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Tail(3)               User Contributed Perl Documentation              Tail(3)

       File::Tail - Perl extension for reading from continously updated files

         use File::Tail;
         while (defined($line=$file->read)) {
             print "$line";

         use File::Tail;
         $file=File::Tail->new(name=>$name, maxinterval=>300, adjustafter=>7);
         while (defined($line=$file->read)) {
             print "$line";

       OR, you could use tie (additional parameters can be passed with the
       name, or can be set using $ref):

           use File::Tail;
           my $ref=tie *FH,"File::Tail",(name=>$name);
           while (<FH>) {
               print "$_";

       Note that the above script will never exit. If there is nothing being
       written to the file, it will simply block.

       You can find more synopsii in the file logwatch, which is included in
       the distribution.

       Note: Select functionality was added in version 0.9, and it required
       some reworking of all routines. ***PLEASE*** let me know if you see
       anything strange happening.

       You can find two way of using select in the file select_demo which is
       included in the ditribution.

       The primary purpose of File::Tail is reading and analysing log files
       while they are being written, which is especialy usefull if you are
       monitoring the logging process with a tool like Tobias Oetiker's MRTG.

       The module tries very hard NOT to "busy-wait" on a file that has little
       traffic. Any time it reads new data from the file, it counts the number
       of new lines, and divides that number by the time that passed since
       data were last written to the file before that. That is considered the
       average time before new data will be written. When there is no new data
       to read, "File::Tail" sleeps for that number of seconds. Thereafter,
       the waiting time is recomputed dynamicaly. Note that "File::Tail" never
       sleeps for more than the number of seconds set by "maxinterval".

       If the file does not get altered for a while, "File::Tail" gets
       suspicious and startschecking if the file was truncated, or moved and
       recreated. If anything like that had happened, "File::Tail" will
       quietly reopen the file, and continue reading. The only way to affect
       what happens on reopen is by setting the reset_tail parameter (see
       below). The effect of this is that the scripts need not be aware when
       the logfiles were rotated, they will just quietly work on.

       Note that the sleep and time used are from Time::HiRes, so this module
       should do the right thing even if the time to sleep is less than one

       The logwatch script (also included) demonstrates several ways of
       calling the methods.

   new ([ ARGS ])
       Creates a "File::Tail". If it has only one paramter, it is assumed to
       be the filename. If the open fails, the module performs a croak. I am
       currently looking for a way to set $! and return undef.

       You can pass several parameters to new:

           This is the name of the file to open. The file will be opened for
           reading.  This must be a regular file, not a pipe or a terminal
           (i.e. it must be seekable).

           The maximum number of seconds (real number) that will be spent
           sleeping.  Default is 60, meaning "File::Tail" will never spend
           more than sixty seconds without checking the file.

           The initial number of seconds (real number) that will be spent
           sleeping, before the file is first checked. Default is ten seconds,
           meaning "File::Tail" will sleep for 10 seconds and then determine,
           how many new lines have appeared in the file.

           The number of "times" "File::Tail" waits for the current interval,
           before adjusting the interval upwards. The default is 10.

           The number of seconds after last change when "File::Tail" decides
           the file may have been closed and reopened. The default is

           The maximum size of the internal buffer. When File::Tail suddenly
           found an enormous ammount of information in the file (for instance
           if the retry parameters were set to very infrequent checking and
           the file was rotated), File::Tail sometimes slurped way too much
           file into memory.  This sets the maximum size of File::Tail's

           Default value is 16384 (bytes).

           A large internal buffer may result in worse performance (as well as
           increased memory usage), since File::Tail will have to do more work
           processing the internal buffer.

           Does not block on read, but returns an empty string if there is
           nothing to read. DO NOT USE THIS unless you know what you are
           doing. If you are using it in a loop, you probably DON'T know what
           you are doing.  If you want to read tails from multiple files, use

               Do not complain if the file doesn't exist when it is first
           opened or when it is to be reopened. (File may be reopened after
           resetafter seconds have passed since last data was found.)

               When first started, read and return C<n> lines from the file.
           If C<n> is zero, start at the end of file. If C<n> is negative,
           return the whole file.

               Default is C<0>.

               Same as tail, but applies after reset. (i.e. after the
           file has been automaticaly closed and reopened). Defaults to
           C<-1>, i.e. does not skip any information present in the
           file when it first checks it.

              Why would you want it otherwise? I've seen files which
           have been cycled like this:

              grep -v lastmonth log >newlog
              mv log archive/lastmonth
              mv newlog log
              kill -HUP logger

           Obviously, if this happens and you have reset_tail set to c<-1>,
           you will suddenly get a whole bunch of lines - lines you already
           saw. So in this case, reset_tail should probably be set to a small
           positive number or even 0.

           Some logging systems change the name of the file they are writing
           to, sometimes to include a date, sometimes a sequence number,
           sometimes other, even more bizarre changes.

           Instead of trying to implement various clever detection methods,
           File::Tail will call the code reference defined in name_changes.
           The code reference should return the string which is the new name
           of the file to try opening.

           Note that if the file does not exist, File::Tail will report a
           fatal error (unless ignore_nonexistant has also been specified).

           Set to nonzero if you want to see more about the inner workings of
           File::Tail. Otherwise not useful.

           Modeled after the methods from Net:Telnet, here you decide how the
           errors should be handled. The parameter can be a code reference
           which is called with the error string as a parameter, an array with
           a code reference as the first parameter and other parameters to be
           passed to handler subroutine, or one of the words:

           return  - ignore any error (just put error message in errmsg).
           warn    - output the error message but continue die     - display
           error message and exit

           Default is die.

       "read" returns one line from the input file. If there are no lines
       ready, it blocks until there are.

       "select" is intended to enable the programmer to simoultaneously wait
       for input on normal filehandles and File::Tail filehandles. Of course,
       you may use it to simply read from more than one File::Tail filehandle
       at a time.

       Basicaly, you call File::Tail::select just as you would normal select,
       with fields for rbits, wbits and ebits, as well as a timeout, however,
       you can tack any number of File::Tail objects (not File::Tail
       filehandles!)  to the end.

       Usage example:

        foreach (@ARGV) {
        while (1) {
          unless ($nfound) {
            # timeout - do something else here, if you need to
          } else {
            foreach (@pending) {
               print $_->{"input"}." (".localtime(time).") ".$_->read;

        # There is a more elaborate example in select_demo in the distribution.

       When you do this, File::Tail's select emulates normal select, with two

       a) it will return if there is input on any of the parameters (i.e.
       normal filehandles) _or_ File::Tails.

       b) In addition to "($nfound, $timeleft)", the return array will also
       contain a list of File::Tail objects which are ready for reading.
       $nfound will contain the correct number of filehandles to be read (i.e.
       both normal and File::Tails).

       Once select returns, when you want to determine which File::Tail
       objects have input ready, you can either use the list of objects select
       returned, or you can check each individual object with
       $object->predict. This returns the ammount of time (in fractional
       seconds) after which the handle expects input. If it returns 0, there
       is input waiting. There is no guarantee that there will be input
       waiting after the returned number of seconds has passed.  However,
       File::Tail won't do any I/O on the file until that time has passed.
       Note that the value of $timeleft may or may not be correct - that
       depends on the underlying operating system (and it's select), so you're
       better off NOT relying on it.

       Also note, if you are determining which files are ready for input by
       calling each individual predict, the $nfound value may be invalid,
       because one or more of File::Tail object may have become ready between
       the time select has returned and the time when you checked it.

       Planned for 1.0: Using $/ instead of \n to separate "lines" (which
       should make it possible to read wtmp type files).  Except that I
       discovered I have no need for that enhancement If you do, feel free to
       send me the patches and I'll apply them - if I feel they don't add too
       much processing time.

       Matija Grabnar, matija.grabnar@arnes.si

       perl(1), tail (1), MRTG

perl v5.12.1                      2010-07-05                           Tail(3)

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