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Re: Syntax for tab character


From: Chet Ramey
Subject: Re: Syntax for tab character
Date: Sat, 25 Apr 2009 14:49:53 -0400
User-agent: Thunderbird 2.0.0.21 (Macintosh/20090302)

Bill Gradwohl wrote:
> On Fri, 2009-04-24 at 18:54 -0400, Chet Ramey wrote:
>> I'm not quite sure what this is supposed to demonstrate.  In the cases
>> where you have specified the tab character correctly ($'\t'), it's
>> converted into a tab before matching is attempted.  
> I respectfully disagree, and I believe I have proof below.
>> You can tell this
>> because in the cases where you have not, either the literal `$' gets
>> removed, or the literal instances of \t in the value of parameter are
>> removed.  Since there are no tab characters in $parameter, no tabs get
>> removed when you specify $'\t' correctly.
>>
> I did a hex dump to prove that there were Hex '09' (tab) characters in
> the original parm field and that field gets reused again and again. The
> original dump also shows the $'s in evidence.

No, there really aren't any tabs in the value of ${parameter}.  For
some reason, you use `echo -e' to send input to `dumpit'.  The whole
reason the -e option to echo exists is to expand backslash escapes,
and it dutifully expands \t to tab.

All of your conclusions result from a faulty premise.

To restate:  the \t in the pattern will remove the literal \t in
$parameter. (As a sort of side effect -- the backslashes and `t's are
matched and removed separately.  You just don't notice because the
rest of the string doesn't contain any `t's.).  The $'\t' doesn't
remove anything, since there are no tabs in $parameter.  The $ '\t'
removes the dollar signs and instances of \t in $parameter.  The space
matches and removes space characters.

I prefer the `recho' program built as part of the bash test suite to
hexdump.  When I change your script to use recho instead of `echo -e'
piped to hexdump, I get the following:

argv[1] = <hello \t and \t $ $good \t bye.>

argv[1] = <helloand$$goodbye.>

argv[1] = <helloand$$goodbye.>

argv[1] = <hello\tand\t$$good\tbye.>

argv[1] = <hello\tand\t$$good\tbye.>

argv[1] = <helloandgoodbye.>

argv[1] = <helloandgoodbye.>

I think that makes the effects clear.

Chet
-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer

Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/




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