This is a CMake Environment Variable. Its initial value is taken from the calling process environment.

Specify the number of tests for CTest to run in parallel. For example, if CTEST_PARALLEL_LEVEL is set to 8, CTest will run up to 8 tests concurrently as if ctest were invoked with the --parallel 8 option.

Changed in version 3.29: The value may be empty, or 0, to let ctest use a default level of parallelism, or unbounded parallelism, respectively, as documented by the ctest --parallel option.

On Windows, environment variables cannot be set to an empty string. CTest will interpret a whitespace-only string as empty.

In CMake 3.28 and earlier, an empty or 0 value was equivalent to 1.

See ctest(1) for more information on parallel test execution.