Run or send a command to a tmux pane in a running tmux session

Each tmux pane is an interface for a single pty (pseudo tty). Non-split windows have a single pane; split windows have multiple panes.

tmux does not supply a way to add extra processes to a pane once it has been started with its initial command. It is up to that initial command’s process (usually a shell) to supply job control¹ for that terminal.

If you want to clobber whatever is currently running in the pane, you can use respawn-pane -k to kill the existing command and replace it with a new one (e.g., respawn-pane -t sessionname:0.4 -k 'some -new command').

But, if you want to maintain whatever is currently running in the pane, then there may be no better option that simply “typing at it” with send-keys.

You might script it like this (attach last, because otherwise the script will just wait for you to detach before continuing):

session=whatever
window=${session}:0
pane=${window}.4
tmux send-keys -t "$pane" C-z 'some -new command' Enter
tmux select-pane -t "$pane"
tmux select-window -t "$window"
tmux attach-session -t "$session"

Note that, on the send-keys command, you should actually type the letters E n t e r, to tell tmux to send a newline key to the window.  (Naturally, every command ends with the Enter key.)

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¹Job control is the arbitration between multiple process groups of a single session that uses the tty as its controlling terminal. I.e., the Ctrl+Z suspend key, and the jobs, fg, and bg shell commands.

You can use C-m instead of Enter. It's the same thing. Also, tmux makes it easy to send to the right / left pane by using -t right or -t left. And if you want to address a pane in another window, just prefix it with the window number and period, like -t 0.right.

Now you can write a simple Bash function to get the sequence of keys as argument and send it over to the target pane easily.