Re: Sus behaviour when cmd string ends with single backslash

[Chet Ramey]

On 2/13/22 3:15 PM, vzvzvz2@gmail.com wrote:

> Bash Version: 5.0
> Patch Level: 17
> Release Status: release
>
> Description:
>
> Background
> ----------
>
> Commit a0c0a00fc419b7bc08202a79134fcd5bc0427071 (bash-4.4) introduced a change 
> in parse.y with following documentation in the change logs:
>
> parse.y
>         - shell_getc: if bash is reading input from a string that ends with an
>           unquoted backslash, add another backslash instead of a newline, since
>           the backslash and newline will disappear in normal processing.  Fixes
>           bug with `bash -c 'eval \\; echo y' ' skipping the eval command and
>           setting incorrect exit status, and `bash -ic 'eval \\; echo y' '
>           seeing EOF on empty line and exiting before the echo.  Keep track of
>           backslash state with last_was_backslash; set in char reading loop.
>           Fixes bug reported by Eduardo A. Bustamante López <dualbus@gmail.com>
>
> The new code in parse.y
>
>           /* Don't add a newline to a string that ends with a backslash if we're
>              going to be removing quoted newlines, since that will eat the
>              backslash.  Add another backslash instead (will be removed by
>              word expansion). */
>           if (bash_input.type == st_string && expanding_alias() == 0 && last_was_backslash 
> && c == EOF && remove_quoted_newline)
>             shell_input_line[shell_input_line_len] = '\\';
>           else
>             shell_input_line[shell_input_line_len] = '\n';
>           shell_input_line[shell_input_line_len + 1] = '\0';
>
>
> This specific change is also there in commit 
> 0385211bb5cb01e0259c64ec2c5cc6337d4e215c on a development branch.
>
> Observed vs. expected behaviour
> -------------------------------
>
> The mentioned bug is indeed fixed by this change. However, in case of another 
> edge case following new behaviour is observable:
>
>   $ bash -c 'echo \'
>   \
>   $ # backslash appears on output

I think the behavior of a word consisting only of a backslash is officially
unspecified.

It's not a quote character -- there's nothing to quote -- so it's not
removed by quote removal.

In general, I think it's better to err on the side of preserving output
here.

Existing shell behavior varies, so you can't really count on anything.

--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/