python实现tail -f命令功能

#!/usr/bin/env python#!encoding:utf-8'''Python-Tail - Unix tail follow implementation in Python.python-tail can be used to monitor changes to a file.Example:    import tail    # Create a tail instance    t = tail.Tail('file-to-be-followed')    # Register a callback function to be called when a new line is found in the followed file.    # If no callback function is registerd, new lines would be printed to standard out.    t.register_callback(callback_function)    # Follow the file with 5 seconds as sleep time between iterations.    # If sleep time is not provided 1 second is used as the default time.    t.follow(s=5) '''# Author - Kasun Herath <kasunh01 at gmail.com># Source - https://github.com/kasun/python-tailimport osimport sysimport timeclass Tail(object):    ''' Represents a tail command. '''    def __init__(self, tailed_file):        ''' Initiate a Tail instance.            Check for file validity, assigns callback function to standard out.            Arguments:                tailed_file - File to be followed. '''        self.check_file_validity(tailed_file)        self.tailed_file = tailed_file        self.callback = sys.stdout.write    def follow(self, s=1):        ''' Do a tail follow. If a callback function is registered it is called with every new line.        Else printed to standard out.        Arguments:            s - Number of seconds to wait between each iteration; Defaults to 1. '''        with open(self.tailed_file) as file_:            # Go to the end of file            file_.seek(0,2)            while True:                curr_position = file_.tell()                line = file_.readline()                if not line:                    file_.seek(curr_position)                else:                    self.callback(line)                time.sleep(s)    def register_callback(self, func):        ''' Overrides default callback function to provided function. '''        self.callback = func    def check_file_validity(self, file_):        ''' Check whether the a given file exists, readable and is a file '''        if not os.access(file_, os.F_OK):            raise TailError("File '%s' does not exist" % (file_))        if not os.access(file_, os.R_OK):            raise TailError("File '%s' not readable" % (file_))        if os.path.isdir(file_):            raise TailError("File '%s' is a directory" % (file_))class TailError(Exception):    def __init__(self, msg):        self.message = msg    def __str__(self):        return self.messageif __name__=='__main__':args = sys.argvif len(args)<2:print 'need one filename parameter'exit(1)t = Tail(args[1])#t.register_callback(print_line)t.follow()