Well you have two sets of files, one for windows and one for linux. If you go ahead and open the nifi.cmd File, you can see that you can run the file with the arguments set-single-user-credentials and with the username and password in quotes:
nifi.cmd set-single-user-credentials "username" "password"
if %RUN_COMMAND% == "set-single-user-credentials" (
rem Set credentials with quoted arguments passed to Java command
set "CREDENTIALS=^"%~2^" ^"%~3^""