How to move a tmux pane to another session without restarting its process?

Short answer

tmux join-pane -s scratch              # active pane of session 'scratch' -> split here
tmux join-pane -h -s scratch:2.1       # a precise pane (window 2, pane 1) of another session
tmux move-window -s scratch:1 -t :     # or the whole window -> new window in this session

join-pane and move-window take a source (-s) in another session of the same tmux server, and the process running in the pane survives the move: tmux re-parents the pane, it does not restart anything. Typical case: a dev server launched in a quick throwaway session, pulled into the main workspace without cutting it.

Details

The source can be a session name alone (scratch, meaning its active pane), a session:window.pane form (scratch:2.1), or a pane id (%7), which is unambiguous and works across sessions too. With move-window, -t : means “this session, next free index”; name the session explicitly (-t main:) when running it from a script rather than from inside tmux.

Once the source session loses its last pane it closes itself, so emptying a scratch session with joins needs no cleanup afterwards.

The limit is the server: sessions of the same server share one process tree, so panes move freely between them. Two different servers (started with different sockets, tmux -L or -S) cannot exchange panes; nothing short of restarting the process crosses that boundary.

This is the cross-session variant of pulling a pane from another window with join-pane, covered earlier in the same cookbook; the targeting rules (qualify the source, prefer pane ids over bare numbers) apply unchanged.

Verified on tmux 3.2a.

Links & references

• Wiki: tmux