python实现tail（考虑到几种特殊情况）
1. 网上有很多种使用python实现tail的方法，大体思想都一样，典型的一种如下：
#!/usr/bin/env python #!-*- coding: utf-8 -*- ################################################################################ # # Copyright (c) 2015 XX.com, Inc. All Rights Reserved # ################################################################################ ################################################################################ # This module provide ... # Third libs those depend on: ################################################################################ """ Compiler Python 2.7.10 Authors: XX(XX@baidu.com) Date: 2015-09-09 12:57:41 Todo: nothing $XX$2015-09-09 """ """SYS LIBS """ import os import re import sys import time import traceback """THIRD LIBS """ try: # import the third libs there. pass except ImportError as e: print e os._exit(-1) """CUSTOM libs Strongly recommend using abs path when using custmo libs. """ # Good behaviors. # It means refusing called like from xxx import * # When `__all__` is [] __all__ = [] reload(sys) sys.setdefaultencoding('utf-8') def send_error(msg): """ Send error to email. """ print msg #******************************************************** #* Global defines start. * #******************************************************** #******************************************************** #* Global defines end. * #******************************************************** class Tail(object): """ 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) """ ''' 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=0.01): """ 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): """ Custom error type. """ def __init__(self, msg): """ Init. """ self.message = msg def __str__(self): """ str. """ return self.message if __name__ == '__main__': pass """ vim: set ts=4 sw=4 sts=4 tw=100 et: """
2. 但是考虑到集中情况，上面的方法会失效（shell中tail命令甚至也会失效）
(1) 正在tail的文件被清空，例如 echo "" > test.log （这种情况下tail依然可以打印，上面程序无法打印）
(2) 日志被logrotate以copy形式切割时，这种情况下tail依然可以打印，上面程序无法打印
(3) 日志被logrotate以新建文件形式切割时，上面两种方法都会失效。
我的应用场景是需要兼容上面的情况的，由于时间匆忙，简单的改良了一下，可以兼容上面的两种情况，同事可以避免由于logrotate切割导致的日志丢失，代码如下：#!/usr/bin/env python
#!-*- coding: utf-8 -*- ################################################################################ # # Copyright (c) 2015 XX.com, Inc. All Rights Reserved # ################################################################################ ################################################################################ # This module provide ... # Third libs those depend on: ################################################################################ """ Compiler Python 2.7.10 Authors: XX(XX@baidu.com) Date: 2015-09-09 12:57:41 Todo: nothing $XX$2015-09-09 """ """SYS LIBS """ import os import re import sys import time import traceback """THIRD LIBS """ try: # import the third libs there. pass except ImportError as e: print e os._exit(-1) """CUSTOM libs Strongly recommend using abs path when using custmo libs. """ # Good behaviors. # It means refusing called like from xxx import * # When `__all__` is [] __all__ = [] reload(sys) sys.setdefaultencoding('utf-8') def send_error(msg): """ Send error to email. """ print msg #******************************************************** #* Global defines start. * #******************************************************** #******************************************************** #* Global defines end. * #******************************************************** class Tail(object): """ 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) """ ''' 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 self.try_count = 0 try: self.file_ = open(self.tailed_file, "r") self.size = os.path.getsize(self.tailed_file) # Go to the end of file self.file_.seek(0, 2) except: raise def reload_tailed_file(self): """ Reload tailed file when it be empty be `echo "" > tailed file`, or segmentated by logrotate. """ try: self.file_ = open(self.tailed_file, "r") self.size = os.path.getsize(self.tailed_file) # Go to the head of file self.file_.seek(0, 1) return True except: return False def follow(self, s=0.01): """ 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. """ while True: _size = os.path.getsize(self.tailed_file) if _size < self.size: while self.try_count < 10: if not self.reload_tailed_file(): self.try_count += 1 else: self.try_count = 0 self.size = os.path.getsize(self.tailed_file) break time.sleep(0.1) if self.try_count == 10: raise Exception("Open %s failed after try 10 times" % self.tailed_file) else: self.size = _size curr_position = self.file_.tell() line = self.file_.readline() if not line: self.file_.seek(curr_position)
elif not line.endswith("\n"):
self.file_.seed(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): """ Custom error type. """ def __init__(self, msg): """ Init. """ self.message = msg def __str__(self): """ str. """ return self.message if __name__ == '__main__': t = Tail("/home/syslog/switch.log") def print_msg(msg): print msg t.register_callback(print_msg) t.follow() """ vim: set ts=4 sw=4 sts=4 tw=100 et: """
上面程序通过简单的测试，可以解决以上几种情况，由于比较匆忙，没有仔细考虑，也希望大家有什么想法尽量贡献哈。
上述红色部分为bugs修复，在使用过程中，发现有的一行内容被截断成两行了，红色部分可以修复该bug：
bugs的原因应该是：readline在读取文件的最后一行时，不会去判断\n这个结束符，有兴趣的同学可以测试验证下。
