Re: Parameter assignment in a redirection word

Regardless of the rest, this construct:

    $ (echo foo) | tee ${myvar:=tmpfile}

is expected to set the var in zsh (and also real AT&T ksh) but not bash.
That's because the right side of a pipe in bash is always in a subshell,
while the left side of the pipe is in the subshell in ksh and zsh.

But yeah, I can see no reason why the assignment would be subshelled in a
normal pipeless redirection from a subshell. Especially since the
subshellless version works, meaning it isn't getting subsumed by the "set
just for this command" behavior of bare assignments.


On Sun, Oct 26, 2025 at 5:17 AM Alexey Sukhoguzov <[email protected]> wrote:

> Hi,
>
> Please help me to clarify this situation:
>
>         $ (echo foo) > ${myvar:=tmpfile}
>         $ echo ${myvar:-still unset}
>         still unset
>
> Why $myvar is set in a subshell instead of a parent process in this
> case?  I was unable to find where it's documented, but I've checked
> some other shells, and of course it turned out that zsh is not alone
> here, at least bash is doing the same.  OTOH, FreeBSD sh, oksh and
> dash all seem to set the variable in a parent.  I was thinking that
> the latter is the only expected behavior, just like in
>
>         $ echo foo > ${myvar:=tmpfile}
>         $ echo ${myvar:-still unset}
>         tmpfile
>
> which, by the way, works identically in all the shells that I've
> tested.  But after I tried
>
>         $ (echo foo) | tee ${myvar:=tmpfile}
>
> and found that $myvar is set by a parent in zsh only, I'm not sure
> what to expect any longer.  Could anyone shed some light on what's
> going on here?
>
>

-- 
Mark J. Reed <[email protected]>