I recently noticed that a test for an expected failure was added around
the time of this bug report[1: ea8c0e1b9e].

[1: ea8c0e1b9e]: 2018-01-29 22:31:50 +0900
  * test/lisp/simple-tests.el (simple-tests-async-shell-command-30280): Add test
  https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=ea8c0e1b9eaa6651919fb4e039e3fcb5a1fa73db

I attach two patches.  The first tries to make this test succeed in
accordance with the resulting bugfix.  The second suggests some
simplifications to the logic in shell-command.  WDYT?

Thanks,

-- 
Basil