Nested shells should not be login (#5247) (#5565)

notebook/terminal/__init__.py

@@ -21,8 +21,13 @@ def initialize(nb_app):
     shell = nb_app.terminado_settings.get('shell_command',
                                           [os.environ.get('SHELL') or default_shell]
                                           )
-    # Enable login mode - to automatically source the /etc/profile script
-    if os.name != 'nt':
+    # Enable login mode - to automatically source the /etc/profile
+    # script, but only for non-nested shells; for nested shells, it's
+    # superfluous and may even be harmful (e.g. on macOS, where login
+    # shells invoke /usr/libexec/path_helper to add entries from
+    # /etc/paths{,.d} to the PATH, reordering it in the process and
+    # potentially overriding virtualenvs and other PATH modifications)
+    if os.name != 'nt' and int(os.environ.get("SHLVL", 0)) < 1:
         shell.append('-l')
     terminal_manager = nb_app.web_app.settings['terminal_manager'] = TerminalManager(
         shell_command=shell,