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Re: read with parameter expansion in here string


From: Chet Ramey
Subject: Re: read with parameter expansion in here string
Date: Fri, 22 Nov 2013 21:25:25 -0500
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:24.0) Gecko/20100101 Thunderbird/24.0.1

On 11/22/13, 9:22 AM, Clint Hepner wrote:

> Bash Version: 4.2
> Patch Level: 45
> Release Status: release
> 
> Description:
> 
>         An unquoted parameter expansion in a here string does not seem to 
> conform with my understanding of how
>         read works with a local IFS override. Personally observed in bash 
> 3.2.51, 4.1.2 as well. I first learned
>         of this possible bug via http://stackoverflow.com/q/20144593/1126841.
> 
> Repeat-By:
>         $ var="hello:world"
> 
>         # Case (1) I would expect "hello:world" in var1 if $var isn't split, 
> or "hello"
>         # if it is. World splitting seems to occur after var1 is set, which 
> doesn't
>         # seem to make sense.
>         $ IFS=: read var1 var2 <<< $var
>         $ echo "$var1"
>         hello world

The read builtin gets "hello:world" as input; $var is not split using the
value of IFS passed to `read'.  You can see this by using something like

IFS=: cat <<< $var


The idea is that read acts as it usually does: it reads a line of input
(hello:world), splits it into words on $IFS (:), resulting in multiple
words (hello world), and assigns the words to each variable in turn,
leaving $var1 as `hello'.

There is a bug in bash-4.2 involving cached values of $IFS that causes
your case (1) to not work right.  It was fixed in January, 2013 and will
be in bash-4.3.  If you want to look at the devel branch changelogs, there
is a more complete explanation.

Chet
-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/



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