Update monitoring of dmesg (/ dev / kmsg) from python on Linux

A sample implementation that monitors the logs that can be confirmed with dmesg, such as tail -F, for tail updates. The ring buffer output by dmesg can be opened as a file by / dev / kmsg or / proc / kmsg. Since it blocks with read on the last line, ʻO_NONBLOCK` is specified only when skipping is processed.


dmesg(/dev/kmsg)Update monitoring

#!/usr/bin/env python2
# cofing: UTF-8

import os
import fcntl
import time

def readLines(path):
    with open(path, "r") as fp:
        stat = os.stat(path)

        where = None

        # =================
        # seek to last line
        # =================
        fd   = fp.fileno()
        flag = fcntl.fcntl(fd, fcntl.F_GETFL)
        fcntl.fcntl(fd, fcntl.F_SETFL, flag | os.O_NONBLOCK)

            # FIXME
            for last in fp:
        except IOError as e:
            if e.errno != 11:

        fcntl.fcntl(fd, fcntl.F_SETFL, flag)

        # =================
        # tailf impl
        # =================
            while True:
                where = fp.tell()
                line  = fp.readline()
                yield line
        except KeyboardInterrupt:

if __name__ == "__main__":
    import sys
    for line in readLines("/dev/kmsg"):
        if str(line).find("stop tailf") >= 0:
            print "### stop watch tailf ###"
            print line,

The execution and its result are as follows.

Execution result and end(Monitoring side)

$ ./tailf.py
### stop watch tailf ###

By redirecting to / dev / kmsg, the output of the driver log is confirmed in a pseudo manner.

Execution result and end(Update side sample)

$ sudo sh -c "echo aaa > /dev/kmsg"
$ sudo sh -c "echo bbb > /dev/kmsg"
$ sudo sh -c "echo ccc > /dev/kmsg"
$ sudo sh -c "echo stop tailf > /dev/kmsg"


I didn't know a good way to seek to the last line of the file in python, so I implemented it by reading everything up to the last line. In the case of a huge file, it may take a long time just to skip it. (When changing the size of the ring buffer, etc.)

for last in fp:


Emerge Technology: Tail-like with Python Output the line containing the specified character string from the text file --Qiita python - What is the most efficient way to get first and last line of a text file? - Stack Overflow Get last n lines of a file with Python, similar to tail - Stack Overflow python Non-block read file - Stack Overflow

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