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Re: [bug] PROMPT_COMMAND is not executed as expected in some situations


From: Franklin, Jason
Subject: Re: [bug] PROMPT_COMMAND is not executed as expected in some situations
Date: Sat, 18 Apr 2020 09:00:44 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0

On 4/16/20 10:11 AM, Chet Ramey wrote:
> On 4/16/20 9:21 AM, Franklin, Jason wrote:
>> On 4/15/20 5:35 PM, Chet Ramey wrote:
>>> These all pretty much all fall into the category of the editor reprinting
>>> the prompt before it returns.
>>
>> Hmmm...
>>
>> So, is there no way to have Bash handle the printing of the prompt after
>> the return of the command and of the editor?
> 
> Bash doesn't have control at that point. It's readline running a bindable
> function. It's as if you hit ^L when the readline line buffer was empty:
> readline reprints the prompt, but should it call back to bash somehow --
> through a hook function that doesn't currently exist -- to execute
> PROMPT_COMMAND?
> 
>> This would allow PROMPT_COMMAND to function as expected.  It is
>> surprising to see status indicators in my prompt NOT be updated when
>> using <C-x><C-e>.
> 
> I don't think that most people expect PROMPT_COMMAND in that situation.
>> I thought that this would be easy.  Is it really not possible to fix?
> 
> I'm saying it's not a bug at all, and doesn't need fixing.

As a programmer, I can understand that this may be a tough fix.
However, as a user, I must say that I expect PROMPT_COMMAND to be run
whenever PS1 is printed to prompt me for a new command.  The bash man
page makes this clear. To quote:

PROMPT_COMMAND
  If set, the value is executed as a command prior to issuing each
  primary prompt.

My PROMPT_COMMAND does a lot of neat stuff.  It works flawlessly too,
but obviously not after using, for example, <C-x><C-e>, because it
doesn't get invoked then.  This means that indicators about the most
recent command are not updated when I open an editor to enter a command
(or use histverify or show completions).

A simple example where the prompt is not updated as expected (this one
doesn't involve PROMPT_COMMAND, but shows why a full prompt update is
desired):

$ bash --norc
$ PS1='[$?] '"$PS1"
# now, exit status is in prompt, which is pretty useful
# use <C-x><C-e> to edit and enter command "false" and save and quit
# the exit status should be [1], but is [0], so the indicator is wrong

I hope this clarifies my point of view on this issue.  It may be a tough
ask, so I don't expect a ton of effort to fix this if others aren't
complaining.

In any case, thanks so much for all of your magnificent work on Bash.
I've been using Bash for years, and I don't plan to stop!  I wish I had
the knowledge of Bash internals to just code up a patch.  I tried, but
it was to no avail. :(

Thanks!

-- 
Jason Franklin



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