How to trace a lost variable value in CMake?

Edit: the accepted answer does not solve my problem yet but it answered the question I've asked - if you can help me with my actual problem described below you might answer this question. I have a CMake-project which makes use of a framework which needs a variable to be set (namely https://github.com/queezythegreat/arduino-cmake, which needs `ARDUINO_SDK_PATH`) Strangely after I set that variable on the command line it first has a value but it looks like it disappears after a while. I'm running ``` cmake -DARDUINO_SDK_PATH=/path/to/sdk .. ``` .. and get an error message which tells me that it's not set. Printing out the value at the top of my `CMakeLists.txt` and deep inside this framework where the variable is being checked gives me something like this: ``` >>> ARDUINO_SDK_PATH (beginning): '/home/me/project/arduino-1.8.2' >>> ARDUINO_SDK_PATH (before check): '/home/me/project/arduino-1.8.2' -- The C compiler identification is GNU 6.2.0 -- The CXX compiler identification is GNU 6.2.0 -- Check for working C compiler: /usr/bin/avr-gcc >>> ARDUINO_SDK_PATH (before check): '' CMake Error at /home/me/project/arduino-cmake/cmake/ArduinoToolchain.cmake:84 (message): Could not find Arduino SDK (set ARDUINO_SDK_PATH)! Call Stack (most recent call first): /home/me/project/build/CMakeFiles/3.6.2/CMakeSystem.cmake:6 (include) /home/me/project/build/CMakeFiles/CMakeTmp/CMakeLists.txt:3 (project) CMake Error: Internal CMake error, TryCompile configure of cmake failed -- Configuring incomplete, errors occurred! See also "/home/me/project/build/CMakeFiles/CMakeOutput.log". ``` So it looks like `ARDUINO_SDK_PATH` looses it's value somehow. I didn't find an actual command where it get's assigned any value so I don't know how to proceed.. I can now of course add code to my CMake project everywhere to print out the value of `ARDUINO_SDK_PATH` but I wonder if there's a builtin way to trace variable values. I tried `cmake --trace ..` and `cmake --trace-expand ..` but the output doesn't seem helpful.. System: Fedora 25 with CMake 3.6.2 Update Thanks to Florian I've added `variable_watch(ARDUINO_SDK_PATH)` as my first line in `CMakeLists.txt` and now my variable trace lines (`message()`) look like this: ``` CMake Debug Log at arduino-cmake/cmake/ArduinoToolchain.cmake:41 (MESSAGE): Variable "ARDUINO_SDK_PATH" was accessed using READ_ACCESS with value "/home/me/project/arduino-1.8.2". Call Stack (most recent call first): /usr/share/cmake/Modules/CMakeDetermineSystem.cmake:98 (include) CMakeLists.txt:10 (project) >>> ARDUINO_SDK_PATH (before check): /home/me/project/arduino-1.8.2 ``` I have about 30 messages like this but there follow a couple of trace lines without the value and without the trace message. So it looks like the variable `ARDUINO_SDK_PATH` gets replaced by a new one which is empty and which is not traced any more.. Reproduce In order to make this behavior reproducible I've uploaded the code: https://github.com/frans-fuerst/trinket_led Note: the provided `CMakeLists.txt` does not contain useful code yet - it just reproduces the error. You need to download the Arduino-SDK, extract it and provide the path on the command line: ``` tar xvf ~/Downloads/arduino-1.8.2-linux64.tar.xz git clone https://github.com/frans-fuerst/trinket_led cd trinket_led git submodules update --init mkdir build cd build cmake -DARDUINO_SDK_PATH=/path/to/arduino-1.8.2 .. ``` Note: there is a `find_path` command in `ArduinoToolchain.cmake` which looks suspicious. But you can remove it with the same result.. CMakeFiles/CMakeOutput.log ``` The target system is: Arduino - - The host system is: Linux - 4.10.12-200.fc25.x86_64+debug - x86_64 Compiling the C compiler identification source file "CMakeCCompilerId.c" succeeded. Compiler: /usr/bin/avr-gcc Build flags: Id flags: The output was: 0 Compilation of the C compiler identification source "CMakeCCompilerId.c" produced "a.out" The C compiler identification is GNU, found in "/home/frans/_HOME/1704_trinket_led/build/CMakeFiles/3.6.2/CompilerIdC/a.out" Compiling the CXX compiler identification source file "CMakeCXXCompilerId.cpp" succeeded. Compiler: /usr/bin/avr-g++ Build flags: Id flags: The output was: 0 Compilation of the CXX compiler identification source "CMakeCXXCompilerId.cpp" produced "a.out" The CXX compiler identification is GNU, found in "/home/frans/_HOME/1704_trinket_led/build/CMakeFiles/3.6.2/CompilerIdCXX/a.out" ```