Creating a git commit id file in the project build folder using CMake - cmake

Question

First, I define the COMMIT_ID variable:
execute_process(COMMAND git rev-parse HEAD
WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
OUTPUT_VARIABLE COMMIT_ID )
If you specify the COMMIT_ID variable the project is not built:
add_custom_command(TARGET ${APP_NAME} POST_BUILD
WORKING_DIRECTORY
$<TARGET_FILE_DIR:${APP_NAME}>
DEPENDS
${COMMIT_ID}
COMMAND
${CMAKE_COMMAND} -E echo ${COMMIT_ID} > ./version.md
COMMENT
"Generating file version.md"
VERBATIM)
But, if you specify a static string the project is built without errors:
add_custom_command(TARGET ${APP_NAME} POST_BUILD
WORKING_DIRECTORY
$<TARGET_FILE_DIR:${APP_NAME}>
DEPENDS
${COMMIT_ID}
COMMAND
${CMAKE_COMMAND} -E echo "COMMIT_ID" > ./version.md
COMMENT
"Generating file version.md"
VERBATIM)

Answer 1

The issue with using the ${COMMIT_ID} variable is that it may contain trailing whitespace or newlines from its creation in execute_process. You can add the OUTPUT_STRIP_TRAILING_WHITESPACE argument to cleanup the output of execute_process before it is used in the variable:
execute_process(COMMAND git rev-parse HEAD
WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
OUTPUT_VARIABLE COMMIT_ID
OUTPUT_STRIP_TRAILING_WHITESPACE
)