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Re: [PATCH] Define $as_echo and $as_echo_n for backward compatibility.

From: Eric Blake
Subject: Re: [PATCH] Define $as_echo and $as_echo_n for backward compatibility.
Date: Fri, 13 Mar 2020 14:32:23 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 3/13/20 2:22 PM, Zack Weinberg wrote:
On Fri, Mar 13, 2020 at 3:13 PM Eric Blake <address@hidden> wrote:

Unpatched bash 5.0 has a bug where calling $as_echo that contains \ can
result in unintended globbing, where the behavior of the expansion is
dependent on the contents of the current directory.  Nasty!

Yikes!  And not just unpatched 5.0.0, either...

$ bash --version
GNU bash, version 5.0.16(1)-release (x86_64-pc-linux-gnu)
$ touch %sn; bash -c 'cmd='\''printf %s\n'\''; $cmd test'

I don't see any practical workaround and I surely hope this gets fixed
*properly* soon in bash (that discussion you linked to doesn't seem
terribly promising, though).

You can always make $as_echo expand to the name of a shell function rather than to something that directly contains \, but then we have to worry about making sure the function is defined before anyone that uses $as_echo...

I'm wondering if Chet has an update on the matter (adding bug-bash). Repeating some context:
was a mail to the POSIX folks last June complaining about how bash 5.0's change to allow \ to trigger globbing has unintended consequences, and breaks many existing configure scripts based on the contents of the current directory. I know the Austin Group finally settled on wording that does indeed explicitly state that unpatched bash 5.0 is buggy:

On page 3749 line 128725 section C.2.13.3, add a new paragraph:

Patterns are matched against existing filenames and pathnames only when the pattern contains a '*', '?' or '[' character that will be treated as special. This prevents accidental removal of backslash characters in variable expansions where generating a list of matching files is not intended and a (usually oddly named) file with a matching name happens to exist. For example, a shell script that tries to be portable to systems that predate the introduction of functions and printf might use this on POSIX systems:

    myecho='printf %s\n'

    to be used as:

    $myecho args...

If <tt>%s\n</tt> were to be matched against existing files, this would not work if a file called <tt>%sn</tt> happened to exist.

but I don't know where things stand in bash proper to incorporate the result of that discussion (is it something fixed in unreleased batch, and we are just missing a formal patch to the 5.0 series, or is it still unwritten, or...?).

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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