InfoMagic Source Code 1993 July/THE_SOURCE_CODE_CD_ROM.iso/languages/perl/info/faq

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Archive-name: perl-faq/part0
Version: $Id: faq,v 1.1 92/11/30 05:12:22 tchrist Exp Locker: tchrist $

This article contains the table of contents to some of the most
frequently asked questions in comp.lang.perl, a newsgroup devoted to
the Perl programming language.  There are two pieces following
this, the general information questions in part1 and the largely
technical opnes in part2.

They're all good questions, but they come up often enough that
substantial net bandwidth can be saved by looking here first before
asking.  Before posting a question, you really should consult the Perl
man page; there's a lot of information packed in there.

Some questions in this group aren't really about Perl, but rather
about system-specific issues.  You might also consult the Most
Frequently Asked Questions list in comp.unix.questions for answers
to this type of question.

The current version of perl is 4.035 (version 4, patchlevel 35).
There haven't actually been 35 updates to perl4; rather, the context
diffs posted to the net have been broken up into 35 news-digestable
chunks.

This list is maintained by Tom Christiansen, and is archived on
convex.com [130.168.1.1] in the file pub/perl/info/faq.  If you
have any suggested additions or corrections to this article, please
send them to Tom at either <[e-mail]> or <convex!tchrist>.
Special thanks to Larry Wall for initially reviewing this list for
accuracy and especially for writing and releasing Perl in the first place.


1.1)  What is Perl?
1.2)  Is Perl hard to learn?
1.3)  Should I program everything in Perl?
1.4)  Where can I get Perl over the Internet?
1.5)  Where can I get Perl via Email?
1.6)  How can I get Perl via UUCP?
1.7)  Where can I get more information on Perl?
1.8)  Can people who aren't on USENET receive comp.lang.perl as a digest?
1.9)  Are archives of comp.lang.perl available?
1.10) How do I get Perl to run on machine FOO?
1.11) Where can I get (info|inter|ora|sql|syb)perl?
1.12) There's an a2p and an s2p; why isn't there a p2c (perl-to-C)?
1.13) Where can I get undump for my machine?
1.14) Where can I get a perl-mode for emacs?
1.15) How can I use Perl interactively?
1.16) Is there a Perl shell?
1.17) Is there a Perl profiler?
1.18) Is there a yacc for Perl?
1.19) How can I use curses with perl?
1.20) How can I use X with Perl?
1.21) What is perl4?  What is perl5?
1.22) How does Perl compare with languages like REXX or TCL?
1.23) Is it a Perl program or a Perl script?
1.24) What's the difference between "Perl" and "perl"?
1.25) What companies use or ship Perl?
1.26) Is there commercial, 3rd-party support for Perl?
1.27) Where can I get a list of the JAPH signature quotes?
1.28) Where can I get a list of Larry Wall witticisms?

2.1)  What are all these $@*%<> signs and how do I know when to use them?
2.2)  Why don't backticks work as they do in shells?
2.3)  How come Perl operators have different precedence than C operators?
2.4)  How come my converted awk/sed/sh script runs more slowly in Perl?
2.5)  How can I call my system's unique C functions from Perl?
2.6)  Where do I get the include files to do ioctl() or syscall()?
2.7)  Why doesn't "local($foo) = <FILE>;" work right?
2.8)  How can I detect keyboard input without reading it?
2.9)  How can I make an array of arrays or other recursive data types?
2.10) How can I quote a variable to use in a regexp?
2.11) Why do setuid Perl scripts complain about kernel problems?
2.12) How do I open a pipe both to and from a command?
2.13) How can I change the first N letters of a string?
2.14) How can I manipulate fixed-record-length files?
2.15) How can I make a file handle local to a subroutine?
2.16) How can I extract just the unique elements of an array?
2.17) How can I call alarm() or usleep() from Perl?
2.18) How can I test whether an array contains a certain element?
2.19) How can I do an atexit() or setjmp()/longjmp() in Perl?
2.20) Why doesn't Perl interpret my octal data octally?
2.21) How do I sort an associative array by value instead of by key?
2.22) How can I capture STDERR from an external command?
2.23) Why doesn't open return an error when a pipe open fails?
2.24) How can I compare two date strings?
2.25) What's the fastest way to code up a given task in perl?
2.26) How can I know how many entries are in an associative array?
2.27) Why can't my perl program read from STDIN after I gave it ^D (EOF) ?
2.28) Do I always/never have to quote my strings or use semicolons?
2.29) How can I translate tildes in a filename?
2.30) How can I convert my shell script to Perl?
2.31) What is variable suicide and how can I prevent it?
2.32) Can I use Perl regular expressions to match balanced text?
2.33) Can I use Perl to run a telnet or ftp session?
2.34) What does "Malformed command links" mean?



1.1) What is Perl?

    A programming language, by Larry Wall <[e-mail]>.

    Here's the beginning of the description from the man page:

    Perl is an interpreted language optimized for scanning arbitrary text
    files, extracting information from those text files, and printing reports
    based on that information.  It's also a good language for many system
    management tasks.  The language is intended to be practical (easy to use,
    efficient, complete) rather than beautiful (tiny, elegant, minimal).  It
    combines (in the author's opinion, anyway) some of the best features of C,
    sed, awk, and sh, so people familiar with those languages should have
    little difficulty with it.  (Language historians will also note some
    vestiges of csh, Pascal, and even BASIC-PLUS.)  Expression syntax
    corresponds quite closely to C expression syntax.  Unlike most Unix
    utilities, Perl does not arbitrarily limit the size of your data--if
    you've got the memory, Perl can slurp in your whole file as a single
    string.  Recursion is of unlimited depth.  And the hash tables used by
    associative arrays grow as necessary to prevent degraded performance.
    Perl uses sophisticated pattern matching techniques to scan large amounts
    of data very quickly.  Although optimized for scanning text, Perl can also
    deal with binary data, and can make dbm files look like associative arrays
    (where dbm is available).  Setuid Perl scripts are safer than C programs
    through a dataflow tracing mechanism which prevents many stupid security
    holes.  If you have a problem that would ordinarily use sed or awk or sh,
    but it exceeds their capabilities or must run a little faster, and you
    don't want to write the silly thing in C, then Perl may be for you.  There
    are also translators to turn your sed and awk scripts into Perl scripts.


1.2) Is Perl hard to learn?

    No, Perl is easy to learn for two reasons.

    The first reason is that most of Perl is derived from existing tools
    and languages, ones that many people who turn to Perl already have
    some familiarity with.  These include the C programming language, the
    UNIX C library, the UNIX shell, sed, and awk.  If you already know
    these somewhat, Perl should be very easy for you.

    The second reason that Perl is easy to learn is that you don't have to
    know every thing there is to know about it in order to get good use
    out of it.  In fact, just a very small subset, mostly borrowed from C,
    the shell, and sed, will be enough for most tasks.  As you feel the
    need or desire to use more sophisticated features (such as C
    structures or networking), you can learn these as you go.  The
    learning curve for Perl is not a steep one, especially if you have
    the headstart of having a background in UNIX.  Rather, its learning
    curve is gentle and gradual, but it *is* admittedly rather long.

    If you don't know C or UNIX at all, it'll be a steeper learning curve,
    but what you then learn from Perl will carry over into other areas,
    like using the C library, UNIX system call, regular expressions, and
    associative arrays, just to name a few.  To know Perl is to know
    UNIX, and vice versa.


1.3) Should I program everything in Perl?

    Of course not.  You should choose the appropriate tool for the task at
    hand.  While it's true that the answer to the question "Can I do (some
    arbitrary task) in Perl?" is almost always "yes", that doesn't mean
    this is necessarily a good thing to do.  For many people, Perl serves
    as a great replacement for shell programming.  For a few people, it
    also serves as a replacement for most of what they'd do in C.  But
    for some things, Perl just isn't the optimal choice, such as tasks
    requiring very complex data structures.


1.4) Where can I get Perl over the Internet?

    From any comp.sources.misc archive.   Initial sources  were posted to
    Volume 18, Issues 19-54 at patchlevel 3.  The Patches 4-10 were posted
    to Volume 20, Issues 56-62.  You can use the archie server
    (see the alt.sources FAQ in news.answers) for ways to find these.

    These machines, at the very least, definitely have it available for
    anonymous FTP:

    ftp.uu.net                137.39.1.2
    archive.cis.ohio-state.edu      128.146.8.52
    jpl-devvax.jpl.nasa.gov         128.149.1.143
    ftp.netlabs.com         192.94.48.152
    prep.ai.mit.edu            18.71.0.38
    archive.cs.ruu.nl        131.211.80.5  (Europe)




1.5) Where can I get Perl via Email?

    If you are in Europe, you might using the following site.  (I'm still
    looking for a domestic site.) This information thanks to "Henk P.
    Penning" <[e-mail]>:  One automated fashion is as follows:

    Email: Send a message to '[e-mail]' containing:
     begin
     path your_email_address
     send help
     send PERL/INDEX
     end
    The path-line may be omitted if your message contains a normal From:-line.
    You will receive a help-file and an index of the directory that contains
    the Perl stuff.

    If all else fails, mail to Larry usually suffices.


1.6) How can I get Perl via UUCP?

    You can get it from the site osu-cis; here is the appropriate info,
    thanks to J Greely <[e-mail]> or <osu-cis!jgreely>.

    E-mail contact:
        osu-cis!uucp
    Get these two files first:
        osu-cis!~/GNU.how-to-get.
        osu-cis!~/ls-lR.Z
    Current Perl distribution:
        osu-cis!~/perl/4.0/kits@10/perl.kitXX.Z (XX=01-37)
    How to reach osu-cis via uucp(L.sys/Systems file lines):
    #
    # Direct Trailblazer
    #
    osu-cis Any ACU 19200 1-614-292-5112 in:--in:--in: Uanon
    #
    # Direct V.32 (MNP 4)
    # dead, dead, dead...sigh.
    #
    #osu-cis Any ACU 9600 1-614-292-1153 in:--in:--in: Uanon
    #
    # Micom port selector, at 1200, 2400, or 9600 bps.
    # Replace ##'s below with 12, 24, or 96 (both speed and phone number).
    #
    osu-cis Any ACU ##00 1-614-292-31## "" \r\c Name? osu-cis nected \c GO \d\r\d\r\d\r in:--in:--in:
     Uanon

    Modify as appropriate for your site, of course, to deal with your
    local telephone system.  There are no limitations concerning the hours
    of the day you may call.

    Another possibility is to use UUNET, although they charge you
    for it.  You have been duly warned.  Here's the advert:

           Anonymous Access to UUNET's Source Archives

                 1-900-GOT-SRCS

     UUNET now provides access to its extensive collection of UNIX
    related sources to non- subscribers.  By  calling  1-900-468-7727
    and  using the login "uucp" with no password, anyone may uucp any
    of UUNET's on line source collection.  Callers will be charged 40
    cents  per  minute.   The charges will appear on their next tele-
    phone bill.

     The  file  uunet!/info/help  contains  instructions.   The  file
    uunet!/index//ls-lR.Z contains a complete list of the files available
    and is updated daily.  Files ending in Z need to be uncompressed
    before being used.   The file uunet!~/compress.tar is a tar
    archive containing the C sources for the uncompress program.

     This service provides a  cost  effective  way  of  obtaining
    current  releases  of sources without having to maintain accounts
    with UUNET or some other service.  All modems  connected  to  the
    900  number  are  Telebit T2500 modems.  These modems support all
    standard modem speeds including PEP, V.32 (9600), V.22bis (2400),
    Bell  212a  (1200), and Bell 103 (300).  Using PEP or V.32, a 1.5
    megabyte file such as the GNU C compiler would cost $10  in  con-
    nect  charges.   The  entire  55  megabyte X Window system V11 R4
    would cost only $370 in connect time.  These costs are less  than
    the  official  tape  distribution fees and they are available now
    via modem.

              UUNET Communications Services
           3110 Fairview Park Drive, Suite 570
             Falls Church, VA 22042
             [phone] (voice)
              [phone] (fax)
                [e-mail]



1.7) Where can I get more information on Perl?

    We'll cover five areas here: USENET (where you're probably reading
    this), publications, the reference guide, examples on the Internet,
    and Perl instructional courses.

    A.  USENET

    You should definitely read the USENET comp.lang.perl newsgrouor
    mailing list for all sorts of discussions regarding the language,
    bugs, features, history, humor, and trivia.  In this respect, it
    functions both as a comp.lang.* style newsgroup and also as a user
    group for the language; in fact, there's a mailing list called
    ``perl-users'' that is bidirectionally gatewayed to the newsgroup; see
    question #38 for details.  Larry Wall is a very frequent poster here,
    as well as many (if not most) of the other seasoned Perl programmers.
    It's the best place for the very latest information on Perl.

    B.  PUBLICATIONS

    If you've been dismayed by the ~80-page troffed Perl man page (or is
    that man treatise?) you should look to ``the Camel Book'', written by
    Larry and Randal L. Schwartz <[e-mail]>, published as a Nutshell
    Handbook by O'Reilly & Associates and entitled _Programming Perl_.
    Besides serving as a reference guide for Perl, it also contains
    tutorial material and is a great source of examples and cookbook
    procedures, as well as wit and wisdom, tricks and traps, pranks and
    pitfalls.  The code examples contained therein are available via
    anonymous FTP from ftp.uu.net in
    /published/oreilly/nutshell/perl/perl.tar.Z for your retrieval.
    Corrections and additions to the book can be found in the Perl man
    page right before the BUGS section under the heading ERRATA AND
    ADDENDA.

    If you can't find the book in your local technical bookstore, the book
    may be ordered directly from O'Reilly by calling 1-800-998-9938 if in
    North America and 1-707-829-0515.  Autographed copies are available
    from TECHbooks by calling 1-503-646-8257 or mailing [e-mail].
    Cost is ~30$US for the regular version, 40$US for the autographed one.
    The book's ISBN is 0-937175-64-1.

    Reasonably substantiated rumor has it that there will be another Perl
    book out pretty soon, this one aimed more at beginners.  Look for it
    from ORA towards the beginning of 93.

    Larry Wall has published a 3-part article on perl in Unix World
    (August through October of 1991), and Rob Kolstad also had a 3-parter
    in Unix Review (May through July of 1990).  Tom Christiansen also has
    a brief overview article in the trade newsletter Unix Technology
    Advisor from November of 1989.  You might also investigate "The Wisdom
    of Perl" by Gordon Galligher from SunExpert magazine;  April 1991
    Volume 2 Number 4.

    The USENIX LISA (Large Installations Systems Adminstration) Conference
    have for several years now included many papers of tools written in
    Perl.  Old proceedings of these conferences are available; look in
    your current issue of ";login:" or send mail to [e-mail]
    for futher information.

    C.  INTERNET

    For other examples of Perl scripts, look in the Perl source directory in
    the eg subdirectory.  You can also find a good deal of them on
    tut.cis.ohio-state.edu in the pub/perl/scripts/ subdirectory.

    Another source for examples, currently only for anonymous FTP, is on
    convex.com [130.168.1.1].  This contains, amongst other things,
    a copy of the newsgroup up through Aug 91, a text retrieval database
    for the newsgroup, a rather old and short troff version of Tom Christiansen's
    perl tutorial (this was the version presented at Washington DC USENIX),
    and quite a few of Tom's scripts.  You can look at the INDEX file
    in /pub/perl/INDEX for a list of what's in that directory.

    The Convex and Ohio State archives are mirrored on uunet
    in /languages/perl/scripts-{convex,osu}.

    D.  REFERENCE GUIDE

    A nice reference guide by Johan Vromans <[e-mail]> is also available;
    It is distributed in LaTeX (source) and PostScript (ready to
    print) forms. Obsolete versions may still be available in TeX and troff
    forms, although these don't print as nicely. The official kit
    includes both LaTeX and PostScript forms, and can be FTP'd from
    archive.cs.ruu.nl [131.211.80.5], file /pub/DOC/perlref-4.035.tar.Z.
    The reference guide comes with the O'Reilly book in a nice, glossy
    card format.

    E.  PERL COURSES

    Various technical conferences, including USENIX, SUG, WCSAS, AUUG,
    FedUnix, and Europen have been sponsoring tutorials of varying lengths
    on Perl at their system administration and general conferences.  You
    might consider attending one of these.  These classes are typically
    taught by Tom Christiansen <[e-mail]>, although both Rob
    Kolstad <[e-mail]> and Randal Schwartz <[e-mail]> also
    teach Perl on occasion.  Special appearances by Tom, Rob, and/or
    Randal may also be negotiated.  Classes can run from one day up to a
    week ranging over a wide range of subject matter (most are two or
    three days), and can include lab time if you want; having lab time
    with exercises is generally of great benefit.  Send us mail if your
    organization is interested in having a Perl class taught at your site.


1.8) Can people who aren't on USENET receive comp.lang.perl as a digest?

    "Perl-Users" is the mailing list version of the comp.lang.perl
    newsgroup.  If you're not lucky enough to be on USENET you can post to
    comp.lang.perl by sending to one of the following addresses.  Which one
    will work best for you depends on which nets your site is hooked into.
    Ask your local network guru if you're not certain.

    Internet: [e-mail]
              [e-mail]

    BitNet: Perl@Virginia

    uucp: ...!uunet!virginia!perl-users

    The Perl-Users list is bidirectionally gatewayed with the USENET
    newsgroup comp.lang.perl.  This means that VIRGINIA functions as a
    reflector.  All traffic coming in from the non-USENET side is
    immediately posted to the newsgroup.  Postings from the USENET side are
    periodically digested and mailed out to the Perl-Users mailing list.  A
    digest is created and distributed at least once per day, more often if
    traffic warrants.

    All requests to be added to or deleted from this list, problems,
    questions, etc., should be sent to:

    Internet: [e-mail]
              [e-mail]

    BitNet: Perl-Req@Virginia

    uucp: ...!uunet!virginia!perl-users-request

    Coordinator: Marc Rouleau <[e-mail]>

1.9) Are archives of comp.lang.perl available?

    Yes, although they're poorly organized.  You can get them from
    the host betwixt.cs.caltech.edu (131.215.128.4) in the directory
    /pub/comp.lang.perl.  They are also to uunet in
    /languages/perl/comp.lang.perl .  It contains these things:

    comp.lang.perl.tar.Z  -- the 5M tarchive in MH/news format
    archives/             -- the unpacked 5M tarchive
    unviewed/             -- new comp.lang.perl messages

    These are currently stored in news- or MH-style format; there are
    subdirectories named things like "arrays", "programs", "taint", and
    "emacs".  Unfortunately, only the first ~1600 or so messages have been
    so categorized, and we're now up to almost 15000.  Furthermore, even
    this categorization was haphazardly done and contains errors.

    A more sophisticated query and retrieval mechanism is desirable.
    Preferably one that allows you to retrieve article using a fast-access
    indices, keyed on at least author, date, subject, thread (as in "trn")
    and probably keywords.  Right now, the MH pick command works for this,
    but it is very slow to select on 15000 articles.

    If you're serious about this, your best bet is probably to retrieve
    the compressed tarchive and play with what you get.  Any suggestions
    how to better sort this all out are extremely welcome.

    Currently the comp.lang.perl archives on convex.com are nearly a year
    behind.  That's because I no longer have room to store them there.  I
    do have them all on-line still, but they are not publicly accessible.
    If you have a special request for a query on the old newsgroup
    postings, and make nice noises in my direction, I can run the query
    and send them to you.  Algebraic queries are like "find me anything
    about this and that and the other thing but not this or whozits".  I
    hope to put this in the form of a mailserver.  Donated software would
    be fine. :-)

    The fast text-retrieval query system for this I'm currently using is
    Liam Quin's excellent lqtext system, available from ftp.toronto.edu
    in /pub/lq-text* .

    Rumor has it that there are WAIS servers out there for comp.lang.perl
    these days, but I haven't used them.


1.10) How do I get Perl to run on machine FOO?

    Perl comes with an elaborate auto-configuration script that allows Perl
    to be painlessly ported to a wide variety of platforms, including many
    non-UNIX ones.  Amiga and MS-DOS binaries are available on
    jpl-devvax.jpl.nasa.gov [128.149.1.143] for anonymous FTP.  Try to bring
    Perl up on your machine, and if you have problems, examine the README
    file carefully, and if all else fails, post to comp.lang.perl;
    probably someone out there has run into your problem and will be able
    to help you.

    In particular, since they're so often asked about, here's some information
    for the MacIntosh from Matthias Ulrich Neeracher <[e-mail]>:

    A port of Perl to the Apple Macintosh is available by anonymous
    ftp to rascal.ics.utexas.edu from the file
    ~ftp/mac/programming/Perl_402_MPW_CPT_bin .

    The file is 1.1M and must be transferred in BINARY mode. Please
    be considerate of RASCAL's users during CDT working hours.
    (And, no, there is no way to get it by email).

    For European users, the file should soon appear on lth.se.

    To make optimal use of all the features of this port, you
    should have MPW, ToolServer, and 5M of memory. There is also a
    standalone version included, but it's currently of very limited
    usefulness.

    This package contains all of the sources for compilation with
    MPW C 3.2

    And here's some VMS information from Rao V. Akella
    <[e-mail]>:  (this appears to be an old port)

    You can pick up Perl for VMS (version 3.0.1.1 patchlevel 4) via
    anonymous ftp from ftp.pitt.edu [130.49.253.1] in the
    software/vms/perl subdirectory (there are two files there:
    perl-pl18.bck and perl-pl4.bck).

    There is also a v3.018 on info.rz.uni-ulm.de [134.60.1.125] or
    vms.huji.ac.il [128.139.4.3] in /pub/VMS/misc (information courtesy
    of Anders Rolff <[e-mail]>).

    And here is a recent version for MS-DOS from Budi Rahard
    <[e-mail]>, who says:

    I am collecting MS-DOS Perl(s) in ftp.ee.umanitoba.ca directory
    /pub/msdos/perl.  Currently I received three versions of Perl v4.019
    and one of 4.010.  (Tommy Thorn <[e-mail]> and Len Reed
    <holos0![e-mail]>)

    There is now a 4.035 for 386 [DOS], Hitoshi Doi <[e-mail]>
    port, is available ftp.ee.umanitoba.ca as /pub/msdos/perl/perl386.zoo .

    Please contact the porters directly in case of questions about
    these ports.


1.11) Where can I get (info|inter|ora|sql|syb)perl?

    Numerous database-oriented extensions to Perl have been written.
    These amount to using the usub mechanism (see the usub/ subdirectory
    in the distribution tree) to link in a database library, allowing
    embedded calls to Informix, Interbase, Oracle, Ingres, and Sybase.
    There is currently a project underway, organized by Buzz Moschetti
    <[e-mail]>, to create a higher level interface
    (DBperl) that will allow you to write your queries in a
    database-independent fashion.  Meanwhile, here are the authors of the
    various extensions:

    What            Target DB       Who
    --------        -----------     ----------------------------------------
    Infoperl        Informix        Kurt Andersen ([e-mail])
    Interperl       Interbase       Buzz Moschetti ([e-mail])
    Oraperl         Oracle          Kevin Stock ([e-mail])
    Sqlperl         Ingres          Ted Lemon ([e-mail])
    Sybperl         Sybase          Michael Peppler ([e-mail])


1.12) There's an a2p and an s2p; why isn't there a p2c (perl-to-C)?

    Because the Pascal people would be upset that we stole their name. :-)

    The dynamic nature of Perl's do and eval operators (and remember that
    constructs like s/$mac_donald/$mac_gregor/eieio count as an eval) would
    make this very difficult.  To fully support them, you would have to put
    the whole Perl interpreter into each compiled version for those scripts
    using them.  This is what undump does right now, if your machine has it.
    If what you're doing will be faster in C than in Perl, maybe it should
    have been written in C in the first place.  For things that ought to be
    written in Perl, the interpreter will be just about as fast, because the
    pattern matching routines won't work any faster linked into a C program.
    Even in the case of simple Perl programs that don't do any fancy evals, the
    major gain would be in compiling the control flow tests, with the rest
    still being a maze of twisty, turny subroutine calls.  Since these are not
    usually the major bottleneck in the program, there's not as much to be
    gained via compilation as one might think.


1.13) Where can I get undump for my machine?

    The undump program comes from the TeX distribution.  If you have TeX, then
    you may have a working undump.  If you don't, and you can't get one,
    *AND* you have a GNU emacs working on your machine that can clone itself,
    then you might try taking its unexec() function and compiling Perl with
    -DUNEXEC, which will make Perl call unexec() instead of abort().  You'll
    have to add unexec.o to the objects line in the Makefile.  If you succeed,
    post to comp.lang.perl about your experience so others can benefit from it.


1.14) Where can I get a perl-mode for emacs?

    In the perl4.0 source directory, you'll find a directory called
    "emacs", which contains several files that should help you.


1.15) How can I use Perl interactively?

    The easiest way to do this is to run Perl under its debugger.
    If you have no program to debug, you can invoke the debugger
    on an `empty' program like this:

        perl -de 0

    (The more positive amongst us prefer "perl -de 1". :-)

    Now you can type in any legal Perl code, and it will be immediately
    evaluated.  You can also examine the symbol table, get stack
    backtraces, check variable Values, and if you want to, set
    breakpoints and do the other things you can do in a symbolic debugger.


1.16) Is there a Perl shell?

   Not really.  Perl is a programming language, not a command
   interpreter.  There is a very simple one called "perlsh"
   included in the Perl source distribution.  It just does this:

    $/ = '';        # set paragraph mode
    $SHlinesep = "\n";
    while ($SHcmd = <>) {
        $/ = $SHlinesep;
        eval $SHcmd; print $@ || "\n";
        $SHlinesep = $/; $/ = '';
    }

   Not very interesting, eh?

   Daniel Smith <[e-mail]> is working on an interactive Perl
   shell called SoftList.  It's currently at version 3.0beta.  SoftList
   3.0 has tcsh-like command line editing, can let you define a file of
   aliases so that you can run chunks of perl or UNIX commands, and so
   on.  You can send mail to him for further information and availability.


1.17) Is there a Perl profiler?

    While there isn't one included with the perl source distribution,
    various folks have written packages that allow you to do at least some
    sort of profiling.  The strategy usually includes modifying the perl
    debugger to handle profiling.  Authors of these packages include

    Wayne Thompson         <[e-mail]>
    Ray Lischner         <[e-mail]>
    Kresten Krab Thorup     <[e-mail]>

    The original articles by these folks containing their
    profilers are available on convex.com in
    /pub/perl/information/profiling.shar via anon ftp.


1.18) Is there a yacc for Perl?

    Yes!! It's a version of Berkeley yacc that outputs Perl code instead
    of C code!  You can get this from ftp.sterling.com [192.124.9.1] in
    /local/perl-byacc1.8.1.tar.Z, or send the author mail for details.


1.19) How can I use curses with perl?

    One way is to build a curseperl binary by linking in your C curses
    library as described in the usub subdirectory of the perl sources.
    This requires a modicum of work, but it will be reasonably fast
    since it's all in C (assuming you consider curses reasonably fast. :-)
    Programs written using this method require the modified curseperl,
    not vanilla perl, to run.  While this is something of a disadvantage,
    experience indicates that it's better to use curseperl than to
    try to roll your own using termcap directly.

    Another possibility is to use Henk Penning's cterm package, a curses
    emulation library written in perl.  cterm is actually a separate
    program with which you communicate via a pipe.  It is available from
    archive.cs.ruu.nl [131.211.80.5] via anonymous ftp in the directory
    pub/PERL.  You may also acquire the package via email in compressed,
    uuencoded form by sending a message to [e-mail]
    containing these lines:

    begin
    send PERL/cterm.shar.Z
    end

    See the question on retrieving perl via mail for more information on
    how to get retrieve other items of interest from the mail server
    there.


1.20) How can I use X with Perl?

    Right now, you have several choices.  You can wait for perl5, use
    the WAFE or STDWIN packages, or try to make your own usub bindings.

    Perl5 is anticipated to be released with bindings for X, called
    guiperl.  An exciting prototype for this, written by Jon Biggar
    <[e-mail]>, Larry's *other* brother-in-law and officemate,
    is already up and running inside of Netlabs.  This program addresses
    the same dynamic gui-building problem space as does tcl/tk.

    If you can't wait or don't think that guiperl will do what you want,
    a stab at Motif bindings was begun by Theodore C. Law
    <[e-mail]> area.  His article about this is
    on convex.com in /pub/perl/info/motif for anon ftp.

    STDWIN is a library written by Guido van Rossum <[e-mail]>
    (author of the Python programming language) that is portable
    between Mac, Dos and X11.  One could write a Perl agent to
    speak to this STDIN server.

    WAFE is a package that implements a symbolic interface to the Athena
    widgets (X11R5). A typical Wafe application consists in our framework
    of two parts: the front-end (we call it Wafe for Widget[Athena]front
    end) and an application program running typically as separate process.
    The application program can be implemented in an arbitrary programming
    language and talks to the front-end via stdio.  Since Wafe (the
    front-end) was developed using the extensible TCL shell (cite John
    Ousterhout), an application program can dynamically submit requests to
    the front-end to build up the graphical user interface; the
    application can even down-load application specific procedures into
    the front-end.  The distribution contains sample application programs
    in Perl, GAWK, Prolog, TCL, and C talking to the same Wafe binary.
    Many of the demo applications are implemented in Perl.  Wafe 0.9 can
    be obtained via anonymous ftp from
    ftp.wu-wien.ac.at:pub/src/X11/wafe-0.9.tar.Z
    (for people without name server: the ip address is 137.208.3.5)


1.21) What is perl4?  What is perl5?

    The answer to what is perl4 is nearly anything you might otherwise
    program in shell or C.  The answer to what is perl5 is basically
    Perl: the Next Generation.  In fact, it's essentially a complete
    rewrite of perl from the bottom up, and back again.

    Larry gave a talk on perl5 at a Bay LISA meeting as well as at the
    most recent USENIX LISA conference in Long Beach in which he timorously
    admitted that perl5 might possibly be beta released in early 1993.
    He enumerated some of the following features.  Note that not only have
    not all these been implemented yet, the ones further down the list
    might well not get done at all.

    a faster, tighter, more flexible interpreter
    very easy GUI Perl applications using X bindings ("guiperl")
    embeddable Perl code in C code: cc prog.c -lperl
    multiple coresident perl interpreters:
        perhaps threading and/or coroutines
    named argument passing:
        some_func( OC => $red, TOF => "\f");
    recursive lists:
        [a, b, [c, d], e] has 4 elts, the 3rd being itself a list
    typed pointers and generalized indirection:
        like @{$aptr} or &{$fptr} or &{ $table[$index] . "func" }().
    merging of list operator and function calling syntax:
        split /pat/, $string;
    subroutines without &'s:   myfunc($arg);
    generalization of dbm binding for assoc arrays to handle
        any generic fetch/store/open/close/flush package.
        (thus allowing both dbm and gdbm at once)
    object oriented programming:
        STDOUT->flush(1);
        give dog $bone;
    lexical scoping
    dynamic loading of C libraries for systems that can
    byte-compiled code for speed and maybe security

    It's tempting to want this stuff soon, since the sooner it comes
    out the sooner we can all build really cool applications.  But the
    longer Larry works on it, the more items from this list will actually
    get done, and the more robust the release will be.  So let's not
    ask him about it too often.


1.22) How does Perl compare with languages like REXX or TCL?

    REXX is an interpreted programming language first seen on IBM systems,
    and TCL is John Ousterhout's embeddable command language.  TCL's most
    intriguing feature for many people is the tcl/tk toolset that allows
    for interpreted X-based tools.

    To avoid any flamage, if you really want to know the answer to this
    question, probably the best thing to do is try to write equivalent
    code to do a set of tasks.  All three have their own newsgroups in
    which you can learn about (but hopefully not argue about) these
    languages.

    To find out more about these or other languages, you might also check
    out David Muir Sharnoff <[e-mail]>'s posting on "Catalog of
    compilers, interpreters, and other language tools" which he posts to
    comp.lang.misc, comp.sources.d, comp.archives.admin, and the
    news.answers newsgroups.  It's a comprehensive treatment of many
    different languages.  (Caveat lector: he considers Perl's syntax
    "unappealing".)   This list is archived on convex.com in
    /pub/perl/info/lang-survey.shar .


1.23) Is it a Perl program or a Perl script?

    Certainly. :-)

    Current UNIX parlance holds that anything interpreted
    is a script, and anything compiled into native machine
    code is a program.  However, others hold that a program
    is a program is a program: after all, one seldom discusses
    scripts written in BASIC or LISP.  Larry considers it
    a program if it's set in stone and you can't change it,
    whereas if you go in and hack on it, then it's a script.

    But doesn't really matter.  The terms are generally
    interchangeable today.


1.24) What's the difference between "Perl" and "perl"?

     32 :-)  [  ord('p') - ord('P')  ]

     Larry now uses "Perl" to signify the language proper and "perl" the
     implementation of it, i.e. the current interpreter.  Hence my quip
     that "Nothing but perl can parse Perl."

     On the other hand, the aesthetic value of casewise parallelism
     in "awk", "sed", and "perl" as much require the lower-case
     version as "C", "Pascal", and "Perl" require the
     upper-case version.  It's also easier to type "Perl" in
     typeset print than to be constantly switching in Courier. :-)

     In other words, it doesn't matter much, especially if all
     you're doing is hearing someone talk about the language;
     case is hard to distingish aurally.


1.25) What companies use or ship Perl?

    At this time, the known list includes at least the following: Convex,
    Netlabs, BSDI, Integraph, Dell, and Kubota Pacific, although the
    latter is in /usr/contrib only.  Many other companies use Perl
    internally for purposes of tools development, systems administration,
    installation scripts, and test suites.  Rumor has it that the large
    workstation vendors (the TLA set) are seriously looking into shipping
    Perl with their standard systems "soon".

    People with support contracts with their vendors are actively
    encouraged to submit enhancement requests that Perl be shipped
    as part of their standard system.  It would, at the very least,
    reduce the FTP load on the Internet. :-)

1.26) Is there commercial, 3rd-party support for Perl?

    No.  Although perl is included in the GNU distribution, at last check,
    Cygnus does not offer support for it.  However, it's unclear whether
    they've ever been offered sufficient financial incentive to do so.

    On the other hand, you do have comp.lang.perl as a totally gratis
    support mechanism.  As long as you ask "interesting" questions,
    you'll probably get plenty of help. :-)

1.27) Where can I get a list of the JAPH signature quotes?

    These are the "just another perl hacker" signatures that
    some people sign their postings with.  About 100 of the
    of the earlier ones are on convex.com in /pib/perl/info/japh.

1.28) Where can I get a list of Larry Wall witticisms?

    Over a hundred quips by Larry, from postings of his or source code,
    can be found on convex.com in /pub/perl/info/lwall-quotes.




2.1) What are all these $@*%<> signs and how do I know when to use them?

    Those are type specifiers: $ for scalar values, @ for indexed arrays,
    and % for hashed arrays.  The * means all types of that symbol name
    and are sometimes used like pointers; the <> are used for inputting
    a record from a filehandle.  See the question on arrays of arrays
    for more about Perl pointers.

    Always make sure to use a $ for single values and @ for multiple ones.
    Thus element 2 of the @foo array is accessed as $foo[2], not @foo[2],
    which is a list of length one (not a scalar), and is a fairly common
    novice mistake.  Sometimes you can get by with @foo[2], but it's
    not really doing what you think it's doing for the reason you think
    it's doing it, which means one of these days, you'll shoot yourself
    in the foot; ponder for a moment what these will really do:
    @foo[0] = `cmd args`;
    @foo[2] = <FILE>;
    Just always say $foo[2] and you'll be happier.

    This may seem confusing, but try to think of it this way:  you use the
    character of the type which you *want back*.  You could use @foo[1..3] for
    a slice of three elements of @foo, or even @foo{A,B,C} for a slice of
    of %foo.  This is the same as using ($foo[1], $foo[2], $foo[3]) and
    ($foo{A}, $foo{B}, $foo{C}) respectively.  In fact, you can even use
    lists to subscript arrays and pull out more lists, like @foo[@bar] or
    @foo{@bar}, where @bar is in both cases presumably a list of subscripts.

    While there are a few places where you don't actually need these type
    specifiers, except for files, you should always use them.  Note that
    <FILE> is NOT the type specifier for files; it's the equivalent of awk's
    getline function, that is, it reads a line from the handle FILE.  When
    doing open, close, and other operations besides the getline function on
    files, do NOT use the brackets.

    Beware of saying:
    $foo = BAR;
    Which wil be interpreted as
    $foo = 'BAR';
    and not as
    $foo = <BAR>;
    If you always quote your strings, you'll avoid this trap.

    Normally, files are manipulated something like this (with appropriate
    error checking added if it were production code):

    open (FILE, ">/tmp/foo.$$");
    print FILE "string\n";
    close FILE;

    If instead of a filehandle, you use a normal scalar variable with file
    manipulation functions, this is considered an indirect reference to a
    filehandle.  For example,

    $foo = "TEST01";
    open($foo, "file");

    After the open, these two while loops are equivalent:

    while (<$foo>) {}
    while (<TEST01>) {}

    as are these two statements:

    close $foo;
    close TEST01;

    but NOT to this:

    while (<$TEST01>) {} # error
        ^
        ^ note spurious dollar sign

    This is another common novice mistake; often it's assumed that

    open($foo, "output.$$");

    will fill in the value of $foo, which was previously undefined.
    This just isn't so -- you must set $foo to be the name of a valid
    filehandle before you attempt to open it.


2.2) Why don't backticks work as they do in shells?

    Several reason.  One is because backticks do not interpolate within
    double quotes in Perl as they do in shells.

    Let's look at two common mistakes:

         $foo = "$bar is `wc $file`";  # WRONG

    This should have been:

     $foo = "$bar is " . `wc $file`;

    But you'll have an extra newline you might not expect.  This
    does not work as expected:

      $back = `pwd`; chdir($somewhere); chdir($back); # WRONG

    Because backticks do not automatically eat trailing or embedded
    newlines.  The chop() function will remove the last character from
    a string.  This should have been:

      chop($back = `pwd`); chdir($somewhere); chdir($back);

    You should also be aware that while in the shells, embedding
    single quotes will protect variables, in Perl, you'll need
    to escape the dollar signs.

    Shell: foo=`cmd 'safe $dollar'`
    Perl:  $foo=`cmd 'safe \$dollar'`;


2.3) How come Perl operators have different precedence than C operators?

    Actually, they don't; all C operators have the same precedence in Perl as
    they do in C.  The problem is with a class of functions called list
    operators, e.g. print, chdir, exec, system, and so on.  These are somewhat
    bizarre in that they have different precedence depending on whether you
    look on the left or right of them.  Basically, they gobble up all things
    on their right.  For example,

    unlink $foo, "bar", @names, "others";

    will unlink all those file names.  A common mistake is to write:

    unlink "a_file" || die "snafu";

    The problem is that this gets interpreted as

    unlink("a_file" || die "snafu");

    To avoid this problem, you can always make them look like function calls
    or use an extra level of parentheses:

    (unlink "a_file") || die "snafu";
    unlink("a_file")  || die "snafu";

    Sometimes you actually do care about the return value:

    unless ($io_ok = print("some", "list")) { }

    Yes, print() return I/O success.  That means

    $io_ok = print(2+4) * 5;

    reutrns 5 times whether printing (2+4) succeeded, and
    print(2+4) * 5;
    returns the same 5*io_success value and tosses it.

    See the Perl man page's section on Precedence for more gory details,
    and be sure to use the -w flag to catch things like this.


2.4) How come my converted awk/sed/sh script runs more slowly in Perl?

    The natural way to program in those languages may not make for the fastest
    Perl code.  Notably, the awk-to-perl translator produces sub-optimal code;
    see the a2p man page for tweaks you can make.

    Two of Perl's strongest points are its associative arrays and its regular
    expressions.  They can dramatically speed up your code when applied
    properly.  Recasting your code to use them can help alot.

    How complex are your regexps?  Deeply nested sub-expressions with {n,m} or
    * operators can take a very long time to compute.  Don't use ()'s unless
    you really need them.  Anchor your string to the front if you can.

    Something like this:
    next unless /^.*%.*$/;
    runs more slowly than the equivalent:
    next unless /%/;

    Note that this:
    next if /Mon/;
    next if /Tue/;
    next if /Wed/;
    next if /Thu/;
    next if /Fri/;
    runs faster than this:
    next if /Mon/ || /Tue/ || /Wed/ || /Thu/ || /Fri/;
    which in turn runs faster than this:
    next if /Mon|Tue|Wed|Thu|Fri/;
    which runs *much* faster than:
    next if /(Mon|Tue|Wed|Thu|Fri)/;

    There's no need to use /^.*foo.*$/ when /foo/ will do.

    Remember that a printf costs more than a simple print.

    Don't split() every line if you don't have to.

    Another thing to look at is your loops.  Are you iterating through
    indexed arrays rather than just putting everything into a hashed
    array?  For example,

    @list = ('abc', 'def', 'ghi', 'jkl', 'mno', 'pqr', 'stv');

    for $i ($[ .. $#list) {
        if ($pattern eq $list[$i]) { $found++; }
    }

    First of all, it would be faster to use Perl's foreach mechanism
    instead of using subscripts:

    foreach $elt (@list) {
        if ($pattern eq $elt) { $found++; }
    }

    Better yet, this could be sped up dramatically by placing the whole
    thing in an associative array like this:

    %list = ('abc', 1, 'def', 1, 'ghi', 1, 'jkl', 1,
         'mno', 1, 'pqr', 1, 'stv', 1 );
    $found += $list{$pattern};

    (but put the %list assignment outside of your input loop.)

    You should also look at variables in regular expressions, which is
    expensive.  If the variable to be interpolated doesn't change over the
    life of the process, use the /o modifier to tell Perl to compile the
    regexp only once, like this:

    for $i (1..100) {
        if (/$foo/o) {
        &some_func($i);
        }
    }

    Finally, if you have a bunch of patterns in a list that you'd like to
    compare against, instead of doing this:

    @pats = ('_get.*', 'bogus', '_read', '.*exit', '_write');
    foreach $pat (@pats) {
        if ( $name =~ /^$pat$/ ) {
        &some_func();
        last;
        }
    }

    If you build your code and then eval it, it will be much faster.
    For example:

    @pats = ('_get.*', 'bogus', '_read', '.*exit', '_write');
    $code = <<EOS
        while () {
            study;
EOS
    foreach $pat (@pats) {
        $code .= <<EOS
        if ( /^$pat\$/ ) {
            &some_func();
            next;
        }
EOS
    }
    $code .= "}\n";
    print $code if $debugging;
    eval $code;



2.5) How can I call my system's unique C functions from Perl?

    If these are system calls and you have the syscall() function, then
    you're probably in luck -- see the next question.  For arbitrary
    library functions, it's not quite so straight-forward.  While you
    can't have a C main and link in Perl routines, if you're
    determined, you can extend Perl by linking in your own C routines.
    See the usub/ subdirectory in the Perl distribution kit for an example
    of doing this to build a Perl that understands curses functions.  It's
    neither particularly easy nor overly-documented, but it is feasible.


2.6) Where do I get the include files to do ioctl() or syscall()?

    These are generated from your system's C include files using the h2ph
    script (once called makelib) from the Perl source directory.  This will
    make files containing subroutine definitions, like &SYS_getitimer, which
    you can use as arguments to your function.

    You might also look at the h2pl subdirectory in the Perl source for how to
    convert these to forms like $SYS_getitimer; there are both advantages and
    disadvantages to this.  Read the notes in that directory for details.

    In both cases, you may well have to fiddle with it to make these work; it
    depends how funny-looking your system's C include files happen to be.

    If you're trying to get at C structures, then you should take a look
    at using c2ph, which uses debugger "stab" entries generated by your
    BSD or GNU C compiler to produce machine-independent perl definitions
    for the data structures.  This allows to you avoid hardcoding
    structure layouts, types, padding, or sizes, greatly enhancing
    portability.  c2ph comes with the perl distribution.  On an SCO
    system, GCC only has COFF debugging support by default, so you'll have
    to build GCC 2.1 with DBX_DEBUGGING_INFO defined, and use -gstabs to
    get c2ph to work there.

    See the file /pub/perl/info/ch2ph on convex.com via anon ftp
    for more traps and tips on this process.


2.7) Why doesn't "local($foo) = <FILE>;" work right?

    Well, it does.  The thing to remember is that local() provides an array
    context, an that the <FILE> syntax in an array context will read all the
    lines in a file.  To work around this, use:

    local($foo);
    $foo = <FILE>;

    You can use the scalar() operator to cast the expression into a scalar
    context:

    local($foo) = scalar(<FILE>);


2.8) How can I detect keyboard input without reading it?

    You should check out the Frequently Asked Questions list in
    comp.unix.* for things like this: the answer is essentially the same.
    It's very system dependent.  Here's one solution that works on BSD
    systems:

    sub key_ready {
        local($rin, $nfd);
        vec($rin, fileno(STDIN), 1) = 1;
        return $nfd = select($rin,undef,undef,0);
    }

    A closely related question is how to input a single character from the
    keyboard.  Again, this is a system dependent operation.  The following
    code that may or may not help you:

    $BSD = -f '/vmunix';
    if ($BSD) {
        system "stty cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system "stty", 'cbreak',
        system "stty", 'eol', "\001";
    }

    $key = getc(STDIN);

    if ($BSD) {
        system "stty -cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system "stty", 'icanon';
        system "stty", 'eol', '^@'; # ascii null
    }
    print "\n";

    You could also handle the stty operations yourself for speed if you're
    going to be doing a lot of them.  This code works to toggle cbreak
    and echo modes on a BSD system:

    sub set_cbreak { # &set_cbreak(1) or &set_cbreak(0)
    local($on) = $_[0];
    local($sgttyb,@ary);
    require 'sys/ioctl.ph';
    $sgttyb_t   = 'C4 S' unless $sgttyb_t;  # c2ph: &sgttyb'typedef()

    ioctl(STDIN,&TIOCGETP,$sgttyb) || die "Can't ioctl TIOCGETP: $!";

    @ary = unpack($sgttyb_t,$sgttyb);
    if ($on) {
        $ary[4] |= &CBREAK;
        $ary[4] &= ~&ECHO;
    } else {
        $ary[4] &= ~&CBREAK;
        $ary[4] |= &ECHO;
    }
    $sgttyb = pack($sgttyb_t,@ary);

    ioctl(STDIN,&TIOCSETP,$sgttyb) || die "Can't ioctl TIOCSETP: $!";
    }

    Note that this is one of the few times you actually want to use the
    getc() function; it's in general way too expensive to call for normal
    I/O.  Normally, you just use the <FILE> syntax, or perhaps the read()
    or sysread() functions.

    For perspectives on more portable solutions, use anon ftp to retrieve
    the file /pub/perl/info/keypress from convex.com.


2.9) How can I make an array of arrays or other recursive data types?

    Remember that Perl isn't about nested data structures (actually,
    perl0 ..  perl4 weren't, but maybe perl5 will be, at least
    somewhat).  It's about flat ones, so if you're trying to do this, you
    may be going about it the wrong way or using the wrong tools.  You
    might try parallel arrays with common subscripts.

    But if you're bound and determined, you can use the multi-dimensional
    array emulation of $a{'x','y','z'}, or you can make an array of names
    of arrays and eval it.

    For example, if @name contains a list of names of arrays, you can
    get at a the j-th element of the i-th array like so:

    $ary = $name[$i];
    $val = eval "\$$ary[$j]";

    or in one line

    $val = eval "\$$name[$i][\$j]";

    You could also use the type-globbing syntax to make an array of *name
    values, which will be more efficient than eval.  Here @name hold
    a list of pointers, which we'll have to dereference through a temporary
    variable.

    For example:

    { local(*ary) = $name[$i]; $val = $ary[$j]; }

    In fact, you can use this method to make arbitrarily nested data
    structures.  You really have to want to do this kind of thing
    badly to go this far, however, as it is notationally cumbersome.

    Let's assume you just simply *have* to have an array of arrays of
    arrays.  What you do is make an array of pointers to arrays of
    pointers, where pointers are *name values described above.  You
    initialize the outermost array normally, and then you build up your
    pointers from there.  For example:

    @w = ( 'ww' .. 'xx' );
    @x = ( 'xx' .. 'yy' );
    @y = ( 'yy' .. 'zz' );
    @z = ( 'zz' .. 'zzz' );

    @ww = reverse @w;
    @xx = reverse @x;
    @yy = reverse @y;
    @zz = reverse @z;

    Now make a couple of array of pointers to these:

    @A = ( *w, *x, *y, *z );
    @B = ( *ww, *xx, *yy, *zz );

    And finally make an array of pointers to these arrays:

    @AAA = ( *A, *B );

    To access an element, such as AAA[i][j][k], you must do this:

    local(*foo) = $AAA[$i];
    local(*bar) = $foo[$j];
    $answer = $bar[$k];

    Similar manipulations on associative arrays are also feasible.

    You could take a look at recurse.pl package posted by Felix Lee
    <[e-mail]>, which lets you simulate vectors and tables (lists and
    associative arrays) by using type glob references and some pretty serious
    wizardry.

    In C, you're used to creating recursive datatypes for operations
    like recursive decent parsing or tree traversal.  In Perl, these
    algorithms are best implemented using associative arrays.  Take an
    array called %parent, and build up pointers such that $parent{$person}
    is the name of that person's parent.  Make sure you remember that
    $parent{'adam'} is 'adam'. :-) With a little care, this approach can
    be used to implement general graph traversal algorithms as well.


2.10) How can I quote a variable to use in a regexp?

    From the manual:

    $pattern =~ s/(\W)/\\$1/g;

    Now you can freely use /$pattern/ without fear of any unexpected
    meta-characters in it throwing off the search.  If you don't know
    whether a pattern is valid or not, enclose it in an eval to avoid
    a fatal run-time error.


2.11) Why do setuid Perl scripts complain about kernel problems?

    This message:

    YOU HAVEN'T DISABLED SET-ID SCRIPTS IN THE KERNEL YET!
    FIX YOUR KERNEL, PUT A C WRAPPER AROUND THIS SCRIPT, OR USE -u AND UNDUMP!

    is triggered because setuid scripts are inherently insecure due to a
    kernel bug.  If your system has fixed this bug, you can compile Perl
    so that it knows this.  Otherwise, create a setuid C program that just
    execs Perl with the full name of the script.


2.12) How do I open a pipe both to and from a command?

    In general, this is a dangerous move because you can find yourself in a
    deadlock situation.  It's better to put one end of the pipe to a file.
    For example:

    # first write some_cmd's input into a_file, then
    open(CMD, "some_cmd its_args < a_file |");
    while (<CMD>) {

    # or else the other way; run the cmd
    open(CMD, "| some_cmd its_args > a_file");
    while ($condition) {
        print CMD "some output\n";
        # other code deleted
    }
    close CMD || warn "cmd exited $?";

    # now read the file
    open(FILE,"a_file");
    while (<FILE>) {

    If you have ptys, you could arrange to run the command on a pty and
    avoid the deadlock problem.  See the chat2.pl package in the
    distributed library for ways to do this.

    At the risk of deadlock, it is theoretically possible to use a
    fork, two pipe calls, and an exec to manually set up the two-way
    pipe.  (BSD system may use socketpair() in place of the two pipes,
    but this is not as portable.)  The open2 library function distributed
    with the current perl release will do this for you.

    It assumes it's going to talk to something like adb, both writing to
    it and reading from it.  This is presumably safe because you "know"
    that commands like adb will read a line at a time and output a line at
    a time.  Programs like sort that read their entire input stream first,
    however, are quite apt to cause deadlock.


2.13) How can I change the first N letters of a string?

    Remember that the substr() function produces an lvalue, that is, it may be
    assigned to.  Therefore, to change the first character to an S, you could
    do this:

    substr($var,0,1) = 'S';

    This assumes that $[ is 0;  for a library routine where you can't know $[,
    you should use this instead:

    substr($var,$[,1) = 'S';

    While it would be slower, you could in this case use a substitute:

    $var =~ s/^./S/;

    But this won't work if the string is empty or its first character is a
    newline, which "." will never match.  So you could use this instead:

    $var =~ s/^[^\0]?/S/;

    To do things like translation of the first part of a string, use substr,
    as in:

    substr($var, $[, 10) =~ tr/a-z/A-Z/;

    If you don't know then length of what to translate, something like
    this works:

    /^(\S+)/ && substr($_,$[,length($1)) =~ tr/a-z/A-Z/;

    For some things it's convenient to use the /e switch of the
    substitute operator:

    s/^(\S+)/($tmp = $1) =~ tr#a-z#A-Z#, $tmp/e

    although in this case, it runs more slowly than does the previous example.


2.14) How can I manipulate fixed-record-length files?

    The most efficient way is using pack and unpack.  This is faster than
    using substr.  Here is a sample chunk of code to break up and put back
    together again some fixed-format input lines, in this case, from ps.

    # sample input line:
    #   15158 p5  T      0:00 perl /mnt/tchrist/scripts/now-what
    $ps_t = 'A6 A4 A7 A5 A*';
    open(PS, "ps|");
    $_ = <PS>; print;
    while (<PS>) {
        ($pid, $tt, $stat, $time, $command) = unpack($ps_t, $_);
        for $var ('pid', 'tt', 'stat', 'time', 'command' ) {
        print "$var: <", eval "\$$var", ">\n";
        }
        print 'line=', pack($ps_t, $pid, $tt, $stat, $time, $command),  "\n";
    }


2.15) How can I make a file handle local to a subroutine?

    You must use the type-globbing *VAR notation.  Here is some code to
    cat an include file, calling itself recursively on nested local
    include files (i.e. those with #include "file", not #include <file>):

    sub cat_include {
        local($name) = @_;
        local(*FILE);
        local($_);

        warn "<INCLUDING $name>\n";
        if (!open (FILE, $name)) {
        warn "can't open $name: $!\n";
        return;
        }
        while (<FILE>) {
        if (/^#\s*include "([^"]*)"/) {
            &cat_include($1);
        } else {
            print;
        }
        }
        close FILE;
    }


2.16) How can I extract just the unique elements of an array?

    There are several possible ways, depending on whether the
    array is ordered and you wish to preserve the ordering.

    a) If @in is sorted, and you want @out to be sorted:

    $prev = 'nonesuch';
    @out = grep($_ ne $prev && (($prev) = $_), @in);

       This is nice in that it doesn't use much extra memory,
       simulating uniq's behavior of removing only adjacent
       duplicates.

    b) If you don't know whether @in is sorted:

    undef %saw;
    @out = grep(!$saw{$_}++, @in);

    c) Like (b), but @in contains only small integers:

    @out = grep(!$saw[$_]++, @in);

    d) A way to do (b) without any loops or greps:

    undef %saw;
    @saw{@in} = ();
    @out = sort keys %saw;  # remove sort if undesired

    e) Like (d), but @in contains only small positive integers:

    undef @ary;
    @ary[@in] = @in;
    @out = sort @ary;


2.17) How can I call alarm() or usleep() from Perl?

    It's available as a built-in as of version 3.038.  If you want finer
    granularity than 1 second (as usleep() provides) and have itimers and
    syscall() on your system, you can use the following.  You could also
    use select().

    It takes a floating-point number representing how long to delay until
    you get the SIGALRM, and returns a floating- point number representing
    how much time was left in the old timer, if any.  Note that the C
    function uses integers, but this one doesn't mind fractional numbers.

    # alarm; send me a SIGALRM in this many seconds (fractions ok)
    # tom christiansen <[e-mail]>
    sub alarm {
    require 'syscall.ph';
    require 'sys/time.ph';

    local($ticks) = @_;
    local($in_timer,$out_timer);
    local($isecs, $iusecs, $secs, $usecs);

    local($itimer_t) = 'L4'; # should be &itimer'typedef()

    $secs = int($ticks);
    $usecs = ($ticks - $secs) * 1e6;

    $out_timer = pack($itimer_t,0,0,0,0);
    $in_timer  = pack($itimer_t,0,0,$secs,$usecs);

    syscall(&SYS_setitimer, &ITIMER_REAL, $in_timer, $out_timer)
        && die "alarm: setitimer syscall failed: $!";

    ($isecs, $iusecs, $secs, $usecs) = unpack($itimer_t,$out_timer);
    return $secs + ($usecs/1e6);
    }


2.18) How can I test whether an array contains a certain element?

    There are several ways to approach this.  If you are going to make
    this query many times and the values are arbitrary strings, the
    fastest way is probably to invert the original array and keep an
    associative array lying about whose keys are the first array's values.

    @blues = ('turquoise', 'teal', 'lapis lazuli');
    undef %is_blue;
    for (@blues) { $is_blue{$_} = 1; }

    Now you can check whether $is_blue{$some_color}.  It might have been
    a good idea to keep the blues all in an assoc array in the first place.

    If the values are all small integers, you could use a simple
    indexed array.  This kind of an array will take up less space:

    @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
    undef @is_tiny_prime;
    for (@primes) { $is_tiny_prime[$_] = 1; }

    Now you check whether $is_tiny_prime[$some_number].

    If the values in question are integers, but instead of strings,
    you can save quite a lot of space by using bit strings instead:

    @articles = ( 1..10, 150..2000, 2017 );
    undef $read;
    grep (vec($read,$_,1) = 1, @articles);

    Now check whether vec($read,$n,1) is true for some $n.


2.19) How can I do an atexit() or setjmp()/longjmp() in Perl?

    Perl's exception-handling mechanism is its eval operator.  You
    can use eval as setjmp and die as longjmp.  Here's an example
    of Larry's for timed-out input, which in C is often implemented
    using setjmp and longjmp:

      $SIG{ALRM} = TIMEOUT;
      sub TIMEOUT { die "restart input\n" }

      do { eval { &realcode } } while $@ =~ /^restart input/;

      sub realcode {
          alarm 15;
          $ans = <STDIN>;
          alarm 0;
      }

   Here's an example of Tom's for doing atexit() handling:

    sub atexit { push(@_exit_subs, @_) }

    sub _cleanup { unlink $tmp }

    &atexit('_cleanup');

    eval <<'End_Of_Eval';  $here = __LINE__;
    # as much code here as you want
    End_Of_Eval

    $oops = $@;  # save error message

    # now call his stuff
    for (@_exit_subs) { &$_() }

    $oops && ($oops =~ s/\(eval\) line (\d+)/$0 .
        " line " . ($1+$here)/e, die $oops);

    You can register your own routines via the &atexit function now.  You
    might also want to use the &realcode method of Larry's rather than
    embedding all your code in the here-is document.  Make sure to leave
    via die rather than exit, or write your own &exit routine and call
    that instead.   In general, it's better for nested routines to exit
    via die rather than exit for just this reason.

    Eval is also quite useful for testing for system dependent features,
    like symlinks, or using a user-input regexp that might otherwise
    blowup on you.


2.20) Why doesn't Perl interpret my octal data octally?

    Perl only understands octal and hex numbers as such when they occur
    as constants in your program.  If they are read in from somewhere
    and assigned, then no automatic conversion takes place.  You must
    explicitly use oct() or hex() if you want this kind of thing to happen.
    Actually, oct() knows to interpret both hex and octal numbers, while
    hex only converts hexadecimal ones.  For example:

    {
        print "What mode would you like? ";
        $mode = <STDIN>;
        $mode = oct($mode);
        unless ($mode) {
        print "You can't really want mode 0!\n";
        redo;
        }
        chmod $mode, $file;
    }

    Without the octal conversion, a requested mode of 755 would turn
    into 01363, yielding bizarre file permissions of --wxrw--wt.

    If you want something that handles decimal, octal and hex input,
    you could follow the suggestion in the man page and use:

    $val = oct($val) if $val =~ /^0/;

2.21) How do I sort an associative array by value instead of by key?

    You have to declare a sort subroutine to do this.  Let's assume
    you want an ASCII sort on the values of the associative array %ary.
    You could do so this way:

    foreach $key (sort by_value keys %ary) {
        print $key, '=', $ary{$key}, "\n";
    }
    sub by_value { $ary{$a} cmp $ary{$b}; }

    If you wanted a descending numeric sort, you could do this:

    sub by_value { $ary{$b} <=> $ary{$a}; }

    You can also inline your sort function, like this:

    foreach $key ( sort { $x{$b} <=> $a{$a} } keys %ary ) {
        print $key, '=', $ary{$key}, "\n";
    }

    If you wanted a function that didn't have the array name hard-wired
    into it, you could so this:

    foreach $key (&sort_by_value(*ary)) {
        print $key, '=', $ary{$key}, "\n";
    }
    sub sort_by_value {
        local(*x) = @_;
        sub _by_value { $x{$a} cmp $x{$b}; }
        sort _by_value keys %x;
    }

    If you want neither an alphabetic nor a numeric sort, then you'll
    have to code in your own logic instead of relying on the built-in
    signed comparison operators "cmp" and "<=>".

    Note that if you're sorting on just a part of the value, such as a
    piece you might extract via split, unpack, pattern-matching, or
    substr, then rather than performing that operation inside your sort
    routine on each call to it, it is significantly more efficient to
    build a parallel array of just those portions you're sorting on, sort
    the indices of this parallel array, and then to subscript your original
    array using the newly sorted indices.  This method works on both
    regular and associative arrays, since both @ary[@idx] and @ary{@idx}
    make sense.  See page 245 in the Camel Book on "Sorting an Array by a
    Computable Field" for a simple example of this.


2.22) How can I capture STDERR from an external command?

    There are three basic ways of running external commands:

    system $cmd;
    $output = `$cmd`;
    open (PIPE, "cmd |");

    In the first case, both STDOUT and STDERR will go the same place as
    the script's versions of these, unless redirected.  You can always put
    them where you want them and then read them back when the system
    returns.  In the second and third cases, you are reading the STDOUT
    *only* of your command.  If you would like to have merged STDOUT and
    STDERR, you can use shell file-descriptor redirection to dup STDERR to
    STDOUT:

    $output = `$cmd 2>&1`;
    open (PIPE, "cmd 2>&1 |");

    Another possibility is to run STDERR into a file and read the file
    later, as in

    $output = `$cmd 2>some_file`;
    open (PIPE, "cmd 2>some_file |");

    Here's a way to read from both of them and know which descriptor
    you got each line from.  The trick is to pipe only STDERR through
    sed, which then marks each of its lines, and then sends that
    back into a merged STDOUT/STDERR stream, from which your Perl program
    then reads a line at a time:

        open (CMD,
          "3>&1 (cmd args 2>&1 1>&3 3>&- | sed 's/^/STDERR:/' 3>&-) 3>&- |");

        while (<CMD>) {
          if (s/^STDERR://)  {
              print "line from stderr: ", $_;
          } else {
              print "line from stdout: ", $_;
          }
        }

    Be apprised that you *must* use Bourne shell redirection syntax
    here, not csh!  In fact, you can't even do these things with csh.
    For details on how lucky you are that perl's system() and backtick
    and pipe opens all use Bourne shell, fetch the file from convex.com
    called /pub/csh.whynot -- and you'll be glad that perl's shell
    interface is the Bourne shell.


2.23) Why doesn't open return an error when a pipe open fails?

    These statements:

    open(TOPIPE, "|bogus_command") || die ...
    open(FROMPIPE, "bogus_command|") || die ...

    will not fail just for lack of the bogus_command.  They'll only
    fail if the fork to run them fails, which is seldom the problem.

    If you're writing to the TOPIPE, you'll get a SIGPIPE if the child
    exits prematurely or doesn't run.  If you are reading from the
    FROMPIPE, you need to check the close() to see what happened.

    If you want an answer sooner than pipe buffering might otherwise
    afford you, you can do something like this:

    $kid = open (PIPE, "bogus_command |");   # XXX: check defined($kid)
    (kill 0, $kid) || die "bogus_command failed";

    This works fine if bogus_command doesn't have shell metas in it, but
    if it does, the shell may well not have exited before the kill 0.  You
    could always introduce a delay:

    $kid = open (PIPE, "bogus_command </dev/null |");
    sleep 1;
    (kill 0, $kid) || die "bogus_command failed";

    but this is sometimes undesirable, and in any event does not guarantee
    correct behavior.  But it seems slightly better than nothing.

    Similar tricks can be played with writable pipes if you don't wish to
    catch the SIGPIPE.


2.24) How can I compare two date strings?

    If the dates are in an easily parsed, predetermined format, then you
    can break them up into their component parts and call &timelocal from
    the distributed perl library.  If the date strings are in arbitrary
    formats, however, it's probably easier to use the getdate program
    from the Cnews distribution, since it accepts a wide variety of dates.
    Note that in either case the return values you will really be
    comparing will be the total time in seconds as return by time().

    Here's a getdate function for perl that's not very efficient; you
    can do better this by sending it many dates at once or modifying
    getdate to behave better on a pipe.  Beware the hardcoded pathname.

    sub getdate {
        local($_) = shift;

        s/-(\d{4})$/+$1/ || s/\+(\d{4})$/-$1/;
        # getdate has broken timezone sign reversal!

        $_ = `/usr/local/lib/news/newsbin/getdate '$_'`;
        chop;
        $_;
    }

    Richard Ohnemus <[e-mail]> actually has a getdate.y
    for use with the Perl yacc.  You can get this from ftp.sterling.com
    [192.124.9.1] in /local/perl-byacc1.8.1.tar.Z, or send the author
    mail for details.


2.25) What's the fastest way to code up a given task in perl?

    Because Perl so lends itself to a variety of different approaches
    for any given task, a common question is which is the fastest way
    to code a given task.  Since some approaches can be dramatically
    more efficient that others, it's sometimes worth knowing which is
    best.  Unfortunately, the implementation that first comes to mind,
    perhaps as a direct translation from C or the shell, often yields
    suboptimal performance.  Not all approaches have the same results
    across different hardware and software platforms.  Furthermore,
    legibility must sometimes be sacrificed for speed.

    While an experienced perl programmer can sometimes eye-ball the code
    and make an educated guess regarding which way would be fastest,
    surprises can still occur.  So, in the spirit of perl programming
    being an empirical science, the best way to find out which of several
    different methods runs the fastest is simply to code them all up and
    time them. For example:

    $COUNT = 10_000; $| = 1;

    print "method 1: ";

        ($u, $s) = times;
        for ($i = 0; $i < $COUNT; $i++) {
        # code for method 1
        }
        ($nu, $ns) = times;
        printf "%8.4fu %8.4fs\n", ($nu - $u), ($ns - $s);

    print "method 2: ";

        ($u, $s) = times;
        for ($i = 0; $i < $COUNT; $i++) {
        # code for method 2
        }
        ($nu, $ns) = times;
        printf "%8.4fu %8.4fs\n", ($nu - $u), ($ns - $s);

    For more specific tips, see the section on Efficiency in the
    ``Other Oddments'' chapter at the end of the Camel Book.


2.26) How can I know how many entries are in an associative array?

    While the number of elements in a @foobar array is simply @foobar when
    used in a scalar, you can't figure out how many elements are in an
    associative array in an analagous fashion.  That's because %foobar in
    a scalar context returns the ratio (as a string) of number of buckets
    filled versus the number allocated.  For example, scalar(%ENV) might
    return "20/32".  While perl could in theory keep a count, this would
    break down on associative arrays that have been bound to dbm files.

    However, while you can't get a count this way, one thing you *can* use
    it for is to determine whether there are any elements whatsoever in
    the array, since "if (%table)" is guaranteed to be false if nothing
    has ever been stored in it.

    So you either have to keep your own count around and increments
    it every time you store a new key in the array, or else do it
    on the fly when you really care, perhaps like this:

    $count++ while each %ENV;

    This preceding method will be faster than extracting the
    keys into a temporary array to count them.

    As of a very recent patch, you can say

    $count = keys %ENV;



2.27) Why can't my perl program read from STDIN after I gave it ^D (EOF) ?

    Because some stdio's set error and eof flags that need clearing.

    Try keeping around the seekpointer and go there, like this:
     $where = tell(LOG);
     seek(LOG, $where, 0);

    If that doesn't work, try seeking to a different part of the file and
    then back.  If that doesn't work, try seeking to a different part of
    the file, reading something, and then seeking back.  If that doesn't
    work, give up on your stdio package and use sysread.  You can't call
    stdio's clearerr() from Perl, so if you get EINTR from a signal
    handler, you're out of luck.  Best to just use sysread() from the
    start for the tty.


2.28) Do I always/never have to quote my strings or use semicolons?

    You don't have to quote strings that can't mean anything else
    in the language, like identifiers with any upper-case letters
    in them.  Therefore, it's fine to do this:

    $SIG{INT} = Timeout_Routine;
    or

    @Days = (Sun, Mon, Tue, Wed, Thu, Fri, Sat, Sun);

    but you can't get away with this:

    $foo{while} = until;

    in place of

    $foo{'while'} = 'until';

    The requirements on semicolons have been increasingly relaxed.  You no
    longer need one at the end of a block, but stylistically, you're
    better to use them if you don't put the curly brace on the same line:

    for (1..10) { print }

    is ok, as is

    @nlist = sort { $a <=> $b } @olist;

    but you probably shouldn't do this:

    for ($i = 0; $i < @a; $i++) {
        print "i is $i\n"  # <-- oops!
    }

    because you might want to add lines later, and anyway,
    it looks funny. :-)


2.29) How can I translate tildes in a filename?

    Perl doesn't expand tildes -- the shell (ok, some shells) do.
    The classic request is to be able to do something like:

    open(FILE, "~/dir1/file1");
    open(FILE, "~tchrist/dir1/file1");

    which doesn't work.  (And you don't know it, because you
    did a system call without an "|| die" clause! :-)

    If you *know* you're on a system with the csh, and you *know*
    that Larry hasn't internalized file globbing, then you could
    get away with

    $filename = <~tchrist/dir1/file1>;

    but that's pretty iffy.

    A better way is to do the translation yourself, as in:

    $filename =~ s#^~(\w+)(/.*)?$#(getpwnam($1))[7].$2#e;

    More robust and efficient versions that checked for error conditions,
    handed simple ~/blah notation, and cached lookups are all reasonable
    enhancements.


2.30) How can I convert my shell script to Perl?

    Larry's standard answer for this is to send your script to me (Tom
    Christiansen) with appropriate supplications and offerings.  :-(
    That's because there's no automatic machine translator.  Even if you
    were, you wouldn't gain a lot, as most of the external programs would
    still get called.  It's the same problem as blind translation into C:
    you're still apt to be bogged down by exec()s.  You have to analize
    the dataflow and algorithm and rethink it for optimal speedup.  It's
    not uncommon to see one, two, or even three orders of magnitude of
    speed difference between the brute-force and the recoded approaches.


2.31) What is variable suicide and how can I prevent it?

    Variable suicide is a nasty sideeffect of dynamic scoping and
    the way variables are passed by reference.  If you say

    $x = 17;
    &munge($x);
    sub munge {
        local($x);
        local($myvar) = $_[0];
        ...
    }

    Then you have just clubbered $_[0]!  Why this is occurring
    is pretty heavy wizardry: the reference to $x stored in
    $_[0] was temporarily occluded by the previous local($x)
    statement (which, you're recall, occurs at run-time, not
    compile-time).  The work around is simple, however: declare
    your formal parameters first:

    sub munge {
        local($myvar) = $_[0];
        local($x);
        ...
    }

    That doesn't help you if you're going to be trying to access
    @_ directly after the local()s.  In this case, careful use
    of the package facility is your only recourse.

    Another manifestation of this problem occurs due to the
    magical nature of the index variable in a foreach() loop.

    @num = 0 .. 4;
    print "num begin  @num\n";
    foreach $m (@num) { &ug }
    print "num finish @num\n";
    sub ug {
        local($m) = 42;
        print "m=$m  $num[0],$num[1],$num[2],$num[3]\n";
    }

    Which prints out the mysterious:

    num begin  0 1 2 3 4
    m=42  42,1,2,3
    m=42  0,42,2,3
    m=42  0,1,42,3
    m=42  0,1,2,42
    m=42  0,1,2,3
    num finish 0 1 2 3 4

    What's happening here is that $m is an alias for each
    element of @num.  Inside &ug, you temporarily change
    $m.  Well, that means that you've also temporarily
    changed whatever $m is an alias to!!  The only workaround
    is to be careful with global variables, using packages,
    and/or just be aware of this potential in foreach() loops.


2.32) Can I use Perl regular expressions to match balanced text?

    No, or at least, not by the themselves.

    Regexps just aren't powerful enough.  Although Perl's patterns aren't
    strictly regular because they do backtracking (the \1 notation), you
    still can't do it.  You need to employ auxiliary logic.  A simple
    approach would involve keeping a bit of state around, something
    vaguely like this (although we don't handle patterns on the same line):

    while(<>) {
        if (/pat1/) {
        if ($inpat++ > 0) { warn "already saw pat1" }
        redo;
        }
        if (/pat2/) {
        if (--$inpat < 0) { warn "never saw pat1" }
        redo;
        }
    }

    A rather more elaborate subroutine to pull out balanced and possibly
    nested single chars, like ` and ', { and }, or ( and ) can be found
    on convex.com in /pub/perl/scripts/pull_quotes.


2.33) Can I use Perl to run a telnet or ftp session?

    Sure, you can connect directly to them using sockets, or you can run a
    session on a pty.  In either case, Randal's chat2 package, which is
    distributed with the perl source, will come in handly.  It address
    much the same problem space as Don Libes's expect package does.  Two
    examples of using managing an ftp session using chat2 can be found on
    convex.com in /pub/perl/scripts/ftp-chat2.shar .

    Caveat lector: chat2 is documented only by example, may not run on
    System V systems, and is subtly machine dependent both in its ideas
    of networking and in pseudottys.


2.34) What does "Malformed command links" mean?

    This is a bug in 4.035.  While in general it's merely a cosmetic
    problem, it often comanifests with a highly undesirable coredumping
    problem.  Programs known to be affected by the fatal coredump include
    plum and pcops.  Since perl5 is prety much a total rewrite, we can
    count on it being fixed then, but if anyone tracks down the coredump
    problem before then, a signifcant portion of the perl world would
    rejoice.