On 04/26/2013 08:58 AM, Cristian Sava wrote: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.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? 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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