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Re: Despite text in gnu bash manual, quote removal appears to be perform

From: Stahlman Family
Subject: Re: Despite text in gnu bash manual, quote removal appears to be performed on case pattern
Date: Wed, 13 Dec 2006 06:49:59 -0600

----- Original Message ----- From: "Chet Ramey" <address@hidden>
To: "Stahlman Family" <address@hidden>
Cc: <address@hidden>; <address@hidden>
Sent: Tuesday, December 12, 2006 2:01 PM
Subject: Re: Despite text in gnu bash manual, quote removal appears to be 
performed on case pattern

Stahlman Family wrote:
Mingw (Msys) Bash 2.04.0(1)-release
Microsoft Windows XP Home Edition Version 2002 Service Pack 2

Perhaps I'm misunderstanding something, but the way I read the manual
text on the case command, if anything is printed in the example below,
it would be "2", since if the quotes around " foo bar " are not removed
in the case pattern, then the first case is testing the string

Not exactly.  Strict quote removal is not performed.  The behavior of
quoted characters is as in pattern matching:  quoted characters match
themselves, even characters are have special meaning in pattern matching.

There is some internal process of turning, for instance, " foo bar "
into \ \f\o\o\ \b\a\r\ , but that is not the same as quote removal.

Ahhh. Thanks. That's the piece I was missing. As I understand it then, a pattern can be quoted with literal quotes and/or backslashes and the output of the pattern pre-processing stage will be a pattern with only backslash-style quoting (a form that is understood by the glob pattern matching code). Although it should have been clear to me in retrospect, I did not realize that the glob pattern matching engine understood backslash escaping. Of course, if it didn't, the implementation would be much messier, since the pattern pre-processing code (or as I thought, the tokenizing code) would need to set flags of some sort while processing patterns (either one for the entire pattern if entire pattern is to be treated as a string, or one for each character in the pattern otherwise) to tell the pattern matching code to inhibit glob special character recognition in a certain pattern or section of a pattern. In fact, it appears that the "flags" are embedded in the pre-processed pattern as backslashes - much more elegant.

Playing with it now, I'm pleased and relieved to see that interpretation of backslashes within a double quoted pattern is consistent with that performed for non-pattern double quoted strings; eg, "\*" is not the same as "*".

This has cleared a number of things up for me.

   Brett S.

That becomes more clear when the pattern to be quoted is "*":  quote
removal would turn that into * rather than \*, so you would not be able
to use "*" to match a literal * if quote removal were performed.

Posix is, as usual, more verbose about this than the bash manual page.

``The lyf so short, the craft so long to lerne.'' - Chaucer
       Live Strong.  No day but today.
Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/

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