nspawn: catch config mistake of specifying -b and args

man/systemd-nspawn.xml

@@ -170,7 +170,9 @@
                                 <listitem><para>Automatically search
                                 for an init binary and invoke it
                                 instead of a shell or a user supplied
-                                program.</para></listitem>
+                                program. A command to execute cannot
+                                be specified in this case.
+                                </para></listitem>
                         </varlistentry>
 
                         <varlistentry>

src/nspawn/nspawn.c

@@ -267,6 +267,11 @@ static int parse_argv(int argc, char *argv[]) {
                 }
         }
 
+        if (optind < argc && arg_boot) {
+                log_error("Cannot specify a command together with '-b'");
+                return -EINVAL;
+        }
+
         return 1;
 }