Using cmake as the configure tool (CONFIGURE_COMMAND) in CMakes "Add_External_Project" function

I am having an issue with CMakes Add_External_Project functionality (more of an annoyance than anything else). Specifically, I do not understand the keys CONFIGURE_COMMAND, BUILD_COMMAND and INSTALL_COMMAND.

In the following (working) example, which downloads Google's test library, the two files at the end of the question will ensure that the third party libraries are downloaded and built (not installed).

However, when I tried to add configure and build commands as "CONFIGURE_COMMAND" and "BUILD_COMMAND" (cmake . and cmake --build) instead of having to do execute_process CMake craps out with the error message:

[ 55%] Performing configure step for 'googletest'
/bin/sh: 1: cmake .: not found

Am I trying to do something that is obviously not within the scope of the Add_External_Project functionality?

Example Files:

CMakeLists.txt

cmake_minimum_required (VERSION 3.0)

project (Test VERSION 0.1.0.0 LANGUAGES CXX)

# Download and unpack googletest at configure time
configure_file("${CMAKE_CURRENT_SOURCE_DIR}/CMakeLists.txt.in" "${CMAKE_BINARY_DIR}/googletest-download/CMakeLists.txt" @ONLY)
execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" . WORKING_DIRECTORY "${CMAKE_BINARY_DIR}/googletest-download" )

execute_process(COMMAND ${CMAKE_COMMAND} --build . WORKING_DIRECTORY "${CMAKE_BINARY_DIR}/googletest-download")

add_subdirectory("${CMAKE_BINARY_DIR}/googletest-src" "${CMAKE_BINARY_DIR}/googletest-build")

CMakeLists.txt.in

cmake_minimum_required(VERSION 3.0)

project(third-party NONE)

include(ExternalProject)
ExternalProject_Add(googletest
  GIT_REPOSITORY    https://github.com/google/googletest.git
  GIT_TAG           master
  SOURCE_DIR        "@CMAKE_BINARY_DIR@/googletest-src"
  BINARY_DIR        "@CMAKE_BINARY_DIR@/googletest-build"
  CONFIGURE_COMMAND ""
  BUILD_COMMAND     ""
  INSTALL_COMMAND   ""
  TEST_COMMAND      ""
)

Solution

If you don't specify CONFIGURE_COMMAND at all, it will assume a CMake project and run the appropriate cmake command for you (by appropriate, I mean it will use the same CMake generator as your main build, etc.). Similarly, if you leave out BUILD_COMMAND, it will also assume a CMake project and do cmake --build for you. So in your case, just leave out those two lines and ExternalProject_Add() should do exactly what you want.

The main reason you might specify these two options as empty strings is to prevent those steps from doing anything at all. This can be useful, for example, to use ExternalProject_Add() simply for its download and unpacking functionality. This exact situation is used in a technique described here for downloading the source of GoogleTest so it can be added to your project via add_subdirectory(), making it part of your build (see also this answer and other answers to that question for some related material). I suspect this might be where your code is derived from, as the structure looks similar.

For completeness, if you find yourself in a situation where you do need to specify a CMake command, don't use a bare cmake to refer to the command to run. Instead, always use ${CMAKE_COMMAND}, which is provided by CMake as the location of the CMake executable currently being used to process the file. Using this variable means cmake doesn't have to be on the user's PATH and also ensures that if the developer chooses to run a different version of CMake other than the one on the PATH, that same cmake will still be used for the command you are adding.