Why 'x' used in [ x"$VAR" = x"VALUE" ]?

I can see in unix shell scripts that following construction used开发者_运维技巧

[ x"$VAR" = x"VALUE" ] 

instead of

[ "$VAR" = "VALUE" ] 

Why?

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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.

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when $VAR is empty it would give a syntax error [ = "VALUE"], thus the usage of x

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This is an old technique to prevent either side of the comparison from being empty. Some shells don't handle empty strings properly.

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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