A simple TMUX script to run (`bin/rails s`, `bin/vite dev` and `sidekiq` in 3 separate top panes and have a bottom pane with a shell)

start_rails.sh

#!/usr/bin/env bash
set -euo pipefail
# PS4='+ ${BASH_SOURCE}:${LINENO}:${FUNCNAME[0]:-main}() '
# set -x

# -- Add to your ~/.tmux.conf --
# set -g mouse on
#   or run
# echo "set -g mouse on" >> ~/.tmux.conf

# --- Config (override via env if needed) ---
BOTTOM_PERCENT="${BOTTOM_PERCENT:-60%}" # bottom row height percentage
RIGHT_PERCENT="${RIGHT_PERCENT:-35%}"   # top right column width percentage (eg: takes 35% of the top row)
MIDDLE_PERCENT="${MIDDLE_PERCENT:-15%}" # top middle column width percentage (eg: takes 15% of the width left in the left column)

SESSION_NAME="${SESSION_NAME:-$(basename "$PWD")}"
# --- Commands to run ---
CMD_LEFT_PANE="${CMD_LEFT_PANE:-bin/rails s}"
CMD_MIDDLE_PANE="${CMD_MIDDLE_PANE:-bin/vite dev}"
CMD_RIGHT_PANE="${CMD_RIGHT_PANE:-sidekiq}" # or "bundle exec sidekiq"



require_tmux() {
  if ! command -v tmux >/dev/null 2>&1; then
    echo "tmux is required. Install with: brew install tmux" >&2
    exit 1
  fi
}

# Build layout in a specific target window (format: <session>:<window-index>)
# and start commands. This function runs tmux commands server-side; it doesn't
# depend on the interactive client being attached.
build_layout_in_window() {
  local TARGET="$1"

  # Ensure we are operating on the right window
  tmux select-window -t "$TARGET"

  # Start from a single pane: kill all other panes in that window
  # (safe even if there's only one)
  tmux kill-pane -a -t "$TARGET" || true

  # Capture the top pane id (the only pane right now)
  local P_TOP
  P_TOP="$(tmux display-message -p -t "$TARGET" '#{pane_id}')"

  # Create bottom free shell
  tmux split-window -v -l "$BOTTOM_PERCENT" -t "$TARGET"

  # Go back to the captured TOP pane explicitly
  tmux select-pane -t "$P_TOP"

  # Split TOP into three columns: left (P_TOP), new right (P_RIGHT1),
  # then split the left again to create the middle (P_MID)
  tmux split-window -h -l "$RIGHT_PERCENT" -t "$P_TOP"
  local P_RIGHT
  P_RIGHT="$(tmux display-message -p -t "$TARGET" '#{pane_id}')"

  tmux split-window -h -l "$MIDDLE_PERCENT" -t "$P_TOP"
  local P_MID
  P_MID="$(tmux display-message -p -t "$TARGET" '#{pane_id}')"

  local P_LEFT="$P_TOP"

  # Fire up commands in precise panes
  tmux send-keys -t "$P_LEFT"  "$CMD_LEFT_PANE"  C-m
  tmux send-keys -t "$P_MID"   "$CMD_MIDDLE_PANE"   C-m
  tmux send-keys -t "$P_RIGHT" "$CMD_RIGHT_PANE" C-m

  # Put focus on the bottom shell (the one that was created by split-window -v)
  # Easiest: select the last pane (bottom) by direction from any top pane
  tmux select-pane -t "$P_LEFT"
  tmux select-pane -D -t "$TARGET" 2>/dev/null || true
}

main() {
  require_tmux

  if [ -n "${TMUX-}" ]; then
    # Already inside tmux: operate on the current window
    local CUR_SESSION CUR_INDEX TARGET
    CUR_SESSION="$(tmux display-message -p '#S')"
    CUR_INDEX="$(tmux display-message -p '#I')"
    TARGET="${CUR_SESSION}:${CUR_INDEX}"
    build_layout_in_window "$TARGET"
  else
    # Not in tmux: create or reuse a session (no new iTerm2 window is spawned)
    if ! tmux has-session -t "$SESSION_NAME" 2>/dev/null; then
      tmux new-session -d -s "$SESSION_NAME" -n dev
    fi
    # Build in window 0 of that session
    build_layout_in_window "${SESSION_NAME}:0"
    # Attach in THIS terminal window
    tmux attach -t "$SESSION_NAME"
  fi
}

main "$@"

Usage :

Move to your Rails folder and run the script above.

# Don't forget to make that file executable: `chmod +x start_rails.sh`
# I personally like to locate it to the parent folder of my rails project(s)
cd my_rails_app
../start_rails.sh