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up vote 6 down vote favorite

This was part of fetchmail deamon script,

if [ ! "x$START_DAEMON" = "xyes" -a ! "$1" = "status" ]; then

What's the point of using x$START_DAEMON = "xyes", why not just use $START_DAEMON = "yes" ?

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1 Answer

up vote 8 down vote accepted

I've seen cases where the x$variable was not quoted, and in that case it's to handle situations where the variable ($START_DAEMON in this case) might be empty. Otherwise, your test would be checking whether ! = "yes", and the test would error. However, as you've pointed out in comments, double-quoting the x$variable takes care of this.

On the other hand, a question on ServerFault led me to this page: http://pubs.opengroup.org/onlinepubs/009695399/utilities/test.html, wherein one is preventing the variable expansion from starting with a character that may confuse [ or test

Even so, I think this is an issue that may only be a problem in older ("Historical" as the opengroup page says) shells, and that newer implementations do the right thing without the x, so long as double-quotes are involved at least. So it may primarily be a coding tradition. I was able to confirm a problem using /bin/sh on SunOS 5.10, though:

 # a="!"; [ "$a" = "yes" ] && echo ok
 test: argument expected
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unset a;[ "$a" = "yes" ] && echo ok that doesn't report error, hmm – warl0ck Oct 8 '12 at 5:19
Right you are. I have seen cases where x$variable was not quoted, and it may have been those situations where problems with test arose. However, using a known character like x does prevent other problems with special characters -- I'll update the answer. – Lars Rohrbach Oct 8 '12 at 5:47
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It can also be a shell quoting issue. For example, Bash 4.2.24 complains bash: !: event not found with your posted example, but avoids history expansion and works fine with a=\!; [[ "$a" = "yes" ]] && echo ok instead. – CodeGnome Oct 8 '12 at 6:40
POSIX mandates the parsing of all correct test expressions up to three tokens, such as this one. Historical Bourne shells are buggy. The x prefix makes the left-to-right parsing unambiguous. The double quotes are necessary in any shell, in case the value of the variable is empty, contains spaces, etc. The double quotes aren't necessary inside [[…]] which is special syntax, not a mere built-in utility like [. – Gilles Oct 8 '12 at 22:51

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