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 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 "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
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 ALSO¶
perl(1), tail (1), MRTG
(
http://ee-staff.ethz.ch/~oetiker/webtools/mrtg/mrtg.html)