Re: ${var@P} expansion includes 0x01 and 0x02

[Dennis Williamson]

On Wed, Oct 28, 2015 at 8:55 AM, Chet Ramey <chet.ramey@case.edu> wrote:

On 10/27/15 5:02 PM, Greg Wooledge wrote:
> I decided to play around with the ${var@P} expansion in 4.4-beta.
>
> imadev:~$ red=$(tput setaf 1) reset=$(tput sgr0) x='\[$red\]\u\[$reset\]@\h:\w\$ '; printf %s "${x@P}" | od -t x1
> 0000000    1  1b  5b  33  31  6d   2  77  6f  6f  6c  65  64  67   1  1b
> 0000020   5b  6d   f   2  40  69  6d  61  64  65  76  3a  7e  24  20
> 0000037
>
> I don't think the "1" and "2" bytes should be printed.  They're for
> internal use only, even if they're usually invisible.

That's what the \[ and \] escape sequences expand to and use to
communicate information to readline about invisible characters in the
prompt (RL_PROMPT_START_IGNORE and RL_PROMPT_END_IGNORE).  If you want to
use the expansion of ${var@P} as, for instance, the prompt passed to
readline when using `read -e -p prompt', those characters need to be there.

--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    chet@case.edu    http://cnswww.cns.cwru.edu/~chet/

To follow on to what Greg said:

The only way I've found to output a string containing non-printing sequence delimiters using the @P transformation is to use read -e -p or to strip the \[ and \] first. All the other prompt escapes work in printf or echo -e when using @P.

To reproduce:

red=$(tput setaf 1)

none=$(tput sgr0)

greet='\[$red\]Hello\[$none\]'

printf '%s\n' "${greet@P}"

echo -e "${greet@P}"

read -e -p "${greet@P}"

Naively stripping the delimiters in this case would leave $redHELLO$none where $redHello is unset or worse.

Granted it makes no sense to have non-printing sequences delimited when readline isn't involved, but it makes things simpler if you're dealing with a pre-existing string.

So if @P needs to output \001 and \002 then perhaps there needs to be an @p which doesn't (unless there's a way to determine whether readline is involved and only output the delimiters in that case).

--

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