Shell Programming and Scripting

Is there some way I can have a script monitor the newest line of a log file until a certain entry gets placed in the log?

Basically I want to start this script, and have it spit up the last 10 lines or so to a browser via AJAX... the AJAX is not my problem though, as I have no idea how to go about perpetually getting the last line of a log... without being a resource hog that is.

I figure it looks something like this:

Code:

start_log-checker();
start_log-generating-app();

start_log-checker() {
    while($log_line ne "Done!") {
        if ($@array < 10) { 
            unshift(@array, $log_line);
        } else {
            pop(@array);
            unshift(@array, $log_line);
        }
        upstream_to_browser(@array);
    }
    if ($log_line eq "Done!") upstream_to_browser(@array);
}

Any help?

popen tail?

From what I can see, there is no such thing as "popen tail" in Perl...

I mean I can backtick or qx the actual command that writes the log in the first place, but that would give me the whole log in a Perl variable, and all I care about is one line at a time - the last line.

I followed the hint, and found a CPAN introduction to using piped opening of files... I'm still wandering aimlessly, but I think I am in the right direction

I have an impression that you can try to use seek() available in Perl to first move the cursor to the last 500 bytes of the file (), and then read the 500 byte block to check whether that already has 10 lines as needed, if not, go back by a further 500 byte block (now 1000 bytes) until you get 10 lines. It is likely faster than having to read the entire file head to toe if your log file is very very long, although you only keep the last 10 lines during the scan.

I guess that is how tail is implemented but I'm not sure. There is no way to be fast if you have to move the file pointer in a gigantic file from head to toe.