Re: How to use PROMPT_COMMAND(S) without breaking other scripts

From	Martijn Dekker <martijn@inlv.org>
Newsgroups	gnu.bash.bug
Subject	Re: How to use PROMPT_COMMAND(S) without breaking other scripts
Date	2020-08-24 17:58 +0100
Message-ID	<mailman.1025.1598288325.2469.bug-bash@gnu.org> (permalink)
References	<CAKOZuevPZ1xwhLmLzXt_a=G+azDTouO52MCA=wvh=-M82wzaSQ@mail.gmail.com> <17a0ba52-32ee-b9bc-72ff-3587b2050fbd@case.edu> <CAFLRLk9vNPM5NtypB2UNcDqMZfG3Lmi3zEMGfPqZNacmHCoAoA@mail.gmail.com> <0c10b76f-a7f0-1c52-77b7-4b208035956b@case.edu> <c904f5b5-1312-ac4c-26bd-bfb97c1f43e8@inlv.org>

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Op 24-08-20 om 15:57 schreef Chet Ramey:
> I sometimes think I should have stuck with converting PROMPT_COMMAND to
> an array. Either way, there's going to be a transition, and maybe that
> would have been the easiest.

Is it too late? I think that would actually be cleaner than adding a 
separate array, per Koichi's report.

One problem is that if a script does the obvious

     PROMPT_COMMAND+=("some command here")

if PROMPT_COMMAND is not yet set, then it starts adding at array index 
0, so a subsequent traditional usage from some other script

     PROMPT_COMMAND="some command here"

would overwrite it. So array usage should not use the 0 index. To avoid 
using the 0 index, one possibility is:

     PROMPT_COMMAND[$(( ${#PROMPT_COMMAND[@]} + 1 ))]="some command here"

which, if PROMPT_COMMAND is unset, starts adding at index 1, not 0, and 
otherwise acts identically. However, 'set -u'/'set -o nounset' kills 
that. That option makes the ${#PROMPT_COMMAND[@]} expansion error out if 
there are no array elements (even though ${#@} works with no positional 
parameters). It's also an unwieldy command. So maybe that idea is not 
the best.

Another way to avoid using the 0 index, which is 'set -u' compatible, 
would be

     PROMPT_COMMAND=${PROMPT_COMMAND-}
     PROMPT_COMMAND+=("some command here")

The first command sets PROMPT_COMMAND[0] to the empty value if it 
doesn't exist yet, and otherwise leaves it unchanged. It's a bit of an 
ugly hack though.

But then, maybe it's best if bash itself just sets PROMPT_COMMAND[0] to 
the empty value on initialisation. IMO that would be a reasonably clean 
and reliable way to ensure a smooth transition.

Just my 2¢,

- M.

-- 
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||	https://github.com/modernish/modernish
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Re: How to use PROMPT_COMMAND(S) without breaking other scripts Martijn Dekker <martijn@inlv.org> - 2020-08-24 17:58 +0100

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