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28.11 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 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 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 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.⁸ Furthermore, note that GNU make will update prerequisites for the Makefile file itself even with -n (see Remaking Makefiles in The GNU Make Manual).
• make SHELL="/bin/bash -vx" can help debug complex rules. See The Make Macro SHELL in The Autoconf Manual, for some portability quirks associated with this construct.
• echo 'print: ; @echo "$(VAR)"' | make -f Makefile -f - print can be handy to examine the expanded value of variables. You may need to use a target other than ‘print’ if that is already used or a file with that name exists.
• http://bashdb.sourceforge.net/remake/ provides a modified GNU make command called remake that copes with complex GNU make-specific Makefiles and allows to trace execution, examine variables, and call rules interactively, much like a debugger.