Re: OR rules

[Paul D. Smith]

%% Daniel Mahler <address@hidden> writes:

  dm> a : b
  dm>   echo a : b
  dm> a : c
  dm>   echo a : c
  dm> a : d
  dm>   echo a : d
  dm> c :
  dm>   echo c

Yes, that's because you can't do that.

But you CAN do this:

  dm> %.tar : %.tar.Z
  dm>       unzip $<
  dm>
  dm> %.tar : %.tar.gz
  dm>       gunzip $<


The difference is that the second one is a pattern rule: a pattern rule
might or might not match.  If the pattern rule doesn't match (say
because there's no prerequisite that already exists and make can't
figure out how to build one) then make will keep looking for another
pattern rule that matches.

So you can, and in fact even the builtin GNU make rules do this, have
multiple rules where the target is the same pattern.


But, with an explicit rule you're giving one and only one way to build
every target.  In an explicit rule if the prerequisites aren't there, it
will fail.  There's no way to change that behavior.

If you have to use explicit rules you'll have to do something like this:

  a.tar: $(wildcard a.tar.gz a.tar.Z)
        case "$<" in
          *.gz) gunzip $< ;;
          *.Z)  unzip $< ;;
        esac

Shouldn't that "unzip" really be "uncompress"?  Actually, though, I
think gunzip will unzip .Z files as well.

-- 
-------------------------------------------------------------------------------
 Paul D. Smith <address@hidden>          Find some GNU make tips at:
 http://www.gnu.org                      http://make.paulandlesley.org
 "Please remain calm...I may be mad, but I am a professional." --Mad Scientist