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 or . 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) = ;" 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 . 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" : One automated fashion is as follows: Email: Send a message to 'mail-server@cs.ruu.nl' 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 or . 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 +1 703 876 5050 (voice) +1 703 876 5059 (fax) info@uunet.uu.net 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 , 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 info@techbook.com. 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 office@usenix.org 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 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 , although both Rob Kolstad and Randal Schwartz 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: PERL-USERS@VIRGINIA.EDU Perl-Users@UVAARPA.VIRGINIA.EDU 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: Perl-Users-Request@Virginia.EDU Perl-Users-Request@uvaarpa.Virginia.EDU BitNet: Perl-Req@Virginia uucp: ...!uunet!virginia!perl-users-request Coordinator: Marc Rouleau 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 : 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 : (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 ). And here is a recent version for MS-DOS from Budi Rahard , 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 and Len Reed ) There is now a 4.035 for 386 [DOS], Hitoshi Doi 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 , 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 (kurt@hpsdid.sdd.hp.com) Interperl Interbase Buzz Moschetti (buzz@fsrg.bear.com) Oraperl Oracle Kevin Stock (kstock@encore.com) Sqlperl Ingres Ted Lemon (mellon@ncd.com) Sybperl Sybase Michael Peppler (mpeppler@itf.ch) 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 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 Ray Lischner Kresten Krab Thorup 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 mail-server@cs.ruu.nl 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 , 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 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 (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 '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] = ; 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 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 = ; 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.$$"); print FILE "string\n"; close FILE; If instead of a filehandle, you use a normal scalar variable with file manipulation functions, this is considered an indirect reference to a filehandle. For example, $foo = "TEST01"; open($foo, "file"); After the open, these two while loops are equivalent: while (<$foo>) {} while () {} as are these two statements: close $foo; close TEST01; but NOT to this: while (<$TEST01>) {} # error ^ ^ note spurious dollar sign This is another common novice mistake; often it's assumed that open($foo, "output.$$"); 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'); $code = <;" work right? Well, it does. The thing to remember is that local() provides an array context, an that the syntax in an array context will read all the lines in a file. To work around this, use: local($foo); $foo = ; You can use the scalar() operator to cast the expression into a scalar context: local($foo) = scalar(); 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 2>&1"; } else { system "stty", 'cbreak', system "stty", 'eol', "\001"; } $key = getc(STDIN); if ($BSD) { system "stty -cbreak /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 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 , 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 /$pattern/ without fear of any unexpected meta-characters in it throwing off the search. If you don't know whether a pattern is valid or not, enclose it in an eval to avoid a fatal run-time error. 2.11) Why do setuid Perl scripts complain about kernel problems? This message: YOU HAVEN'T DISABLED SET-ID SCRIPTS IN THE KERNEL YET! FIX YOUR KERNEL, PUT A C WRAPPER AROUND THIS SCRIPT, OR USE -u AND UNDUMP! is triggered because setuid scripts are inherently insecure due to a kernel bug. If your system has fixed this bug, you can compile Perl so that it knows this. Otherwise, create a setuid C program that just execs Perl with the full name of the script. 2.12) How do I open a pipe both to and from a command? In general, this is a dangerous move because you can find yourself in a deadlock situation. It's better to put one end of the pipe to a file. For example: # first write some_cmd's input into a_file, then open(CMD, "some_cmd its_args < a_file |"); while () { # or else the other way; run the cmd open(CMD, "| some_cmd its_args > a_file"); while ($condition) { print CMD "some output\n"; # other code deleted } close CMD || warn "cmd exited $?"; # now read the file open(FILE,"a_file"); while () { If you have ptys, you could arrange to run the command on a pty and avoid the deadlock problem. See the chat2.pl package in the distributed library for ways to do this. At the risk of deadlock, it is theoretically possible to use a fork, two pipe calls, and an exec to manually set up the two-way pipe. (BSD system may use socketpair() in place of the two pipes, but this is not as portable.) The open2 library function distributed with the current perl release will do this for you. It assumes it's going to talk to something like adb, both writing to it and reading from it. This is presumably safe because you "know" that commands like adb will read a line at a time and output a line at a time. Programs like sort that read their entire input stream first, however, are quite apt to cause deadlock. 2.13) How can I change the first N letters of a string? Remember that the substr() function produces an lvalue, that is, it may be assigned to. Therefore, to change the first character to an S, you could do this: substr($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|"); $_ = ; print; while () { ($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, $command), "\n"; } 2.15) How can I make a file handle local to a subroutine? You must use the type-globbing *VAR notation. Here is some code to cat an include file, calling itself recursively on nested local include files (i.e. those with #include "file", not #include ): sub cat_include { local($name) = @_; local(*FILE); local($_); warn "\n"; if (!open (FILE, $name)) { warn "can't open $name: $!\n"; return; } while () { 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[$_]++, @in); d) A way to do (b) without any loops or greps: undef %saw; @saw{@in} = (); @out = sort keys %saw; # remove sort if undesired e) Like (d), but @in contains only small positive integers: undef @ary; @ary[@in] = @in; @out = sort @ary; 2.17) How can I call alarm() or usleep() from Perl? It's available as a built-in as of version 3.038. If you want finer granularity than 1 second (as usleep() provides) and have itimers and syscall() on your system, you can use the following. You could also use select(). It takes a floating-point number representing how long to delay until you get the SIGALRM, and returns a floating- point number representing how much time was left in the old timer, if any. Note that the C function uses integers, but this one doesn't mind fractional numbers. # alarm; send me a SIGALRM in this many seconds (fractions ok) # tom christiansen sub alarm { require 'syscall.ph'; require 'sys/time.ph'; local($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 = ; 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 = ; $mode = oct($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 () { 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 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. [Fixed in 4.036--lwall]