CMake check that a local file exists

In my CMake script I want to see if I have a file on my system, and if it is there do something with it, otherwise do something with a default file. Here is the code:

find_file(
          ${project_name}_${customer}_config 
          ${ROOT}/configuration/${customer}/configuration.${project_name}.xml
)

if(NOT ${${project_name}_${customer}_config} STREQUAL
   ${project_name}_${customer}_config-NOTFOUND )
        configure_file(${ROOT}/configuration/${customer}/configuration.${project_name}.xml
                       ${CMAKE_CURRENT_BINARY_DIR}/conf/configuration.xml)
else()
    configure_file(${FAPP_ROOT}/configuration/Default/configuration.${project_name}.xml
                   ${CMAKE_CURRENT_BINARY_DIR}/conf/configuration.xml)
endif()

But it seems, this is not working.

What is the proper way of checking if a file exists in CMake?

The proper way to check if a file exists, if you already know the full path name to the file is simply:

if(EXISTS "${ROOT}/configuration/${customer}/configuration.${project_name}.xml")
   ...
else()
   ...
endif()

You should be able to just use

if(NOT ${project_name}_${customer}_config)

From the docs:

if(<constant>)

True if the constant is 1, ON, YES, TRUE, Y, or a non-zero number. False if the constant is 0, OFF, NO, FALSE, N, IGNORE, "", or ends in the suffix '-NOTFOUND'.

However, if the file is found using find_file, the value is cached, and subsequent runs of CMake will not try to find it again. To force a recheck on every run, before the find_file call do:

unset(${project_name}_${customer}_config CACHE)