systemd/man/common-variables.xml

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<refsect1>
  <title>Environment</title>

  <variablelist class='environment-variables'>
    <varlistentry id='log-level'>
      <term><varname>$SYSTEMD_LOG_LEVEL</varname></term>

      <listitem><para id='log-level-body'>The maximum log level of emitted messages (messages with a higher
      log level, i.e. less important ones, will be suppressed). Either one of (in order of decreasing
      importance) <constant>emerg</constant>, <constant>alert</constant>, <constant>crit</constant>,
      <constant>err</constant>, <constant>warning</constant>, <constant>notice</constant>,
      <constant>info</constant>, <constant>debug</constant>, or an integer in the range 0…7. See
      <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
      for more information.</para>
      </listitem>
    </varlistentry>

    <varlistentry id='log-color'>
      <term><varname>$SYSTEMD_LOG_COLOR</varname></term>

      <listitem><para id='log-color-body'>A boolean. If true, messages written to the tty will be colored
      according to priority.</para>

      <para>This setting is only useful when messages are written directly to the terminal, because
      <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> and
      other tools that display logs will color messages based on the log level on their own.</para>
      </listitem>
    </varlistentry>

    <varlistentry id='log-time'>
      <term><varname>$SYSTEMD_LOG_TIME</varname></term>

      <listitem><para id='log-time-body'>A boolean. If true, log messages will be prefixed with a
      timestamp.</para>

      <para>This setting is only useful when messages are written directly to the terminal or a file, because
      <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> and
      other tools that display logs will attach timestamps based on the entry metadata on their own.</para>
      </listitem>
    </varlistentry>

    <varlistentry id='log-location'>
      <term><varname>$SYSTEMD_LOG_LOCATION</varname></term>

      <listitem><para id='log-location-body'>A boolean. If true, messages will be prefixed with a filename
      and line number in the source code where the message originates.</para>

      <para>Note that the log location is often attached as metadata to journal entries anyway. Including it
      directly in the message text can nevertheless be convenient when debugging programs.</para>
      </listitem>
    </varlistentry>

    <varlistentry id='log-tid'>
      <term><varname>$SYSTEMD_LOG_TID</varname></term>

      <listitem><para id='log-tid-body'>A boolean. If true, messages will be prefixed with the current
      numerical thread ID (TID).</para>

      <para>Note that the this information is attached as metadata to journal entries anyway. Including it
      directly in the message text can nevertheless be convenient when debugging programs.</para>
      </listitem>
    </varlistentry>

    <varlistentry id='log-target'>
      <term><varname>$SYSTEMD_LOG_TARGET</varname></term>

      <listitem><para id='log-target-body'>The destination for log messages. One of
      <constant>console</constant> (log to the attached tty), <constant>console-prefixed</constant> (log to
      the attached tty but with prefixes encoding the log level and "facility", see <citerefentry
      project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <constant>kmsg</constant> (log to the kernel circular log buffer), <constant>journal</constant> (log to
      the journal), <constant>journal-or-kmsg</constant> (log to the journal if available, and to kmsg
      otherwise), <constant>auto</constant> (determine the appropriate log target automatically, the default),
      <constant>null</constant> (disable log output).</para>
      <!-- <constant>syslog</constant>, <constant>syslog-or-kmsg</constant> are deprecated -->
      </listitem>
    </varlistentry>

    <varlistentry id='pager'>
      <term><varname>$SYSTEMD_PAGER</varname></term>

      <listitem><para>Pager to use when <option>--no-pager</option> is not given; overrides
      <varname>$PAGER</varname>. If neither <varname>$SYSTEMD_PAGER</varname> nor <varname>$PAGER</varname> are set, a
      set of well-known pager implementations are tried in turn, including
      <citerefentry project='man-pages'><refentrytitle>less</refentrytitle><manvolnum>1</manvolnum></citerefentry> and
      <citerefentry project='man-pages'><refentrytitle>more</refentrytitle><manvolnum>1</manvolnum></citerefentry>, until one is found. If
      no pager implementation is discovered no pager is invoked. Setting this environment variable to an empty string
      or the value <literal>cat</literal> is equivalent to passing <option>--no-pager</option>.</para></listitem>
    </varlistentry>

    <varlistentry id='less'>
      <term><varname>$SYSTEMD_LESS</varname></term>

      <listitem><para>Override the options passed to <command>less</command> (by default
      <literal>FRSXMK</literal>).</para>

      <para>Users might want to change two options in particular:</para>

      <variablelist>
        <varlistentry>
          <term><option>K</option></term>

          <para>This option instructs the pager to exit immediately when
          <keycombo><keycap>Ctrl</keycap><keycap>C</keycap></keycombo> is pressed. To allow
          <command>less</command> to handle <keycombo><keycap>Ctrl</keycap><keycap>C</keycap></keycombo>
          itself to switch back to the pager command prompt, unset this option.</para>

          <para>If the value of <varname>$SYSTEMD_LESS</varname> does not include <literal>K</literal>,
          and the pager that is invoked is <command>less</command>,
          <keycombo><keycap>Ctrl</keycap><keycap>C</keycap></keycombo> will be ignored by the
          executable, and needs to be handled by the pager.</para>
        </varlistentry>

        <varlistentry>
          <term><option>X</option></term>

          <para>This option instructs the pager to not send termcap initialization and deinitialization
          strings to the terminal. It is set by default to allow command output to remain visible in the
          terminal even after the pager exits. Nevertheless, this prevents some pager functionality from
          working, in particular paged output cannot be scrolled with the mouse.</para>
        </varlistentry>
      </variablelist>

      <para>See
      <citerefentry project='man-pages'><refentrytitle>less</refentrytitle><manvolnum>1</manvolnum></citerefentry>
      for more discussion.</para></listitem>
    </varlistentry>

    <varlistentry id='lesscharset'>
      <term><varname>$SYSTEMD_LESSCHARSET</varname></term>

      <listitem><para>Override the charset passed to <command>less</command> (by default <literal>utf-8</literal>, if
      the invoking terminal is determined to be UTF-8 compatible).</para></listitem>
    </varlistentry>

    <varlistentry id='lesssecure'>
      <term><varname>$SYSTEMD_PAGERSECURE</varname></term>

      <listitem><para>Takes a boolean argument. When true, the "secure" mode of the pager is enabled; if
      false, disabled. If <varname>$SYSTEMD_PAGERSECURE</varname> is not set at all, secure mode is enabled
      if the effective UID is not the same as the owner of the login session, see
      <citerefentry project='man-pages'><refentrytitle>geteuid</refentrytitle><manvolnum>2</manvolnum></citerefentry>
      and <citerefentry><refentrytitle>sd_pid_get_owner_uid</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
      In secure mode, <option>LESSSECURE=1</option> will be set when invoking the pager, and the pager shall
      disable commands that open or create new files or start new subprocesses. When
      <varname>$SYSTEMD_PAGERSECURE</varname> is not set at all, pagers which are not known to implement
      secure mode will not be used. (Currently only
      <citerefentry project='man-pages'><refentrytitle>less</refentrytitle><manvolnum>1</manvolnum></citerefentry>
      implements secure mode.)</para>

      <para>Note: when commands are invoked with elevated privileges, for example under <citerefentry
      project='man-pages'><refentrytitle>sudo</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
      <citerefentry
      project='die-net'><refentrytitle>pkexec</refentrytitle><manvolnum>1</manvolnum></citerefentry>, care
      must be taken to ensure that unintended interactive features are not enabled. "Secure" mode for the
      pager may be enabled automatically as describe above. Setting <varname>SYSTEMD_PAGERSECURE=0</varname>
      or not removing it from the inherited environment allows the user to invoke arbitrary commands. Note
      that if the <varname>$SYSTEMD_PAGER</varname> or <varname>$PAGER</varname> variables are to be
      honoured, <varname>$SYSTEMD_PAGERSECURE</varname> must be set too. It might be reasonable to completely
      disable the pager using <option>--no-pager</option> instead.</para></listitem>
    </varlistentry>

    <varlistentry id='colors'>
      <term><varname>$SYSTEMD_COLORS</varname></term>

      <listitem><para>Takes a boolean argument. When true, <command>systemd</command> and related utilities
      will use colors in their output, otherwise the output will be monochrome. Additionally, the variable can
      take one of the following special values: <literal>16</literal>, <literal>256</literal> to restrict the use
      of colors to the base 16 or 256 ANSI colors, respectively. This can be specified to override the automatic
      decision based on <varname>$TERM</varname> and what the console is connected to.</para></listitem>
    </varlistentry>

    <!-- This is not documented on purpose, because it is not clear if $NO_COLOR will become supported
         widely enough. So let's provide support, but without advertising this.
    <varlistentry id='no-color'>
      <term><varname>$NO_COLOR</varname></term>

      <listitem><para>If set (to any value), and <varname>$SYSTEMD_COLORS</varname> is not set, equivalent to
      <option>SYSTEMD_COLORS=0</option>. See <ulink url="https://no-color.org/">no-color.org</ulink>.</para>
      </listitem>
    </varlistentry>
    -->

    <varlistentry id='urlify'>
      <term><varname>$SYSTEMD_URLIFY</varname></term>

      <listitem><para>The value must be a boolean. Controls whether clickable links should be generated in
      the output for terminal emulators supporting this. This can be specified to override the decision that
      <command>systemd</command> makes based on <varname>$TERM</varname> and other conditions.</para>
      </listitem>
    </varlistentry>

  </variablelist>
</refsect1>