Move a tmux pane to another session

Short of doing this, is it possible to move a running command in a pane to another tmux session?

Yes it's pretty easy, use the move-pane tmux command. For more info run

man tmux | less -I '+/mOVE-pane'

The tricky part is just knowing how tmux references sessions, windows and panes in the commands to manipulate panes etc.

Step one

Move into the pane you want to move to the other session

Step two

Get the name of the target session by running <prefix>s.

On my system that produces:

 (0) + 0: 8 windows (attached)  
 (1) + 1: 1 windows 
       ^
       |
    This is the session name. It could be different 
    on your system and it could be a word (not just a number).

Step 3 - run command

<prefix>:move-pane -t <session_name>:<window number>
                      ^^^^^^^^^^^^^^ the name from above goes in here

So on my system I moved into the pane with the running process, ran

<prefix>:move-pane -t 1:1

and the pane with the process in it moved to window one on the session 1.

It gets inserted as a split, so usually you would want to run window on the other session as a throwaway window.

Note: it seems that the target window does need to exist, else the move-pane command will issue an error.

For more background on the format for specifying sessions, windows and panes within a tmux command, see the section COMMANDS in man tmux.

I can't comment, so I will place it here: for tmux 2.1-3build1~precise the command should be:

<prefix>:move-pane -t session_name:window_number

-- you need a "-t" before session name (not necessarily a number), and the window with a given number must exist.

All the rest is as described by the_velour_fog in his answer.

P.S. If you type only a session_name without window_number like so:

<prefix>:move-pane -t session_name:,

then your pane will be moved into current window of destination session (and this window will be split).

P.P.S. If you just want to move your window (and not just your pane), then there a good answer on how to Move window between tmux clients:

<prefix>:move-window [-d] [-s src-window] [-t dst-window]

(alias: movew)

where src-window and dst-window have the form session:window.pane.

For Version tmux 3.1b - I just found out about the shortcut: <prefix>. (a dot)

<prefix>. then input 6<enter> - move current window to position 6 (in the current session) <prefix>. then input 2:6<enter> - move current window to position 6 in session 2 (see als session names/numbers with <prefix>s or <prefix>:-ls<enter>

(also new for me: <prefix>! to move a pane (from a split) to a new window)