On Mon, 2011-03-28 at 11:15 -0700, givemecode wrote:
> I modified a for loop in a makefile that looked like this:
>
> @for f in $(DRV_GEN_HEADER) ; do echo "#include \""$$f"\"" >> $@ ; done
>
> which puts the output of echo into a new file I sepcified.
>
> changed to this:
>
> @for f in $(INCLUDE_DIR)/%.h); do \
> echo "#include \""$$f"\""; \
> done > '$@'
>
> which does the same thing but the writing is done after the loop completes
> (I think). Can someone please explain the difference and why it works?
This is a question about shell programming, not about makefiles and is
probably better asked in help list for shell hacking.
However, I can answer it for you :-)
Note that the latter does NOT do all the writing after the loop
completes. The file is written to, one line at a time, just as in the
first example.
The first example says for each file, echo this one line and append it
(hence ">>" which is append) to the file $@.
The second example says, for each file echo this one line, and redirect
all output from all commands run inside the for-loop to overwrite the
file $@ (hence ">" which is overwrite--i.e., truncate and write from
scratch).
Some differences between the two:
1. If the file $@ exists before the command is invoked, then the
first version will append the new content to the
already-existing content... this is probably not what you want
in this situation. The second version will always recreate the
file from scratch.
2. If you wanted to print some content to stdout and some to the
file, then you have to use the first version because the second
version will put all stdout from all commands in the for loop
into the file (well, you can play fancy tricks with file
descriptors but it's hardly worth it).
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