Provided by: libfile-tail-perl_1.3-8_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 especially 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
SEE ALSO
perl(1), tail (1), MRTG (http://ee-staff.ethz.ch/~oetiker/webtools/mrtg/mrtg.html)