F18, F19 webalizer problem?

[Jonathan Kamens]

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

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