[CMake] stderr and stdout in cmake

[luxInteg]

Greetings

I have   a  bash script (from an auto-tools Makefile )   to translate   to 
cmake


My 'translation might  go something like thios

execute_process(COMMAND ${some_command}  '2>&1'
                INPUT_FILE        ${someFile2}
                OUTPUT_FILE        /dev/null
                WORKING_DIRECTORY ${some_dir} )

I am unconfortable with the '2>&1'  and I tried  reading up on 

file(WRITE ) and 
file(APPEND )   

but I am unsure how to proceed; for example how to use cmake -P  or cmake --
build  or using /dev/std* device files

test [ ~ ]$ ls /dev/std*
/dev/stderr  /dev/stdin  /dev/stdout
test [ ~ ]$ 


Advice would be  appreciated

Yours sincerely
luxInteg