Re: What is the difference between > and >> in a makefile.

[givemecode]

Paul Smith-20 wrote:
> 
> 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 address@hidden
> 
> 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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> 
> 

thanks to both of you. so the second version doesn't only write after the
for loop is complete? i guess that's just how i am reading it since it is
after the "done" keyword. what about this line tells the redirect to only
print the one line above it: "done > '$@'" ? 

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