dash: echo builtin interprets backslash escape sequences

[Edward Crichton]

In dash, backslashes are parsed within single quotes, contradicting POSIX and it's own man page.
POSIX: http://www.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_02_02

The following should print 2 backslashes:
echo '\\'

[Yann Leprince (sciyann)]

Thank you for taking the time to report.
When I run your test script with bash (under karmic), I get the backslash as it should be. However when I run it with dash (the default /bin/sh since edgy, 6.10) the backslash does not get displayed.
Please run 'ls -l /bin/sh' so that we can decide if this is really a bash bug (dpkg -L containing /bin/sh does not mean that your /bin/sh is actually bash) or a dash bug.

[Edward Crichton (edwardcrichton)]

Hi,

ls -l /bin/sh

/bin/sh -> dash

Aah. It's plugged into dash (as is the feisty version which is working).

[Yann Leprince (sciyann)]

it appears that this is a bug in dash and not bash

[Nonconventionally Creative (br-longbons)]

There is a much simpler test: does
echo '\\'
print one backslash or 2?

The man page says:
   Single Quotes
     Enclosing characters in single quotes preserves the literal meaning of all the characters (except sin‐
     gle quotes, making it impossible to put single-quotes in a single-quoted string).

This bug should be SEVERE because it makes it practically impossible to write portable shell scripts.

[Jilles Tjoelker (jilles)]

The issue is not with the single quotes, but with dash's echo builtin. Apparently, the way they are compiled and configured by default on Ubuntu, dash's echo builtin interprets backslash escape sequences while bash's does not.

According to POSIX, this interpretation is permitted; with the XSI option (following traditional System V), it is required.

Practically, the echo utility can only be used portably if both backslashes and initial hyphens (which could be interpreted as options) are omitted. The printf utility is a good substitute.

I don't see much point in changing dash here.

[Edward Crichton (edwardcrichton)]

Is this, then, the same as the bug I reported originally - that was later replaced by echo '\\':

BACKSLASH='\u'
SQUARE='\u[]'
echo $BACKSLASH
echo $SQUARE

gives
\u
u[] on ubuntu and \u[] everywhere else

So it is the echo command on Ubuntu dash only?
A work around would be to use printf in place of echo when using ubuntu? This would work reliably on other versions of sh?

> Date: Fri, 14 May 2010 22:55:17 +0000
> From: <email address hidden>
> To: <email address hidden>
> Subject: [Bug 259671] Re: dash parses backslashes in single quotes
>
> The issue is not with the single quotes, but with dash's echo builtin.
> Apparently, the way they are compiled and configured by default on
> Ubuntu, dash's echo builtin interprets backslash escape sequences while
> bash's does not.
>
> According to POSIX, this interpretation is permitted; with the XSI
> option (following traditional System V), it is required.
>
> Practically, the echo utility can only be used portably if both
> backslashes and initial hyphens (which could be interpreted as options)
> are omitted. The printf utility is a good substitute.
>
> I don't see much point in changing dash here.
>
> --
> dash parses backslashes in single quotes
> https://bugs.launchpad.net/bugs/259671
> You received this bug notification because you are a direct subscriber
> of the bug.
>
> Status in Bourne Again Shell: Invalid
> Status in “dash” package in Ubuntu: Confirmed
> Status in “dash” package in Debian: New
>
> Bug description:
> In dash, backslashes are parsed within single quotes, contradicting POSIX and it's own man page.
> POSIX: http://www.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_02_02
>
> The following should print 2 backslashes:
> echo '\\'
>
>
>
> To unsubscribe from this bug, go to:
> https://bugs.launchpad.net/bash/+bug/259671/+subscribe

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[Jilles Tjoelker (jilles)]

The Debian dash maintainer considers this a WONTFIX: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=550399

It seems unlikely that Ubuntu will want to override this.

[Richard Hansen (rhansen)]

This is not a bug -- http://pubs.opengroup.org/onlinepubs/9699919799/utilities/echo.html says that echo should process the backslashes. It would be a bug if echo did not process the backslashes.

If you don't want backslashes processed, use printf:

    printf %s\\n '\\'