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