bug#25485: 25.1; Having trouble setting --time-style in dired-listing-switches

[Eli Zaretskii]

> From: Chunyang Xu <mail@xuchunyang.me>
> Cc: 25485@debbugs.gnu.org
> Date: Fri, 20 Jan 2017 01:20:45 +0800
> 
> 
> Eli Zaretskii writes:
> 
> >> From: Chunyang Xu <mail@xuchunyang.me>
> >> Date: Thu, 19 Jan 2017 22:34:31 +0800
> >>
> >> I want to custom the file timestamps used in dired through the option:
> >>
> >>   --time-style='+%_m月 %d %H:%M'
> >>
> >> then I adjust `dired-listing-switches' accordingly:
> >>
> >>   (setq dired-listing-switches "-al --time-style='+%_m月 %d %H:%M'")
> >>
> >> However, it doesn't work, it signals an error, here is the Backtrace:
> >
> > Isn't --time-style a GNU ls extension?  You are on Darwin, so do you
> > have GNU ls installed?  What happens if you use that option from the
> > shell prompt?
> 
> Yes, and yes, and I have set shell and Emacs to use GNU ls, thus in
> shell (via M-x shell) it works like expected:

Your setting of dired-listing-switches assumes that the 'ls' command
will be run by a shell: you use shell quoting.  But that's not what
Dired does, it in most cases invokes 'ls' directly, and for that it
needs to split the switches into individual options.  It does that by
calling split-string on the value of dired-listing-switches, which of
course knows nothing about the '..' quoting.  That's why 'ls' fails:
it gets the --time-style option split between several separate
command-line arguments.

What this means is that currently Dired doesn't support switches that
include embedded whitespace at all.  One way of lifting at least some
of that limitation is the simple patch below; it will require you to
format your switches like this:

  (setq dired-listing-switches "-al \"--time-style=+%_m月 %d %H:%M\"")

The function combine-and-quote-strings can help in producing the
correct format.  Given that we document this, is this an acceptable
solution?  (It won't work on MS-Windows, but the default there is not
to call external programs at all, so I think it's not a terrible loss,
for those Windows users who set up their systems to use 'ls'.)

Here's the proposed patch; comments are welcome:

diff --git a/lisp/files.el b/lisp/files.el
index 25392fd..0a7b6a2 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -6572,7 +6572,7 @@ insert-directory
                              (unless (equal switches "")
                                ;; Split the switches at any spaces so we can
                                ;; pass separate options as separate args.
-                               (split-string switches)))
+                               (split-string-and-unquote switches)))
                            ;; Avoid lossage if FILE starts with `-'.
                            '("--")
                            (progn