I can see in unix shell scripts that following construction used
[ x"$VAR" = x"VALUE" ]
instead of
[ "$VAR" = "VALUE" ]
Why?
views:
77answers:
5
+4
A:
when $VAR is empty it would give a syntax error [ = "VALUE"], thus the usage of x
knittl
2010-06-22 13:23:57
Beat me by 9 seconds.
Paul Tomblin
2010-06-22 13:24:30
Yup. If you're interested in learning a lot of little things like this in advance, rather than as you stumble across them, I'd personally recommend the O'Reilly book *Classic Shell Scripting*, see http://oreilly.com/catalog/9780596005955 . A lot of the more obscure bits of shell scripts became a lot more comprehensible once I'd read this.
Matt Gibson
2010-06-22 13:30:34
A:
Because if either $VAR or VALUE is empty, you get a nasty error if you don't use the "x".
Paul Tomblin
2010-06-22 13:24:06
+5
A:
Because shells were not always entirely well-behaved if one of the variables was empty.
Consider if $VAR was empty/null and $VALUE is "foo", your two constructs expand to:
[ x = xfoo ]
and
[ = foo ]
The latter would cause an error in some shells due to being an illegal construct, while the former is valid in any case. This is not a problem in recent version of bash (perhaps even old versions of bash) but that's where it comes from historically - and asides from the occasional moment of puzzlement from people in your situation, there's little reason not to do it for compatibility with a wider range of shells.
Andrzej Doyle
2010-06-22 13:24:28
There are quotes in my example.I said [ "x" = x"foo" ] not [ x = xfoo ]
Vladimir Bezugliy
2010-06-22 13:41:34
With quotes (and modern shells), you don't need the 'x' -- `[ "$var" = "val" ]` becomes, when var is empty: `[ "" = "val" ]` which is simply false with no errors.
glenn jackman
2010-06-22 15:02:09
+1
A:
This is an old technique to prevent either side of the comparison from being empty. Some shells don't handle empty strings properly.
Frerich Raabe
2010-06-22 13:25:28
@Vladimir: According to http://tldp.org/LDP/abs/html/comparison-ops.html (see the footnote), some older Bash versions don't handle empty strings correctly.
Frerich Raabe
2010-06-22 13:55:11
A:
you have got your explanations, but here's another approach, using case/esac instead of if/else
case "$VAR" in
"$VALUE" ) echo 'yes';;
* ) echo 'no' ;;
esac
ghostdog74
2010-06-22 14:00:57