>NoNewPrivileges=</varname></term>

                                <listitem><para>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.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>SystemCallFilter=</varname></term>

                                <listitem><para>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
                                <constant>SIGSYS</constant> signal
                                (whitelisting). If the first character
                                of the list is <literal>~</literal>,
                                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,
                                <varname>NoNewPrivileges=yes</varname>
                                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
                                <function>execve</function>,
                                <function>rt_sigreturn</function>,
                                <function>sigreturn</function>,
                                <function>exit_group</function>,
                                <function>exit</function> 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.</para>

                                <para>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
                                <function>read</function> and
                                <function>write</function>, and right
                                after it add a blacklisting of
                                <function>write</function>, then
                                <function>write</function> will be
                                removed from the set.)
                                </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>SystemCallErrorNumber=</varname></term>

                                <listitem><para>Takes an
                                <literal>errno</literal> error number
                                name to return when the system call
                                filter configured with
                                <varname>SystemCallFilter=</varname>
                                is triggered, instead of terminating
                                the process immediately. Takes an
                                error name such as
                                <constant>EPERM</constant>,
                                <constant>EACCES</constant> or
                                <constant>EUCLEAN</constant>. When this
                                setting is not used, or when the empty
                                string is assigned, the process will be
                                terminated immediately when the filter
                                is triggered.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>SystemCallArchitectures=</varname></term>

                                <listitem><para>Takes a space
                                separated list of architecture
                                identifiers to include in the system
                                call filter. The known architecture
                                identifiers are
                                <constant>x86</constant>,
                                <constant>x86-64</constant>,
                                <constant>x32</constant>,
                                <constant>arm</constant> as well as
                                the special identifier
                                <constant>native</constant>. 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
                                <constant>native</constant> 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,
                                <varname>NoNewPrivileges=yes</varname>
                                is implied. Note that setting this
                                option to a non-empty list implies
                                that <constant>native</constant> is
                                included too. By default, this option
                                is set to the empty list, i.e. no
                                architecture system call filtering is
                                applied.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>RestrictAddressFamilies=</varname></term>

                                <listitem><para>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
                                <constant>AF_UNIX</constant>,
                                <constant>AF_INET</constant> or
                                <constant>AF_INET6</constant>. When
                                prefixed with <constant>~</constant>
                                the listed address families will be
                                applied as blacklist, otherwise as
                                whitelist. Note that this restricts
                                access to the
                                <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                system call only. Sockets passed into
                                the process by other means (for
                                example, by using socket activation
                                with socket units, see
                                <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>)
                                are unaffected. Also, sockets created
                                with <function>socketpair()</function>
                                (which creates connected AF_UNIX
                                sockets only) are unaffected. Note
                                that this option has no effect on
                                32-bit x86 and is ignored (but works
                                correctly on x86-64). If running in user
                                mode and this option is used,
                                <varname>NoNewPrivileges=yes</varname>
                                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.</para>

                                <para>Use this option to limit
                                exposure of processes to remote
                                systems, in particular via exotic
                                network protocols. Note that in most
                                cases, the local
                                <constant>AF_UNIX</constant> address
                                family should be included in the
                                configured whitelist as it is
                                frequently used for local
                                communication, including for
                                <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                logging.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>Personality=</varname></term>

                                <listitem><para>Controls which
                                kernel architecture
                                <citerefentry><refentrytitle>uname</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                                shall report, when invoked by unit
                                processes. Takes one of
                                <constant>x86</constant> and
                                <constant>x86-64</constant>. This is
                                useful when running 32-bit services on
                                a 64-bit host system. If not specified,
                                the personality is left unmodified and
                                thus reflects the personality of the
                                host system's
                                kernel.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>RuntimeDirectory=</varname></term>
                                <term><varname>RuntimeDirectoryMode=</varname></term>

                                <listitem><para>Takes a list of
                                directory names. If set, one or more
                                directories by the specified names
                                will be created below
                                <filename>/run</filename> (for system
                                services) or below
                                <varname>$XDG_RUNTIME_DIR</varname>
                                (for user services) when the unit is
                                started, and removed when the unit is
                                stopped. The directories will have the
                                access mode specified in
                                <varname>RuntimeDirectoryMode=</varname>,
                                and will be owned by the user and
                                group specified in
                                <varname>User=</varname> and
                                <varname>Group=</varname>. 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
                                <literal>/</literal>, i.e. must refer
                                to simple directories to create or
                                remove. This is particularly useful
                                for unprivileged daemons that cannot
                                create runtime directories in
                                <filename>/run</filename> 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
                                <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
                        </varlistentry>

                </variablelist>
        </refsect1>

        <refsect1>
                <title>Environment variables in spawned processes</title>

                <para>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.
                </para>

                <variablelist class='environment-variables'>
                        <varlistentry>
                                <term><varname>$PATH</varname></term>

                                <listitem><para>Colon-separated list
                                of directiories to use when launching
                                executables. Systemd uses a fixed
                                value of
                                <filename>/usr/local/sbin</filename>:<filename>/usr/local/bin</filename>:<filename>/usr/sbin</filename>:<filename>/usr/bin</filename>:<filename>/sbin</filename>:<filename>/bin</filename>.
                                </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$LANG</varname></term>

                                <listitem><para>Locale. Can be set in
                                <citerefentry><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
                                or on the kernel command line (see
                                <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                                and
                                <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>).
                                </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$USER</varname></term>
                                <term><varname>$LOGNAME</varname></term>
                                <term><varname>$HOME</varname></term>
                                <term><varname>$SHELL</varname></term>

                                <listitem><para>User name (twice), home
                                directory, and the login shell.
                                The variables are set for the units that
                                have <varname>User=</varname> set,
                                which includes user
                                <command>systemd</command> instances.
                                See
                                <citerefentry><refentrytitle>passwd</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
                                </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$XDG_RUNTIME_DIR</varname></term>

                                <listitem><para>The directory for volatile
                                state. Set for the user <command>systemd</command>
                                instance, and also in user sessions.
                                See
                                <citerefentry><refentrytitle>pam_systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
                                </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$XDG_SESSION_ID</varname></term>
                                <term><varname>$XDG_SEAT</varname></term>
                                <term><varname>$XDG_VTNR</varname></term>

                                <listitem><para>The identifier of the
                                session, the seat name, and
                                virtual terminal of the session. Set
                                by
                                <citerefentry><refentrytitle>pam_systemd</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                                for login sessions.
                                <varname>$XDG_SEAT</varname> and
                                <varname>$XDG_VTNR</varname> will
                                only be set when attached to a seat and a
                                tty.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$MAINPID</varname></term>

                                <listitem><para>The PID of the units
                                main process if it is known. This is
                                only set for control processes as
                                invoked by
                                <varname>ExecReload=</varname> and
                                similar.  </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$MANAGERPID</varname></term>

                                <listitem><para>The PID of the user
                                <command>systemd</command> instance,
                                set for processes spawned by it.
                                </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$LISTEN_FDS</varname></term>
                                <term><varname>$LISTEN_PID</varname></term>

                                <listitem><para>Information about file
                                descriptors passed to a service for
                                socket activation.  See
                                <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
                                </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$TERM</varname></term>

                                <listitem><para>Terminal type, set
                                only for units connected to a terminal
                                (<varname>StandardInput=tty</varname>,
                                <varname>StandardOutput=tty</varname>,
                                or
                                <varname>StandardError=tty</varname>).
                                See
                                <citerefentry project='man-pages'><refentrytitle>termcap</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
                                </para></listitem>
                        </varlistentry>
                </variablelist>

                <para>Additional variables may be configured by the
                following means: for processes spawned in specific
                units, use the <varname>Environment=</varname> and
                <varname>EnvironmentFile=</varname> options above; to
                specify variables globally, use
                <varname>DefaultEnvironment=</varname> (see
                <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>)
                or the kernel option
                <varname>systemd.setenv=</varname> (see
                <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>). Additional
                variables may also be set through PAM,
                cf. <citerefentry project='man-pages'><refentrytitle>pam_env</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
        </refsect1>

        <refsect1>
                  <title>See Also</title>
                  <para>
                          <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                          <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                          <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
                          <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                          <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                          <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                          <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                          <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                          <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                          <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                          <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
                          <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                          <citerefentry project='man-pages'><refentrytitle>exec</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                  </para>
        </refsect1>

</refentry>