Tail(3) User Contributed Perl Documentation Tail(3)NAMEFile::Tail - Perl extension for reading from continously 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 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
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 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:
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 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
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 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>.
reset_tail
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.
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 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.
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 ALSOperl(1), tail (1), MRTG
(http://ee-staff.ethz.ch/~oetiker/webtools/mrtg/mrtg.html)
perl v5.10.0 2009-02-21 Tail(3)
