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Re: Handling of Zero-Width escapes in PS1


From: Dennis Williamson
Subject: Re: Handling of Zero-Width escapes in PS1
Date: Sat, 4 May 2019 18:00:00 -0500

On Sat, May 4, 2019, 3:39 PM <address@hidden> wrote:

> Configuration Information [Automatically generated, do not change]:
> Machine: x86_64
> OS: linux-gnu
> Compiler: gcc
> Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64'
> -DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-pc-linux-gnu'
> -DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL
> -DHAVE_CONFIG_H   -I.  -I../. -I.././include -I.././lib  -Wdate-time
> -D_FORTIFY_SOURCE=2 -g -O2
> -fdebug-prefix-map=/build/bash-vEMnMR/bash-4.4.18=.
> -fstack-protector-strong -Wformat -Werror=format-security -Wall
> -Wno-parentheses -Wno-format-security
> uname output: Linux desktop 5.0.3-050003-generic #201903191333 SMP Tue Mar
> 19 13:35:24 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
> Machine Type: x86_64-pc-linux-gnu
>
> Bash Version: 4.4
> Patch Level: 19
> Release Status: release
>
> Description:
>         When setting up a complex PS1 prompt where parts of the
>         prompt is expanded on every new prompt displayed by bash
>         the escape sequences that are generated by that dynamic
>         part are handled improperly (read: not at all).
>
>         The desired behaviour should treat such escapes for e.g.
>         the sequences \[ and \] equal both when the prompt expands
>         to stuff\[stuff\]stuff, regardless of whether this was
>         reached by "stuff$(echo \[stuff\])stuff" (\[\] handled) or
>         reached by 'stuff$(echo \[stuff\])stuff' (not handled) being
>         set as the value of PS1.
>
> Repeat-By:
>         To demonstrate this odd behaviour assume some command
>         that displays changing information at each invokation:
>
>         __test1(){ echo -e "Random is \\[\e[37;1m\\]$RANDOM\e[0m"; }
>
>         Now let's assume two cases:
>
>         Case 1:
>         PS1='Test$(__test1)Test\[\e[0m\]$ '
>         -> Shows \[\] markers, includes output of $(__test1) dynamically
>
>         Case 2:
>         PS1="Test$(__test1)Test\[\e[0m\]$ "
>         -> Displays correct, but statically includes output of $(__test1)
>
>         The desired behaviour is dynamic inclusion of $(__test1) as with
>         case 1 while having the \[\] markers respected as seen in case 2.
>
>         Splitting the output of __test1 to separate sections with
>         \[\] markers and such which don't need them is not possible,
>         as the number of such sections may vary and the limited syntax
>         of PS1 does not allow for loops and other conditional constructs
>         on its top level.
>
>         Also leaving out the \[\] markers from __test1 will cause the
>         displayed prompt to misbehave.
>
> Fix:
>         Move handling of \[ and \] markers to be done on the fully
>         expanded, representation of the PS1 variable after variable
>         substitutions and subshell output have been inserted.
>

I compute complex strings and assign PS1 within PROMPT_COMMAND rather than
expecting PS1's promptvars command substitution to do that - since
backslash escapes are done earlier. See "PROMPTING" in the man page where
the order of processing is described.

>
>


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