cmake ExternalProject_Add how to download URL non-tar file?

How does one set up an External project to download a link that isn't a .tgz file?

For example Catch provides a release that is a single header distribution. I would like to just download this rather than the git repo or the .tgz release. But I haven't figured out how to tell CMake to do this.

I would like to do something like:

Include(ExternalProject)
ExternalProject_Add(
    catch
    PREFIX ${CMAKE_BINARY_DIR}/catch
    URL https://github.com/philsquared/Catch/releases/download/v1.9.6/catch.hpp ${CMAKE_BINARY_DIR}/catch
    CONFIGURE_COMMAND ""
    BUILD_COMMAND ""
    INSTALL_COMMAND ""
    LOG_DOWNLOAD ON
)

This fails as URL assumes that url is a compressed tar file. I have tried various variants of the DOWNLOAD_COMMAND without any success either.

Solution

Use DOWNLOAD_NO_EXTRACT option of ExternalProject_Add:

Just download the file and do not extract it; the full path to the downloaded file is available as <DOWNLOADED_FILE>.

Code example:

ExternalProject_Add(
    catch
    PREFIX ${CMAKE_BINARY_DIR}/catch
    URL https://github.com/philsquared/Catch/releases/download/v1.9.6/catch.hpp
    DOWNLOAD_NO_EXTRACT 1
    CONFIGURE_COMMAND ""
    BUILD_COMMAND ""
    INSTALL_COMMAND ""
    )

File will be downloaded into <prefix>/src directory.

If use LOG_DOWNLOAD option alongside with DOWNLOAD_NO_EXTRACT, you need CMake 3.9 for work correctly. See these bugreports: https://gitlab.kitware.com/cmake/cmake/issues/16544, https://gitlab.kitware.com/cmake/cmake/issues/17046.