How to copy a file if modified and avoid rebuilding executables and libraries when running make all? - cmake

Question

I want to copy a file from the source directory to the binary directory when running make or make all if the file has been modified. So if only this file has been modified, then no libraries or executables should be rebuilt when running make. Only the file should be copied. I tried several approaches, for example:
cmake_minimum_required(VERSION 3.18)
project(copy-file VERSION 1.0 DESCRIPTION "testing copy file if modified on make all")
set(FILE_PATH "some_dir/file.txt")
add_executable(hello hello.c)
#add_custom_command(
# OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${FILE_PATH}
# COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/${FILE_PATH} ${CMAKE_CURRENT_BINARY_DIR}/${FILE_PATH}
# MAIN_DEPENDENCY ${CMAKE_CURRENT_SOURCE_DIR}/${FILE_PATH})
add_custom_command(
TARGET hello
POST_BUILD
COMMAND ${CMAKE_COMMAND} -E copy
${CMAKE_CURRENT_SOURCE_DIR}/${FILE_PATH} ${CMAKE_CURRENT_BINARY_DIR}/${FILE_PATH})
#configure_file(${FILE_PATH} ${FILE_PATH} COPYONLY)
Unfortunately, this only copies file.txt (when running make) if hello.c has been modified. If hello.c has not been modified, but file.txt has been modified, nothing happens when I run make (whereas I expected the file to be copied from the source directory to the binary directory)
Here is a link to the source files I used for this minimal example.
Any ideas what I am missing?

Answer 1

Do not use POST_BUILD on a custom target. Specify input and output and let CMake take care of the dependency.
add_custom_command(
OUTPUT
${CMAKE_CURRENT_BINARY_DIR}/${FILE_PATH}
DEPENDS
${CMAKE_CURRENT_SOURCE_DIR}/${FILE_PATH}
COMMAND
${CMAKE_COMMAND} -E copy_if_different
${CMAKE_CURRENT_SOURCE_DIR}/${FILE_PATH}
${CMAKE_CURRENT_BINARY_DIR}/${FILE_PATH}
)
add_custom_target(copy DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/${FILE_PATH})

Answer 2

Following the suggestion of #vre and adding copy_if_different instead of copy and then adding add_custom_target() that can be used with the custom command like this seems to work now:
cmake_minimum_required(VERSION 3.18)
project(copy-file VERSION 1.0 DESCRIPTION "testing copy file if modified on make all")
set(FILE_PATH "some_dir/file.txt")
add_executable(hello hello.c)
add_custom_target(copy ALL )
add_custom_command(
TARGET copy
PRE_BUILD
COMMAND ${CMAKE_COMMAND} -E copy_if_different
${CMAKE_CURRENT_SOURCE_DIR}/${FILE_PATH} ${CMAKE_CURRENT_BINARY_DIR}/${FILE_PATH})