Copy file from source directory to binary directory using CMake

You may consider using configure_file with the COPYONLY option:

configure_file(<input> <output> COPYONLY)

Unlike file(COPY ...) it creates a file-level dependency between input and output, that is:

If the input file is modified the build system will re-run CMake to re-configure the file and generate the build system again.

both option are valid and targeting two different steps of your build:

1. file(COPY ... copies the file in configuration step and only in this step. When you rebuild your project without having changed your cmake configuration, this command won't be executed.
2. add_custom_command is the preferred choice when you want to copy the file around on each build step.

The right version for your task would be:

add_custom_command(
        TARGET foo POST_BUILD
        COMMAND ${CMAKE_COMMAND} -E copy
                ${CMAKE_SOURCE_DIR}/test/input.txt
                ${CMAKE_CURRENT_BINARY_DIR}/input.txt)

you can choose between PRE_BUILD, PRE_LINK, POST_BUILD best is you read the documentation of add_custom_command

an example on how to use the first version can be found here: Use CMake add_custom_command to generate source for another target