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unset
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-----
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Unset a variable, cache variable, or environment variable.
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::
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unset(<variable> [CACHE | PARENT_SCOPE])
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Removes a normal variable from the current scope, causing it
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to become undefined. If ``CACHE`` is present, then a cache variable
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is removed instead of a normal variable. Note that when evaluating
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:ref:`Variable References` of the form ``${VAR}``, CMake first searches
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for a normal variable with that name. If no such normal variable exists,
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CMake will then search for a cache entry with that name. Because of this
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unsetting a normal variable can expose a cache variable that was previously
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hidden. To force a variable reference of the form ``${VAR}`` to return an
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empty string, use ``set(<variable> "")``, which clears the normal variable
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but leaves it defined.
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If ``PARENT_SCOPE`` is present then the variable is removed from the scope
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above the current scope. See the same option in the :command:`set` command
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for further details.
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``<variable>`` can be an environment variable such as:
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::
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unset(ENV{LD_LIBRARY_PATH})
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in which case the variable will be removed from the current
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environment.
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