- main process of the daemon. The
- command line accepts '<literal>%</literal>'
- specifiers as described in
- <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+ main process of the daemon.</para>
+
+ <para>The command line accepts
+ '<literal>%</literal>' specifiers as
+ described in
+ <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. Note
+ that the first argument of the command
+ line (i.e. the program to execute) may
+ not include specifiers.</para>
+
+ <para>Basic environment variable
+ substitution is supported. Use
+ <literal>${FOO}</literal> as part of a
+ word, or as a word of its own on the
+ command line, in which case it will be
+ replaced by the value of the
+ environment variable including all
+ whitespace it contains, resulting in a
+ single argument. Use
+ <literal>$FOO</literal> as a separate
+ word on the command line, in which
+ case it will be replaced by the value
+ of the environment variable split up
+ at whitespace, resulting in zero or
+ more arguments. Note that the first
+ argument (i.e. the program to execute)
+ may not be a variable, since it must
+ be a literal and absolute path
+ name.</para>