Re: IFS not fixing carriage returns

[Eric Blake <eblake at redhat dot com>]

On 03/25/2011 10:35 AM, Tim McDaniel wrote:
> Although it's not documented in "man bash" on the latest Cygwin, I did
> find "set -o igncr" and it seems to work well.

I documented it in the cygwin bash release notes, so it is in
/usr/share/doc/Cygwin/bash.README.  Patches welcome if you want to see
it somewhere else like the man page, because that takes more effort.

> 
> But I'm just curious about why my first attempt didn't work.

IFS only affects word splitting _after words have been parsed and
expansions performed_.  Bash does _not_ ignore CR in the input stream
while parsing words, regardless of the IFS settings.  Let's use a
simpler example:

If you set IFS=., then echo a.b will still only echo the one word "a.b"
and not split into two words "a" and "b", because '.' is not special in
determining word boundaries (a.b is a single word), and there was no
expansion going on.

And it would be prohibitively expensive to make shells honor random IFS
while parsing out words, so bash _only_ uses space, tab, and newline to
determine word boundaries, not carriage return.

That's why igncr is more powerful than IFS - it strips the CR prior to
the point that bash is even trying to parse the line into words.

-- 
Eric Blake   eblake@redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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