I'm not sure what you're trying to say here.

With GNU cp on Linux, the commands

  cp <( ( echo foo; sleep 1; echo bar) | sponge ) foobar
  cp =( ( echo foo; sleep 1; echo bar) | sponge ) foobar

both have the same result: the file "foobar" containing the string "foo\nbar\n".

I didn't test it. You're right, I'm simply mistaken.

Here's a better (but awkward) illustration of the problem:

  :~$ python3 -c 'import sys; import shutil; shutil.copy(sys.argv[1], "foobar")' <
  (echo foo ; sleep 1 ; echo bar)
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/opt/homebrew/Cellar/python@3.9/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/shutil.py", line 427, in copy
      copyfile(src, dst, follow_symlinks=follow_symlinks)
    File "/opt/homebrew/Cellar/python@3.9/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/shutil.py", line 257, in copyfile
      raise SpecialFileError("`%s` is a named pipe" % fn)
  shutil.SpecialFileError: `/dev/fd/11` is a named pipe
  :~$ python3 -c 'import sys; import shutil; shutil.copy(sys.argv[1], "foobar")' =(echo foo ; sleep 1 ; echo bar)

Notice the difference:

  :~$ echo =(echo foo ; sleep 1; echo bar)
  /tmp/zshbusU5D
  :~$ echo <(echo foo ; sleep 1; echo bar)
  /dev/fd/11

/dev/fd/11 is a named pipe, whereas /tmp/zshbEF1D9 is a plain file. There are situations where named pipes fail, where you actually do need a plain file, and I can't think of a single one offhand. :)

"I've run into it in the past" is all I can say. And the above snippet at least gives one specific example of a hypothetical case.

> There are situations where named pipes fail, where you actually do need a plain file, and I can't think of a single one offhand. :)

My go-to use-case for =() is

  okular =(dot -Tpdf foo.dot)

PDF-viewers and its ilk typically don't work on non-files.

> There are situations where named pipes fail, where you actually do need a plain file

In general: If the command you are passing the named pipe to expects a file which it can randomly access, a named pipe won’t work.

I ran into this today using a python tool, yamldiff. I was trying to diff a local yaml file with one from a cluster obtained via kubectl, eg:

yamldiff localManifest.yaml <(kubectl get … —output yaml)

and it failed with python telling me the file wasn’t seekable. Then I remembered this thread which I had initially read a few days ago and tried =() (since I use shell) — and it worked!