man: clarify quoting of `$` in command lines (#39494)

When the special executable prefix `:` is used, `$$` yield the literal `$$`.

(cherry picked from commit bfb365d92445f8ac0bb5c57106a8598dc5eb56d4)

diff --git a/man/systemd.service.xml b/man/systemd.service.xml
index 68663e9ed81685f15ba5e63dc79123dfc011dd00..b84db0f70b15bf187e0c4614b72ffbadf417fafd 100644 (file)
--- a/man/systemd.service.xml
+++ b/man/systemd.service.xml
@@ -1483,7 +1483,8 @@ ExecStart=/bin/echo $ONE $TWO $THREE</programlisting>
     <literal>too</literal>.
     </para>
 
-    <para>To pass a literal dollar sign, use <literal>$$</literal>.
+    <para>Unless for commands with the special executable prefix <literal>:</literal>,
+    to pass a literal dollar sign, use <literal>$$</literal>.
     Variables whose value is not known at expansion time are treated
     as empty strings. Note that the first argument (i.e. the program
     to execute) may not be a variable.</para>