CPU Temperature Monitoring on Python Xlib

CPU Temperature Overlay on X11 with Python: Cross-Platform Monitoring

A ~120-line Python script monitors CPU temperature and overlays text directly onto the X11 desktop. It uses the python-xlib library to draw on the root window or Desktop window without requiring elevated privileges. Supports FreeBSD (via sysctl with coretemp/amdtemp modules) and Linux (/sys/class/thermal). Updates every 15 seconds and clears the display area on Ctrl-C.

Reading Sensor Data

On FreeBSD, temperature is accessible via sysctl after loading kernel modules:

• Intel: kldload coretemp, read sysctl dev.cpu |grep temperature
• AMD: kldload amdtemp, read sysctl dev.amdtemp.0 |grep core

On Linux, use sysfs:

cat /sys/class/thermal/thermal_zone*/temp

Values are in millidegrees Celsius (divide by 1000). The script parses output from subprocess.Popen, formats temperatures into a comma-separated string.

Working with the X Server

The key trick: drawing in other windows without notification. Xlib enables:

• Retrieve the root window or Desktop window via _NET_CLIENT_LIST.
• Create a GC (graphics context) with white text on black background.
• Clear the area with fill_rectangle before rendering.
• Draw text using a fixed-width font with draw_text.

The get_root_window function scans top-level windows, looks for 'Desktop' in WM_NAME (KDE/Xfce). Fallback is the root window.

Full Source Code

Self-executing script (.sh with shebang). Only dependency: python-xlib.

#!/usr/bin/env python3

import Xlib
from Xlib import display, X   # display and X are not imported automatically
import subprocess,time,logging

# logging configuration
logging.basicConfig(level=logging.INFO)
#logging.basicConfig(level=logging.DEBUG)

# screen coordinates for overlay
POS_X = 150
POS_Y = 50
# for FreeBSD and AMD systems
PATTERN = 'sysctl dev.amdtemp.0 |grep core'
# for FreeBSD with coretemp module
#PATTERN = 'sysctl dev.cpu |grep temperature'
# for Linux
#PATTERN = "cat /sys/class/thermal/thermal_zone*/temp | awk '{ print "temp: " ($1 / 1000) "C" }'"
# font
FONT = '-misc-fixed-medium-r-normal--13-120-75-75-c-70-iso8859-1'
# refresh interval
REFRESH_SECS = 15

last_dim = [0,0]

def get_root_window(d):
    
    screen = d.screen()
    root = screen.root

    windowIDs = root.get_full_property(d.intern_atom('_NET_CLIENT_LIST'), 
                                                  X.AnyPropertyType).value
    logging.debug('Found %d windows.',len(windowIDs))

    for windowID in windowIDs:
        window = d.create_resource_object('window', windowID)

        try:
            window_name = window.get_wm_name()
            if window_name:
                if 'Desktop' in window_name:
                    logging.debug("Found desktop ID: %d - Name: %s",
                                                 windowID,window_name)
                    return window
            else:
                logging.debug("ID: %d - Name: None (no WM_NAME property)",
                                                 windowID)

        except X.BadWindow:
            logging.debug("ID: %d - Window no longer exists",windowID)
    return root


def clear_rect(msg):
    global last_dim
    if last_dim[0] > 0:    
     root.fill_rectangle(gc2, POS_X,POS_Y-last_dim[1], 
                          last_dim[0]-20, last_dim[1]+5)
    else:
        text_extents = font.query_text_extents(msg)
        tw = text_extents.overall_width
        th = text_extents.font_ascent + text_extents.font_descent
        root.fill_rectangle(gc2, POS_X, POS_Y-th, tw-20, th+5)
        last_dim = [tw,th]


def draw_message(msg):
    clear_rect(msg)
    root.draw_text(gc, POS_X, POS_Y, msg)
    display.flush()


display = Xlib.display.Display()
screen = display.screen()
root = get_root_window(display)

root_id = root.id
logging.debug("Root window ID: %d",root_id)
root.change_attributes(event_mask=X.ExposureMask)
gc = root.create_gc(foreground = screen.white_pixel, 
                    background = screen.black_pixel)
colormap = screen.default_colormap
color = colormap.alloc_named_color('black') 
gc2 = root.create_gc(foreground=color.pixel)

try:
    font = display.open_font(FONT)
    gc.font = font.id
except Exception as e:
    logging.exception(e)
    exit(1)

try:
    while 1:
            process = subprocess.Popen(PATTERN, 
                    shell=True, text=True,
                    stdout=subprocess.PIPE)
            stdout_list = process.communicate()[0].split('\n')

            out = ''
            for s in stdout_list:
                if ':' not in s: continue
                kv = s.split(':')
                if len(out) > 0: out+= ','
                out+= kv[1]

            logging.debug(out)
            draw_message(out.encode())
            time.sleep(REFRESH_SECS)
            
except KeyboardInterrupt:
        x = POS_X // 2
        y = POS_Y // 2
        root.clear_area(x,y,last_dim[0]+x,last_dim[1]+y,True)
        display.flush()

Customizing Settings

All constants are at the top of the script:

• POS_X, POS_Y: overlay position on screen.
• PATTERN: sensor command (adjust per platform).
• FONT: X11 font in XLFD format.
• REFRESH_SECS: update frequency.
• Logging: set to INFO or DEBUG.

Install python-xlib:

• FreeBSD: pkg install py311-python-xlib
• Linux: install python3-xlib via apt/yum

Key Highlights

• Cross-platform: Works on FreeBSD/Linux, Intel/AMD, no Wayland.
• Minimalist: Just 120 lines, no OOP/threads, single dependency.
• X11 hack: Draws on root/Desktop without permissions or visual indicators.
• Cleanup: Uses fill_rectangle and clear_area on exit.
• Parsing: Relies on subprocess + split for sysctl/sysfs data.

— Editorial Team