Implement the 'dirs' builtin (#55)

Add error checking to the 'pushd' and 'popd' builtins.

core/builtin.py

@@ -482,10 +482,12 @@ def Cd(argv, mem):
  state.SetGlobalString(mem, 'PWD', dest_dir)
  return 0

def PrintDirStack(dir_stack):
  if len(dir_stack) > 0:
    sys.stdout.write(' '.join(dir_stack) + '\n');

def Pushd(argv, dir_stack):
  num_args = len(argv)

  if num_args <= 0:
    util.error('pushd: no other directory')
    return 1
@@ -499,8 +501,9 @@ def Pushd(argv, dir_stack):
  except OSError as e:
    util.error("pushd: %r: %s", dest_dir, os.strerror(e.errno))
    return 1

  dir_stack.append(os.getcwd())
  PrintDirStack(dir_stack)

  return 0


@@ -513,15 +516,36 @@ def Popd(argv, dir_stack):

  try:
    os.chdir(dest_dir)
    PrintDirStack(dir_stack)
  except OSError as e:
    util.error("popd: %r: %s", dest_dir, os.strerror(e.errno))
    return 1

  return 0


DIRS_SPEC = _Register('dirs')
DIRS_SPEC.ShortFlag('-c')
DIRS_SPEC.ShortFlag('-l')
DIRS_SPEC.ShortFlag('-p')
DIRS_SPEC.ShortFlag('-v')

def Dirs(argv, dir_stack):
  print(dir_stack)
  arg, i = DIRS_SPEC.Parse(argv)
  if arg.l:
    util.warn('*** dirs -l not implemented ***')
  # Following `bash` behavior for order of operations
  if arg.c:
    del dir_stack[:]
  elif arg.v:
    for i, entry in enumerate(dir_stack):
      print('%2d %s' % (i, entry))
  elif arg.p:
    for entry in dir_stack:
      print(entry)
  else:
    PrintDirStack(dir_stack)

  return 0

spec/builtins2.test.sh

@@ -24,3 +24,31 @@ echo status=$?
# BUG bash stdout-json: "echo\nmyfunc\nfor\nstatus=0\n"
# BUG dash stdout-json: "echo\nstatus=0\n"
# OK mksh stdout-json: "echo\nmyfunc\nstatus=1\n"

### dirs builtin
dirs
# stdout-json: ""
# status: 0

### dirs -c
pushd /
dirs
dirs -c
dirs
# stdout-json: "/\n/\n"
# status: 0

### dirs -v
pushd /
dirs -v
pushd /
dirs -v
# stdout-json: "/\n 0 /\n/ /\n 0 /\n 1 /\n"
# status: 0

### dirs -p
pushd /
dirs -p
pushd /
dirs -p
# stdout-json: "/\n/\n/ /\n/\n/\n"