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shellunix

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

I can see in unix shell scripts that following construction used 

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

instead of 

[ "$VAR" = "VALUE" ]

Why?

Solution

  • 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.