Copy file example

Consider that we want a build step which copies and renames a file, and we only want it to be done when needed - i.e. if and only if:

• there exists a target being built which depends on the output file; AND
• either:
  • the output file doesn't exist; OR
  • the output file is older than the input file according to the file timestamps

This can be achieved using add_custom_command such that it adds a custom command to produce an output:

add_custom_command(
	OUTPUT out.txt 
	DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/in.txt
	COMMENT "copying file" 
	COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/in.txt out.txt)

Note that we cannot simply use a copy command on a custom target, because a custom target is always considered out of date.

cmake doesn't analyse the COMMAND parameter to work out the dependencies. Instead the dependencies have to be specified explicitly. In this case:

• out.txt is declared to be an output of a custom command
• The command which generates out.txt is declared to depend on the file named in.txt

Note that out.txt is only generated if there is some downstream target that depends on it. For example, this could be achieved by declaring a custom target named T which is always built and depends on out.txt:

add_custom_target(T ALL DEPENDS out.txt)

This adds the following dependencies: