Re: suggestion: $LS_ARGS

[Eric Blake]

On 02/27/2018 01:38 AM, Harald Dunkel wrote:

> please consider
>
> - there are tons of magic vars in wide use today: LANG and
>    LC_xxx,

These are required by POSIX.  They are well-documented, and portable 
scripts already know they exist.

> LD_LIBRARY_PATH,

This one is already relatively well-known, and is frequently mentioned 
as something that must be sanitized.

> LS_COLORS,

This one does NOT change default behavior.  Rather, it changes the 
behavior when a non-default option is specified.

> TAPE, etc. Some of them
>    are pretty close to a vulnerability, but they are in daily
>    use, still.

Our complaint is that we shouldn't be making the list longer, by 
introducing NEW variables.  Existing variables are harder to eradicate, 
but we don't want to make the mess bigger.

> - AFAICS coreutils' ls (as of 8.29) is not compatible to the
>    "standard" ls anyway, already changing its behavior on the
>    fly, unless (!) you define QUOTING_STYLE.

Changing interactive output on the fly does not violate POSIX, or we 
would not have made the change.  Setting QUOTING_STYLE indeed risks 
making ls change what it produces in non-interactive output, but only 
for filenames that are not portable to POSIX in the first place.

> Sample:
>
>      % touch /tmp/a\ b
>      % ls -1 /tmp/a\ b
>      '/tmp/a b'
>      % src/ls -1 /tmp/a\ b | cut -d\' -f1
>      /tmp/a b
>      % export QUOTING_STYLE=shell
>      % src/ls -1 /tmp/a\ b | cut -d\' -f1
>
>      %
>
> When did pipes become unreliable?

You just demonstrated why environment variables should NOT be relied on 
to change default behaviors - the fact that the pipe produced different 
output according to QUOTING_STYLE means that QUOTING_STYLE can silently 
trip up scripts, and therefore scripts should probably sanitize 
QUOTING_STYLE if they expect to parse ls output (although parsing ls 
output is already a questionable activity).

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org