Ubuntu Manpage: File::Tail - Perl extension for reading from continuously updated files

Provided by: libfile-tail-perl_1.3-3_all

NAME

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

SYNOPSIS

         use File::Tail;
         $file=File::Tail->new("/some/log/file");
         while (defined($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.

DESCRIPTION

The primary purpose of File::Tail is reading and analysing log files while they are being written, which
is especialy useful 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 second.

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

CONSTRUCTOR

   new ([ ARGS ])
       Creates a "File::Tail". If it has only one parameter, 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:

       name
           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).

       maxinterval
           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.

       interval
           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.

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

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

       maxbuf
           The maximum size of the internal buffer. When File::Tail suddenly found an enormous amount 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 buffer.

           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.

       nowait
           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 select.

       ignore_nonexistant
           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.)

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

           Default is 0.

       reset_tail
           Same as tail, but applies after reset. (i.e. after the file has been automatically closed and
           reopened). Defaults to "-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 "-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.

       name_changes
           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).

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

       errmode
           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.

METHODS

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

   select
       "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) {
            push(@files,File::Tail->new(name=>"$_",debug=>$debug));
        }
        while (1) {
          ($nfound,$timeleft,@pending)=
                    File::Tail::select(undef,undef,undef,$timeout,@files);
          unless ($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 exceptions:

       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 amount 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.

TO BE DONE

       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.

AUTHOR

       Matija Grabnar, matija.grabnar@arnes.si