How to activate the STL import/export addon from the commandline? From the commandline documentation, activating an addon should be:
$: blender --addons &lt;list of addons to activate&gt;

But I can't seem to find the right syntax.
Example with the STL import/export:

$: blender -b --addons io_mesh_stl
Exception in module register(): '/usr/share/blender/scripts/addons/io_mesh_stl/__init__.py'
Traceback (most recent call last):
  File "/usr/share/blender/scripts/modules/addon_utils.py", line 312, in enable
  mod.register()
  File "/usr/share/blender/scripts/addons/io_mesh_stl/__init__.py", line 260, in register
bpy.utils.register_module(__name__)
  File "/usr/share/blender/scripts/modules/bpy/utils.py", line 609, in register_module
    raise Exception("register_module(%r): defines no classes" % module)
Exception: register_module('io_mesh_stl'): defines no classes
bpy.ops.export_mesh.stl() will be present without explicitly enabling the add-on. io_mesh_stl is enabled already by default, re-enabling it seems to be the problem here.

Exactly why you get that error is not something I'd spend much time on as a user, considering the fact that it can be avoided by not re-enabling the add-on.