Change arithmetic translation to use the trivial $shExpr(...).

tools/osh2oil.py

@@ -638,7 +638,12 @@ def DoCommand(self, node, local_symbols, at_top_level=False):
    elif node.tag == command_e.DParen:
      # Just change (( )) to ( )
      # Test it with while loop
      self.DoArithExpr(node.child, local_symbols)

      # NOTE: I have a (( n++ )) in one script.  That can be 'set n++' or
      # 'set n += 1'.

      #self.DoArithExpr(node.child, local_symbols)
      raise NotImplementedError('DParen')

    elif node.tag == command_e.DBracket:
      # [[ 1 -eq 2 ]] to (1 == 2)
@@ -1194,19 +1199,22 @@ def DoWordPart(self, node, local_symbols, quoted=False):
      # change to $[echo hi]

    elif node.tag == word_part_e.ArithSubPart:
      # We're not bothering to translate the arithmetic language.
      # Just turn $(( x ? 0 : 1 )) into $shExpr('x ? 0 : 1').

      left_spid, right_spid = node.spids

      # Skip over left bracket and write our own.
      self.f.write('$(')
      self.f.write("$shExpr('")
      self.cursor.SkipUntil(left_spid + 1)

      # NOTE: This doesn't do anything yet.
      self.DoArithExpr(node.anode, local_symbols)
      #self.DoArithExpr(node.anode, local_symbols)
      # Placeholder for now
      self.cursor.PrintUntil(right_spid - 1)

      # Skip over right bracket and write our own.
      self.f.write(')')
      self.f.write("')")
      self.cursor.SkipUntil(right_spid + 1)

    elif node.tag == word_part_e.EmptyPart:
@@ -1215,28 +1223,13 @@ def DoWordPart(self, node, local_symbols, quoted=False):
    else:
      raise AssertionError(node.__class__.__name__)

  def DoArithExpr(self, node, local_symbols):
    if node.tag == arith_expr_e.ArithBinary:
      # Maybe I should just write the left span and right span for each word?
      #self.f.write(str(node.left))

      if node.op_id == Id.Arith_Plus:
        # NOTE: Right isn't necessarily a word!
        r_id = word.LeftMostSpanForWord(node.right.w)
        #self.cursor.SkipUntil(r_id)
        #self.f.write('PLUS')

      #self.f.write(str(node.right))
    elif node.tag == arith_expr_e.ArithWord:
      self.DoWordInCommand(node.w, local_symbols)

    else:
      raise AssertionError(node.__class__.__name__)

  def DoBoolExpr(self, node):
    # TODO: switch on node.tag
    # TODO:
    # - Some are turned into '( x ~ *.py )'
    # - Some are turned into 'test x -lt y'
    pass


# WordPart?

# array_item