From: Brian Candler <B.Candler@...>
Date: 2007-03-25T21:59:35+09:00
Subject: Re: Prcoessing outoput from tail

On Sun, Mar 25, 2007 at 07:30:33AM +0900, Phy Prabab wrote:
> I am trying to figure out how to perpetually process the output from a
> file.  That is I continuously want to process output that is appended to a
> file and that file could get rolled out at any time.

I guess this is a follow-on to this thread:
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/240367

Have a look at using "tail -F=<filename>" or "tail -f=<filename> --retry"
within popen.

Screen 1:
$ tail -F /tmp/foo
tail: cannot open `/tmp/foo' for reading: No such file or directory

Screen 2:
$ echo "abc" >/tmp/foo; sleep 3; mv /tmp/foo /tmp/foo2; echo "def" >/tmp/foo

Now Screen 1 shows:

tail: `/tmp/foo' has appeared; following end of new file
abc
tail: `/tmp/foo' has been replaced; following end of new file
def

(read 'man tail' for more details. The above example is using tail 5.93 from
GNU coreutils, which comes with Ubuntu Linux 6.06)

> I know that I could probably open the file, tack location, and watch
> the inode to make sure the file has not been rolled, but that amounted to
> a lot of mistakes and I could never get it work properly.

Let 'tail -F=<filename>' do that for you :-)

Brian.