Re: conditional prerequisite

[Boris Kolpackov]

address@hidden writes:

> Suppose, I have a dependency that extracts compilable information 
> from a data source, and another rule that retrieves the data source 
> e.g. from the web:
> 
> info.h:       info.txt
>       sed -f info.sed info.txt > info.h
> 
> info.txt:
>       wget http://blabla/info.txt
> 
> Now of course I want info.h to be made if info.txt exists and 
> is newer.
> But it is also possible that info.txt is not available but info.h 
> is still there. In this case I want it to be done and not try to 
> retrieve info.txt (so I might actually leave out the second rule).

You can do something along these lines:

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


.PHONY: FORCE

info.txt.tmp: FORCE
        wget -N -O $@ http://blabla/info.txt || echo >$@    

info.txt: info.txt.tmp
        if [ ( -s $< ) -a ( $< -nt $@ ) ]; then cp $< $@; fi


First, we unconditionally (FORCE) execute wget to obtain `info.txt'
if it is newer (-N). Note `|| echo >$@' part - we want empty file
if wget fails.

Second, if `info.txt.tmp' is not empty (-s $<) and newer than
`info.txt' ($< -nt $@) we copy it to `info.txt'. Note that make
will check timestamp of `info.txt' after executing the rule and
if it hasn't changed will not rebuild `info.h' (unless `info.h'
does not exist).


-boris