To run in background use & at the end of the program, for example:
$ my_prog input.txt > result.txt &
[1] 6790
The number is the Process ID or PID.
To check what process we have running
$ jobs
[1]+ Running my_prog input.txt > results.txt
To send the first process of the stack in foreground
$ fg
my_prog input.txt > result.txt
to send any process
$ fg %<num>
where num is the is the number of the process in the stack. In our example is 1.
To send in background a process use Ctrl-z