Re: Interpretation of escapes in expansions in pattern matching contexts

[Chris F.A. Johnson]

On Sat, 6 Apr 2013, Chet Ramey wrote:

> On 4/6/13 4:48 AM, Dan Douglas wrote:
> > I couldn't find anything obvious in POSIX that implies which interpretation is
> > correct. Assuming it's unspecified.
> >
> > Bash (4.2.45) uniquely does interpret such escapes for [[, which makes me
> > think this test should say "no":
> >
> >     x=\\x; if [[ x == $x ]]; then echo yes; else echo no; fi
> >
> > bash: yes
> > ksh:  no
> > mksh: no
> > zsh:  no
> >
> > However, ksh93 (AJM 93v- 2013-03-17) is unique in that it flips the result
> > depending on "[[ ]]" or "case..esac" (bug?), but otherwise it looks like a
> > fairly random spread:
> >
> >     x=\\x; case x in $x) echo yes;; *) echo no; esac
>
> These two cases should not be different.  They undergo the same expansions,
> except that the conditional command adds quote removal, which doesn't
> matter in this case.  In both cases, you ask the pattern matching code
> whether or not the string `x' matches the pattern `\x'.
>
> You invoke the same pattern matching code on the same patterns, why would
> you not get the same answer?

  In bash, the expansion differs when in  [[ ... ]]:

$ x=\\x; if [[ x == $x ]]; then echo yes; else echo no; fi
yes
$ x=\\x; if [ x == $x ]; then echo yes; else echo no; fi
no

  But not in ksh93:

$ x=\\x; if [[ x == $x ]]; then echo yes; else echo no; fi
no
$ x=\\x; if [ x == $x ]; then echo yes; else echo no; fi
no



--
   Chris F.A. Johnson, <http://cfajohnson.com/>
   Author:
   Pro Bash Programming: Scripting the GNU/Linux Shell (2009, Apress)
   Shell Scripting Recipes: A Problem-Solution Approach (2005, Apress)