RE: conditional prerequisite

[Alexander.Farber]

How about (untested):

info.h: info.txt
        sed -f info.sed $< > $@
 
info.txt:
        test -e info.h || wget -N http://blabla/info.txt -O $@

Regards
Alex

> -----Original Message-----
> From: address@hidden
> [mailto:address@hidden Behalf
> Of ext address@hidden
> Sent: Friday, January 28, 2005 6:09 PM
> To: address@hidden
> Subject: conditional prerequisite
> 
> 
> Hello,
> 
> I had asked this before and Boris Kolpackov was so kind to give some 
> hints. Unfortunately, they did not exactly address the problem and 
> I'm trying to explain once more, hopefully better, what I'm trying 
> to achieve:
> 
> I have the following two rules.
> 
> info.h:       info.txt
>       sed -f info.sed info.txt > info.h
> 
> info.txt:
>       wget -N http://blabla/info.txt
> 
> The purpose is:
> * The dependency of info.h from info.txt extracts compilable 
>   information from a data source.
> * The rule for info.txt would retrieve that data source from 
> the Internet.
> -> However, I only want the second rule (wget) to be executed if the 
>    first target needs to be built (info.h does not exist).
>    If info.h already exists, I want the wget to be suppressed.
>    If info.h does not exist, I want wget to retrieve info.txt so 
>    that info.h can be generated.
>    (As a side-case, if info.txt does exist AND is newer than info.h, 
>    it would be reasonable to regenerate info.h as well, but actually 
>    that is of minor importance.)
>    The reason is that in a production environment, I cannot assume 
>    that Internet access is always available (or fast enough, as the 
>    files are quite large). So downloading should only be triggered if 
>    unavoidable.
>    (For an explicit update, another target might delete info.h, or 
>    might activate the info.txt rule explicitly.)
> 
> The intended behaviour in detail is:
> 
> info.h is newer than info.txt:
>       nothing happens
> 
> info.h is older than info.txt:
>       sed is invoked
> 
> info.h exists but info.txt does not exist:
>       nothing happens!
> 
> info.h does not exist but info.txt exists:
>       sed is invoked
> 
> info.h and info.txt both do not exist:
>       wget is invoked, then sed
> 
> Is this possible?
> 
> Thanks and kind regards,
> Thomas Wolff
> 
> 
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