Re: Unexpected behaviour when using process substitution with stdout and stderr

[Alex fxmbsw7 Ratchev]

you printf basically to stdout from stderr, ..maybe its that
if you'd change your stderr printf to >&2 maybe works, ..just saying, maybe
or change the order to 2> and then >
..maybe..

peace

On Sun, Jul 11, 2021, 12:09 earnestly <zibeon@googlemail.com> wrote:

> GNU bash, version 5.1.8(1)-release (x86_64-pc-linux-gnu)
>
> I have attempted to use process substitution in order to feed the
> output of a command into two filters, one for handling stdout and the
> other for stderr.
>
> Prior to this I was using POSIX sh and named pipes to achieve this but
> decided to try bash with its >() and <() notation.
>
> Perhaps I am misunderstanding how this works because I found them to be
> unusable in this context.
>
> What appears to be happening is that the output from standard error is
> being mixed into the function handling standard out, even more
> surprisingly that xtrace output is also being consumed and filtered as
> well.
>
> I don't quite know how to describe what's happening so I have provided a
> small demonstration/reproducer instead:
>
> #!/bin/bash --
>
> generate() {
>     for ((i=0; i<10; ++i)); do
>         if ((RANDOM % 2)); then
>             printf 'generate to stdout\n'
>         else
>             printf 'generate to stderr\n' >&2
>         fi
>     done
> }
>
> stdout() {
>     local line
>
>     while read -r line; do
>         printf 'from stdout: %s\n' "$line"
>     done
> }
>
> stderr() {
>     local line
>
>     while read -r line; do
>         printf 'from stderr: %s\n' "$line"
>     done
> }
>
> # Using process substitution.
> unexpected() {
>     # This is particularly dangerous when the script is executed under
> `bash -x'
>     # as the xtrace output is read as standard input to the `stdout'
> function.
>     generate > >(stdout) 2> >(stderr)
>     wait
>
>     # Example output:
>     # from stdout: generate to stdout
>     # from stdout: generate to stdout
>     # from stdout: from stderr: generate to stderr
>     # from stdout: from stderr: generate to stderr
>     # from stdout: from stderr: generate to stderr
>     # from stdout: generate to stdout
>     # from stdout: from stderr: generate to stderr
>     # from stdout: generate to stdout
>     # from stdout: from stderr: generate to stderr
>     # from stdout: from stderr: generate to stderr
> }
>
> # Using named pipes.
> expected() {
>     mkfifo a b
>     trap 'rm a b' EXIT
>
>     generate > a 2> b &
>
>     stdout < a &
>     stderr < b &
>     wait
>
>     # Example output:
>     # from stdout: generate to stdout
>     # from stderr: generate to stderr
>     # from stdout: generate to stdout
>     # from stderr: generate to stderr
>     # from stdout: generate to stdout
>     # from stdout: generate to stdout
>     # from stderr: generate to stderr
>     # from stdout: generate to stdout
>     # from stdout: generate to stdout
>     # from stderr: generate to stderr
> }
>
>