How can I attach to the the first unattached tmux session, or create a new session if there are no unattached sessions?

Question

How can I attach to the the first tmux session that has no attached clients or, if there are no unattached sessions, create a new session and attach to that?

(The use case is for a command for my terminal emulator to run when opening a new terminal window. I don't want it to create a new session each time I open a window, if there are detached sessions lying around. Nor do I want it to re-attach to the same named session every time, if there's already a window attached to that session. I want it to recycle existing unattached sessions but create new sessions when there are none unattached.)

Answer 1

There may be a simpler way to do this using tmux itself, without resorting to shell scripts. But I got it working by combining a couple of scripts.

The first script prints out the name of the first unattached session. This is ,tmux-first-unattached-session:

#!/usr/bin/env sh
# Print the name of the first tmux session that has no clients attached.
tmux ls -F '#{session_name}|#{?session_attached,attached,not attached}' 2>/dev/null | grep 'not attached$' | tail -n 1 | cut -d '|' -f1

The second script attaches to the first unattached session or a new session:

#!/usr/bin/env sh
# Attach to the first tmux session that has no attached clients.
# If there are no unattached sessions, then create a new session.
tmux attach -t `,tmux-first-unattached-session` 2> /dev/null || tmux

Answer 2

I have something like this which is pretty simple.

1. Detect if there is an un-attached session, try to attach
2. Else start a clean session

# Start TMUX first; try to reattach a session
if [[ -z $TMUX ]]; then
  ATTACH_OPT=$(tmux ls | grep -vq attached && echo "attach -d")
  exec eval "tmux $ATTACH_OPT"
fi

Answer 3

I created a tmux2 script to handle this problem:

#!/bin/bash
#
# tmux2 - attach to first unattached session or
#         create new session if none are found
#
N=$(tmux ls | grep -v attached | head -1 | cut -d: -f1)

if [[ ! -z $N ]]
then
    ATTACH_OPTS="attach -t $N"
fi

exec tmux -CC $ATTACH_OPTS

In my case I'm using this for an iterm connection from my Mac. The connection is set up like this:

Now if my terminal session drops, I just reconnect and it picks up where I left things.