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Config::Watch(3) User Contributed Perl Documentation Config::Watch(3)

Log::Log4perl::Config::Watch - Detect file changes

    use Log::Log4perl::Config::Watch;

    my $watcher = Log::Log4perl::Config::Watch->new(
                          file            => "/data/my.conf",
                          check_interval  => 30,

    while(1) {
        if($watcher->change_detected()) {
            print "Change detected!\n";

This module helps detecting changes in files. Although it comes with the "Log::Log4perl" distribution, it can be used independently.

The constructor defines the file to be watched and the check interval in seconds. Subsequent calls to "change_detected()" will

  • return a false value immediately without doing physical file checks if "check_interval" hasn't elapsed.
  • perform a physical test on the specified file if the number of seconds specified in "check_interval" have elapsed since the last physical check. If the file's modification date has changed since the last physical check, it will return a true value, otherwise a false value is returned.

Bottom line: "check_interval" allows you to call the function "change_detected()" as often as you like, without paying the performing a significant performance penalty because file system operations are being performed (however, you pay the price of not knowing about file changes until "check_interval" seconds have elapsed).

The module clearly distinguishes system time from file system time. If your (e.g. NFS mounted) file system is off by a constant amount of time compared to the executing computer's clock, it'll just work fine.

To disable the resource-saving delay feature, just set "check_interval" to 0 and "change_detected()" will run a physical file test on every call.

If you already have the current time available, you can pass it on to "change_detected()" as an optional parameter, like in


which then won't trigger a call to "time()", but use the value provided.

Instead of polling time and file changes, "new()" can be instructed to set up a signal handler. If you call the constructor like

    my $watcher = Log::Log4perl::Config::Watch->new(
                          file    => "/data/my.conf",
                          signal  => 'HUP'

then a signal handler will be installed, setting the object's variable "$self->{signal_caught}" to a true value when the signal arrives. Comes with all the problems that signal handlers go along with.

To trigger a physical file check on the next call to "change_detected()" regardless if "check_interval" has expired or not, call


on the watcher object.

The watcher can also be used to detect files that have moved. It will not only detect if a watched file has disappeared, but also if it has been replaced by a new file in the meantime.

    my $watcher = Log::Log4perl::Config::Watch->new(
        file           => "/data/my.conf",
        check_interval => 30,

    while(1) {
        if($watcher->file_has_moved()) {
            print "File has moved!\n";

The parameters "check_interval" and "signal" limit the number of physical file system checks, similarily as with "change_detected()".

Copyright 2002-2013 by Mike Schilli <> and Kevin Goess <>.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

Please contribute patches to the project on Github:

Send bug reports or requests for enhancements to the authors via our

MAILING LIST (questions, bug reports, suggestions/patches):

Authors (please contact them via the list above, not directly): Mike Schilli <>, Kevin Goess <>

Contributors (in alphabetical order): Ateeq Altaf, Cory Bennett, Jens Berthold, Jeremy Bopp, Hutton Davidson, Chris R. Donnelly, Matisse Enzer, Hugh Esco, Anthony Foiani, James FitzGibbon, Carl Franks, Dennis Gregorovic, Andy Grundman, Paul Harrington, Alexander Hartmaier David Hull, Robert Jacobson, Jason Kohles, Jeff Macdonald, Markus Peter, Brett Rann, Peter Rabbitson, Erik Selberg, Aaron Straup Cope, Lars Thegler, David Viner, Mac Yang.

2020-07-22 perl v5.32.1

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