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Date: Mon, 29 Jun 2009 06:01:32 -0600
From: Eric Blake <ebb9 AT byu DOT net>
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To: cygwin AT cygwin DOT com, grvsinghal AT gmail DOT com
Subject: Re: Problem with single quotes
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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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