Re: conditional prerequisite

[Paul D. Smith]

%% address@hidden writes:

  tw> info.h:   info.txt
  tw>   sed -f info.sed info.txt > info.h

  tw> info.txt:
  tw>   wget -N http://blabla/info.txt

  tw> The intended behaviour in detail is:

  tw> info.h is newer than info.txt:
  tw>   nothing happens

  tw> info.h is older than info.txt:
  tw>   sed is invoked

  tw> info.h exists but info.txt does not exist:
  tw>   nothing happens!

  tw> info.h does not exist but info.txt exists:
  tw>   sed is invoked

  tw> info.h and info.txt both do not exist:
  tw>   wget is invoked, then sed

  tw> Is this possible?

To do this you'll need to be tricky, because you want to know what the
state of the directory is before any rules are invoked; you can find
that out like this:

    FILES := $(wildcard info.h info.txt)

Now you can check which ones existed.  So for example, if you only
wanted to run the wget if neither existed, you could do something like
this (there are obviously many ways to do it):

 info.txt:
 ifndef FILES
        wget -N http://blabla/info.txt
 endif

or whatever.

-- 
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