feat(shell): Detect TMUX, if exists (#854)

Author: tony

CHANGES

@@ -19,6 +19,8 @@ $ pipx install --suffix=@next 'tmuxp' --pip-args '\--pre' --force
 
 <!-- Maintainers, insert changes / features for the next release here -->
 
+- `tmuxp shell` now detects current `server` via `TMUX` (#854)
+
 ## tmuxp 1.21.0 (2022-12-27)
 
 *Maintenance only, no bug fixes or features*

src/tmuxp/cli/shell.py

@@ -1,5 +1,6 @@
 import argparse
 import os
+import pathlib
 import typing as t
 
 from libtmux.server import Server
@@ -140,6 +141,17 @@ def command_shell(
     - :attr:`libtmux.Server.attached_sessions`, :attr:`libtmux.Session.attached_window`,
       :attr:`libtmux.Window.attached_pane`
     """
+    # If inside a server, detect socket_path
+    env_tmux = os.getenv("TMUX")
+    if env_tmux is not None and isinstance(env_tmux, str):
+        env_socket_path = pathlib.Path(env_tmux.split(",")[0])
+        if (
+            env_socket_path.exists()
+            and args.socket_path is None
+            and args.socket_name is None
+        ):
+            args.socket_path = str(env_socket_path)
+
     server = Server(socket_name=args.socket_name, socket_path=args.socket_path)
 
     server.raise_if_dead()