# phpman > perldoc > find

## Found in /usr/share/perl/5.34/pod/perlfaq3.pod
  How do I find which modules are installed on my system?
    From the command line, you can use the "cpan" command's "-l" switch:

        $ cpan -l

    You can also use "cpan"'s "-a" switch to create an autobundle file that
    "CPAN.pm" understands and can use to re-install every module:

        $ cpan -a

    Inside a Perl program, you can use the [ExtUtils::Installed](https://www.chedong.com/phpMan.php/perldoc/ExtUtils%3A%3AInstalled/markdown) module to
    show all installed distributions, although it can take awhile to do its
    magic. The standard library which comes with Perl just shows up as
    "Perl" (although you can get those with [Module::CoreList](https://www.chedong.com/phpMan.php/perldoc/Module%3A%3ACoreList/markdown)).

        use [ExtUtils::Installed](https://www.chedong.com/phpMan.php/perldoc/ExtUtils%3A%3AInstalled/markdown);

        my $inst    = [ExtUtils::Installed](https://www.chedong.com/phpMan.php/perldoc/ExtUtils%3A%3AInstalled/markdown)->new();
        my @modules = $inst->modules();

    If you want a list of all of the Perl module filenames, you can use
    [File::Find::Rule](https://www.chedong.com/phpMan.php/perldoc/File%3A%3AFind%3A%3ARule/markdown):

        use [File::Find::Rule](https://www.chedong.com/phpMan.php/perldoc/File%3A%3AFind%3A%3ARule/markdown);

        my @files = [File::Find::Rule](https://www.chedong.com/phpMan.php/perldoc/File%3A%3AFind%3A%3ARule/markdown)->
            extras({follow => 1})->
            file()->
            name( '*.pm' )->
            in( @INC )
            ;

    If you do not have that module, you can do the same thing with
    [File::Find](https://www.chedong.com/phpMan.php/perldoc/File%3A%3AFind/markdown) which is part of the standard library:

        use [File::Find](https://www.chedong.com/phpMan.php/perldoc/File%3A%3AFind/markdown);
        my @files;

        find(
            {
            wanted => sub {
                push @files, $[File::Find::fullname](https://www.chedong.com/phpMan.php/perldoc/File%3A%3AFind%3A%3Afullname/markdown)
                if -f $[File::Find::fullname](https://www.chedong.com/phpMan.php/perldoc/File%3A%3AFind%3A%3Afullname/markdown) && /\.pm$/
            },
            follow => 1,
            follow_skip => 2,
            },
            @INC
        );

        print join "\n", @files;

    If you simply need to check quickly to see if a module is available, you
    can check for its documentation. If you can read the documentation the
    module is most likely installed. If you cannot read the documentation,
    the module might not have any (in rare cases):

        $ perldoc [Module::Name](https://www.chedong.com/phpMan.php/perldoc/Module%3A%3AName/markdown)

    You can also try to include the module in a one-liner to see if perl
    finds it:

        $ perl -[MModule::Name](https://www.chedong.com/phpMan.php/perldoc/MModule%3A%3AName/markdown) -e1

    (If you don't receive a "Can't locate ... in @INC" error message, then
    Perl found the module name you asked for.)

## Found in /usr/share/perl/5.34/pod/perlfaq4.pod
  How do I find the day or week of the year?
    The day of the year is in the list returned by the "localtime" function.
    Without an argument "localtime" uses the current time.

        my $day_of_year = (localtime)[7];

    The POSIX module can also format a date as the day of the year or week
    of the year.

        use POSIX qw/strftime/;
        my $day_of_year  = strftime "%j", localtime;
        my $week_of_year = strftime "%W", localtime;

    To get the day of year for any date, use POSIX's "mktime" to get a time
    in epoch seconds for the argument to "localtime".

        use POSIX qw/mktime strftime/;
        my $week_of_year = strftime "%W",
            localtime( mktime( 0, 0, 0, 18, 11, 87 ) );

    You can also use [Time::Piece](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3APiece/markdown), which comes with Perl and provides a
    "localtime" that returns an object:

        use [Time::Piece](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3APiece/markdown);
        my $day_of_year  = localtime->yday;
        my $week_of_year = localtime->week;

    The [Date::Calc](https://www.chedong.com/phpMan.php/perldoc/Date%3A%3ACalc/markdown) module provides two functions to calculate these, too:

        use [Date::Calc](https://www.chedong.com/phpMan.php/perldoc/Date%3A%3ACalc/markdown);
        my $day_of_year  = Day_of_Year(  1987, 12, 18 );
        my $week_of_year = Week_of_Year( 1987, 12, 18 );

  How do I find the current century or millennium?
    Use the following simple functions:

        sub get_century    {
            return int((((localtime(shift || time))[5] + 1999))/100);
        }

        sub get_millennium {
            return 1+int((((localtime(shift || time))[5] + 1899))/1000);
        }

    On some systems, the POSIX module's "strftime()" function has been
    extended in a non-standard way to use a %C format, which they sometimes
    claim is the "century". It isn't, because on most such systems, this is
    only the first two digits of the four-digit year, and thus cannot be
    used to determine reliably the current century or millennium.

  How can I compare two dates and find the difference?
    (contributed by brian d foy)

    You could just store all your dates as a number and then subtract. Life
    isn't always that simple though.

    The [Time::Piece](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3APiece/markdown) module, which comes with Perl, replaces localtime with a
    version that returns an object. It also overloads the comparison
    operators so you can compare them directly:

        use [Time::Piece](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3APiece/markdown);
        my $date1 = localtime( $some_time );
        my $date2 = localtime( $some_other_time );

        if( $date1 < $date2 ) {
            print "The date was in the past\n";
        }

    You can also get differences with a subtraction, which returns a
    [Time::Seconds](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3ASeconds/markdown) object:

        my $date_diff = $date1 - $date2;
        print "The difference is ", $date_diff->days, " days\n";

    If you want to work with formatted dates, the [Date::Manip](https://www.chedong.com/phpMan.php/perldoc/Date%3A%3AManip/markdown), [Date::Calc](https://www.chedong.com/phpMan.php/perldoc/Date%3A%3ACalc/markdown),
    or DateTime modules can help you.

  How can I find the Julian Day?
    (contributed by brian d foy and Dave Cross)

    You can use the [Time::Piece](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3APiece/markdown) module, part of the Standard Library, which
    can convert a date/time to a Julian Day:

        $ perl -[MTime::Piece](https://www.chedong.com/phpMan.php/perldoc/MTime%3A%3APiece/markdown) -le 'print localtime->julian_day'
        2455607.7959375

    Or the modified Julian Day:

        $ perl -[MTime::Piece](https://www.chedong.com/phpMan.php/perldoc/MTime%3A%3APiece/markdown) -le 'print localtime->mjd'
        55607.2961226851

    Or even the day of the year (which is what some people think of as a
    Julian day):

        $ perl -[MTime::Piece](https://www.chedong.com/phpMan.php/perldoc/MTime%3A%3APiece/markdown) -le 'print localtime->yday'
        45

    You can also do the same things with the DateTime module:

        $ perl -MDateTime -le'print DateTime->today->jd'
        2453401.5
        $ perl -MDateTime -le'print DateTime->today->mjd'
        53401
        $ perl -MDateTime -le'print DateTime->today->doy'
        31

    You can use the [Time::JulianDay](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3AJulianDay/markdown) module available on CPAN. Ensure that
    you really want to find a Julian day, though, as many people have
    different ideas about Julian days (see
    <<http://www.hermetic.ch/cal_stud/jdn.htm>> for instance):

        $  perl -[MTime::JulianDay](https://www.chedong.com/phpMan.php/perldoc/MTime%3A%3AJulianDay/markdown) -le 'print local_julian_day( time )'
        55608

  How do I find yesterday's date?
    (contributed by brian d foy)

    To do it correctly, you can use one of the "Date" modules since they
    work with calendars instead of times. The DateTime module makes it
    simple, and give you the same time of day, only the day before, despite
    daylight saving time changes:

        use DateTime;

        my $yesterday = DateTime->now->subtract( days => 1 );

        print "Yesterday was $yesterday\n";

    You can also use the [Date::Calc](https://www.chedong.com/phpMan.php/perldoc/Date%3A%3ACalc/markdown) module using its "Today_and_Now"
    function.

        use [Date::Calc](https://www.chedong.com/phpMan.php/perldoc/Date%3A%3ACalc/markdown) qw( Today_and_Now Add_Delta_DHMS );

        my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );

        print "@date_time\n";

    Most people try to use the time rather than the calendar to figure out
    dates, but that assumes that days are twenty-four hours each. For most
    people, there are two days a year when they aren't: the switch to and
    from summer time throws this off. For example, the rest of the
    suggestions will be wrong sometimes:

    Starting with Perl 5.10, [Time::Piece](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3APiece/markdown) and [Time::Seconds](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3ASeconds/markdown) are part of the
    standard distribution, so you might think that you could do something
    like this:

        use [Time::Piece](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3APiece/markdown);
        use [Time::Seconds](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3ASeconds/markdown);

        my $yesterday = localtime() - ONE_DAY; # WRONG
        print "Yesterday was $yesterday\n";

    The [Time::Piece](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3APiece/markdown) module exports a new "localtime" that returns an object,
    and [Time::Seconds](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3ASeconds/markdown) exports the "ONE_DAY" constant that is a set number of
    seconds. This means that it always gives the time 24 hours ago, which is
    not always yesterday. This can cause problems around the end of daylight
    saving time when there's one day that is 25 hours long.

    You have the same problem with [Time::Local](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3ALocal/markdown), which will give the wrong
    answer for those same special cases:

        # contributed by Gunnar Hjalmarsson
         use [Time::Local](https://www.chedong.com/phpMan.php/perldoc/Time%3A%3ALocal/markdown);
         my $today = timelocal 0, 0, 12, ( localtime )[3..5];
         my ($d, $m, $y) = ( localtime $today-86400 )[3..5]; # WRONG
         printf "Yesterday: %d-%02d-%02d\n", $y+1900, $m+1, $d;

  How do I find matching/nesting anything?
    To find something between two single characters, a pattern like
    "/x([^x]*)x/" will get the intervening bits in $1. For multiple ones,
    then something more like "/alpha(.*?)omega/" would be needed. For nested
    patterns and/or balanced expressions, see the so-called (?PARNO)
    construct (available since perl 5.10). The CPAN module [Regexp::Common](https://www.chedong.com/phpMan.php/perldoc/Regexp%3A%3ACommon/markdown)
    can help to build such regular expressions (see in particular
    [Regexp::Common::balanced](https://www.chedong.com/phpMan.php/perldoc/Regexp%3A%3ACommon%3A%3Abalanced/markdown) and [Regexp::Common::delimited](https://www.chedong.com/phpMan.php/perldoc/Regexp%3A%3ACommon%3A%3Adelimited/markdown)).

    More complex cases will require to write a parser, probably using a
    parsing module from CPAN, like [Regexp::Grammars](https://www.chedong.com/phpMan.php/perldoc/Regexp%3A%3AGrammars/markdown), [Parse::RecDescent](https://www.chedong.com/phpMan.php/perldoc/Parse%3A%3ARecDescent/markdown),
    [Parse::Yapp](https://www.chedong.com/phpMan.php/perldoc/Parse%3A%3AYapp/markdown), [Text::Balanced](https://www.chedong.com/phpMan.php/perldoc/Text%3A%3ABalanced/markdown), or [Marpa::R2](https://www.chedong.com/phpMan.php/perldoc/Marpa%3A%3AR2/markdown).

  How do I find the soundex value of a string?
    (contributed by brian d foy)

    You can use the "[Text::Soundex](https://www.chedong.com/phpMan.php/perldoc/Text%3A%3ASoundex/markdown)" module. If you want to do fuzzy or close
    matching, you might also try the [String::Approx](https://www.chedong.com/phpMan.php/perldoc/String%3A%3AApprox/markdown), and [Text::Metaphone](https://www.chedong.com/phpMan.php/perldoc/Text%3A%3AMetaphone/markdown),
    and [Text::DoubleMetaphone](https://www.chedong.com/phpMan.php/perldoc/Text%3A%3ADoubleMetaphone/markdown) modules.

  How do I find the first array element for which a condition is true?
    To find the first array element which satisfies a condition, you can use
    the "first()" function in the [List::Util](https://www.chedong.com/phpMan.php/perldoc/List%3A%3AUtil/markdown) module, which comes with Perl
    5.8. This example finds the first element that contains "Perl".

        use [List::Util](https://www.chedong.com/phpMan.php/perldoc/List%3A%3AUtil/markdown) qw(first);

        my $element = first { /Perl/ } @array;

    If you cannot use [List::Util](https://www.chedong.com/phpMan.php/perldoc/List%3A%3AUtil/markdown), you can make your own loop to do the same
    thing. Once you find the element, you stop the loop with last.

        my $found;
        foreach ( @array ) {
            if( /Perl/ ) { $found = $_; last }
        }

    If you want the array index, use the "firstidx()" function from
    "[List::MoreUtils](https://www.chedong.com/phpMan.php/perldoc/List%3A%3AMoreUtils/markdown)":

        use [List::MoreUtils](https://www.chedong.com/phpMan.php/perldoc/List%3A%3AMoreUtils/markdown) qw(firstidx);
        my $index = firstidx { /Perl/ } @array;

    Or write it yourself, iterating through the indices and checking the
    array element at each index until you find one that satisfies the
    condition:

        my( $found, $index ) = ( undef, -1 );
        for( $i = 0; $i < @array; $i++ ) {
            if( $array[$i] =~ /Perl/ ) {
                $found = $array[$i];
                $index = $i;
                last;
            }
        }

## Found in /usr/share/perl/5.34/pod/perlfaq7.pod
  How can I find out my current or calling package?
    (contributed by brian d foy)

    To find the package you are currently in, use the special literal
    "__PACKAGE__", as documented in perldata. You can only use the special
    literals as separate tokens, so you can't interpolate them into strings
    like you can with variables:

        my $current_package = __PACKAGE__;
        print "I am in package $current_package\n";

    If you want to find the package calling your code, perhaps to give
    better diagnostics as Carp does, use the "caller" built-in:

        sub foo {
            my @args = ...;
            my( $package, $filename, $line ) = caller;

            print "I was called from package $package\n";
            );

    By default, your program starts in package "main", so you will always be
    in some package.

    This is different from finding out the package an object is blessed
    into, which might not be the current package. For that, use "blessed"
    from [Scalar::Util](https://www.chedong.com/phpMan.php/perldoc/Scalar%3A%3AUtil/markdown), part of the Standard Library since Perl 5.8:

        use [Scalar::Util](https://www.chedong.com/phpMan.php/perldoc/Scalar%3A%3AUtil/markdown) qw(blessed);
        my $object_package = blessed( $object );

    Most of the time, you shouldn't care what package an object is blessed
    into, however, as long as it claims to inherit from that class:

        my $is_right_class = eval { $object->isa( $package ) }; # true or false

    And, with Perl 5.10 and later, you don't have to check for an
    inheritance to see if the object can handle a role. For that, you can
    use "DOES", which comes from "UNIVERSAL":

        my $class_does_it = eval { $object->DOES( $role ) }; # true or false

    You can safely replace "isa" with "DOES" (although the converse is not
    true).

## Found in /usr/share/perl/5.34/pod/perlfaq8.pod
  How do I find out which operating system I'm running under?
    The $^O variable ($OSNAME if you use "English") contains an indication
    of the name of the operating system (not its release number) that your
    perl binary was built for.

  How do I find out if I'm running interactively or not?
    (contributed by brian d foy)

    This is a difficult question to answer, and the best answer is only a
    guess.

    What do you really want to know? If you merely want to know if one of
    your filehandles is connected to a terminal, you can try the "-t" file
    test:

        if( -t STDOUT ) {
            print "I'm connected to a terminal!\n";
        }

    However, you might be out of luck if you expect that means there is a
    real person on the other side. With the Expect module, another program
    can pretend to be a person. The program might even come close to passing
    the Turing test.

    The [IO::Interactive](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3AInteractive/markdown) module does the best it can to give you an answer.
    Its "is_interactive" function returns an output filehandle; that
    filehandle points to standard output if the module thinks the session is
    interactive. Otherwise, the filehandle is a null handle that simply
    discards the output:

        use [IO::Interactive](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3AInteractive/markdown);

        print { is_interactive } "I might go to standard output!\n";

    This still doesn't guarantee that a real person is answering your
    prompts or reading your output.

    If you want to know how to handle automated testing for your
    distribution, you can check the environment. The CPAN Testers, for
    instance, set the value of "AUTOMATED_TESTING":

        unless( $ENV{AUTOMATED_TESTING} ) {
            print "Hello interactive tester!\n";
        }

## Found in /usr/share/perl/5.34/pod/perlfaq9.pod
  How do I find the user's mail address?
    Ask them for it. There are so many email providers available that it's
    unlikely the local system has any idea how to determine a user's email
    address.

    The exception is for organization-specific email (e.g.
    <foo@yourcompany.com>) where policy can be codified in your program. In
    that case, you could look at $ENV{USER}, $ENV{LOGNAME}, and getpwuid($<)
    in scalar context, like so:

      my $user_name = getpwuid($<)

    But you still cannot make assumptions about whether this is correct,
    unless your policy says it is. You really are best off asking the user.

  How do I find out my hostname, domainname, or IP address?
    (contributed by brian d foy)

    The [Net::Domain](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADomain/markdown) module, which is part of the Standard Library starting
    in Perl 5.7.3, can get you the fully qualified domain name (FQDN), the
    host name, or the domain name.

        use [Net::Domain](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADomain/markdown) qw(hostname hostfqdn hostdomain);

        my $host = hostfqdn();

    The [Sys::Hostname](https://www.chedong.com/phpMan.php/perldoc/Sys%3A%3AHostname/markdown) module, part of the Standard Library, can also get the
    hostname:

        use [Sys::Hostname](https://www.chedong.com/phpMan.php/perldoc/Sys%3A%3AHostname/markdown);

        $host = hostname();

    The [Sys::Hostname::Long](https://www.chedong.com/phpMan.php/perldoc/Sys%3A%3AHostname%3A%3ALong/markdown) module takes a different approach and tries
    harder to return the fully qualified hostname:

      use [Sys::Hostname::Long](https://www.chedong.com/phpMan.php/perldoc/Sys%3A%3AHostname%3A%3ALong/markdown) 'hostname_long';

      my $hostname = hostname_long();

    To get the IP address, you can use the "gethostbyname" built-in function
    to turn the name into a number. To turn that number into the dotted
    octet form (a.b.c.d) that most people expect, use the "inet_ntoa"
    function from the Socket module, which also comes with perl.

        use Socket;

        my $address = inet_ntoa(
            scalar gethostbyname( $host || 'localhost' )
        );