CMake add_custom_command does not find file

I have a example project with the structure below:

--SampleMinimal
----CMakeLists.txt
----main.cpp
----sample.cpp

and the CMakeLists.txt:

cmake_minimum_required(VERSION 3.20)
project(sample-minimal)
set(CMAKE_CXX_STANDARD 11)

add_custom_command(OUTPUT sample2.cpp
        COMMAND cp sample.cpp sample2.cpp
        DEPENDS sample.cpp)
add_executable(grpc-app-main main.cpp sample2.cpp)

While I build the target, I get these errors:

$cmake --S . -B build
$ cmake --build build
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/SL/Projects/cpp/sample_minimal/build
[ 25%] Generating sample2.cpp
cp: sample.cpp: No such file or directory
make[2]: *** [sample2.cpp] Error 1
make[1]: *** [CMakeFiles/grpc-app-main.dir/all] Error 2
make: *** [all] Error 2

There exists in the project, Why does it could not find the file, why doesn't it copy the sample into the cmake-build-debug folder like other source files?

The default working directory for running the command of add_custom_command is the directory in the build tree corresponding to the current CMakeLists.txt. This means the command isn't be able to find the file via the relative path sample.cpp and it wouldn't copy the file to a location where it can be found by the executable target as sample2.cpp.

Generating files to the source tree is not a good choice anyways, since it prevents you from setting up multiple build dirs based on the same source dir in a way that has all of the projects work properly. (In this case it happens to not cause any problems, since the logic for generating the file is independent of the configuration.)

Here's what I'd use

set(GENERATED_SOURCE ${CMAKE_CURRENT_BINARY_DIR}/sample2.cpp)

add_custom_command(OUTPUT sample2.cpp
        COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/sample.cpp ${GENERATED_SOURCE}
        DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/sample.cpp)
add_executable(grpc-app-main main.cpp ${GENERATED_SOURCE})

Note: I also changed the use of cp to using cmake -E copy to be platform independent here.

If sample.cpp isn't in /Users/AAA/clion/BigPro/cmake-build-debug/grpc-demo, you either have to adjust the WORKING_DIRECTORY of the custom command, or use absolute paths for the files. For example, instead of COMMAND cp sample.cpp sample2.cpp, do COMMAND cp "${PROJECT_SOURCE_DIR}/sample.cpp" sample2.cpp.