"echo (ls)" 在 bash 中有什么作用？

Question

当我echo (ls)在 bash 中运行时，它返回：

-bash: syntax error near unexpected token `ls'

我知道我应该转义括号或引用以获得纯文本输出。但是如果括号意味着在子shell环境中运行命令序列，结果对我来说仍然没有意义。

我的环境：bash 4.3（自制软件安装），OS X El Capitan

Answer 1

它本质上是一个通用的语法错误，与ls令牌没有特别的关系。 bash使用 yacc 解析器，它yyerror()在任何问题上调用 common ，在产生的错误处理中，它继续尝试查明错误。消息来自这个块（参见源代码）：

  /* If the line of input we're reading is not null, try to find the       
     objectionable token.  First, try to figure out what token the
     parser's complaining about by looking at current_token. */
  if (current_token != 0 && EOF_Reached == 0 && (msg = error_token_from_token (current_token)))
    {
      if (ansic_shouldquote (msg))
    {
      p = ansic_quote (msg, 0, NULL);
      free (msg);
      msg = p;
    }
      parser_error (line_number, _("syntax error near unexpected token `%s'"), msg);
      free (msg);

      if (interactive == 0)
    print_offending_line ();

      last_command_exit_value = parse_and_execute_level ? EX_BADSYNTAX : EX_BADUSAGE;
      return;
    }

换句话说，它已经被 混淆了'('，并且提前查看上下文正在报告ls.

A(在命令的开头是合法的，但不能嵌入。每个手册页：

   Compound Commands                                                       
       A compound command is one of the following:

       (list) list  is  executed in a subshell environment (see COMMAND EXECU?
              TION ENVIRONMENT below).  Variable assignments and builtin  com?
              mands  that  affect  the  shell's  environment  do not remain in
              effect after the command completes.  The return  status  is  the
              exit status of list.

进一步阅读：

• 3.2.4.3 分组命令（Bash 参考手册）
• 3.5.4 命令替换（Bash 参考手册）