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.

Implementation

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)
        fp.seek(stat[6])

        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)

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

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

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

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

• In the case of normal file update monitoring, it is necessary to correct it because it does not stop at fp.readline () when the last line is reached.

The execution and its result are as follows.

Execution result and end(Monitoring side)

$ ./tailf.py
12,642,9584294872,-;aaa
12,643,9588998703,-;bbb
12,644,9593362017,-;ccc
### 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"

TODO

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.)

# FIXME
for last in fp:
    pass

reference

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