This works, most of the time:

Prerequisites: have reptyr and tmux/screen installed; you'll be able to find them with apt-get or yum, depending on your platform.

1. Use Ctrl+Z to suspend the process.

2. Resume the process in the background with bg

3. Find the process ID of the background process with jobs -l

   You'll see something similar to this:

   [1]+ 11475 Stopped (signal) yourprocessname
   

4. Disown the job from the current parent (shell) with disown yourprocessname

5. Start tmux (preferred), or screen.

6. Reattach the process to the tmux/screen session with reptyr:

   reptyr 11475
   

7. Now you can detach the multiplexer (default Ctrl+B, D for tmux, or Ctrl+A, D for screen), and disconnect SSH while your process continues in tmux/screen.

8. Later when you connect with SSH again, you can then attach to your multiplexer (e.g. tmux attach).

You can revoke “ownership” of the program from the shell with the disown built-in:

# press Ctrl+Z to suspend the program
bg
disown

However this only tells the shell not to send a SIGHUP signal to the program when the shell exits. The program will retain any connection it has with the terminal, usually as standard input, output and error streams. There is no way to reattach those to another terminal. (Screen works by emulating a terminal for each window, so the programs are attached to the screen window.)

It is possible to reattach the filedescriptors to a different file by attaching the program in a debugger (i.e. using ptrace) and making it call open, dup and close. There are a few tools that do this; this is a tricky process, and sometimes they will crash the process instead. The possibilities include (links collected from answers to How can I disown a running process and associate it to a new screen shell? and Can I nohup/screen an already-started process?):

• grab (and the more ambitious cryopid)
• neercs
• reredirect
• reptyr
• retty

You can not re-attach a process id. You need to reattach the corresponding tmux session.

So do tmux ls. Pick whatever session you want to re-attach. Then do tmux attach -d -t <session id> to re-attach it to a new tmux instance and release it from the old one.

There is no portable way to do this. There is a program https://github.com/nelhage/reptyr that can do this on Linux (it uses ptrace and interposes all the system calls), but I don't know how reliable it is or if it is still maintained. YMMV.

You can list tmux sessions when reconnecting to the remote server:

tmux ls
0: 4 windows (created Tue Oct 15 07:55:07 2019) [255x62]
1: 2 windows (created Tue Oct 15 07:55:07 2019) [255x62]

You can then reattach to a target session:

tmux attach-session -t 1

This would attach to the second tmux instance listed above.

If you just have one tmux session running, then you can use the shorthand:

tmux a

to reattach the first session.