1Tail(3)               User Contributed Perl Documentation              Tail(3)
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NAME

6       File::Tail - Perl extension for reading from continously updated files
7

SYNOPSIS

9         use File::Tail;
10         $file=File::Tail->new("/some/log/file");
11         while (defined($line=$file->read)) {
12             print "$line";
13         }
14
15         use File::Tail;
16         $file=File::Tail->new(name=>$name, maxinterval=>300, adjustafter=>7);
17         while (defined($line=$file->read)) {
18             print "$line";
19         }
20
21       OR, you could use tie (additional parameters can be passed with the
22       name, or can be set using $ref):
23
24           use File::Tail;
25           my $ref=tie *FH,"File::Tail",(name=>$name);
26           while (<FH>) {
27               print "$_";
28           }
29
30       Note that the above script will never exit. If there is nothing being
31       written to the file, it will simply block.
32
33       You can find more synopsii in the file logwatch, which is included in
34       the distribution.
35
36       Note: Select functionality was added in version 0.9, and it required
37       some reworking of all routines. ***PLEASE*** let me know if you see
38       anything strange happening.
39
40       You can find two way of using select in the file select_demo which is
41       included in the ditribution.
42

DESCRIPTION

44       The primary purpose of File::Tail is reading and analysing log files
45       while they are being written, which is especialy usefull if you are
46       monitoring the logging process with a tool like Tobias Oetiker's MRTG.
47
48       The module tries very hard NOT to "busy-wait" on a file that has little
49       traffic. Any time it reads new data from the file, it counts the number
50       of new lines, and divides that number by the time that passed since
51       data were last written to the file before that. That is considered the
52       average time before new data will be written. When there is no new data
53       to read, "File::Tail" sleeps for that number of seconds. Thereafter,
54       the waiting time is recomputed dynamicaly. Note that "File::Tail" never
55       sleeps for more than the number of seconds set by "maxinterval".
56
57       If the file does not get altered for a while, "File::Tail" gets
58       suspicious and startschecking if the file was truncated, or moved and
59       recreated. If anything like that had happened, "File::Tail" will
60       quietly reopen the file, and continue reading. The only way to affect
61       what happens on reopen is by setting the reset_tail parameter (see
62       below). The effect of this is that the scripts need not be aware when
63       the logfiles were rotated, they will just quietly work on.
64
65       Note that the sleep and time used are from Time::HiRes, so this module
66       should do the right thing even if the time to sleep is less than one
67       second.
68
69       The logwatch script (also included) demonstrates several ways of
70       calling the methods.
71

CONSTRUCTOR

73   new ([ ARGS ])
74       Creates a "File::Tail". If it has only one paramter, it is assumed to
75       be the filename. If the open fails, the module performs a croak. I am
76       currently looking for a way to set $! and return undef.
77
78       You can pass several parameters to new:
79
80       name
81           This is the name of the file to open. The file will be opened for
82           reading.  This must be a regular file, not a pipe or a terminal
83           (i.e. it must be seekable).
84
85       maxinterval
86           The maximum number of seconds (real number) that will be spent
87           sleeping.  Default is 60, meaning "File::Tail" will never spend
88           more than sixty seconds without checking the file.
89
90       interval
91           The initial number of seconds (real number) that will be spent
92           sleeping, before the file is first checked. Default is ten seconds,
93           meaning "File::Tail" will sleep for 10 seconds and then determine,
94           how many new lines have appeared in the file.
95
96       adjustafter
97           The number of "times" "File::Tail" waits for the current interval,
98           before adjusting the interval upwards. The default is 10.
99
100       resetafter
101           The number of seconds after last change when "File::Tail" decides
102           the file may have been closed and reopened. The default is
103           adjustafter*maxinterval.
104
105       maxbuf
106           The maximum size of the internal buffer. When File::Tail suddenly
107           found an enormous ammount of information in the file (for instance
108           if the retry parameters were set to very infrequent checking and
109           the file was rotated), File::Tail sometimes slurped way too much
110           file into memory.  This sets the maximum size of File::Tail's
111           buffer.
112
113           Default value is 16384 (bytes).
114
115           A large internal buffer may result in worse performance (as well as
116           increased memory usage), since File::Tail will have to do more work
117           processing the internal buffer.
118
119       nowait
120           Does not block on read, but returns an empty string if there is
121           nothing to read. DO NOT USE THIS unless you know what you are
122           doing. If you are using it in a loop, you probably DON'T know what
123           you are doing.  If you want to read tails from multiple files, use
124           select.
125
126       ignore_nonexistant
127               Do not complain if the file doesn't exist when it is first
128           opened or when it is to be reopened. (File may be reopened after
129           resetafter seconds have passed since last data was found.)
130
131       tail
132               When first started, read and return C<n> lines from the file.
133           If C<n> is zero, start at the end of file. If C<n> is negative,
134           return the whole file.
135
136               Default is C<0>.
137
138       reset_tail
139               Same as tail, but applies after reset. (i.e. after the
140           file has been automaticaly closed and reopened). Defaults to
141           C<-1>, i.e. does not skip any information present in the
142           file when it first checks it.
143
144              Why would you want it otherwise? I've seen files which
145           have been cycled like this:
146
147              grep -v lastmonth log >newlog
148              mv log archive/lastmonth
149              mv newlog log
150              kill -HUP logger
151
152           Obviously, if this happens and you have reset_tail set to c<-1>,
153           you will suddenly get a whole bunch of lines - lines you already
154           saw. So in this case, reset_tail should probably be set to a small
155           positive number or even 0.
156
157       name_changes
158           Some logging systems change the name of the file they are writing
159           to, sometimes to include a date, sometimes a sequence number,
160           sometimes other, even more bizarre changes.
161
162           Instead of trying to implement various clever detection methods,
163           File::Tail will call the code reference defined in name_changes.
164           The code reference should return the string which is the new name
165           of the file to try opening.
166
167           Note that if the file does not exist, File::Tail will report a
168           fatal error (unless ignore_nonexistant has also been specified).
169
170       debug
171           Set to nonzero if you want to see more about the inner workings of
172           File::Tail. Otherwise not useful.
173
174       errmode
175           Modeled after the methods from Net:Telnet, here you decide how the
176           errors should be handled. The parameter can be a code reference
177           which is called with the error string as a parameter, an array with
178           a code reference as the first parameter and other parameters to be
179           passed to handler subroutine, or one of the words:
180
181           return  - ignore any error (just put error message in errmsg).
182           warn    - output the error message but continue die     - display
183           error message and exit
184
185           Default is die.
186

METHODS

188   read
189       "read" returns one line from the input file. If there are no lines
190       ready, it blocks until there are.
191
192   select
193       "select" is intended to enable the programmer to simoultaneously wait
194       for input on normal filehandles and File::Tail filehandles. Of course,
195       you may use it to simply read from more than one File::Tail filehandle
196       at a time.
197
198       Basicaly, you call File::Tail::select just as you would normal select,
199       with fields for rbits, wbits and ebits, as well as a timeout, however,
200       you can tack any number of File::Tail objects (not File::Tail
201       filehandles!)  to the end.
202
203       Usage example:
204
205        foreach (@ARGV) {
206            push(@files,File::Tail->new(name=>"$_",debug=>$debug));
207        }
208        while (1) {
209          ($nfound,$timeleft,@pending)=
210                    File::Tail::select(undef,undef,undef,$timeout,@files);
211          unless ($nfound) {
212            # timeout - do something else here, if you need to
213          } else {
214            foreach (@pending) {
215               print $_->{"input"}." (".localtime(time).") ".$_->read;
216          }
217        }
218
219        #
220        # There is a more elaborate example in select_demo in the distribution.
221        #
222
223       When you do this, File::Tail's select emulates normal select, with two
224       exceptions:
225
226       a) it will return if there is input on any of the parameters (i.e.
227       normal filehandles) _or_ File::Tails.
228
229       b) In addition to "($nfound, $timeleft)", the return array will also
230       contain a list of File::Tail objects which are ready for reading.
231       $nfound will contain the correct number of filehandles to be read (i.e.
232       both normal and File::Tails).
233
234       Once select returns, when you want to determine which File::Tail
235       objects have input ready, you can either use the list of objects select
236       returned, or you can check each individual object with
237       $object->predict. This returns the ammount of time (in fractional
238       seconds) after which the handle expects input. If it returns 0, there
239       is input waiting. There is no guarantee that there will be input
240       waiting after the returned number of seconds has passed.  However,
241       File::Tail won't do any I/O on the file until that time has passed.
242       Note that the value of $timeleft may or may not be correct - that
243       depends on the underlying operating system (and it's select), so you're
244       better off NOT relying on it.
245
246       Also note, if you are determining which files are ready for input by
247       calling each individual predict, the $nfound value may be invalid,
248       because one or more of File::Tail object may have become ready between
249       the time select has returned and the time when you checked it.
250

TO BE DONE

252       Planned for 1.0: Using $/ instead of \n to separate "lines" (which
253       should make it possible to read wtmp type files).  Except that I
254       discovered I have no need for that enhancement If you do, feel free to
255       send me the patches and I'll apply them - if I feel they don't add too
256       much processing time.
257

AUTHOR

259       Matija Grabnar, matija.grabnar@arnes.si
260

SEE ALSO

262       perl(1), tail (1), MRTG
263       (http://ee-staff.ethz.ch/~oetiker/webtools/mrtg/mrtg.html)
264
265
266
267perl v5.36.0                      2022-07-22                           Tail(3)
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