Re: Prefixed backslash removed when completing command

[Mikael Auno <auno@kth.se>]

On 2010-10-27 12:02, Peter Stephenson wrote:
> On Wed, 27 Oct 2010 11:06:16 +0200
> Mikael Auno<auno@kth.se>  wrote:
>> If I am not completely misunderstanding your reply, I think you
>> misunderstood my issue. My issue has nothing to do with either single
>> or double quotes (they are only there in the original message to
>> separate the examples from the rest of the text), but with the
>> backslash.
>
> No, you've misunderstood my reply.
>
>> When I try to tab complete something starting with a leading
>> backslash, e.g. \mpla, then it is completed to mplayer (without a
>> leading backslash) instead of \mplayer (with a leading backslash) as
>> I would expect.
>
> Yes, I realise that.  Here's my reply rephrased.
>
> The internals of completion are complicated.  There is no prospect of it
> ever being able to preserve quotations in a general way.  In particular
> there is no prospect of it ever being able to retain backslashes that
> have no basic syntactic effect.
>
> Slight digression on what I mean by a "basic syntactic effect".  Aliases
> are a special case as they are expanded on input, so things that are
> usually ignored in the usual shell grammar become significant;
> completion doesn't know anything about this.  Completion just tries to
> muddle through and produce an expression that (ignoring the alias
> problem) is quoted in an appropriate way for the shell to process the
> word on the command line.
>
> If quoting is done using backslashes the only backslashes left will be
> those needed for special characters.  Completion makes no attempt to try
> to remember how the original word looked and reconstruct it.
>
> However, if quoting is done with a single or double quote character at
> the start of the word, that form of quoting is preserved even if it's
> not (apparently) needed to quote special characters.  In this case,
> completion remembers the whole word is quoted.  This is much simpler
> than trying to remember what happens character by character.
>
> So if you really insist on having a backslash, you are stuck, as far as
> completion is concerned.  (You still have the possibility of writing
> line editor, non-completion, functions to quote the word how you want.)
>
> However, *if* it happens to be the case that you are simply trying to
> ensure the word is quoted, somehow, you can get away with starting it
> with a single quote, and forget about the backslash.  The single quote
> is preserved during completion, unlike the backslash, and a closing
> quote appended.  So it all works, and you have completed a quoted word.
>
> I'm not aware of anywhere in the shell that requires you to quote a word
> as \mplayer rather than 'mplayer'.  They should have the same effect in
> every case except the bizarre one where you have defined 'mplayer',
> including the quotes, as an alias.  So if you started with a single
> quote (I know that's not what you asked about) it should do the right
> thing, or I'd like to hear why it doesn't.

You are completely correct. I misunderstood your reply as I did not know 
that the backslash was at all related to quoting or that ordinary 
quoting had the same result as prefixing a command with a backslash with 
respect to suppressing alias expansion. Prefixing with single or double 
quotes works great, especially as the closing quote is appended 
automatically.

Thank you for this insight,
Mikael Auno