broadcast input to multiple hosts simultaneously with tmux & zsh. idea from https://www.reddit.com/r/tmux/comments/lqp9sd/send_keystrokes_to_multiple_panes_like_iterm2/

multi.sh

# usage: multi ssh the-host{0,1,2}

function multi {
  cmd=$1
  shift

  if [ -z "$TMUX" ]; then
    session="multi-${cmd}-$(head -c 4 /dev/urandom | base64 | tr '/+' '_-' | tr -d '=')"
    tmux new -d -s "${session}"
  else
    session=$(tmux display-message -p '#S')
  fi
  window="${session}$(tmux display-message -p ':#I')"

  tmux send-keys -t ${window}.0 "$cmd ${@[1]}"
  for ((pane = 2; pane <= ${#@[@]}; pane++)); do
    tmux splitw -h -t $window
    tmux send-keys -t ${window}.$((pane-1)) "$cmd ${@[pane]}"
    tmux select-layout -t $window tiled > /dev/null
  done
  tmux set-window-option -t $window synchronize-panes on > /dev/null
  tmux set-window-option -t $window pane-active-border-style fg=red > /dev/null
  tmux set-window-option -t $window pane-border-style fg=yellow > /dev/null
  tmux send-keys -t ${window}.0 Enter
  tmux new -A -s ${session}
}