Bash – Start shell with tmux and attach only to a session if it is unattached

bashshelltmuxzsh

I am currently starting tmux when opening my shell, which the following config for my shell

[[ $TMUX == "" ]] && tmux new-session

Is there any way to start my shell and attach tmux to the (first) unattached session, if one exists? If a shell is already attached to that session I don't want to attach to that.

Best Answer

I think you can achieve what you're looking for by using an appropriate format with tmux list-session and parsing the output:

if [ -z "$TMUX" ]; then
    attach_session=$(tmux 2> /dev/null ls -F \
        '#{session_attached} #{?#{==:#{session_last_attached},},1,#{session_last_attached}} #{session_id}' |
        awk '/^0/ { if ($2 > t) { t = $2; s = $3 } }; END { if (s) printf "%s", s }')

    if [ -n "$attach_session" ]; then
        tmux attach -t "$attach_session"
    else
        tmux
    fi
fi

The format here for tmux ls gives, for each session, the number of clients attached, time when last attached (or 1 if never attached before, such as after tmux new -d), and session ID. The AWK script uses this information to find the most recently attached session with no currently attached clients and outputs its ID. We then either attach to that session or create a new one if no such session is found, like when the server isn't started or every existing session is attached.

Related Solutions

Bash – Why might tmux only be capable of attaching once per shell session

In my Shell's config file (I use zsh, so its in my .zshrc, should work in a .bashrc) I have the following:

# Default to TMUX
if [ -z "$TMUX" ]; then
  base_session=$USER"_session"
  # Create the base session if it doesn't exist
  tmux has-session -t $base_session || tmux new-session -d -s $base_session
  # Get a count of clients connected
  client_cnt=$(tmux list-clients | wc -l)
  if [ $client_cnt -ge 1 ]; then
    # Make a unique session name
    session_name=$base_session"-"$client_cnt
    # Create the new session based on the base_session
    tmux new-session -d -t $base_session -s $session_name
    # Launch the connection with a few caveats (kill the session when the client goes away)
    tmux -2 attach-session -t $session_name \; set-option destroy-unattached
  else
    tmux -2 attach-session -t $base_session
  fi
fi

Now I can connect many times using PuTTY to a single server and see the same session in all windows (and change one connection to display a different window). I would also like to

Edit

I thought this was working once before, then I just learned that I must have still been in screen, so I have fixed this to do separate sessions. I commented, so let me know if you have any questions. Now there is one downside to this, it will choose the smallest size for both windows. That isn't helpful...I am working on finding a fix currently.

Edit 2

In order to fix the resizing issue, add the following line to your .tmux.conf file

set-option -g aggressive-resize on

Tmux: Set default session for attach

It is time sensitive.

From the man page:

The target-session rules for attach-session are slightly adjusted: if tmux needs to select the most recently used session, it will prefer the most recently used unattached session.

Your shell alias is the way to go for this, it is not a workaround, but the way Unix/Linux works. Small tools to do small things to produce great results.

Best Answer

Related Solutions

Bash – Why might tmux only be capable of attaching once per shell session

Tmux: Set default session for attach

Related Question