use interactive shells and bind to work around issue in bash (#506)

* use interactive shells and bind to work around issue in bash

* fix bash version

* Ignore warning about non-interactive mode, instead of using an interactive subshell.

argcomplete/completers.py

@@ -59,13 +59,17 @@ def __call__(self, prefix, **kwargs):
         completion = []
         if self.allowednames:
             if self.directories:
-                files = _call(["bash", "-c", "compgen -A directory -- '{p}'".format(p=prefix)])
+                # Using 'bind' in this and the following commands is a workaround to a bug in bash
+                # that was fixed in bash 5.3 but affects older versions. Environment variables are not treated
+                # correctly in older versions and calling bind makes them available. For details, see
+                # https://savannah.gnu.org/support/index.php?111125
+                files = _call(["bash", "-c", "bind; compgen -A directory -- '{p}'".format(p=prefix)], stderr=subprocess.DEVNULL)
                 completion += [f + "/" for f in files]
             for x in self.allowednames:
-                completion += _call(["bash", "-c", "compgen -A file -X '!*.{0}' -- '{p}'".format(x, p=prefix)])
+                completion += _call(["bash", "-c", "bind; compgen -A file -X '!*.{0}' -- '{p}'".format(x, p=prefix)], stderr=subprocess.DEVNULL)
         else:
-            completion += _call(["bash", "-c", "compgen -A file -- '{p}'".format(p=prefix)])
-            anticomp = _call(["bash", "-c", "compgen -A directory -- '{p}'".format(p=prefix)])
+            completion += _call(["bash", "-c", "bind; compgen -A file -- '{p}'".format(p=prefix)], stderr=subprocess.DEVNULL)
+            anticomp = _call(["bash", "-c", "bind; compgen -A directory -- '{p}'".format(p=prefix)], stderr=subprocess.DEVNULL)
             completion = list(set(completion) - set(anticomp))
 
             if self.directories: