I am using `&`: why isn't the process running in the background?

When you close a terminal window, the terminal emulator sends a SIGHUP to the process it is running, your shell. Your shell then forwards that SIGHUP to everything it's running. On your local system, this is the ssh. The ssh then forwards the SIGHUP to what it's running, the remote shell. So your remote shell then sends a SIGHUP to all its processes, your backgrounded program.

There are 2 ways around this.

1. Disassociate the backgrounded program from your shell.
   1. Use the disown command after backgrounding your process. This will make the shell forget about it.
   2. Prefix your command with nohup (nohup $python program.py &). This accomplishes the same thing, but by using an intermediate process. Basically it ignores the SIGHUP signal, and then forks & executes your program which inherits the setting, and then exits. Because it forked, the program being launched is not a child of the shell, and the shell doesn't know about it. And unless it installs a signal handler for SIGHUP, it keeps the ignore action anyway.
2. Use logout instead of closing the terminal window. When you use logout, this isn't a SIGHUP, and so the shell won't send a SIGHUP to any of its children.

Additionally you must make sure that your program doesn't write to the terminal through STDOUT or STDERR, as both of those will no longer exist once the terminal exits. If you don't redirect them to something like /dev/null, the program will still run, but if it tries to write to them, it'll get a SIGPIPE, and the default action of SIGPIPE is to kill the process).

The process is running in the background in the terminal, but the output from stdout (and stderr) is still being sent to the terminal. To stop this, add > /dev/null 2>&1 before the & to redirect both the outputs to /dev/null - adding disown also makes sure the process is not killed after you close the terminal:

COMMAND > /dev/null 2>&1 & disown

In your case this would be:

python program.py > /dev/null 2>&1 & disown

The & operator separates commands to run in parallel, just as ; separates commands to run in series. Both kinds of commands will still run as a child of the shell process.

So, when you close the shell which started those children, the children will be closed also.

What you appear to want is a daemon process, which is significantly more tricky because it needs to dissociate entirely from the parent process. The shell usually doesn't have a simple way of doing that.