How to monitor only the last n lines of a log file?

I have a growing log file for which I want to display only the last 15 lines. Here is what I know I can do:

tail -n 15 -F mylogfile.txt

As the log file is filled, tail appends the last lines to the display.

I am looking for a solution that only displays the last 15 lines and get rid of the lines before the last 15 after it has been updated. Would you have an idea?

Answers:

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Method 1

It might suffice to use watch:

$ watch tail -n 15 mylogfile.txt

Method 2

If you use watch, try the -n option to control the interval between each update.

Thus, the following would call tail every 2 seconds

$ watch -n 2 tail -n 15 mylogfile.txt

while this one polls it every 1 second

$ watch -n 1 tail -n 15 mylogfile.txt

Method 3

You could stream the logfile running less and pressing SHIFT + F that will stream the file using less.
$ less mylogfile.txt
Then just press SHIFT + F and it will stream. I think it is convenient for monitoring log files that update.

Method 4

Maybe you find the -d param handy.

man watch

-d
Highlight the differences between successive updates. Option will read optional argument that changes highlight to be
permanent, allowing to see what has changed at least once since first
iteration.

Method 5

In Solaris, AIX or HPUX or UNIX-like (including Linux) you can use scripts to monitoring logs or anything like that:

while true; 
    clear; 
    do date; 
    echo ;
    echo "MONITORING LOG IN "/path/to/file.log": "; 
    echo "Obs.: Last 20 lines of a logfile:
    echo ;
    tail -20 /path/to/file.log;
    echo ;
sleep 5; 
done

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0