Re: How to pipe just stderr to stdin in a pipeline?

[Koichi Murase]

2020-05-12 1:25 Peng Yu <address@hidden>:
> Hi Chet, These walkarounds seem to be cumbersome.
>
> Given | and |& are available, for the reason of symmetry, could an
> operator be introduced to bash to make piping stderr only in a
> pipeline easy?

I think there is not so much demands to support it as a builtin
feature like `|&'.  Even if it is cumbersome to write them directly,
one can always write a shell function to wrap these complicated
redirections and pipes rather than request them to be builtin
features.

2020-05-10 12:24 Peng Yu <address@hidden>:
>
> Thanks. The original stdout must be redirected back to stdout (see
> "2>&1"). I have to use something like this to do it. This looks quite
> cumbersome.
>
> $ { { echo error 1>&2; echo out; } 3>&1 1>&2 2>&3 | awk -e '{ print
> "awk:", $0 }'; } 2>&1 | awk -e '{ print "awk2: " $0 }'
> awk2: out
> awk2: awk: error

If you want to achieve the above pipe structures, you can actually use
"pipesubst" in the following post.

https://lists.gnu.org/archive/html/help-bash/2020-04/msg00017.html

  $ cat a.sh
  f1() {
    echo stderr 1>&2
    echo stdout
  }

  echo cumbersome
  { f1 3>&1 1>&2 2>&3 |
    awk -e '{ print "awk1: ", $0 }'; } 2>&1 |
    awk -e '{ print "awk2: " $0 }'

  echo pipesubst
  source pipesubst.sh
  pipesubst 'f1 2>&$fd1' \
            $'> awk \'{ print "awk1: ", $0 }\'' |
    awk '{ print "awk2: " $0 }'

  $ bash a.sh
  cumbersome
  awk2: stdout
  awk2: awk1:  stderr
  pipesubst
  awk2: stdout
  awk2: awk1:  stderr

----------------------------------------------------------------------

>> On Sun, May 10, 2020 at 09:43:48PM +0200, Andreas Kusalananda Kähäri wrote:
>>> On Sun, May 10, 2020 at 01:58:16PM -0500, Peng Yu wrote:
>>> > That is not a pipeline. And, different from a pipeline, the status in
>>> > >(cmd) can not be gotten easily.
>>>
>>> Maybe you should have stated you requirements from the start?

I think these questions are related the following threads.  Peng
doesn't explain the requirements explicitly, but one can guess them to
some extent.  Nevertheless these background discussions should have
been listed and summarized first.

https://lists.gnu.org/archive/html/help-bash/2020-03/msg00071.html
https://lists.gnu.org/archive/html/help-bash/2020-04/msg00006.html
https://lists.gnu.org/archive/html/help-bash/2020-04/msg00014.html

--
Koichi