How to run --verb commands ?

I am trying to run some commands to do some simple treatment on a file. Basically just put it in grayscale.

However if I run without the -g flag, I get errors saying the verbs are ignored without gui :

inkscape -f file_1454436983_4.svg --verb EditSelectAll --verb org.inkscape.color.grayscale --verb FileSave --verb FileClose

Gives :

WARNING: ignoring verb EditSelectAll - GUI required for this verb.

** (inkscape:26217): CRITICAL **: static void Inkscape::EditVerb::perform(SPAction*, void*): assertion 'ensure_desktop_valid(action)' failed
WARNING: ignoring verb org.inkscape.color.grayscale - GUI required for this verb.

** (inkscape:26217): CRITICAL **: static void Inkscape::Extension::Effect::EffectVerb::perform(SPAction*, void*): assertion 'ensure_desktop_valid(action)' failed
WARNING: ignoring verb FileSave - GUI required for this verb.

** (inkscape:26217): CRITICAL **: static void Inkscape::FileVerb::perform(SPAction*, void*): assertion 'ensure_desktop_valid(action)' failed
WARNING: ignoring verb FileClose - GUI required for this verb.

** (inkscape:26217): CRITICAL **: static void Inkscape::FileVerb::perform(SPAction*, void*): assertion 'ensure_desktop_valid(action)' failed
terminate called after throwing an instance of 'Inkscape::Extension::Output::save_failed'

Emergency save activated!
Emergency save completed. Inkscape will close now.
If you can reproduce this crash, please file a bug at www.inkscape.org
with a detailed description of the steps leading to the crash, so we can fix it.
** Message: Error: Inkscape encountered an internal error and will close now.

Aborted

So I searched for this error and discover you can force the gui with -g. However, I'm running this on a VPS which doesn't have a graphical interface and if I run with -g I get this :

(inkscape:26237): Gtk-WARNING **: cannot open display:

I tried running :

sudo yum groupinstall "X Window System" "Desktop" "Desktop Platform"

But that did not help. Still the same error.

inkscape -V

Gives :

Inkscape 0.91 r13725 (Jan 26 2016)

What could be missing for this to work ? Is the GUI really necessary for this ?