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Re: Feature request: PROMPT_COMMANDS array variable

From: Egmont Koblinger
Subject: Re: Feature request: PROMPT_COMMANDS array variable
Date: Sat, 3 Feb 2018 14:57:33 +0100


I'd like to second this feature request.

As Daniel mentioned, the biggest problem with the current single
variable approach that it's hardly usable due to lack of coordination
between distributions as well as apps that wish to modify
PROMPT_COMMAND. Let's look at it at more details.

There isn't a single entity somewhere who decides what should go into
this variable. Instead, various utilities all want to append their
stuff. My two open source hobby projects happen to be GNOME Terminal
(VTE) and Midnight Commander, both of them tamper with this variable.
VTE (from /etc/profile.d/vte-2.91.sh) adds the emission of OSC 7 to
notify the terminal emulator about the current working directory (and
also emits OSC 0 to set the window title which it probably shouldn't
do), see [1] and [2] for some pending (stalled) refactorings/bugfixes
around this area. Midnight Commander uses it for some subshell magic,
see [3] for a recent change.

So the use case is: Someone wants to _add_ something to
PROMPT_COMMAND. How to do that?

The obvious approach is to append to this, using semicolon as the
separator. Now, I don't think this can work. Is a non-empty
PROMPT_COMMAND supposed to contain a trailing semicolon or not? If you
expect parties to append to this variable without any coordination,
you cannot know which one others go for. If it doesn't contain a
trailing semicolon, you obviously MUST add one in order to append. It
if contains one, you MUST NOT add it, since two semicolons next to
each other (even with whitespaces between) is a syntax error. So shall
we figure out if there's already one?? Strip trailing whitespaces,
then see if there's a semicolon, then see if it's backslash-escaped,
then see if that backslash is backslash-escaped...?? Practically

So what can you do instead? You can append using newline as the
separator, works reliably, the slight drawback is that the output of a
simple, unquoted «echo $PROMPT_COMMAND» is pretty confusing. And, of
course, this is not a solution that suddenly occurs to everyone as
trivially as appending to an array. (Mind you, the previous problem is
probably not a problem that occurs to everyone, either.)

Or you can reliably prepend. As a consequence, if let's say you do it
from /etc/profile.d/*.sh, in the rare case when the order of files
there does matter (and hence the filenames are properly prefixed by
numbers), the order will be the exact opposite of the desired one.

Now, let's say a distribution sets up PROMPT_COMMAND to do a couple of
things, and as a user you don't like one of them. For example, you
don't like gnome-terminal setting up the window title for you (hell I
don't like that), or there are a few folks who don't like new windows
opening in the previous one's directory. So they'd want to _remove_ an
entry from PROMPT_COMMAND, ideally without having to manually
duplicate (and maintain through the years, through
installation/removal of packages etc.) the rest.

If whoever adds something to PROMPT_COMMAND thinks about it, and
solely invokes a function whose only purpose is to be invoked from
PROMPT_COMMAND and isn't used anywhere else, the user can redefine
this function to be empty. Without any lack of coordination, I don't
think many providers would think about it. So then you'd need to
remove an exact entire entry, between two unescaped separators
(semicolons or newlines) or the beginning/end of the string, hell no.

Or, just remove an entry from an array, which is much simpler.

Looking at an entire ecosystem, like a distribution with its shell
initialization files, and various apps shipping extensions to it,
maintaining a single scalar is just a total nightmare, maintaining an
array would be the clean way to solve this.

Let alone the case when someone adds something with syntax error, then
the rest (further additions) won't work either. I don't think it's an
important issue to worry about, but still.

(Sorry for not cc'ing those earlier on the thread, and for this email
not showing up nicely in the thread index, I have no clue how to
properly reply to an email just by seeing it in the web archives.)


[1] https://bugzilla.gnome.org/show_bug.cgi?id=704960
[2] https://bugzilla.gnome.org/show_bug.cgi?id=743073
[3] http://midnight-commander.org/ticket/3534

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