F18, F19 webalizer problem?

[Cristian Sava]

On Fri, 2013-04-26 at 09:09 -0400, Jonathan Kamens wrote:
> On 04/26/2013 08:58 AM, Cristian Sava wrote:
> > Webalizer is enabled via /etc/sysconfig/webalizer but I don't understand
> > why /etc/cron.daily/00webalizer has this line:
> > 
> > [ "z$WEBALIZER_CRON" != "zyes" ] && exit 0
> > 
> > that I think it should be (and it is working this way)
> > 
> > [ "$WEBALIZER_CRON" != "yes" ] && exit 0
> > 
> > What is the mistery?
> > 
> The two versions you quoted are functionally equivalent on Linux. If
> there's a z before the variable expansion and a z  before the word
> "yes", then the two z's cancel out and don't affect the string
> comparison.
> 
> As for why they are there, it's an old shell programmers' trick to
> avoid causing syntax errors when doing string comparisons to empty
> strings. Let me explain...
> 
> If you do this:
> 
> [ $WEBALIZER_CRON != yes ]
> 
> and the variable WEBALIZER_CRON is empty, then the command above will
> result in an error, because when it's evaluated, it will look to the
> shell as if there is nothing to the left of the '!=' and therefore
> there's a missing expression in the statement.
> 
> This, however, is fine:
> 
> [ "$WEBALIZER_CRON" != yes ]
> 
> because the quotes ensure that the statement will be evaluated with an
> expression to the left of the != even if the expression is just an
> empty string.
> 
> This is fine too:
> 
> [ z$WEBALIZER_CRON != zyes ]
> 
> because if the variable is empty, the expression to the left will be
> "z" rather than an empty string.
> 
> It's unnecessary to use both the quotes and the extra character on
> both sides of the expression. I'm not sure, but I think it may have
> been necessary to use both at some point in the past, because I think
> the middle example above, with just the quotes, might actually have
> been buggy in some old versions of the "test" program that evaluates
> such expressions. This caused shell programmers to get into the habit
> of using both the quotes and the extra character. I doubt it's been
> necessary to do that for many years, though for all I know webalizer
> may support some UNIX versions that are so old that they still have
> problems in this area. Given that possibility, there's certainly no
> harm in writing the test the way it's written in the file you looked
> at.
> 
>   jik
> 
Thank you very much for your explanation,
C.Sava