Generate script during build with cmake - cmake

Question

I have a cmake project with a single source file called main.c. I want to additionally provide a wrapper script which calls main with specific parameters.
My CMakeLists.txt looks as follows:
cmake_minimum_required(VERSION 3.1...3.16)
file(WRITE ${CMAKE_BINARY_DIR}/wrapper "#!/usr/bin/env bash\n")
file(APPEND ${CMAKE_BINARY_DIR}/wrapper "./main options\n")
add_executable(main main.c)
add_custom_target(wrapper_target
ALL DEPENDS wrapper)
add_custom_target(main_target
ALL DEPENDS main wrapper_target)
add_dependencies(main wrapper_target)
install(
TARGETS main
RUNTIME DESTINATION bin/)
install(
PROGRAMS wrapper
DESTINATION bin/)
If I run cmake --install ., the script wrapper is installed together with the binary main. Running cmake --build . produces the script wrapper, but it is not marked as executable (on Linux).
How can I tell cmake to also generate wrapper during build and mark it as executable?
Note: I need this for an automated build system which runs build and not install, and expects a specific file to be available on build.

Answer 1

Try:
file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/wrapper.tmp
"#!/usr/bin/env bash
# Note that './main' is relative from whatever directory you are in
# Use just main assuming the install prefix is in your bath
# Or use $<TARGET_FILE:main>
# Or maybe ${CMAKE_INSTALL_PREFIX}/bin/main
./main options
")
# add execute permissions
file(
COPY ${CMAKE_CURRENT_BINARY_DIR}/wrapper.tmp
DESTINATION ${CMAKE_CURRENT_BINARY_DIR}
FILE_PERMISSIONS OWNER_READ OWNER_WRITE OWNER_EXECUTE GROUP_READ GROUP_EXECUTE WORLD_READ WORLD_EXECUTE
)
# Rename the file
file(RENAME
${CMAKE_CURRENT_BINARY_DIR}/wrapper.tmp
${CMAKE_CURRENT_BINARY_DIR}/wrapper
)
How can I tell cmake to also generate wrapper during build and mark it as executable?
file( is a in a cmake script - it is executed during configuration phase, when cmake is executed. To generate the file during build use add_custom_command, the most "portable" way in cmake sense would be to run a cmake script inside add_custom_command:
add_custom_command
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/wrapper
COMMAND $(CMAKE_COMMAND)
-D CMAKE_CURRENT_BINARY_DIR=${CMAKE_CURRENT_BINARY_DIR}
-P ${CMAKE_CURRENT_SOURCE_DIR}/the_cmake_script.cmake
DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/the_cmake_script.cmake
COMMAND Generating wrapper script...
VERBATIM
)
then inside the_cmake_script.cmake you could do the script above - add_custom_command will execute the command cmake -P <the script> during build of you project. That way you can DEPEND properly on the wrapper script.

Answer 2

CMake 3.20 added support for FILE_PERMISSIONS attribute to the FILE command. So one could simply:
FILE(GENERATE OUTPUT ${CMAKE_BINARY_DIR}/wrapper
CONTENT
"#!/usr/bin/env bash
./main options
"
FILE_PERMISSIONS OWNER_EXECUTE OWNER_WRITE OWNER_READ)