Can I create at tmux split which is a sub shell of the current pane? - tmux

Question

The question is pretty self explanatory... I have a pane in tmux running bash, and I want to split it, but instead of running a fresh shell I want the new pane to be a sub shell of the first.

Answer 1

Here's a way using reptyr that has limitations which may not work for your case. You should expect to find this utility in a package for your system. I used the following script to test it out:
xterm -e 'tmux new-session -s mysession' &
sleep 1
tmux split-window -v -t 0
sleep 1
tmux send-keys -t 0 'cd /tmp;V=1' Enter
tmux send-keys -t 1 'cd /usr;V=3' Enter
sleep 1
tmux send-keys -t 0 'V=2 bash' Enter
sleep 1
tmux send-keys -t 0 'echo $$; echo $$ >/tmp/pid' Enter
sleep 1
tmux send-keys -t 1 'exec reptyr $(</tmp/pid)' Enter
sleep 2
tmux send-keys -t 1 'echo $(pwd) $$ V=$V' Enter
tmux send-keys -t 0 'echo $(pwd) $$ V=$V' Enter
We start by splitting a window running bash into two.
In the top pane (-t 0) we change the directory, and set an environment variable V to value 1, and in the bottom pane we change to a different directory and set V to 3, just to track what happens.
In the top pane we then start a child bash with V=2 in its environment, and echo the pid into a file.
In the bottom pane we run reptyr to attach to that pid, and then in both panes we see where we are.
The resulting output terminal looks like this:
$ cd /tmp;V=1
$ V=2 bash
$ echo $$; echo $$ >/tmp/pid
27236
$
[1]+ Stopped V=2 bash
$ echo $(pwd) $$ V=$V
/tmp 27184 V=1
$
----------------------------------------
$ cd /usr;V=3
$ exec reptyr $(</tmp/pid)
[-] Timed out waiting for child stop.
$ echo $(pwd) $$ V=$V
/tmp 27236 V=2
$
Clearly, the bottom pane is now process 27236 we launched in the top pane, with V=2 and the appropriate directory, so this has worked.