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Date: Mon, 29 Jun 2009 11:03:07 -0700 (PDT)
From: grvs <grvsinghal AT gmail DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Problem with single quotes
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Thanks Eric for your reply.
Yeah you are right, I am actually new to shell scripting and I downloaded
cygwin only to learn shell scripting. And this script was one of the
assignment question of the book I am reading to learn shell scripting. 
Actually now it may seem funny but I mistakenly used single quotes instead
of back quotes (key left to 1 in the keyboard) which I figured out just now.
If I use back quotes, my code also works. :)

And thanks again, I got to know many new things from your such a exhaustive
and well framed reply, and a work around of the problem I was trying to
solve.

Regards



Eric Blake wrote:
> 
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> According to grvs on 6/29/2009 1:00 AM:
>> Hi all
>> I am new in cygwin as well as linux and I am trying to learn shell
>> scripting
>> I tried to write following script which doesn't give me appropriate
>> result.
> 
> Your question is not cygwin-specific.  You would be better off getting a
> good shell scripting tutorial rather than trying to learn shell scripting
> from this list.
> 
>> 
>> x=3
>> y='[ $x -eq 10 ]'
> 
> Here, the single quoting tells the variable assignment that the variable
> contains text that would otherwise be split into multiple words by the
> shell.  But the single quotes are not assigned to the variable.  So y
> holds the text "[ $x -eq 10 ]", not "'[ $x -eq 10 ]'".
> 
>> z='[ $x -lt 10 ]'
>> echo x=&x y=$y z=$z
>> 
>> and the output is:
>> x=5 y=[ $x -eq 10 ] z=[ $x -lt 10 ]
> 
> Correct (although you got lucky that you didn't have consecutive
> whitespace, which would have been eaten by your underquoted echo
> statement).
> 
>> 
>> I expected
>> x=5 y=0 z=1
>> or x=5 y=1 z=0 ( I am not sure till now that whether 0 is true or 1 is
>> true)
> 
> In shell scripting, true tests return 0 (success), false tests return
> non-zero (usually 1, but can be 2-255).  Yes, that's backwards from C
> conventions.
> 
> Oh, so you wanted indirect evaluation and command substitution, and you
> wanted the exit status of running the command.  Use eval, as in:
> 
> eval echo x="$x" y='$('"$y"'; echo $?)' z='$('"$z"'; echo $?)'
> 
> Be careful, though - indirect evaluation, if used incorrectly, is a big
> cause of security holes in shell scripts.
> 
> - --
> Don't work too hard, make some time for fun as well!
> 
> Eric Blake             ebb9 AT byu DOT net
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