$ unset HGUSER

  $ EMAIL="My Name <myname@example.com>"

  $ export EMAIL

  $ hg init test

  $ cd test

  $ touch asdf

  $ hg add asdf

  $ hg commit -m commit-1

  $ hg tip

  changeset:   0:53f268a58230

  tag:         tip

  user:        My Name <myname@example.com>

  date:        Thu Jan 01 00:00:00 1970 +0000

  summary:     commit-1

  $ unset EMAIL

  $ echo 1234 > asdf

  $ hg commit -u "foo@bar.com" -m commit-1

  $ hg tip

  changeset:   1:3871b2a9e9bf

  tag:         tip

  user:        foo@bar.com

  date:        Thu Jan 01 00:00:00 1970 +0000

  summary:     commit-1

  $ echo "[ui]" >> .hg/hgrc

  $ echo "username = foobar <foo@bar.com>" >> .hg/hgrc

  $ echo 12 > asdf

  $ hg commit -m commit-1

  $ hg tip

  changeset:   2:8eeac6695c1c

  tag:         tip

  user:        foobar <foo@bar.com>

  date:        Thu Jan 01 00:00:00 1970 +0000

  summary:     commit-1

  $ echo 1 > asdf

  $ hg commit -u "foo@bar.com" -m commit-1

  $ hg tip

  changeset:   3:957606a725e4

  tag:         tip

  user:        foo@bar.com

  date:        Thu Jan 01 00:00:00 1970 +0000

  summary:     commit-1

  $ echo 123 > asdf

  $ echo "[ui]" > .hg/hgrc

  $ echo "username = " >> .hg/hgrc

  $ hg commit -m commit-1

  abort: no username supplied (see "hg help config")

  [255]

  $ rm .hg/hgrc

  $ hg commit -m commit-1 2>&1

  no username found, using '[^']*' instead (re)

  $ echo space > asdf

  $ hg commit -u ' ' -m commit-1

  transaction abort!

  rollback completed

  abort: empty username!

  [255]

  $ cd ..