opencv的人脸检测功能在一般场合还是不错的。而ubuntu正好提供了python-opencv这个包，用它可以方便地实现人脸检测的代码。
写代码之前应该先安装python-opencv：
代码如下:
$ sudo apt-get install python-opencv
具体原理就不多说了，可以参考一下这篇文章。直接上源码。
代码如下:
#!/usr/bin/python
# -*- coding: utf-8 -*-
# face_detect.py
# face detection using opencv. based on sample code from:
# http://python.pastebin.com/m76db1d6b
# usage: python face_detect.py
import sys, os
from opencv.cv import *
from opencv.highgui import *
from pil import image, imagedraw
from math import sqrt
def detectobjects(image):
    converts an image to grayscale and prints the locations of any faces found
    grayscale = cvcreateimage(cvsize(image.width, image.height), 8, 1)
    cvcvtcolor(image, grayscale, cv_bgr2gray)
    storage = cvcreatememstorage(0)
    cvclearmemstorage(storage)
    cvequalizehist(grayscale, grayscale)
    cascade = cvloadhaarclassifiercascade(
        '/usr/share/opencv/haarcascades/haarcascade_frontalface_default.xml',
        cvsize(1,1))
    faces = cvhaardetectobjects(grayscale, cascade, storage, 1.1, 2,
        cv_haar_do_canny_pruning, cvsize(20,20))
    result = []
    for f in faces:
        result.append((f.x, f.y, f.x+f.width, f.y+f.height))
    return result
def grayscale(r, g, b):
    return int(r * .3 + g * .59 + b * .11)
def process(infile, outfile):
    image = cvloadimage(infile);
    if image:
        faces = detectobjects(image)
    im = image.open(infile)
    if faces:
        draw = imagedraw.draw(im)
        for f in faces:
            draw.rectangle(f, outline=(255, 0, 255))
        im.save(outfile, jpeg, quality=100)
    else:
        print error: cannot detect faces on %s % infile
if __name__ == __main__:
    process('input.jpg', 'output.jpg')