解释这个 bash 重定向行为

Question

I was experimenting with redirections and pipes and did not understand some behaviour. I have this snippet to generate one line on each of stderr and stdout:

(echo stdout; echo stderr 1>&2)

It seems to work correctly, (try >/devnull and 2>/dev/null).

But the behaviour of these three commands confuses me:

(echo stdout; echo stderr 1>&2) > >(wc -l)
stderr
1
(echo stdout; echo stderr 1>&2) 2> >(wc -l)
stdout
1
(echo stdout; echo stderr 1>&2) > >(wc -l) 2> >(wc -l)
2
(empty)

Why does the last command combine the two streams? Or what else is happening to break my brain?

Answer 1

because the second wc -l derives stdout from current command (stdout already redirect to fist wc -l ), its output is also passed to first wc -l .

 IN   +-----------+   1>    +---------+         OUT
+-+--->echo stdout+----+---->  wc -l  +------------->
  |   +-----------+    ^    +---------+
  |                    |
  |                    |
  |                    +<------------------+
  |                                        |
  |                                        |
  |   +-----------+   2>    +---------+    |
  +--->echo stderr+--------->  wc -l  +----+
      +-----------+         +---------+

Answer 2

It is something like

$ { echo stdout; echo stderr 3>&2 2>&1 1>&3 >&2 | wc -l; } | wc -l
2