unset
-----

Unset a variable, cache variable, or environment variable.

Unset Normal Variable or Cache Entry
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. 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.

Unset Environment Variable
^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: cmake

  unset(ENV{<variable>})

Removes ``<variable>`` from the currently available
:manual:`Environment Variables <cmake-env-variables(7)>`.
Subsequent calls of ``$ENV{<variable>}`` will return the empty string.

This command affects only the current CMake process, not the process
from which CMake was called, nor the system environment at large,
nor the environment of subsequent build or test processes.