systemd/man/less-variables.xml

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<refsect1>
  <title>Environment</title>

  <variablelist class='environment-variables'>
    <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><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 completly
      disable the pager using <option>--no-pager</option> instead.</para></listitem>
    </varlistentry>

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

      <listitem><para>The value must be a boolean. Controls whether colorized output should be
      generated. This can be specified to override the decision that <command>systemd</command> makes 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>