Reading data from a log file as a separate application is writing to it
Question
I would like to monitor a log file that is being written to by an application. I want to process the file line by line as, or shortly after, it is written. I have not found a way of detecting that a file has been extended after reaching eof.
The code needs to work on Mac and PC, and can be in any language, though I am most familiar with C++ and Perl.
Does anybody have a suggestion for the best way to do it?
Solution
In Perl, the File::Tail module does exactly what you need.
OTHER TIPS
A generic enough answer:
Most languages, on EOF, return that no data were read. You can re-try reading after an interval, and if the file has grown since, this time the operating system will return data.
The essense of tail -f
is the following loop:
open IN, $file;
while(1) {
my $line = <IN>;
if($line) {
#process line...
} else {
sleep(1);
seek(IN,0,1);
}
}
close IN;
The seek call is to clear the EOF flag.
You should be able to use read the standard io from tail -f
I'd have thought outputting the actions via tee, and thence tail'ing (or using the loop above) the file created by tee some use.
For Java see this excellent article
http://www.informit.com/guides/content.aspx?g=java&seqNum=226