Re: Backslash mysteriously disappears in command expansion when unescaping would reference an existing file

[Charles-Henri Gros]

On 5/22/19 5:43 AM, Greg Wooledge wrote:
> On Wed, May 22, 2019 at 05:25:43PM +0700, Robert Elz wrote:
>>     Date:        Tue, 21 May 2019 22:11:20 +0000
>>     From:        Charles-Henri Gros <Charles-Henri.Gros@synopsys.com>
>>     Message-ID:  
>> <B51A83F93B459B479B6C4C3FBFE3F9310156F52725@us01wembx1.internal.synopsys.com>
>>
>>   | The existence or not of the file should not have any effect.
>>
>> But it does, and is intended to.   If the mattern matches a file
>> (when patyhname expanded as a result of the unquoted command substitution)
>> you get the file name produced.   If it does not match a file,
>> the pattern is left untouched.   That is the way that things are
>> supposed to work.
> With glob metacharacters, sure.  But none of the characters in his
> variable are glob metacharacters.
>
> There is definitely something weird happening here.
>
> wooledg:/tmp/x$ echo "$BASH_VERSION"
> 5.0.3(1)-release
> wooledg:/tmp/x$ touch 'a$.class'
> wooledg:/tmp/x$ i='a\$.class'; echo {$i} "{$i}"
> {a\$.class} {a\$.class}
> wooledg:/tmp/x$ i='a\$.class'; echo $i "{$i}"
> a$.class {a\$.class}
>
> Other versions of bash, plus ksh and dash, don't behave this way.
>
> wooledg:/tmp/x$ bash-2.05b
> wooledg:/tmp/x$ i='a\$.class'; echo $i "{$i}"
> a\$.class {a\$.class}
>
> wooledg:/tmp/x$ bash-4.4
> wooledg:/tmp/x$ i='a\$.class'; echo $i "{$i}"
> a\$.class {a\$.class}
>
> wooledg:/tmp/x$ ksh
> $ i='a\$.class'; echo $i "{$i}"
> a\$.class {a\$.class}
>
> wooledg:/tmp/x$ dash
> $ i='a\$.class'; echo $i "{$i}"
> a\$.class {a\$.class}
>
> It seems to be unique to bash 5.  If it's a bug fix, then I'm not
> understanding the rationale.  Backslashes shouldn't be consumed during
> glob expansion.
>
> This is also not limited to $ alone.  It happens with letters too.
>
> wooledg:/tmp/x$ touch i
> wooledg:/tmp/x$ i='\i' j='\j'
> wooledg:/tmp/x$ echo $i $j
> i \j
>
> Standard disclaimers apply.  Stop using unquoted variables and these
> bugs will stop affecting you.  Nevertheless, Chet may want to take a
> peek.

What unquoted variables? Are you talking about the "$()" expansion?

The problem I'm trying to solve is to iterate over regex-escaped file
names obtained from a "find" command. I don't know how to make this
work. It works with other versions of bash and with other shells.

The original is closer to something like this:

for file in $(find ... | sed 's/\$/\\$/g'); do grep -e "$file"
someinput; done

It used to work. Now it doesn't. I do not know how to make it work again.


-- 
Charles-Henri Gros