'cd' without arguments should change to the home directory.

Addresses issue #24.

core/builtin.py

@@ -401,9 +401,15 @@ def Shift(argv, mem):
  return mem.Shift(n)


def _Cd(argv, mem):
def Cd(argv, mem):
  # TODO: Parse flags, error checking, etc.
  dest_dir = argv[0]
  try:
    dest_dir = argv[0]
  except IndexError:
    # NOTE: This is equivalent to 'cd ~', but bash/mksh only seem to respect
    # $HOME, and not /etc/passwd.  OSH behavior is different but better.
    dest_dir = util.GetHomeDir()

  if dest_dir == '-':
    old = mem.GetVar('OLDPWD', scope.GlobalOnly)
    if old.tag == value_e.Undef:

core/cmd_exec.py

@@ -211,7 +211,7 @@ def _RunBuiltin(self, builtin_id, argv):
      status = builtin.Shift(argv, self.mem)

    elif builtin_id == EBuiltin.CD:
      status = builtin._Cd(argv, self.mem)
      status = builtin.Cd(argv, self.mem)

    elif builtin_id == EBuiltin.SET:
      status = builtin.Set(argv, self.exec_opts, self.mem)

spec/builtins.test.sh

@@ -48,6 +48,11 @@ echo "old: $OLDPWD"
cd -
# stdout-json: "old: /\n/\n"

### cd with no arguments
cd
test $(pwd) = ~ && echo OK
# stdout: OK

### pushd/popd
set -o errexit
cd /

test/sh_spec.py

@@ -858,6 +858,8 @@ def main(argv):
  env = {
    'TMP': opts.tmp_env,
    'PATH': opts.path_env,
    # The normal case for ~ and 'cd' is that $HOME is defined.
    'HOME': os.environ['HOME'],
  }
  stats = RunCases(cases, case_predicate, shell_pairs, env, out)
  out.EndCases(stats)