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I'm trying to write a Makefile which should download some sources if and only if they are missing.
Something like:
hello: hello.c
gcc -o hello hello.c
hello.c:
wget -O hello.c http://example.org/hello.c
But of course this causes hello.c to be downloaded every time make command is run. I would like hello.c to be downloaded by this Makefile only if it is missing. Is this possible with GNU make and how to do this if it is?
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$@ is the name of the target being generated, and $< the first prerequisite (usually a source file). You can find a list of all these special variables in the GNU Make manual.
A rule appears in the makefile and says when and how to remake certain files, called the rule's targets (most often only one per rule). It lists the other files that are the prerequisites of the target, and the recipe to use to create or update the target.
The Cleanup Rule clean: rm *.o prog3 This is an optional rule. It allows you to type 'make clean' at the command line to get rid of your object and executable files. Sometimes the compiler will link or compile files incorrectly and the only way to get a fresh start is to remove all the object and executable files.
When you type make or make [target] , the Make will look through your current directory for a Makefile. This file must be called makefile or Makefile . Make will then look for the corresponding target in the makefile. If you don't provide a target, Make will just run the first target it finds.
My guess is that wget doesn't update the timestamp on hello.c, but retains the remote timestamp. This causes make to believe that hello.c is old and attempts to download it again. Try
hello.c:
wget ...
touch $@
EDIT: The -N option to wget will prevent wget from downloading anything unless the remote file is newer (but it'll still check the timestamp of the remote file, of course.)
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