Below is the function I use as part of a generalized wrapper around ssh and
tmux that does this. The $new_window_command variable is set via an option
to the script (which is named "work") that contains this function. For
example,

work -N "vim ~/docs/snippet" ToDo


Will attach to an existing session named ToDo. If one doesn't exist it will
create a new session and run the command specified by the -N option.

function attach_session {
    local session="$1"
    local rows cols

    # Try to get the size of the tmux virtual terminal so we can resize
    # the local window to match.
    fields='#{session_height} #{session_width}'
    cmd="tmux list-windows -t '$session' -F '$fields'"
    ssh -x -t -p $port $host "$cmd" 2> /dev/null |
        read rows cols
    if [[ $rows == [0-9]## ]] ; then
        rows=$((rows + 1))  # to account for the tmux status line
        size $rows $cols
        cmd="exec tmux attach-session -t \"$session\""
    else
        cmd="exec tmux new-session -s \"$session\""
        if [[ $new_window_cmd != '' ]] ; then
            p1="tmux send-keys -t \"$session\" \"$new_window_cmd\""
            p2="tmux send-keys -t \"$session\" C-m"
            cmd="(sleep 2; $p1; $p2) &; $cmd"
        fi
    fi

    ssh -x -t -p $port $host "exec /bin/zsh -l -c '$cmd'"
}



On Thu, Feb 6, 2014 at 4:31 PM, Kurtis Rader <krader@skepticism.us> wrote:

> It's pretty easy with tmux (via "tmux send-keys") but you need to consider
> unwanted side-effects if the session isn't at a shell prompt awaiting
> input. What happens if, say, a command like vim is the foreground task when
> you try to execute that command? You're just asking for trouble trying to
> do it that way. The only time I use that capability is when I know I'm
> establishing a new session and want to do some custom initialization (e.g.,
> cd'ing to a particular directory and launching my editor).
>
>
> On Thu, Feb 6, 2014 at 10:27 AM, Thorsten Kampe <thorsten@thorstenkampe.de
> > wrote:
>
>> Hi,
>>
>> tough question: can I automatically execute a command on reattaching
>> a detached byobu (screen or tmux) session?
>>
>> Every time I login, /etc/motd is displayed. Since I hardly ever log
>> out and in, I'd like to have the same functionality when reattaching
>> to my detached session. This could be a simple cat /etc/motd kind of
>> "autostart".
>>
>> Thorsten
>>
>>
>
>
> --
> Kurtis Rader
> Caretaker of the exceptional canines Junior and Hank
>



-- 
Kurtis Rader
Caretaker of the exceptional canines Junior and Hank