The current command line rendering mechanism is direct rendering into a file. The driver is selected by setting the GRASS_RENDER_IMMEDIATE variable or by running d.mon module.

List of available display drivers:

NOTES

GRASS_RENDER_COMMAND

If environmental variable GRASS_RENDER_COMMAND is defined, rendering is redirected by display library to the given external command defined by this variable. Currently only Python scrips are supported.

Lets start with simple example of Python script called render.py: 

#!/usr/bin/env python3

import os
import sys

import grass.script as gs
from grass.script import task as gtask

os.environ['GRASS_RENDER_IMMEDIATE'] = 'default'
os.environ['GRASS_RENDER_FILE'] = 'output.png'

cmd, dcmd = gtask.cmdstring_to_tuple(sys.argv[1])

gs.run_command('d.text', text="Test of GRASS_RENDER_COMMAND redirection")

os.environ['GRASS_RENDER_FILE_READ'] = 'TRUE'
gs.run_command(cmd, **dcmd)
After defining GRASS_RENDER_COMMAND variable (example for Bash): 
export GRASS_RENDER_COMMAND=render.py
Display GRASS modules like d.rast or d.vect will be executed by render.py program. For example the command 
d.vect roadsmajor
produces output PNG file output.png which will contain rendered features from vector map roadsmajor and sample text "Test of GRASS_RENDER_COMMAND redirection".

SEE ALSO

d.mon, variables