Create CMake target that copies files/dirs but excludes some items - c++

Question

I am trying to generate a CMake target for a C++ project using add_custom_target that copies the contents of the directory in which the CMakeLists.txt resides into ${CMAKE_BINARY_DIR}, but excludes a given list of files.
While it looks like a quite easy task, I'm facing problems here. My attempts so far:
1) Generate a list of all files/dirs, remove the items to be excluded and use the CMake copy command:
file(GLOB files_to_copy LIST_DIRECTORIES TRUE *)
list(REMOVE_ITEM files_to_copy
${CMAKE_CURRENT_SOURCE_DIR}/file_to_exclude.txt
${CMAKE_CURRENT_SOURCE_DIR}/dir_to_exclude
# ...
)
add_custom_target(my-target
COMMAND ${CMAKE_COMMAND} -E copy ${files_to_copy} ${CMAKE_BINARY_DIR}
)
Problems:
Removing items this way works on a string comparison level, which could lead to problems when using symbolic links, for example
The copy command line tool apparently supports copying directories, however it doesn't seem to work on my machine, therefore directories do not get copied.
2) Use the file command to copy the files, excluding some files
file(COPY "${CMAKE_CURRENT_SOURCE_DIR}/" DESTINATION ${CMAKE_BINARY_DIR}
PATTERN "file_to_exclude.txt" EXCLUDE
PATTERN "dir_to_exclude" EXCLUDE
)
Problems:
This does exactly what I want, except that the command is not bound to a target and therefore is always executed. But I only need the copy operation to be a target.
Is there any possibility to bind the file(COPY ...) command to a target? Or any other straightforward solution to achieve what I want?
Please note that I only want to use CMake built-in tools and not execute any OS-specific shell commands (working on a cross-platform project).

Answer 1

CMake scripting commands work only in CMake context and are executed immediately.
But a COMMAND in add_custom_command (or add_custom_target) is executed in the context of a build tool (e.g. Makefile), not in CMake context.
However, you may put CMake scripting commands into separate CMake script, and call this script from the COMMAND. This solution has the same platform-independent properties as CMakeLists.txt itself.
You may parameterize separate script either:
With configure_file CMake command.
By passing -D parameters to CMake when call the script.
The first approach is quite simple: you write the script as would you write CMakeLists.txt itself. But it generates additional files for every parametrization set.
The second approach is useful for multi-purpose (or multi-usable) scripts, as it doesn't create additional copy of the script for every usage. But it requires some design of the parameters.
Using 'configure_file'
copy_sources.cmake.in (as if commands are written in CMakeLists.txt):
file(COPY "${CMAKE_CURRENT_SOURCE_DIR}/" DESTINATION ${CMAKE_BINARY_DIR}
PATTERN "file_to_exclude.txt" EXCLUDE
PATTERN "dir_to_exclude" EXCLUDE
)
CMakeLists.txt:
# Instantiate the parameterized script in the binary directory.
configure_file(copy_sources.cmake.in copy_sources.cmake)
add_custom_target(my-target
COMMAND ${CMAKE_COMMAND} -P ${CMAKE_BINARY_DIR}/copy_sources.cmake
)
Using '-D' parameters
copy_sources.cmake:
# Expect the script to be called from the source directory.
# This saves one parameter.
#
# DEST_DIR denotes the directory for copy to.
file(COPY "${CMAKE_SOURCE_DIR}/" DESTINATION ${DEST_DIR}
PATTERN "file_to_exclude.txt" EXCLUDE
PATTERN "dir_to_exclude" EXCLUDE
)
CMakeLists.txt:
add_custom_target(my-target
COMMAND ${CMAKE_COMMAND}
-DDEST_DIR=${CMAKE_BINARY_DIR} # all '-D' options should preceed '-P' one
-P ${CMAKE_CURRENT_SOURCE_DIR}/copy_sources.cmake
WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} # The script assumes proper current directory
)