Re: [RFA/RFC] Problem with '!' escaping with zsh/bash/ksh

[Daniel Jacobowitz <drow@mvista.com>]

On Sat, May 03, 2003 at 09:51:09AM -0700, Joel Brobecker wrote:
> Daniel said:
> > By the time it gets to execve, we have:
> > execve("/home/drow/foo/foo\\!bar/ls", ["/home/drow/foo/foo\\!bar/ls"],
> > 
> > i.e. the backslash has been escaped too!
> 
> Unfortunately, this is not what's happening:
> 
> (top-gdb) p shell_command
> $1 = 0xbffff2f0 "exec '/home/brobecke/tmp/GEO_ENV\\!9.159/foo' "
> 
> The value printed for shell_command is misleading (the double backslash),
> when you dump the memory at the address string, I only see one backslash:
> 
> (top-gdb) x /50c 0xbffff2f0
> 0xbffff2f0:     101 'e' 120 'x' 101 'e' 99 'c'  32 ' '  39 '\'' 47 '/'  104 'h'
> 0xbffff2f8:     111 'o' 109 'm' 101 'e' 47 '/'  98 'b'  114 'r' 111 'o' 98 'b'
> 0xbffff300:     101 'e' 99 'c'  107 'k' 101 'e' 47 '/'  116 't' 109 'm' 112 'p'
> 0xbffff308:     47 '/'  71 'G'  69 'E'  79 'O'  95 '_'  69 'E'  78 'N'  86 'V'
> 0xbffff310:     92 '\\' 33 '!'  57 '9'  46 '.'  49 '1'  53 '5'  57 '9'  47 '/'
>                 ^^^^^^^
> 0xbffff318:     102 'f' 111 'o' 111 'o' 39 '\'' 32 ' '  0 '\0'  -1 'ÿ'  -65 '¿'
> 0xbffff320:     102 'f' -48 'Ð'

Ah, OK.  strace apparently does the same thing.  Shame on me.

> Eli said:
> > Are you saying that zsh doesn't support escaping of arbitrary
> > characters with a backslash?  That is, under zsh, "\a" is not the same
> > as "a"?  I'd be surprised.
> 
> If I restrict myself to using a zsh shell alone, outside of GDB, here is
> the behavior I get:
> 
>     With the backlash:
>     % zsh
>     % exec '/home/brobecke/tmp/GEO_ENV\!9.159/foo'
>     zsh: no such file or directory: /home/brobecke/tmp/GEO_ENV\!9.159/foo
>     %
> 
>     Without the backslash
>     % zsh
>     % exec '/home/brobecke/tmp/GEO_ENV!9.159/foo'
>     %
> 
> I think you are right to say that "\a" is equivalent to "a" in general.
> However, in our case, the argument is quoted, specifically single-quoted.
> And it seems to make a big difference: with single quotes, the
> expression is no longer evaluated. That's why the backslash becomes
> harmful.
> 
> Daniel said:
> > By the way... what would the general reaction be to supporting exec'ing
> > the program directly instead of through the shell?  At least as an
> > option, since it would be a bit of an interface/quoting change?
> 
> I think that'd be very nice, actually. Can somebody tell me what the
> advantage of forking via a shell is?

Globbing, primarily; and it handles some complexities of quoting (but
introduces others!).

Me, I think on modern systems we can just do argument splitting
and globbing ourselves if we want to.  It's more efficient and less
fragile.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer