python代码实现linux下的tail功能

#coding=utf-8
'''
    author:the5fire
    blog:http://www.the5fire.com
    date:2012-07-03
'''
import sys
already_print_num = 0

def get_last_line(filepath):
    '''
    获取未输入的行
    '''
    global already_print_num
    import  os
    if not os.path.exists(filepath):
        print 'no such file %s' % filepath
        sys.exit()
        return
    readfile = open(filepath, 'r')
    lines = readfile.readlines()
    if len(lines) > 20 and already_print_num == 0:
        #last_num = 20  #首次输出最多输出20行
        #经nonoob指正，修改如下
        already_print_num = len(lines) - 20

    if already_print_num < len(lines):
        print_lines = lines[already_print_num - len(lines):]
        for line in print_lines:
            print len(lines), already_print_num, line.replace('\n','')
        already_print_num = len(lines)
    readfile.close()



def timer(filename):
    '''
    每隔1秒执行一次
    '''
    while True:
        get_last_line(filename)
        import time
        time.sleep(1)

if __name__ == '__main__':
    if len(sys.argv) < 2:
        print 'illegal params'
    else:
        filename = sys.argv[1]
        timer(filename)

#coding=utf-8
'''
	author:the5fire
	blog:http://www.the5fire.com
	date:2012-07-03
'''

import time

def writefile(filename):
    counter = 0
    while True:
        writefile = open(filename, 'a')
        writefile.write('test' + str(counter) + '\n')
        writefile.close()
        counter += 1
        print  counter
        time.sleep(10)

if __name__ == '__main__':
    import sys
    if len(sys.argv) == 2:
        filename = sys.argv[1]
        writefile(filename)