What does "echo (ls)" do in bash?

Question

When I run echo (ls) in bash it returns:

-bash: syntax error near unexpected token `ls'

I understand that I should escape parenthesis or quote to get plain text output. But the result still does not make sense to me if parenthesis means running the command sequence in a subshell environment.

My environment: bash 4.3 (installed by homebrew), OS X El Capitan

What does echo (ls) do? Generate an error. What does echo $(ls) do? Pretty much the same thing as ls, making the echo and command substitution redundant. If you're using it for testing, stop: don't try to parse ls output. — Blacklight Shining, Dec 31 '15 at 10:38

Thomas Dickey · Answer 1 · 2015-12-27T18:22:32.297

It is essentially a generic syntax error, not specifically related to the ls token. bash uses a yacc parser, which calls a common yyerror() on any problem, Within the resulting error-handling, it proceeds to try to pinpoint the error. The message is coming from this chunk (see source):

  /* 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;
    }

In other words, it's already confused by the '(', and having looked-ahead for context is reporting the ls.

A ( would be legal at the beginning of a command, but not embedded. Per manual page:

   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.

Further reading:

3.2.4.3 Grouping Commands (Bash Reference Manual)
3.5.4 Command Substitution (Bash Reference Manual)

instead echols ` will work , where ls is inside back-ticks characters , command subsitution — Ijaz Ahmad, Dec 27 '15 at 18:07
I'm aware of that: I was explaining the error message, which was the point of the question. — Thomas Dickey, Dec 27 '15 at 18:17
a ( is also legal when defining a function. if the ls wasn't inside of the (), bash would've waited for more input needed for the definition. — llua, Dec 27 '15 at 21:44

score 3 · Answer 2 · answered Dec 27 '15 at 18:10

3

ls should either be inside backtiks or you should try like this:

    echo $(ls)

answered Dec 27 '15 at 18:10

Ijaz Ahmad

7,202

Good info to know. However, the question is more about the behavior of the parenthesis without the usage of the dollar sign. Therefore, this good-to-know information is technically off-topic of the question that is actually being asked. – TOOGAM Dec 28 '15 at 07:19

What does "echo (ls)" do in bash?

2 Answers2