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Re: [Help-bash] Expand a prompt string to calculate its length?

From: Greg Wooledge
Subject: Re: [Help-bash] Expand a prompt string to calculate its length?
Date: Wed, 19 Jun 2019 15:21:34 -0400
User-agent: Mutt/1.10.1 (2018-07-13)

On Wed, Jun 19, 2019 at 11:57:28AM -0700, Andy Chu wrote:
> 1. colors and ANSI (compensated for by \[ and \] as mentioned)
> 2. Unicode characters.  All shells appear use wcwidth() or some
> variant to compute this.
> So depending on the complexity of the prompt, there are 2 issues to overcome.
> AFAIK there's not an easy pure-bash solution for either problem, short
> of reimplementing PS1, which is probably technically possible in bash
> but something you don't want to do.  I did this when implementing
> bash-compatible prompts for Oil [1].

The @P operator in bash 4.4 overcomes the first, and bash itself already
handles the second.

wooledg:~$ shopt -s extglob
wooledg:~$ glob=$'\x01*([!\x02])\x02'
wooledg:~$ NOTPS1='foo\[xyz\]ñä'
wooledg:~$ tmp=${NOTPS1@P}; tmp2=${tmp//$glob/}
wooledg:~$ echo "<$tmp> <${#tmp}> <$tmp2> <${#tmp2}>"
<fooxyzñä> <10> <fooñä> <5>

I didn't test with characters outside the Unicode BMP, but this case
seems to work.  Note that the <$tmp> piece actually contains two
invisible characters (01 and 02), hence the length of 10.  <$tmp2>
contains 3 ASCII characters and 2 UTF-8 characters, and is counted with
a length of 5.

Is it pretty?  No.  But it's not totally ridiculous.

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