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 2ยข,
- M.
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