Status message redirection:

  $ hg init empty

 status messages are sent to stdout by default:

  $ hg outgoing -R t empty -Tjson 2>/dev/null

  comparing with empty

  searching for changes

  [

   {

    "bookmarks": [],

    "branch": "default",

    "date": [0, 0],

    "desc": "test",

    "node": "acb14030fe0a21b60322c440ad2d20cf7685a376",

    "parents": ["0000000000000000000000000000000000000000"],

    "phase": "draft",

    "rev": 0,

    "tags": ["tip"],

    "user": "test"

   }

  ]

 which can be configured to send to stderr, so the output wouldn't be

 interleaved:

  $ cat <<'EOF' >> "$HGRCPATH"

  > [ui]

  > message-output = stderr

  > EOF

  $ hg outgoing -R t empty -Tjson 2>/dev/null

  [

   {

    "bookmarks": [],

    "branch": "default",

    "date": [0, 0],

    "desc": "test",

    "node": "acb14030fe0a21b60322c440ad2d20cf7685a376",

    "parents": ["0000000000000000000000000000000000000000"],

    "phase": "draft",

    "rev": 0,

    "tags": ["tip"],

    "user": "test"

   }

  ]

  $ hg outgoing -R t empty -Tjson >/dev/null

  comparing with empty

  searching for changes

 this option should be turned off by HGPLAIN= since it may break scripting use:

  $ HGPLAIN= hg outgoing -R t empty -Tjson 2>/dev/null

  comparing with empty

  searching for changes

  [

   {

    "bookmarks": [],

    "branch": "default",

    "date": [0, 0],

    "desc": "test",

    "node": "acb14030fe0a21b60322c440ad2d20cf7685a376",

    "parents": ["0000000000000000000000000000000000000000"],

    "phase": "draft",

    "rev": 0,

    "tags": ["tip"],

    "user": "test"

   }

  ]

 but still overridden by --config:

  $ HGPLAIN= hg outgoing -R t empty -Tjson --config ui.message-output=stderr \

  > 2>/dev/null

  [

   {

    "bookmarks": [],

    "branch": "default",

    "date": [0, 0],

    "desc": "test",

    "node": "acb14030fe0a21b60322c440ad2d20cf7685a376",

    "parents": ["0000000000000000000000000000000000000000"],

    "phase": "draft",

    "rev": 0,

    "tags": ["tip"],

    "user": "test"

   }

  ]

Invalid ui.message-output option:

  $ hg log -R t --config ui.message-output=bad

  abort: invalid ui.message-output destination: bad

  [255]

Underlying message streams should be updated when ui.fout/ferr are set:

  $ cat <<'EOF' > capui.py

  > from mercurial import pycompat, registrar

  > cmdtable = {}

  > command = registrar.command(cmdtable)

  > @command(b'capui', norepo=True)

  > def capui(ui):

  >     out = ui.fout

  >     ui.fout = pycompat.bytesio()

  >     ui.status(b'status\n')

  >     ui.ferr = pycompat.bytesio()

  >     ui.warn(b'warn\n')

  >     out.write(b'stdout: %s' % ui.fout.getvalue())

  >     out.write(b'stderr: %s' % ui.ferr.getvalue())

  > EOF

  $ hg --config extensions.capui=capui.py --config ui.message-output=stdio capui

  stdout: status

  stderr: warn