Feature #18367


Stop the interpreter from escaping error messages

Added by mame (Yusuke Endoh) 6 months ago. Updated 3 months ago.

Target version:



At the present time, the Ruby interpreter escapes some characters (*1) in error messages when an uncaught error is printed. I'd like to propose stopping this escaping behavior.

class MyError < StandardError
  def message

raise MyError
#=> current:  test.rb:7: in `<main>': foo\\bar (MyError)
#=> excepted: test.rb:7: in `<main>': foo\bar (MyError)

*1: Escaped characters are any control characters except \t and \n, and a backslash \\.


This behavior prevents us from adding an attribution (color, underline, etc.) to the error message because it escapes escape sequences. Nowadays, such a rich presentation of terminal output is more and more important.

$ ruby -e 'raise "\e[31mRed\x1b[0m error"'
-e:1:in `<main>': \e[31mRed\x1b[0m error (RuntimeError)

Also, the behavior in question leads to rather confusing error printing. See the error output of "\\".no_method:

$ ruby -e '"\\".no_method'
-e:1:in `<main>': undefined method `no_method' for "\\\\":String (NoMethodError)


The two occurrences of "\\\\" must be "\\". Worse, the output of error_highlight ^^^^ points wrong position.

Note that this issue is never specific to error_highlight. The receiver of NoMethodError, "\\\\":String, is also wrongly escaped. It must be "\\":String.

Why the escaping behavior was introduced

AFAIK, the behavior was introduced because of a security concern. It is considered harmful for an attacker to be able to print arbitrary escape sequences to victim's terminal. (See this article in detail.)

However, I believe it is rare to see the error logs of an application that may be exposed to attacks (i.e. in production mode) in a terminal, as the error output of the Ruby interpreter.

Even if that is the case, I think such escaping should be done as a responsibility of the application, and not implicitly by the interpreter. I briefly surveyed other major languages than Ruby, and I could find no language that escapes error messages. This is the transcript of Python and Node.js.

$ python3 -c 'raise Exception("\x1b[31mRed\x1b[0m error")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
Exception: Red error

$ node -e 'throw("\x1b[31mRed\x1b[0m error")'

throw("\x1b[31mRed\x1b[0m error")
Red error
(Use `node --trace-uncaught ...` to show where the exception was thrown)

Just in case, I reported these behaviors to the security contacts of Python and Node.js, and both responded to me that this is not a securty issue. I think their decisions are quite reasonable.


It would be a good idea to first make the following behavior as a migration path.

  • When an error message does not include a control character, no escaping is applied.
  • When an error message does include a control character, "Warning: this error message is currently escaped because it includes a control character(s), but this will not be escaped in Ruby 3.X" is printed, and the escaping is applied.

Related issues 1 (0 open1 closed)

Related to Ruby master - Feature #18370: Call Exception#full_message to print exceptions reaching the top-levelClosedActions

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