[cmake] Using CMake with GNU Make: How can I see the exact commands?

I use CMake with GNU Make and would like to see all commands exactly (for example how the compiler is executed, all the flags etc.).

GNU make has --debug, but it does not seem to be that helpful are there any other options? Does CMake provide additional flags in the generated Makefile for debugging purpose?

cmake --build . --verbose

On Linux and with Makefile generation, this is likely just calling make VERBOSE=1 under the hood, but cmake --build can be more portable for your build system, e.g. working across OSes or if you decide to do e.g. Ninja builds later on:

mkdir build
cd build
cmake ..
cmake --build . --verbose

Its documentation also suggests that it is equivalent to VERBOSE=1:

--verbose, -v

Enable verbose output - if supported - including the build commands to be executed.

This option can be omitted if VERBOSE environment variable or CMAKE_VERBOSE_MAKEFILE cached variable is set.

It is convenient to set the option in the CMakeLists.txt file as:

set(CMAKE_VERBOSE_MAKEFILE ON)

Or simply export VERBOSE environment variable on the shell like this: export VERBOSE=1

If you use the CMake GUI then swap to the advanced view and then the option is called CMAKE_VERBOSE_MAKEFILE.

I was trying something similar to ensure the -ggdb flag was present.

Call make in a clean directory and grep the flag you are looking for. Looking for debug rather than ggdb I would just write.

make VERBOSE=1 | grep debug

The -ggdb flag was obscure enough that only the compile commands popped up.