27.11 Debugging Make Rules

The rules and dependency trees generated by automake can get rather complex, and leave the developer head-scratching when things don't work as expected. Besides the debug options provided by the make command (see (make)Options Summary section `Options Summary' in The GNU Make Manual), here's a couple of further hints for debugging makefiles generated by automake effectively:

• If less verbose output has been enabled in the package with the ‘silent-rules’ option (see section Changing Automake's Behavior), you can use make V=1 to see the commands being executed.
• make -n can help show what would be done without actually doing it. Note however, that this will still execute commands prefixed with ‘+’, and, when using GNU make, commands that contain the strings ‘$(MAKE)’ or ‘${MAKE}’ (see (make)Instead of Execution section `Instead of Execution' in The GNU Make Manual). Typically, this is helpful to show what recursive rules would do, but it means that, in your own rules, you should not mix such recursion with actions that change any files.(7) Furthermore, note that GNU make will update prerequisites for the ‘Makefile’ file itself even with ‘-n’ (see (make)Remaking Makefiles section `Remaking Makefiles' in The GNU Make Manual).
• make SHELL="/bin/bash -vx" can help debug complex rules. See (autoconf)The Make Macro SHELL section `The Make Macro SHELL' in The Autoconf Manual, for some portability quirks associated with this construct.