def init_command(self, action, flags=None):
    '''return the initial Singularity command with any added flags.
        
       Parameters
       ==========
       action: the main action to perform (e.g., build)
       flags: one or more additional flags (e.g, volumes) 
              not implemented yet.
    '''

    if not isinstance(action, list):
        action = [action]      
    cmd = ['singularity'] + action

    if self.quiet is True:
        cmd.insert(1, '--quiet')
    if self.debug is True:
        cmd.insert(1, '--debug')

    return cmd