Re: why generate output to tmp file and mv to actual file?

[Bob Friesenhahn]

On Fri, 1 Jun 2018, John Calcote wrote:

> I recently ran across some sample code in section 19.4 of the Autoconf
> manual (modified slightly to reduce example):
>
> $(TESTSUITE): $(srcdir)/testsuite.at $(srcdir)/package.m4
>        autom4te --language=autotest -I '$(srcdir)' -o address@hidden 
> address@hidden
>        mv address@hidden $@
>
> This question isn't about autotest, but rather about the two commands in
> this rule - why generate the output into address@hidden and then mv 
> address@hidden into address@hidden
> Is there some power mv has over autom4te that allows it better access to
> the target under some conditions?

A reason to do such a thing is to avoid leaving a 
partial/corrupt output file in place due to the command writing it 
encountering an error.  The assumption is the the 'mv' command would 
not be executed if the previous command encountered an error.

Another reason would be in a multi-processing environment (e.g. 
parallel compile) where the file content (or its timestamp) might be 
consumed before it is ready.  The 'mv' is an atomic operation so once 
the file is in its final location, it is assured to be completely 
written.

Bob
--
Bob Friesenhahn
address@hidden, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/