Enhance shell fallback mechanism and improve environment variable handling

- Updated the listener to ensure a default shell of /bin/sh is used if the shell lookup fails or returns an empty value.
- Modified the session to set a default TERM environment variable to "xterm" if none is provided, improving compatibility across different environments.

rnsh/listener.py

@@ -158,7 +158,10 @@ async def listen(configdir, command, identitypath=None, service_name=None, verbo
         except Exception as e:
             log.error(f"Error looking up shell: {e}")
         log.info(f"Using {shell} for default command.")
-        _cmd = [shell] if shell else None
+        # Ensure a sane shell default. Fall back to /bin/sh if lookup fails.
+        if not shell or len(shell) == 0:
+            shell = "/bin/sh"
+        _cmd = [shell]
     else:
         log.info(f"Using command {shlex.join(_cmd)}")
 

rnsh/session.py

@@ -311,7 +311,7 @@ class ListenerSession:
 
         try:
             self.process = process.CallbackSubprocess(argv=self.cmdline,
-                                                      env={"TERM": self.term or os.environ.get("TERM", None),
+                                                      env={"TERM": self.term or os.environ.get("TERM") or "xterm",
                                                             "RNS_REMOTE_IDENTITY": (RNS.prettyhexrep(self.remote_identity.hash)
                                                                 if self.remote_identity and self.remote_identity.hash else "")},
                                                       loop=self.loop,