Richard Stallman <rms@gnu.org> writes:

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> [[[ whether defending the US Constitution against all enemies,     ]]]
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>
>   > > If two different shells will try to write history into one single file,
>   > > are they doomed to give bad results, one way or another...
>
>   > Not necessarily.  If both shells use a single write() syscall on an
>   > O_APPEND file, they should work as expected to my awareness.
>
> We are miscommunicating.  The way you expect it to work is, in my
> opinion, a bad result -- various histories interspersed.
>
> It seems to me that the crucial thing is for each Bash process
> to have its own separate history.
>
> Do you think that behavior would be bad?

Bad?  No.  It's not what I'd prefer, though.

Note that, with either separate or interspersed histories, history
should never be lost, so, if a file is being shared by multiple shells
(even if it is not continuously re-read), care should be taken not to
lose data.

>   > If a bash process decides to rotate the history file as a result of
>   > HISTSIZE, and another bash process decides to do the same, one of their
>   > new history entries would be lost due to the other one overriding it.
>   > This would be a bug.
>
> Only if they share one single history file.  If each has its own
> history file, each can handle it as if it were your only Bash process.

Indeed.  How would these histories be recalled, though?  Which file does
a new shell read?
-- 
Arsen Arsenović