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Re: Tailing a series of log files

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On 6/14/13 6:06 PM, Eric Sosman wrote:
> On 6/14/2013 7:00 PM, Arved Sandstrom wrote:
>> On 06/12/2013 09:04 AM, SamXiao wrote:
>>> Hi all,
>>>
>>> I am working on a solution to keep track of a series of log files and
>>> read the tail lines. At the very beginning, there is only one log file
>>> named “TEST-YYYYMMDD-00.log” (where YYYYMMDD is the current date).
>>> When the size of this log reaches to 10 MB, another log file
>>> “TEST-YYYYMMDD-01.log” will be generated and so forth. After midnight
>>> (12:00), a new log file with the next date starting with 00 again will
>>> be generated.
>>>
>>> I know Apache Commons IO containing tail.java. However, seems it is
>>> for tailing a single log file only. How could I keep tailing a series
>>> of log files?
>>>
>>> Any help would be highly appreciated. Thanks.
>>>
>> For what it's worth, a sane logging system with a rollover policy has a
>> base log file which always has the same name, like TEST.log. Decent text
>> editors can follow a filename, UNIX command line "tail" doesn't follow a
>> filename but rather the precise file in question. But point being, if
>> you're looking to follow an "active" log file it may help if it's always
>> got the same name. It's the rolled-over logs that have the suffixes.
>
>      This is true, but I don't think it's going to help the
> O.P. much.  Argument: The tailer (or any program) will use
> the name to open the file, but subsequent accesses will use a
> name-independent means -- "vnode," perhaps, or "handle."  If
> the name-to-vnode/handle mapping changes after the program has
> created its connection to the file, that's unlikely to affect
> the existing connection: The program will, most likely, just
> keep on tailing the original, oblivious to the name change.
>
>      A tailer might, I suppose, open TEST.log anew for each
> access: open, tail, close, pause, open, tail, close, pause,
> and so on.  That approach would catch the renames, but would
> also risk missing the last few lines of the pre-rename file:
>
>      Tailer: open, tail, close, pause, open, tail, close, ...
>      Writer: ... write ... write ... rename ... write ...
>      Upshot:               ^^^^^
>                     this batch isn't tailed
>
>      If the log writer switches from file to file occasionally,
> I think the only reliable approach is to have the log reader be
> explicitly aware of the fact.
>

from "man tail" on OS X:

>      -F      The -F option implies the -f option, but tail will also check to see if the file being followed has been renamed or rotated.  The file is closed and
>              reopened when tail detects that the filename being read from has a new inode number.  The -F option is ignored if reading from standard input rather than a
>              file.


So, it will reopen the file if it sees that it has a new inode. If you 
are renaming the files frequently enough, that might miss a rename, but 
that seems to be an unlikely situation.

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Thread

Tailing a series of log files SamXiao <foolsmart2005@gmail.com> - 2013-06-12 05:04 -0700
  Re: Tailing a series of log files Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-06-12 08:56 -0400
  Re: Tailing a series of log files Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-06-14 20:00 -0300
    Re: Tailing a series of log files Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-06-14 21:06 -0400
      Re: Tailing a series of log files Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2013-06-14 21:11 -0700
        Re: Tailing a series of log files Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-06-15 09:20 -0400
          Re: Tailing a series of log files JLP <JLP@jlp.com> - 2013-06-15 18:59 +0200
            Re: Tailing a series of log files JLP <JLP@jlp.com> - 2013-06-15 19:15 +0200
      Re: Tailing a series of log files Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-06-16 16:43 -0300

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