Your shell script question answered

[Gregory Nowak]

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Thanks for your great explanation. I suspected that what was happening
was happening because the script ended up comparing nothing to 1. When
I saw the other examples in this thread, I realized that it might be
because of what you describe. When I changed my script to run the
command, and then check $? in the if statement on the next line,
things worked as i expected. Yes, as I said, the fragment I provided
was to illustrate the problem. A script that expects /bin/true to
return 1 wouldn't be very useful after all.

Thanks again to everyone for your help.

Greg


On Thu, Mar 29, 2012 at 09:00:04PM -0600, D. Curtis Willoughby wrote:
> Hi Greg
> 
> Others have told you other ways to skin the cat.  I will try to explain
> why what you did did what it did, and suggest a way to make what
> you did work.
> 
> I have to say, your script is a little strange, but I assume you did it to
> illustrate something which you are doing in another script.
> 
> The true command produces no output on standard out, but only
> returns an exit status of zero.
> 
> Since its exit status is zero, the "echo $?" after the || is
> not evaluated.  (not executed)
> 
> Thus there is no output from the command(s) between the grave accent (`)
> characters.
> 
> When the shell evaluates the null string standing alone, it is
> treated as nothing at all, and thus the first non-blank character
> after the open bracket ([) is the equal sign (=).  Your error message
> is a complaint from the shell that an equal sign is not allowed
> as the first character inside an open bracket.
> 
> When I run your script fragment, i get:
> 
> unary operator expected
> 
> which means the same thing.
> 
> If you want the shell to see that null string as a separate string,
> you must enclose it in quotation marks like this:
> 
> if [ "`/bin/true || echo $?`" = 1 ] ; then
> echo "returned 1"
> fi
> 
> which does in fact produce no output.
> 
> Of course, if you want the script to tell the truth, you should
> do this:
> 
> if [ "`/bin/true || echo $?`" = 1 ] ; then
> echo "returned 0" i.e. true
> else
> echo "returned 1" i.e. false
> fi
> 
> The zero from the true command is in fact not equal to 1.
> 
> I hope this is more informative than confusing.
> 
> D. Curtis Willoughby
> 


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