run: add --expand-environment=no to disable server-side envvar expansion

This uses StartExecEx to get the equivalent of ExecStart=:. StartExecEx was
added in b3d593673c5b8b0b7d781fd26ab2062ca6e7dbdb, so this will not work with
older systemds.

A hint is emitted if we get an error indicating lack of support. PID1 returns
SD_BUS_ERROR_PROPERTY_READ_ONLY, but I'm checking for
SD_BUS_ERROR_UNKNOWN_PROPERTY too for safety.

1 files changed, 25 insertions, 1 deletions

diff --git a/man/systemd-run.xml b/man/systemd-run.xml
index cd9e50d5b8..2ad68d8884 100644
--- a/man/systemd-run.xml
+++ b/man/systemd-run.xml
@@ -92,6 +92,11 @@
     Consider using the <option>exec</option> service type (i.e. <option>--property=Type=exec</option>) to
     ensure that <command>systemd-run</command> returns successfully only if the specified command line has
     been successfully started.</para>
+
+    <para>After <command>systemd-run</command> passes the command to the service manager, the manager
+    performs variable expansion. This means that dollar characters (<literal>$</literal>) which should not be
+    expanded need to be escaped as <literal>$$</literal>. Expansion can also be disabled using
+    <varname>--expand-environment=no</varname>.</para>
   </refsect1>
 
   <refsect1>
@@ -170,6 +175,24 @@
       </varlistentry>
 
       <varlistentry>
+        <term><option>--expand-environment=<replaceable>BOOL</replaceable></option></term>
+
+        <listitem><para>Expand environment variables in command arguments. If enabled (the default), the
+        service manager that spawns the actual command will expand variables specified as
+        <literal>${<replaceable>VARIABLE</replaceable>}</literal> in the same way as in commands specied via
+        <varname>ExecStart=</varname> in units. Note that this is similar to, but not the same as variable
+        expansion in
+        <citerefentry project='man-pages'><refentrytitle>bash</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+        and other shells.</para>
+
+        <para>See
+        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+        for a description of variable expansion. Disabling variable expansion is useful if the specified
+        command includes or may include a <literal>$</literal> sign.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
         <term><option>-r</option></term>
         <term><option>--remain-after-exit</option></term>
 
@@ -533,7 +556,8 @@ There is a screen on:
 
       <programlisting>$ systemd-run --user --wait true
 $ systemd-run --user --wait -p SuccessExitStatus=11 bash -c 'exit 11'
-$ systemd-run --user --wait -p SuccessExitStatus=SIGUSR1 bash -c 'kill -SIGUSR1 $$$$'</programlisting>
+$ systemd-run --user --wait -p SuccessExitStatus=SIGUSR1 --expand-environment=no \
+      bash -c 'kill -SIGUSR1 $$'</programlisting>
 
       <para>Those three invocations will succeed, i.e. terminate with an exit code of 0.</para>
     </example>