Re: Arithmetic expansion with increments and output redirection

[Ian Neal]

>     Date:        Tue, 23 Apr 2019 15:49:18 -0600
>     From:        Ian Neal <wopr@wopr.tech>
>     Message-ID:  <CADBEB3qA0J1-j=iO5t_63h-Ksea1h6YGRnuU=
> 1_Z7uy+7rvycw@mail.gmail.com>
>
>   | When using arithmetic expansion with variable pre- and
>   | post-increments/decrements in the output redirection file path,
>   | specifically on external executables (not builtins or functions), the
> state
>   | of the variable being incremented/decremented is not persisted in the
>   | environment.
>
> That is as it should be, redirects are eveluated in the sub-shell
> context.   If anything the bug you showed is that redirects when the
> shell does not fork are being evaluated in the context of the shell
> (but I think that might be an unspecified case).
>
> In general it is best not to even consider using any evaluation with
> side effects in any redirection, and unless you really consider what
> you're doing, not in var-assigns either (there, if you're not expecting
> the side effects to be visible in the same, or any other, assignemnt
> in the same command you should be OK).
>
> kre
>

At what point is a subshell being invoked? There's no pipeline, command
substitution, coprocess, background process, or explicit () subshell here,
which are the only cases a subshell should be created. Otherwise, the
entire operation should be evaluated by the main shell. Arithmetic
expansion is not in that list.

>