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@@ -1,2102 +0,0 @@ -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 <tchrist@convex.com> 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 <lwall@netlabs.com>. - - 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" <henkp@cs.ruu.nl>: 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 <jgreely@cis.ohio-state.edu> 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 - +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 <merlyn@ora.com>, 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 <jv@mh.nl> 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 <tchrist@usenix.com>, although both Rob - Kolstad <kolstad@usenix.org> and Randal Schwartz <merlyn@ora.com> 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 <mer6g@VIRGINIA.EDU> - -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 <neeri@iis.ethz.ch>: - - 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 - <rao@moose.cccs.umn.edu>: (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 <rolff@scotty.eurokom.ie>). - - And here is a recent version for MS-DOS from Budi Rahard - <rahard@ee.UManitoba.CA>, 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 <tthorn@daimi.aau.dk> and Len Reed - <holos0!lbr@gatech.edu>) - - There is now a 4.035 for 386 [DOS], Hitoshi Doi <doi@jrd.december.com> - 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 - <buzz@toxicavenger.bear.com>, 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 <dansmith@autodesk.com> 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 <me@anywhere.EBay.Sun.COM> - Ray Lischner <lisch@sysserver1.mentor.com> - Kresten Krab Thorup <krab@iesd.auc.dk> - - 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 - <jon@netlabs.com>, 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 - <TEDLAW@TOROLAB6.VNET.IBM.COM> 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 <guido@cwi.nl> - (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 <muir@tfs.com>'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 - <flee@cs.psu.edu>, 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[$_]++, @in); - - d) A way to do (b) without any loops or greps: - - undef %saw; - @saw{@in} = (); - @out = sort keys %saw; # remove sort if undesired - - e) Like (d), but @in contains only small positive integers: - - undef @ary; - @ary[@in] = @in; - @out = sort @ary; - - -2.17) How can I call alarm() or usleep() from Perl? - - It's available as a built-in as of version 3.038. If you want finer - granularity than 1 second (as usleep() provides) and have itimers and - syscall() on your system, you can use the following. You could also - use select(). - - It takes a floating-point number representing how long to delay until - you get the SIGALRM, and returns a floating- point number representing - how much time was left in the old timer, if any. Note that the C - function uses integers, but this one doesn't mind fractional numbers. - - # alarm; send me a SIGALRM in this many seconds (fractions ok) - # tom christiansen <tchrist@convex.com> - sub alarm { - require 'syscall.ph'; - require 'sys/time.ph'; - - local($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 <rick@IMD.Sterling.COM> 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] |