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 <mail@xxxxxxxxx> 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?