You should have read section 2 of this faq. There you would have learned that comp.lang.perl.misc is the appropriate place to go for free advice. If your question is really important and you require a prompt and correct answer, you should hire a consultant.
Furthermore, you may include this document in any distribution of the full Perl source or binaries, in its verbatim documentation, or on a complete dump of the CPAN archive, providing that the three stipulations given above continue to be met.
Added new question on Perl BNF on the perlfaq7 manpage.
In particular, the core development team (known as the Perl Porters) are a rag-tag band of highly altruistic individuals committed to producing better software for free than you could hope to purchase for money. You may snoop on pending developments via news://genetics.upenn.edu/perl.porters-gw/ and http://www.frii.com/~gnat/perl/porters/summary.html.
While the GNU project includes Perl in its distributions, there's no such thing as ``GNU Perl''. Perl is not produced nor maintained by the Free Software Foundation. Perl's licensing terms are also more open than GNU software's tend to be.
You can get commercial support of Perl if you wish, although for most users the informal support will more than suffice. See the answer to ``Where can I buy a commercial version of perl?'' for more information.
5 release of Perl'', but some people have interpreted this to
mean there's a language called ``perl5'', which isn't the case. Perl5 is
merely the popular name for the fifth major release (October 1994), while
perl4 was the fourth major release (March 1991). There was also a perl1 (in
January 1988), a perl2 (June 1988), and a perl3 (October 1989).
The 5.0 release is, essentially, a complete rewrite of the perl source code from the ground up. It has been modularized, object-oriented, tweaked, trimmed, and optimized until it almost doesn't look like the old code. However, the interface is mostly the same, and compatibility with previous releases is very high.
To avoid the ``what language is perl5?'' confusion, some people prefer to simply use ``perl'' to refer to the latest version of perl and avoid using ``perl5'' altogether. It's not really that big a deal, though.
Larry and the Perl development team occasionally make changes to the internal core of the language, but all possible efforts are made toward backward compatibility. While not quite all perl4 scripts run flawlessly under perl5, an update to perl should nearly never invalidate a program written for an earlier version of perl (barring accidental bug fixes and the rare new keyword).
Most tasks only require a small subset of the Perl language. One of the guiding mottos for Perl development is ``there's more than one way to do it'' (TMTOWTDI, sometimes pronounced ``tim toady''). Perl's learning curve is therefore shallow (easy to learn) and long (there's a whole lot you can do if you really want).
Finally, Perl is (frequently) an interpreted language. This means that you can write your programs and test them without an intermediate compilation step, allowing you to experiment and test/debug quickly and easily. This ease of experimentation flattens the learning curve even more.
Things that make Perl easier to learn: Unix experience, almost any kind of programming experience, an understanding of regular expressions, and the ability to understand other people's code. If there's something you need to do, then it's probably already been done, and a working example is usually available for free. Don't forget the new perl modules, either. They're discussed in Part 3 of this FAQ, along with the CPAN, which is discussed in Part 2.
Probably the best thing to do is try to write equivalent code to do a set of tasks. These languages have their own newsgroups in which you can learn about (but hopefully not argue about) them.
If you have a library that provides an API, you can make any component of it available as just another Perl function or variable using a Perl extension written in C or C++ and dynamically linked into your main perl interpreter. You can also go the other direction, and write your main program in C or C++, and then link in some Perl code on the fly, to create a powerful application.
That said, there will always be small, focused, special-purpose languages dedicated to a specific problem domain that are simply more convenient for certain kinds of problems. Perl tries to be all things to all people, but nothing special to anyone. Examples of specialized languages that come to mind include prolog and matlab.
Actually, one good reason is when you already have an existing application written in another language that's all done (and done well), or you have an application language specifically designed for a certain task (e.g. prolog, make).
For various reasons, Perl is probably not well-suited for real-time embedded systems, low-level operating systems development work like device drivers or context-switching code, complex multithreaded shared-memory applications, or extremely large applications. You'll notice that perl is not itself written in Perl.
The new native-code compiler for Perl may reduce the limitations given in the previous statement to some degree, but understand that Perl remains fundamentally a dynamically typed language, and not a statically typed one. You certainly won't be chastized if you don't trust nuclear-plant or brain-surgery monitoring code to it. And Larry will sleep easier, too -- Wall Street programs not withstanding. :-)
In ``standard terminology'' a program has been compiled to physical machine code once, and can then be be run multiple times, whereas a script must be translated by a program each time it's used. Perl programs, however, are usually neither strictly compiled nor strictly interpreted. They can be compiled to a bytecode form (something of a Perl virtual machine) or to completely different languages, like C or assembly language. You can't tell just by looking whether the source is destined for a pure interpreter, a parse-tree interpreter, a byte-code interpreter, or a native-code compiler, so it's hard to give a definitive answer here.
If you have a project which has a bottleneck, especially in terms of translation, or testing, Perl almost certainly will provide a viable, and quick solution. In conjunction with any persuasion effort, you should not fail to point out that Perl is used, quite extensively, and with extremely reliable and valuable results, at many large computer software and/or hardware companies throughout the world. In fact, many Unix vendors now ship Perl by default, and support is usually just a news-posting away, if you can't find the answer in the comprehensive documentation, including this FAQ.
If you face reluctance to upgrading from an older version of perl, then point out that version 4 is utterly unmaintained and unsupported by the Perl Development Team. Another big sell for Perl5 is the large number of modules and extensions which greatly reduce development time for any given task. Also mention that the difference between version 4 and version 5 of Perl is like the difference between awk and C++. (Well, ok, maybe not quite that distinct, but you get the idea.) If you want support and a reasonable guarantee that what you're developing will continue to work in the future, then you have to run the supported version. That probably means running the 5.004 release, although 5.003 isn't that bad (it's just one year and one release behind). Several important bugs were fixed from the 5.000 through 5.002 versions, though, so try upgrading past them if possible.
Although it's rumored that the (imminent) 5.004 release may build on Windows NT, this is yet to be proven. Binary distributions for 32-bit Microsoft systems and for Apple systems can be found http://www.perl.com/CPAN/ports/ directory. Because these are not part of the standard distribution, they may and in fact do differ from the base Perl port in a variety of ways. You'll have to check their respective release notes to see just what the differences are. These differences can be either positive (e.g. extensions for the features of the particular platform that are not supported in the source release of perl) or negative (e.g. might be based upon a less current source release of perl).
A useful FAQ for Win32 Perl users is http://www.endcontsw.com/people/evangelo/Perl_for_Win32_FAQ.html
make install. Most other approaches are doomed to failure.
One simple way to check that things are in the right place is to print out
the hard-coded @INC which perl is looking for.
perl -e 'print join("\n",@INC)'
If this command lists any paths which don't exist on your system, then you may need to move the appropriate libraries to these locations, or create symlinks, aliases, or shortcuts appropriately.
CPAN/path/... is a naming convention for files available on CPAN sites. CPAN indicates the base directory of a CPAN mirror, and the rest of the path is the path from that directory to the file. For instance, if you're using ftp://ftp.funet.fi/pub/languages/perl/CPAN as your CPAN site, the file CPAN/misc/japh file is downloadable as ftp://ftp.funet.fi/pub/languages/perl/CPAN/misc/japh .
Considering that there are hundreds of existing modules in the archive, one probably exists to do nearly anything you can think of. Current categories under CPAN/modules/by-category/ include perl core modules; development support; operating system interfaces; networking, devices, and interprocess communication; data type utilities; database interfaces; user interfaces; interfaces to other languages; filenames, file systems, and file locking; internationalization and locale; world wide web support; server and daemon utilities; archiving and compression; image manipulation; mail and news; control flow utilities; filehandle and I/O; Microsoft Windows modules; and miscellaneous modules.
man perl if you're on a system resembling Unix. This will lead you to other
important man pages. If you're not on a Unix system, access to the
documentation will be different; for example, it might be only in HTML
format. But all proper perl installations have fully-accessible
documentation.
You might also try perldoc perl in case your system doesn't have a proper man command, or it's been
misinstalled. If that doesn't work, try looking in /usr/local/lib/perl5/pod
for documentation.
If all else fails, consult the CPAN/doc directory, which contains the complete documentation in various formats, including native pod, troff, html, and plain text. There's also a web page at http://www.perl.com/perl/info/documentation.html that might help.
It's also worth noting that there's a PDF version of the complete documentation for perl available in the CPAN/authors/id/BMIDD directory.
Many good books have been written about Perl -- see the section below for more details.
comp.lang.perl.announce Moderated announcement group
comp.lang.perl.misc Very busy group about Perl in general
comp.lang.perl.modules Use and development of Perl modules
comp.lang.perl.tk Using Tk (and X) from Perl
comp.infosystems.www.authoring.cgi Writing CGI scripts for the Web.
There is also USENET gateway to the mailing list used by the crack Perl development team (perl5-porters) at news://genetics.upenn.edu/perl.porters-gw/ .
The incontestably definitive reference book on Perl, written by the creator of Perl and his apostles, is now in its second edition and fourth printing.
Programming Perl (the "Camel Book"):
Authors: Larry Wall, Tom Christiansen, and Randal Schwartz
ISBN 1-56592-149-6 (English)
ISBN 4-89052-384-7 (Japanese)
(French and German translations in progress)
Note that O'Reilly books are color-coded: turquoise (some would call it teal) covers indicate perl5 coverage, while magenta (some would call it pink) covers indicate perl4 only. Check the cover color before you buy!
What follows is a list of the books that the FAQ authors found personally useful. Your mileage may (but, we hope, probably won't) vary.
If you're already a hard-core systems programmer, then the Camel Book just might suffice for you to learn Perl from. But if you're not, check out the ``Llama Book''. It currently doesn't cover perl5, but the 2nd edition is nearly done and should be out by summer 97:
Learning Perl (the Llama Book):
Author: Randal Schwartz, with intro by Larry Wall
ISBN 1-56592-042-2 (English)
ISBN 4-89502-678-1 (Japanese)
ISBN 2-84177-005-2 (French)
ISBN 3-930673-08-8 (German)
Another stand-out book in the turquoise O'Reilly Perl line is the ``Hip Owls'' book. It covers regular expressions inside and out, with quite a bit devoted exclusively to Perl:
Mastering Regular Expressions (the Cute Owls Book): Author: Jeffrey Friedl ISBN 1-56592-257-3
You can order any of these books from O'Reilly & Associates, 1-800-998-9938. Local/overseas is 1-707-829-0515. If you can locate an O'Reilly order form, you can also fax to 1-707-829-0104. See http://www.ora.com/ on the Web.
Recommended Perl books that are not from O'Reilly are the following:
Cross-Platform Perl, (for Unix and Windows NT)
Author: Eric F. Johnson
ISBN: 1-55851-483-X
How to Set up and Maintain a World Wide Web Site, (2nd edition) Author: Lincoln Stein, M.D., Ph.D. ISBN: 0-201-63462-7
CGI Programming in C & Perl, Author: Thomas Boutell ISBN: 0-201-42219-0
Note that some of these address specific application areas (e.g. the Web) and are not general-purpose programming books.
Beyond this, two other magazines that frequently carry high-quality articles on Perl are Web Techniques (see http://www.webtechniques.com/) and Unix Review (http://www.unixreview.com/).
http://www.perl.com/CPAN (redirects to another mirror) http://www.perl.org/CPAN ftp://ftp.funet.fi/pub/languages/perl/CPAN/ http://www.cs.ruu.nl/pub/PERL/CPAN/ ftp://ftp.cs.colorado.edu/pub/perl/CPAN/
If you subscribe to a mailing list, it behooves you to know how to unsubscribe from it. Strident pleas to the list itself to get you off will not be favorably received.
Also see Matthias Neeracher's (the creator and maintainer of MacPerl) webpage at http://www.iis.ee.ethz.ch/~neeri/macintosh/perl.html for many links to interesting MacPerl sites, and the applications/MPW tools, precompiled.
subscribe Perl-Win32-Users
The list software, also written in perl, will automatically determine your address, and subscribe you automatically. To unsubscribe, email the following in the message body to the same address like so:
unsubscribe Perl-Win32-Users
You can also check http://www.activeware.com/ and select ``Mailing Lists'' to join or leave this list.
subscribe perl-packrats
The list software, also written in perl, will automatically determine your address, and subscribe you automatically. To unsubscribe, simple prepend the same command with an ``un'', and mail to the same address like so:
unsubscribe perl-packrats
ftp.cis.ufl.edu:/pub/perl/comp.lang.perl.*/monthly has an almost complete collection dating back to 12/89 (missing 08/91 through 12/93). They are kept as one large file for each month.
You'll probably want more a sophisticated query and retrieval mechanism than a file listing, preferably one that allows you to retrieve articles using a fast-access indices, keyed on at least author, date, subject, thread (as in ``trn'') and probably keywords. The best solution the FAQ authors know of is the MH pick command, but it is very slow to select on 18000 articles.
If you have, or know where can be found, the missing sections, please let perlfaq-suggestions@perl.com know.
However, these answers may not suffice for managers who require a purchase order from a company whom they can sue should anything go wrong. Or maybe they need very serious hand-holding and contractual obligations. Shrink-wrapped CDs with perl on them are available from several sources if that will help.
Or you can purchase a real support contract. Although Cygnus historically provided this service, they no longer sell support contracts for Perl. Instead, the Paul Ingram Group will be taking up the slack through The Perl Clinic. The following is a commercial from them:
``Do you need professional support for Perl and/or Oraperl? Do you need a support contract with defined levels of service? Do you want to pay only for what you need?
``The Paul Ingram Group has provided quality software development and support services to some of the world's largest corporations for ten years. We are now offering the same quality support services for Perl at The Perl Clinic. This service is led by Tim Bunce, an active perl porter since 1994 and well known as the author and maintainer of the DBI, DBD::Oracle, and Oraperl modules and author/co-maintainer of The Perl 5 Module List. We also offer Oracle users support for Perl5 Oraperl and related modules (which Oracle is planning to ship as part of Oracle Web Server 3). 20% of the profit from our Perl support work will be donated to The Perl Institute.''
For more information, contact the The Perl Clinic:
Tel: +44 1483 424424
Fax: +44 1483 419419
Web: http://www.perl.co.uk/
Email: perl-support-info@perl.co.uk or Tim.Bunce@ig.co.uk
If you are posting a bug with a non-standard port (see the answer to ``What platforms is Perl available for?''), a binary distribution, or a non-standard module (such as Tk, CGI, etc), then please see the documentation that came with it to determine the correct place to post bugs.
Read the perlbug man page (perl5.004 or later) for more information.
The perl.com domain is Tom Christiansen's domain. He created it as a public service long before perl.org came about. It's the original PBS of the Perl world, a clearinghouse for information about all things Perlian, accepting no paid advertisements, glossy gifs, or (gasp!) java applets on its pages.
Objects perlref, perlmod, perlobj, perltie Data Structures perlref, perllol, perldsc Modules perlmod, perlsub Regexps perlre, perlfunc, perlop Moving to perl5 perltrap, perl Linking w/C perlxstut, perlxs, perlcall, perlguts, perlembed Various http://www.perl.com/CPAN/doc/FMTEYEWTK/index.html (not a man-page but still useful)
the perltoc manpage provides a crude table of contents for the perl man page set.
perldebug man page, on an ``empty'' program, like this:
perl -de 42
Now just 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, set breakpoints, and other operations typically found in symbolic debuggers
-w?
Have you tried use strict?
Did you check the returns of each and every system call?
Did you read the perltrap manpage?
Have you tried the Perl debugger, described in the perldebug manpage?
perl -MO=Xref[,OPTIONS] foo.pl
indent
will do for C. The complex feedback between the scanner and the parser
(this feedback is what confuses the vgrind and emacs programs) makes it
challenging at best to write a stand-alone Perl parser.
Of course, if you simply follow the guidelines in the perlstyle manpage, you shouldn't need to reformat.
Your editor can and should help you with source formatting. The perl-mode for emacs can provide a remarkable amount of help with most (but not all) code, and even less programmable editors can provide significant assistance.
If you are using to using vgrind program for printing out nice code to a laser printer, you can take a stab at this using http://www.perl.com/CPAN/doc/misc/tips/working.vgrind.entry, but the results are not particularly satisfying for sophisticated code.
In the perl source directory, you'll find a directory called ``emacs'', which contains a cperl-mode that color-codes keywords, provides context-sensitive help, and other nifty things.
Note that the perl-mode of emacs will have fits with ``main'foo'' (single quote), and mess up the indentation and hilighting. You should be using ``main::foo'', anyway.
Other approaches include autoloading seldom-used Perl code. See the AutoSplit and AutoLoader modules in the standard distribution for that. Or you could locate the bottleneck and think about writing just that part in C, the way we used to take bottlenecks in C code and write them in assembler. Similar to rewriting in C is the use of modules that have critical sections written in C (for instance, the PDL module from CPAN).
In some cases, it may be worth it to use the backend compiler to produce byte code (saving compilation time) or compile into C, which will certainly save compilation time and sometimes a small amount (but not much) execution time. See the question about compiling your Perl programs.
If you're currently linking your perl executable to a shared libc.so, you can often gain a 10-25% performance benefit by rebuilding it to link with a static libc.a instead. This will make a bigger perl executable, but your Perl programs (and programmers) may thank you for it. See the INSTALL file in the source distribution for more information.
Unsubstantiated reports allege that Perl interpreters that use sfio outperform those that don't (for IO intensive applications). To try this, see the INSTALL file in the source distribution, especially the ``Selecting File IO mechanisms'' section.
The undump program was an old attempt to speed up your Perl program by storing the already-compiled form to disk. This is no longer a viable option, as it only worked on a few architectures, and wasn't a good solution anyway.
In some cases, using substr or vec to simulate
arrays can be highly beneficial. For example, an array of a thousand
booleans will take at least 20,000 bytes of space, but it can be turned
into one 125-byte bit vector for a considerable memory savings. The
standard Tie::SubstrHash module can also help for certain types of data
structure. If you're working with specialist data structures (matrices, for
instance) modules that implement these in C may use less memory than
equivalent Perl modules.
Another thing to try is learning whether your Perl was compiled with the
system malloc or with Perl's built-in malloc. Whichever one it is, try
using the other one and see whether this makes a difference. Information
about malloc is in the INSTALL file in the source distribution. You can find out whether you are using
perl's malloc by typing perl -V:usemymalloc.
sub makeone {
my @a = ( 1 .. 10 );
return \@a;
}
for $i ( 1 .. 10 ) {
push @many, makeone();
}
print $many[4][5], "\n";
print "@many\n";
However, judicious use of my on your variables will help make
sure that they go out of scope so that Perl can free up their storage for
use in other parts of your program. (NB: my variables also
execute about 10% faster than globals.) A global variable, of course, never
goes out of scope, so you can't get its space automatically reclaimed,
although undefing and/or deleteing it will
achieve the same effect. In general, memory allocation and de-allocation
isn't something you can or should be worrying about much in Perl, but even
this capability (preallocation of data types) is in the works.
There are at least two popular ways to avoid this overhead. One solution involves running the Apache HTTP server (available from http://www.apache.org/) with either of the mod_perl or mod_fastcgi plugin modules. With mod_perl and the Apache::* modules (from CPAN), httpd will run with an embedded Perl interpreter which pre-compiles your script and then executes it within the same address space without forking. The Apache extension also gives Perl access to the internal server API, so modules written in Perl can do just about anything a module written in C can. With the FCGI module (from CPAN), a Perl executable compiled with sfio (see the INSTALL file in the distribution) and the mod_fastcgi module (available from http://www.fastcgi.com/) each of your perl scripts becomes a permanent CGI daemon processes.
Both of these solutions can have far-reaching effects on your system and on the way you write your CGI scripts, so investigate them with care.
First of all, however, you can't take away read permission, because the source code has to be readable in order to be compiled and interpreted. (That doesn't mean that a CGI script's source is readable by people on the web, though.) So you have to leave the permissions at the socially friendly 0755 level.
Some people regard this as a security problem. If your program does insecure things, and relies on people not knowing how to exploit those insecurities, it is not secure. It is often possible for someone to determine the insecure things and exploit them without viewing the source. Security through obscurity, the name for hiding your bugs instead of fixing them, is little security indeed.
You can try using encryption via source filters (Filter::* from CPAN). But crackers might be able to decrypt it. You can try using the byte-code compiler and interpreter described below, but crackers might be able to de-compile it. You can try using the native-code compiler described below, but crackers might be able to disassemble it. These pose varying degrees of difficulty to people wanting to get at your code, but none can definitively conceal it (this is true of every language, not just Perl).
If you're concerned about people profiting from your code, then the bottom line is that nothing but a restrictive licence will give you legal security. License your software and pepper it with threatening statements like ``This is unpublished proprietary software of XYZ Corp. Your access to it does not give you permission to use it blah blah blah.'' We are not lawyers, of course, so you should see a lawyer if you want to be sure your licence's wording will stand up in court.
Please understand that merely compiling into C does not in and of itself guarantee that your code will run very much faster. That's because except for lucky cases where a lot of native type inferencing is possible, the normal Perl run time system is still present and thus will still take just as long to run and be just as big. Most programs save little more than compilation time, leaving execution no more than 10-30% faster. A few rare programs actually benefit significantly (like several times faster), but this takes some tweaking of your code.
Malcolm will be in charge of the 5.005 release of Perl itself to try to unify and merge his compiler and multithreading work into the main release.
You'll probably be astonished to learn that the current version of the
compiler generates a compiled form of your script whose executable is just
as big as the original perl executable, and then some. That's because as
currently written, all programs are prepared for a full eval
statement. You can tremendously reduce this cost by building a shared
libperl.so library and linking against that. See the
INSTALL podfile in the perl source distribution for details. If you link your main
perl binary with this, it will make it miniscule. For example, on one
author's system, /usr/bin/perl is only 11k in size!
extproc perl -S -your_switches
as the first line in *.cmd file (-S due to a bug in cmd.exe's `extproc' handling). For DOS one should first
invent a corresponding batch file, and codify it in ALTERNATIVE_SHEBANG (see the
INSTALL file in the source distribution for more information).
The Win95/NT installation, when using the Activeware port of Perl, will modify the Registry to associate the .pl extension with the perl interpreter. If you install another port, or (eventually) build your own Win95/NT Perl using WinGCC, then you'll have to modify the Registry yourself.
Macintosh perl scripts will have the the appropriate Creator and Type, so that double-clicking them will invoke the perl application.
IMPORTANT!: Whatever you do, PLEASE don't get frustrated, and just throw the perl interpreter into your cgi-bin directory, in order to get your scripts working for a web server. This is an EXTREMELY big security risk. Take the time to figure out how to do it correctly.
# sum first and last fields
perl -lane 'print $F[0] + $F[-1]'
# identify text files
perl -le 'for(@ARGV) {print if -f && -T _}' *
# remove comments from C program
perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c
# make file a month younger than today, defeating reaper daemons
perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' *
# find first unused uid
perl -le '$i++ while getpwuid($i); print $i'
# display reasonable manpath
echo $PATH | perl -nl -072 -e '
s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}'
Ok, the last one was actually an obfuscated perl entry. :-)
For example:
# Unix
perl -e 'print "Hello world\n"'
# DOS, etc.
perl -e "print \"Hello world\n\""
# Mac
print "Hello world\n"
(then Run "Myscript" or Shift-Command-R)
# VMS
perl -e "print ""Hello world\n"""
The problem is that none of this is reliable: it depends on the command interpreter. Under Unix, the first two often work. Under DOS, it's entirely possible neither works. If 4DOS was the command shell, I'd probably have better luck like this:
perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
Under the Mac, it depends which environment you are using. The MacPerl shell, or MPW, is much like Unix shells in its support for several quoting variants, except that it makes free use of the Mac's non-ASCII characters as control characters.
I'm afraid that there is no general solution to all of this. It is a mess, pure and simple.
[Some of this answer was contributed by Kenneth Albanowski.]
The Idiot's Guide to Solving Perl/CGI Problems, by Tom Christiansen http://www.perl.com/perl/faq/idiots-guide.html
Frequently Asked Questions about CGI Programming, by Nick Kew ftp://rtfm.mit.edu/pub/usenet/news.answers/www/cgi-faq http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml
Perl/CGI programming FAQ, by Shishir Gundavaram and Tom Christiansen http://www.perl.com/perl/faq/perl-cgi-faq.html
The WWW Security FAQ, by Lincoln Stein http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html
World Wide Web FAQ, by Thomas Boutell http://www.boutell.com/faq/
make test TEST_VERBOSE=1 along with perl -V.
perl program 2>diag.out
splain [-v] [-p] diag.out
or change your program to explain the messages for you:
use diagnostics;
or
use diagnostics -verbose;
oct or hex if you want the values converted.
oct interprets both hex (``0x350'') numbers and octal ones
(``0350'' or even without the leading ``0'', like ``377''), while
hex only converts hexadecimal ones, with or without a leading
``0x'', like ``0x255'', ``3A'', ``ff'', or ``deadbeef''.
This problem shows up most often when people try using chmod,
mkdir, umask, or sysopen, which all
want permissions in octal.
chmod(644, $file); # WRONG -- perl -w catches this
chmod(0644, $file); # right
sprintf or
printf is usually the easiest route.
The POSIX module (part of the standard perl distribution) implements
ceil, floor, and a number of other mathematical
and trigonometric functions.
The Math::Complex module (part of the standard perl distribution) defines a number of mathematical functions that can also work on real numbers. It's not as efficient as the POSIX library, but the POSIX library can't work with complex numbers.
Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself.
pack function (documented in
pack):
$decimal = pack('B8', '10110110');
Here's an example of going the other way:
$binary_string = join('', unpack('B*', "\x29"));
@results = map { my_func($_) } @array;
For example:
@triple = map { 3 * $_ } @single;
To call a function on each element of an array, but ignore the results:
foreach $iterator (@array) {
&my_func($iterator);
}
To call a function on each integer in a (small) range, you can use:
@results = map { &my_func($_) } (5 .. 25);
but you should be aware that the .. operator creates an array of all integers in the range. This can take a lot
of memory for large ranges. Instead use:
@results = ();
for ($i=5; $i < 500_005; $i++) {
push(@results, &my_func($i));
}
You should also check out the Math::TrulyRandom module from CPAN.
localtime (see
localtime):
$day_of_year = (localtime(time()))[7];
or more legibly (in 5.004 or higher):
use Time::localtime;
$day_of_year = localtime(time())->yday;
You can find the week of the year by dividing this by 7:
$week_of_year = int($day_of_year / 7);
Of course, this believes that weeks start at zero.
When gmtime and localtime are used in a scalar
context they return a timestamp string that contains a fully-expanded year.
For example,
$timestamp = gmtime sets $timestamp to ``Tue Nov 13 01:00:00 2001''. There's no
year 2000 problem here.
s/\\(.)/$1/g;
Note that this won't expand \n or \t or any other special escapes.
s/(.)\1/$1/g;
print "My sub returned @{[mysub(1,2,3)]} that time.\n";
If you prefer scalar context, similar chicanery is also useful for arbitrary expressions:
print "That yields ${\($n + 5)} widgets\n";
/xx/ will get the intervening bits in $1. For multiple ones, then something more
like /alphaomega/ would be needed. But none of these deals with nested patterns, nor can
they. For that you'll have to write a parser.
reverse in a scalar context, as documented in
reverse.
$reversed = reverse $string;
1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
Or you can just use the Text::Tabs module (part of the standard perl distribution).
use Text::Tabs;
@expanded_lines = expand(@lines_with_tabs);
use Text::Wrap;
print wrap("\t", ' ', @paragraphs);
$first_byte = substr($a, 0, 1);
If you want to modify part of a string, the simplest way is often to use
substr as an lvalue:
substr($a, 0, 3) = "Tom";
Although those with a regexp kind of thought process will likely prefer
$a =~ s/^.../Tom/;
$count = 0;
s{((whom?)ever)}{
++$count == 5 # is it the 5th?
? "${2}soever" # yes, swap
: $1 # renege and leave it there
}igex;
$string = "ThisXlineXhasXsomeXx'sXinXit":
$count = ($string =~ tr/X//);
print "There are $count X charcters in the string";
This is fine if you are just looking for a single character. However, if
you are trying to count multiple character substrings within a larger
string, tr/// won't work. What you can do is wrap a while loop around a
global pattern match. For example, let's count negative integers:
$string = "-9 55 48 -2 23 -76 4 14 -44";
while ($string =~ /-\d+/g) { $count++ }
print "There are $count negative numbers in the string";
$line =~
s/\b(\w)/\U$1/g;
To make the whole line upper case: $line = uc;
To force each word to be lower case, with the first letter upper case:
$line =~ s/(\w+)/\u\L$1/g;
SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
Due to the restriction of the quotes, this is a fairly complex problem. Thankfully, we have Jeffrey Friedl, author of a highly recommended book on regular expressions, to handle these for us. He suggests (assuming your string is contained in $text):
@new = ();
push(@new, $+) while $text =~ m{
"([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
| ([^,]+),?
| ,
}gx;
push(@new, undef) if substr($text,-1,1) eq ',';
Alternatively, the Text::ParseWords module (part of the standard perl distribution) lets you say:
use Text::ParseWords;
@new = quotewords(",", 0, $text);
$string =~ s/^\s*(.*?)\s*$/$1/;
It would be faster to do this in two steps:
$string =~ s/^\s+//;
$string =~ s/\s+$//;
Or more nicely written as:
for ($string) {
s/^\s+//;
s/\s+$//;
}
substr or unpack, both documented in the perlfunc manpage.
$text = 'this has a $foo in it and a $bar';
$text =~ s/\$(\w+)/${$1}/g;
Before version 5 of perl, this had to be done with a double-eval substitution:
$text =~ s/(\$\w+)/$1/eeg;
Which is bizarre enough that you'll probably actually need an EEG afterwards. :-)
If you get used to writing odd things like these:
print "$var"; # BAD
$new = "$old"; # BAD
somefunc("$var"); # BAD
You'll be in trouble. Those should (in 99.8% of the cases) be the simpler and more direct:
print $var;
$new = $old;
somefunc($var);
Otherwise, besides slowing you down, you're going to break code when the thing in the scalar is actually neither a string nor a number, but a reference:
func(\@array);
sub func {
my $aref = shift;
my $oref = "$aref"; # WRONG
}
You can also get into subtle problems on those few operations in Perl that
actually do care about the difference between a string and a number, such
as the magical ++ autoincrement operator or the syscall function.
Sometimes it doesn't make a difference, but sometimes it does. For example, compare:
$good[0] = `some program that outputs several lines`;
with
@bad[0] = `same program that outputs several lines`;
The -w flag will warn you about these matters.
$prev = 'nonesuch';
@out = grep($_ ne $prev && ($prev = $_), @in);
This is nice in that it doesn't use much extra memory, simulating
uniq's behavior of removing only adjacent duplicates.
undef %saw;
@out = grep(!$saw{$_}++, @in);
@out = grep(!$saw[$_]++, @in);
undef %saw;
@saw{@in} = ();
@out = sort keys %saw; # remove sort if undesired
undef @ary;
@ary[@in] = @in;
@out = @ary;
@blues = qw/azure cerulean teal turquoise 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 a hash 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 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 is true for some $n.
Please do not use
$is_there = grep $_ eq $whatever, @array;
or worse yet
$is_there = grep /$whatever/, @array;
These are slow (checks every element even if the first matches), inefficient (same reason), and potentially buggy (what if there are regexp characters in $whatever?).
@union = @intersection = @difference = ();
%count = ();
foreach $element (@array1, @array2) { $count{$element}++ }
foreach $element (keys %count) {
push @union, $element;
push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
}
for ($i=0; $i < @array; $i++) {
if ($array[$i] eq "Waldo") {
$found_index = $i;
last;
}
}
Now $found_index has what you want.
If you really, really wanted, you could use structures as described in the perldsc manpage or the perltoot manpage and do just what the algorithm book tells you to do.
unshift(@array, pop(@array)); # the last shall be first
push(@array, shift(@array)); # and vice versa
srand;
@new = ();
@old = 1 .. 10; # just a demo
while (@old) {
push(@new, splice(@old, rand @old, 1));
}
For large arrays, this avoids a lot of the reshuffling:
srand;
@new = ();
@old = 1 .. 10000; # just a demo
for( @old ){
my $r = rand @new+1;
push(@new,$new[$r]);
$new[$r] = $_;
}
for/foreach:
for (@lines) {
s/foo/bar/;
tr[a-z][A-Z];
}
Here's another; let's compute spherical volumes:
for (@radii) {
$_ **= 3;
$_ *= (4/3) * 3.14159; # this will be constant folded
}
rand function (see rand):
srand; # not needed for 5.004 and later
$index = rand @array;
$element = $array[$index];
permut
function should work on any list:
#!/usr/bin/perl -n
# permute - tchrist@perl.com
permut([split], []);
sub permut {
my @head = @{ $_[0] };
my @tail = @{ $_[1] };
unless (@head) {
# stop recursing when there are no elements in the head
print "@tail\n";
} else {
# for all elements in @head, move one from @head to @tail
# and call permut() on the new @head and @tail
my(@newhead,@newtail,$i);
foreach $i (0 .. $#head) {
@newhead = @head;
@newtail = @tail;
unshift(@newtail, splice(@newhead, $i, 1));
permut([@newhead], [@newtail]);
}
}
}
sort (described in sort):
@list = sort { $a <=> $b } @list;
The default sort function is cmp, string comparison, which would sort into . <=>, used above, is the numerical comparison operator.
If you have a complicated function needed to pull out the part you want to sort on, then don't do it inside the sort function. Pull it out first, because the sort BLOCK can be called many times for the same element. Here's an example of how to pull out the first word after the first number on each item, and then sort those words case-insensitively.
@idx = ();
for (@data) {
($item) = /\d+\s*(\S+)/;
push @idx, uc($item);
}
@sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
Which could also be written this way, using a trick that's come to be known as the Schwartzian Transform:
@sorted = map { $_->[0] }
sort { $a->[1] cmp $b->[1] }
map { [ $_, uc((/\d+\s*(\S+) )[0] ] } @data;
If you need to sort on several fields, the following paradigm is useful.
@sorted = sort { field1($a) <=> field1($b) ||
field2($a) cmp field2($b) ||
field3($a) cmp field3($b)
} @data;
This can be conveniently combined with precalculation of keys as given above.
See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about this approach.
See also the question below on sorting hashes.
pack and unpack, or else vec and
the bitwise operations.
For example, this sets $vec to have bit N set if $ints[N] was set:
$vec = '';
foreach(@ints) { vec($vec,$_,1) = 1 }
And here's how, given a vector in $vec, you can get those bits into your
@ints array:
sub bitvec_to_list {
my $vec = shift;
my @ints;
# Find null-byte density then select best algorithm
if ($vec =~ tr/\0// / length $vec > 0.95) {
use integer;
my $i;
# This method is faster with mostly null-bytes
while($vec =~ /[^\0]/g ) {
$i = -9 + 8 * pos $vec;
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
}
} else {
# This method is a fast general algorithm
use integer;
my $bits = unpack "b*", $vec;
push @ints, 0 if $bits =~ s/^(\d)// && $1;
push @ints, pos $bits while($bits =~ /1/g);
}
return \@ints;
}
This method gets faster the more sparse the bit vector is. (Courtesy of Tim Bunce and Winfried Koenig.)
each function (see each) if you don't care whether it's sorted:
while (($key,$value) = each %hash) {
print "$key = $value\n";
}
If you want it sorted, you'll have to use foreach on the
result of sorting the keys as shown in an earlier question.
%by_value = reverse %by_key;
$key = $by_value{$value};
That's not particularly efficient. It would be more space-efficient to use:
while (($key, $value) = each %by_key) {
$by_value{$value} = $key;
}
If your hash could have repeated values, the methods above will only find one of the associated keys. This may or may not worry you.
keys function:
$num_keys = scalar keys %hash;
In void context it just resets the iterator, which is faster for tied hashes.
@keys = sort keys %hash; # sorted by key
@keys = sort {
$hash{$a} cmp $hash{$b}
} keys %hash; # and by value
Here we'll do a reverse numeric sort by value, and if two keys are identical, sort by length of key, and if that fails, by straight ASCII comparison of the keys (well, possibly modified by your locale -- see the perllocale manpage).
@keys = sort {
$hash{$b} <=> $hash{$a}
||
length($b) <=> length($a)
||
$a cmp $b
} keys %hash;
tie using the
$DB_BTREE hash bindings as documented in In Memory Databases.
$key is present in the array, exists will return true. The value for a given key can be undef, in which case $array{$key} will be
undef while $exists{$key} will return true. This corresponds to ($key, undef) being in the hash.
Pictures help... here's the %ary table:
keys values +------+------+ | a | 3 | | x | 7 | | d | 0 | | e | 2 | +------+------+
And these conditions hold
$ary{'a'} is true
$ary{'d'} is false
defined $ary{'d'} is true
defined $ary{'a'} is true
exists $ary{'a'} is true (perl5 only)
grep ($_ eq 'a', keys %ary) is true
If you now say
undef $ary{'a'}
your table now reads:
keys values +------+------+ | a | undef| | x | 7 | | d | 0 | | e | 2 | +------+------+
and these conditions now hold; changes in caps:
$ary{'a'} is FALSE
$ary{'d'} is false
defined $ary{'d'} is true
defined $ary{'a'} is FALSE
exists $ary{'a'} is true (perl5 only)
grep ($_ eq 'a', keys %ary) is true
Notice the last two: you have an undef value, but a defined key!
Now, consider this:
delete $ary{'a'}
your table now reads:
keys values +------+------+ | x | 7 | | d | 0 | | e | 2 | +------+------+
and these conditions now hold; changes in caps:
$ary{'a'} is false
$ary{'d'} is false
defined $ary{'d'} is true
defined $ary{'a'} is false
exists $ary{'a'} is FALSE (perl5 only)
grep ($_ eq 'a', keys %ary) is FALSE
See, the whole entry is gone!
EXISTS and
DEFINED methods differently. For example, there isn't the
concept of undef with hashes that are tied to DBM* files. This means the
true/false tables above will give different results when used on such a
hash. It also means that exists and defined do the same thing with a DBM*
file, and what they end up doing is not what they do with ordinary hashes.
keys %hash in a scalar context returns the number of keys in the hash and resets the iterator associated with the hash. You may need to do this if
you use last to exit a loop early so that when you re-enter it, the hash iterator has
been reset.
%seen = ();
for $element (keys(%foo), keys(%bar)) {
$seen{$element}++;
}
@uniq = keys %seen;
Or more succinctly:
@uniq = keys %{{%foo,%bar}};
Or if you really want to save space:
%seen = ();
while (defined ($key = each %foo)) {
$seen{$key}++;
}
while (defined ($key = each %bar)) {
$seen{$key}++;
}
@uniq = keys %seen;
somefunc($hash{"nonesuch key here"});
Then that element ``autovivifies''; that is, it springs into existence
whether you store something there or not. That's because functions get
scalars passed in by reference. If somefunc modifies $_[0], it has to be ready to write it back into the caller's version.
This has been fixed as of perl5.004.
Normally, merely accessing a key's value for a nonexistent key does not cause that key to be forever there. This is different than awk's behavior.
if (`cat /vmunix` =~ /gzip/) {
print "Your kernel is GNU-zip enabled!\n";
}
On some systems, however, you have to play tedious games with ``text'' versus ``binary'' files. See binmode.
If you're concerned about 8-bit ASCII data, then see the perllocale manpage.
If you want to deal with multi-byte characters, however, there are some gotchas. See the section on Regular Expressions.
warn "has nondigits" if /\D/;
warn "not a whole number" unless /^\d+$/;
warn "not an integer" unless /^-?\d+$/; # reject +3
warn "not an integer" unless /^[+-]?\d+$/;
warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
warn "not a C float"
unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
Or you could check out http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz
instead. The POSIX module (part of the standard Perl distribution) provides
the strtol and strtod for converting strings to double and longs, respectively.
use FreezeThaw qw(freeze thaw);
$new = thaw freeze $old;
Where $old can be (a reference to) any kind of data structure
you'd like. It will be deeply copied.
print or
write in Perl, you go though this buffering.
syswrite circumvents stdio and buffering.
In most stdio implementations, the type of buffering and the size of the buffer varies according to the type of device. Disk files are block buffered, often with a buffer size of more than 2k. Pipes and sockets are often buffered with a buffer size between 1/2 and 2k. Serial devices (e.g. modems, terminals) are normally line-buffered, and stdio sends the entire line when it gets the newline.
Perl does not support truly unbuffered output (except insofar as you can syswrite). What it does instead support is ``command buffering'', in which a physical write is performed after every output command. This isn't as hard on your system as unbuffering, but does get the output where you want it when you want it.
If you expect characters to get to your device when you print them there, you'll want to autoflush its handle, as in the older:
use FileHandle;
open(DEV, "<+/dev/tty"); # ceci n'est pas une pipe
DEV->autoflush(1);
or the newer IO::* modules:
use IO::Handle;
open(DEV, ">/dev/printer"); # but is this?
DEV->autoflush(1);
or even this:
use IO::Socket; # this one is kinda a pipe?
$sock = IO::Socket::INET->new(PeerAddr => 'www.perl.com',
PeerPort => 'http(80)',
Proto => 'tcp');
die "$!" unless $sock;
$sock->autoflush();
$sock->print("GET /\015\012");
$document = join('', $sock->getlines());
print "DOC IS: $document\n";
Note the hardcoded carriage return and newline in their octal equivalents. This is the ONLY way (currently) to assure a proper flush on all platforms, including Macintosh.
You can use select and the $| variable to control autoflushing (see $| and select):
$oldh = select(DEV);
$| = 1;
select($oldh);
You'll also see code that does this without a temporary variable, as in
select((select(DEV), $| = 1)[0]);
(There are exceptions in special circumstances. Replacing a sequence of
bytes with another sequence of the same length is one. Another is using the $DB_RECNO array bindings as documented in the DB_File manpage. Yet another is manipulating files with all lines the same length.)
The general solution is to create a temporary copy of the text file with the changes you want, then copy that over the original.
$old = $file;
$new = "$file.tmp.$$";
$bak = "$file.bak";
open(OLD, "< $old") or die "can't open $old: $!";
open(NEW, "> $new") or die "can't open $new: $!";
# Correct typos, preserving case
while (<OLD>) {
s/\b(p)earl\b/${1}erl/i;
(print NEW $_) or die "can't write to $new: $!";
}
close(OLD) or die "can't close $old: $!";
close(NEW) or die "can't close $new: $!";
rename($old, $bak) or die "can't rename $old to $bak: $!";
rename($new, $old) or die "can't rename $new to $old: $!";
Perl can do this sort of thing for you automatically with the -i
command-line switch or the closely-related $^I variable (see
the perlrun manpage for more details). Note that
-i may require a suffix on some non-Unix systems; see the platform-specific
documentation that came with your port.
# Renumber a series of tests from the command line
perl -pi -e 's/(^\s+test\s+)\d+/ $1 . ++$count /e' t/op/taint.t
# form a script
local($^I, @ARGV) = ('.bak', glob("*.c"));
while (<>) {
if ($. == 1) {
print "This line should appear at the top of each file\n";
}
s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
print;
close ARGV if eof; # Reset $.
}
If you need to seek to an arbitrary line of a file that changes infrequently, you could build up an index of byte positions of where the line ends are in the file. If the file is large, an index of every tenth or hundredth line end would allow you to seek and read fairly efficiently. If the file is sorted, try the look.pl library (part of the standard perl distribution).
In the unique case of deleting lines at the end of a file, you can use
tell and truncate. The following code snippet
deletes the last line of a file without making a copy or reading the whole
file into memory:
open (FH, "+< $file");
while ( <FH> ) { $addr = tell(FH) unless eof(FH) }
truncate(FH, $addr);
Error checking is left as an exercise for the reader.
$lines = 0;
open(FILE, $filename) or die "Can't open `$filename': $!";
while (sysread FILE, $buffer, 4096) {
$lines += ($buffer =~ tr/\n//);
}
close FILE;
BEGIN {
use IO::File;
use Fcntl;
my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP} || $ENV{TEMP};
my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
sub temp_file {
my $fh = undef;
my $count = 0;
until (defined($fh) || $count > 100) {
$base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
$fh = IO::File->new($base_name, O_WRONLY|O_EXCL|O_CREAT, 0644)
}
if (defined($fh)) {
return ($fh, $base_name);
} else {
return ();
}
}
}
Or you could simply use IO::Handle::new_tmpfile.
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 the output of a normal, Berkeley-style ps:
# sample input line:
# 15158 p5 T 0:00 perl /home/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 $var (qw!pid tt stat time command!) {
print "$var: <$$var>\n";
}
print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
"\n";
}
local(*FH);
But while still supported, that isn't the best to go about getting local
filehandles. Typeglobs have their drawbacks. You may well want to use the FileHandle module, which creates new filehandles for you (see the FileHandle manpage):
use FileHandle;
sub findme {
my $fh = FileHandle->new();
open($fh, "</etc/hosts") or die "no /etc/hosts: $!";
while (<$fh>) {
print if /\b127\.(0\.0\.)?1\b/;
}
# $fh automatically closes/disappears here
}
Internally, Perl believes filehandles to be of class IO::Handle. You may use that module directly if you'd like (see Handle), or one of its more specific derived classes.
swrite function.
sub commify {
local $_ = shift;
1 while s/^(-?\d+)(\d{3})/$1,$2/;
return $_;
}
$n = 23659019423.2331;
print "GOT: ", commify($n), "\n";
GOT: 23,659,019,423.2331
You can't just:
s/^(-?\d+)(\d{3})/$1,$2/g;
because you have to put the comma in and then recalculate your position.
Within Perl, you may use this directly:
$filename =~ s{
^ ~ # find a leading tilde
( # save this in $1
[^/] # a non-slash character
* # repeated 0 or more times (0 means me)
)
}{
$1
? (getpwnam($1))[7]
: ( $ENV{HOME} || $ENV{LOGDIR} )
}ex;
open(FH, "+> /path/name"); # WRONG
Whoops. You should instead use this, which will fail if the file doesn't exist.
open(FH, "+< /path/name"); # open for update
If this is an issue, try:
sysopen(FH, "/path/name", O_RDWR|O_CREAT, 0644);
Error checking is left as an exercise for the reader.
<> operator performs a globbing operation (see above). By default
glob forks csh to do the actual glob expansion,
but csh can't handle more than 127 items and so gives the error message
Argument list too long. People who installed tcsh as csh won't have this problem, but their users
may be surprised by it.
To get around this, either do the glob yourself with Dirhandles and patterns, or use a module like Glob::KGlob, one that doesn't use the
shell to do globbing.
glob function or its angle-bracket alias in a scalar
context, you may cause a leak and/or unpredictable behavior. It's best
therefore to use glob only in list context.
sub safe_filename {
local $_ = shift;
return m#^/#
? "$_\0"
: "./$_\0";
}
$fn = safe_filename("<<<something really wicked ");
open(FH, "> $fn") or "couldn't open $fn: $!";
You could also use the sysopen function (see sysopen).
rename function. But that
may not work everywhere, in particular, renaming files across file systems.
If your operating system supports a mv program or its moral
equivalent, this works:
rename($old, $new) or system("mv", $old, $new);
It may be more compelling to use the File::Copy module instead. You just
copy to the new file to the new name (checking return values), then delete
the old one. This isn't really the same semantics as a real
rename, though, which preserves metainformation like
permissions, timestamps, inode info, etc.
flock function (see the perlfunc manpage for details) will call flock if that exists,
fcntl if it doesn't (on perl version 5.004 and later), and
lockf if neither of the two previous system calls exists. On
some systems, it may even use a different form of native locking. Here are
some gotchas with Perl's flock:
lockf does not provide shared locking, and requires that the
filehandle be open for writing (or appending, or read/writing).
flock can't lock files over a network (e.g.
on NFS file systems), so you'd need to force the use of fcntl
when you build Perl. See the flock entry of the perlfunc manpage, and the INSTALL
file in the source distribution for information on building Perl to do
this.
flock can't lock network files.
sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
open(LCK, "> file.lock"); # THIS BROKEN CODE
This is a classic race condition: you take two steps to do something which must be done in one. That's why computer hardware provides an atomic test-and-set instruction. In theory, this ``ought'' to work:
sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT, 0644) or die "can't open file.lock: $!":
except that lamentably, file creation (and deletion) is not atomic over
NFS, so this won't work (at least, not every time) over the net. Various
schemes involving involving link have been suggested, but
these tend to involve busy-wait, which is also subdesirable.
Anyway, this is what to do:
use Fcntl;
sysopen(FH, "numfile", O_RDWR|O_CREAT, 0644) or die "can't open numfile: $!";
flock(FH, 2) or die "can't flock numfile: $!";
$num = <FH> || 0;
seek(FH, 0, 0) or die "can't rewind numfile: $!";
truncate(FH, 0) or die "can't truncate numfile: $!";
(print FH $num+1, "\n") or die "can't write numfile: $!";
# DO NOT UNLOCK THIS UNTIL YOU CLOSE
close FH or die "can't close numfile: $!";
Here's a much better web-page hit counter:
$hits = int( (time() - 850_000_000) / rand(1_000) );
If the count doesn't impress your friends, then the code might. :-)
perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
However, if you have fixed sized records, then you might do something more like this:
$RECSIZE = 220; # size of record, in bytes
$recno = 37; # which record to update
open(FH, "+<somewhere") || die "can't update somewhere: $!";
seek(FH, $recno * $RECSIZE, 0);
read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
# munge the record
seek(FH, $recno * $RECSIZE, 0);
print FH $record;
close FH;
Locking and error checking are left as an exercise for the reader. Don't forget them, or you'll be quite sorry.
Don't forget to set binmode under DOS-like platforms when
operating on files that have anything other than straight text in them. See
the docs on open and on binmode for more details.
localtime, gmtime, or POSIX::strftime() to
convert this into human-readable form.
Here's an example:
$write_secs = (stat($file))[9];
print "file $file updated at ", scalar(localtime($file)), "\n";
If you prefer something more legible, use the File::stat module (part of the standard distribution in version 5.004 and later):
use File::stat;
use Time::localtime;
$date_string = ctime(stat($file)->mtime);
print "file $file updated at $date_string\n";
Error checking is left as an exercise for the reader.
utime function documented in utime. By way of example, here's a little program that copies the read and write
times from its first argument to all the rest of them.
if (@ARGV < 2) {
die "usage: cptimes timestamp_file other_files ...\n";
}
$timestamp = shift;
($atime, $mtime) = (stat($timestamp))[8,9];
utime $atime, $mtime, @ARGV;
Error checking is left as an exercise for the reader.
Note that utime currently doesn't work correctly with Win95/NT
ports. A bug has been reported. Check it carefully before using it on those
platforms.
for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
To connect up to one filehandle to several output filehandles, it's easiest
to use the tee program if you have it, and let it take care of
the multiplexing:
open (FH, "| tee file1 file2 file3");
Otherwise you'll have to write your own multiplexing print function -- or your own tee program -- or use Tom Christiansen's, at http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz, which is written in Perl.
In theory a IO::Tee class could be written, but to date we haven't seen such.
$\ variable (see the perlvar manpage for details). You can either set it to "" to eliminate empty paragraphs ("abc\n\n\n\ndef", for instance, gets treated as two paragraphs and not three), or
"\n\n" to accept empty paragraphs.
If your system supports POSIX, you can use the following code, which you'll note turns off echo processing as well.
#!/usr/bin/perl -w
use strict;
$| = 1;
for (1..4) {
my $got;
print "gimme: ";
$got = getone();
print "--> $got\n";
}
exit;
BEGIN {
use POSIX qw(:termios_h);
my ($term, $oterm, $echo, $noecho, $fd_stdin);
$fd_stdin = fileno(STDIN);
$term = POSIX::Termios->new(); $term->getattr($fd_stdin); $oterm = $term->getlflag();
$echo = ECHO | ECHOK | ICANON; $noecho = $oterm & ~$echo;
sub cbreak {
$term->setlflag($noecho);
$term->setcc(VTIME, 1);
$term->setattr($fd_stdin, TCSANOW);
}
sub cooked {
$term->setlflag($oterm);
$term->setcc(VTIME, 0);
$term->setattr($fd_stdin, TCSANOW);
}
sub getone {
my $key = '';
cbreak();
sysread(STDIN, $key, 1);
cooked();
return $key;
}
}
END { cooked() }
The Term::ReadKey module from CPAN may be easier to use:
use Term::ReadKey;
open(TTY, "</dev/tty");
print "Gimme a char: ";
ReadMode "raw";
$key = ReadKey 0, *TTY;
ReadMode "normal";
printf "\nYou said %s, char number %03d\n",
$key, ord $key;
For DOS systems, Dan Carson
To put the PC in ``raw'' mode, use ioctl with some magic numbers gleaned from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes across the net every so often):
$old_ioctl = ioctl(STDIN,0,0); # Gets device info
$old_ioctl &= 0xff;
ioctl(STDIN,1,$old_ioctl | 32); # Writes it back, setting bit 5
Then to read a single character:
sysread(STDIN,$c,1); # Read a single character
And to put the PC back to ``cooked'' mode:
ioctl(STDIN,1,$old_ioctl); # Sets it back to cooked mode.
So now you have $c. If ord == 0, you have a two byte code, which means you hit a special key. Read another
byte with sysread, and that value tells you what combination it was according to this table:
# PC 2-byte keycodes = ^@ + the following:
# HEX KEYS
# --- ----
# 0F SHF TAB
# 10-19 ALT QWERTYUIOP
# 1E-26 ALT ASDFGHJKL
# 2C-32 ALT ZXCVBNM
# 3B-44 F1-F10
# 47-49 HOME,UP,PgUp
# 4B LEFT
# 4D RIGHT
# 4F-53 END,DOWN,PgDn,Ins,Del
# 54-5D SHF F1-F10
# 5E-67 CTR F1-F10
# 68-71 ALT F1-F10
# 73-77 CTR LEFT,RIGHT,END,PgDn,HOME
# 78-83 ALT 1234567890-=
# 84 CTR PgUp
This is all trial and error I did a long time ago, I hope I'm reading the file that worked.
sub key_ready {
my($rin, $nfd);
vec($rin, fileno(STDIN), 1) = 1;
return $nfd = select($rin,undef,undef,0);
}
You should look into getting the Term::ReadKey extension from CPAN.
sysopen:
use Fcntl;
sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
or die "can't open /tmp/somefile: $!":
sysopen:
use Fcntl;
sysopen(FH, "/tmp/somefile", O_WRONLY|O_EXCL|O_CREAT, 0644)
or die "can't open /tmp/somefile: $!":
Be warned that neither creation nor deletion of files is guaranteed to be an atomic operation over NFS. That is, two processes might both successful create or unlink the same file!
tail -f in perl?
seek(GWFILE, 0, 1);
The statement seek doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next <GWFILE> makes Perl try again to read something.
If that doesn't work (it relies on features of your stdio implementation), then you need something more like this:
for (;;) {
for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
# search for some stuff and put it into files
}
# sleep for a while
seek(GWFILE, $curpos, 0); # seek to where we had been
}
If this still doesn't work, look into the POSIX module. POSIX defines the
clearerr method, which can remove the end of file condition on
a filehandle. The method: read until end of file, clearerr,
read some more. Lather, rinse, repeat.
open should do
the trick. For example:
open(LOG, ">>/tmp/logfile");
open(STDERR, ">&LOG");
Or even with a literal numeric descriptor:
$fd = $ENV{MHCONTEXTFD};
open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
Error checking has been left as an exercise for the reader.
close function is
to be used for things that Perl opened itself, even if it was a dup of a
numeric descriptor, as with MHCONTEXT above. But if you really have to, you
may be able to do this:
require 'sys/syscall.ph';
$rc = syscall(&SYS_close, $fd + 0); # must force numeric
die "can't sysclose $fd: $!" unless $rc == -1;
Either single-quote your strings, or (preferably) use forward slashes.
Since all DOS and Windows versions since something like MS-DOS 2.0 or so
have treated / and \ the same in a path, you might as well use the one that doesn't clash with
Perl -- or the POSIX shell, ANSI C and C++, awk, Tcl, Java, or Python, just
to mention a few.
-i clobber protected files? Isn't this a bug in Perl?
The executive summary: learn how your filesystem works. The permissions on a file say what can happen to the data in that file. The permissions on a directory say what can happen to the list of files in that directory. If you delete a file, you're removing its name from the directory (so the operation depends on the permissions of the directory, not of the file). If you try to write to the file, the permissions of the file govern whether you're allowed to.
srand;
rand($.) < 1 && ($line = $_) while <>;
This has a significant advantage in space over reading the whole file in.
# turn the line into the first word, a colon, and the
# number of characters on the rest of the line
s/^(\w+)(.*)/ lc($1) . ":" . length($2) /ge;
/x modifier causes whitespace to be ignored in a regexp pattern (except in a
character class), and also allows you to use normal comments there, too. As
you can imagine, whitespace and comments help a lot.
/x lets you turn this:
s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs;
into this:
s{ < # opening angle bracket
(?: # Non-backreffing grouping paren
[^>'"] * # 0 or more things that are neither > nor ' nor "
| # or else
".*?" # a section between double quotes (stingy match)
| # or else
'.*?' # a section between single quotes (stingy match)
) + # all occurring one or more times
> # closing angle bracket
}{}gsx; # replace with nothing, i.e. delete
It's still not quite so clear as prose, but it is very useful for describing the meaning of each part of the pattern.
/
characters, they can be delimited by almost any character. the perlre manpage
describes this. For example, the s/// above uses braces as delimiters. Selecting another delimiter can avoid
quoting the delimiter within the pattern:
s/\/usr\/local/\/usr\/share/g; # bad delimiter choice
s#/usr/local#/usr/share#g; # better
modifier on your pattern.
There are many ways to get multiline data into a string. If you want it to happen automatically while reading input, you'll want to set $/ (probably to '' for paragraphs or undef for the whole file) to allow you to read more than one line at a time.
Read the perlre manpage to help you decide which of /s and /m (or both) you might want to use: /s allows dot to include newline, and /m allows caret and dollar to match next to a newline, not just at the end of the string. You do need to make sure that you've actually got a multiline string in there.
For example, this program detects duplicate words, even when they span line breaks (but not paragraph ones). For this example, we don't need /s because we aren't using dot in a regular expression that we want to cross line boundaries. Neither do we need /m because we aren't wanting caret or dollar to match at any point inside the record next to newlines. But it's imperative that $/ be set to something other than the default, or else we won't actually ever have a multiline record read in.
$/ = ''; # read in more whole paragraph, not just one line
while ( <> ) {
while ( /\b(\w\S+)(\s+\1)+\b/gi ) {
print "Duplicate $1 at paragraph $.\n";
}
}
Here's code that finds sentences that begin with ``From '' (which would be mangled by many mailers):
$/ = ''; # read in more whole paragraph, not just one line
while ( <> ) {
while ( /^From /gm ) { # /m makes ^ match next to \n
print "leading from in paragraph $.\n";
}
}
Here's code that finds everything between START and END in a paragraph:
undef $/; # read in whole file, not just one line or paragraph
while ( <> ) {
while ( /START(.*?)END/sm ) { # /s makes . cross line boundaries
print "$1\n";
}
}
.. operator (documented in
the perlop manpage):
perl -ne 'print if /START/ .. /END/' file1 file2 ...
If you wanted text and not lines, you would use
perl -0777 -pe 'print "$1\n" while /START(.*?)END/gs' file1 file2 ...
But if you want nested occurrences of START through END, you'll run up against the problem described in the question in this
section on matching balanced text.
Actually, you could do this if you don't mind reading the whole file into
undef $/;
@records = split /your_pattern/, <FH>;
# Original by Nathan Torkington, massaged by Jeffrey Friedl
#
sub preserve_case($$)
{
my ($old, $new) = @_;
my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc
my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new));
my ($len) = $oldlen < $newlen ? $oldlen : $newlen;
for ($i = 0; $i < $len; $i++) {
if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) {
$state = 0;
} elsif (lc $c eq $c) {
substr($new, $i, 1) = lc(substr($new, $i, 1));
$state = 1;
} else {
substr($new, $i, 1) = uc(substr($new, $i, 1));
$state = 2;
}
}
# finish up with any remaining new (for when new is longer than old)
if ($newlen > $oldlen) {
if ($state == 1) {
substr($new, $oldlen) = lc(substr($new, $oldlen));
} elsif ($state == 2) {
substr($new, $oldlen) = uc(substr($new, $oldlen));
}
}
return $new;
}
$a = "this is a TEsT case";
$a =~ s/(test)/preserve_case($1, "success")/gie;
print "$a\n";
This prints:
this is a SUcCESS case
\w match accented characters?
/[a-zA-Z]/?
/[^\W\d_]/, no matter what locale you're in. Non-alphabetics would be /[\W\d_]/ (assuming you don't consider an underscore a letter).
$variable and
@variable references in regular expressions unless the
delimiter is a single quote. Remember, too, that the right-hand side of a s/// substitution is considered a double-quoted string (see the perlop manpage for more details). Remember also that any regexp special characters will be
acted on unless you precede the substitution with \Q. Here's an example:
$string = "to die?";
$lhs = "die?";
$rhs = "sleep no more";
$string =~ s/\Q$lhs/$rhs/;
# $string is now "to sleep no more"
Without the \Q, the regexp would also spuriously match ``di''.
/o really for?
/o modifier locks in the regexp the first time it's used. This always happens
in a constant regular expression, and in fact, the pattern was compiled
into the internal format at the same time your entire program was.
Use of /o is irrelevant unless variable interpolation is used in the pattern, and if
so, the regexp engine will neither know nor care whether the variables
change after the pattern is evaluated the very
first time.
/o is often used to gain an extra measure of efficiency by not performing
subsequent evaluations when you know it won't matter (because you know the
variables won't change), or more rarely, when you don't want the regexp to
notice if they do.
For example, here's a ``paragrep'' program:
$/ = ''; # paragraph mode
$pat = shift;
while (<>) {
print if /$pat/o;
}
perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c
will work in many but not all cases. You see, it's too simple-minded for certain kinds of C programs, in particular, those with what appear to be comments in quoted strings. For that, you'd need something like this, created by Jeffrey Friedl:
$/ = undef;
$_ = <>;
s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|\n+|.[^/"'\\]*)#$2#g;
print;
This could, of course, be more legibly written with the /x modifier, adding whitespace and comments.
\1 and its ilk), they still aren't powerful enough. You still need to use
non-regexp techniques to parse balanced text, such as the text enclosed
between matching parentheses or braces, for example.
An elaborate subroutine (for 7-bit ASCII only) to pull out balanced and
possibly nested single chars, like ` and ', { and }, or ( and ) can be found in http://www.perl.com/CPAN/authors/id/TOMC/scripts/pull_quotes.gz
.
The C::Scan module from CPAN contains such subs for internal usage, but they are undocumented.
?, *, +,
{}) that are greedy rather than the whole pattern; Perl prefers local greed
and immediate gratification to overall greed. To get non-greedy versions of
the same quantifiers, use (??, *?, +?, {}?).
An example:
$s1 = $s2 = "I am very very cold";
$s1 =~ s/ve.*y //; # I am cold
$s2 =~ s/ve.*?y //; # I am very cold
Notice how the second substitution stopped matching as soon as it
encountered ``y ''. The *? quantifier effectively tells the regular expression engine to find a match
as quickly as possible and pass control on to whatever is next in line,
like you would if you were playing hot potato.
while (<>) {
foreach $word ( split ) {
# do something with $word here
}
}
Note that this isn't really a word in the English sense; it's just chunks of consecutive non-whitespace characters.
To work with only alphanumeric sequences, you might consider
while (<>) {
foreach $word (m/(\w+)/g) {
# do something with $word here
}
}
while (<>) {
while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'"
$seen{$1}++;
}
}
while ( ($word, $count) = each %seen ) {
print "$count $word\n";
}
If you wanted to do the same thing for lines, you wouldn't need a regular expression:
while (<>) {
$seen{$_}++;
}
while ( ($line, $count) = each %seen ) {
print "$count $line";
}
If you want these output in a sorted order, see the section on Hashes.
while (<FH>) {
foreach $pat (@patterns) {
if ( /$pat/ ) {
# do something
}
}
}
Instead, you either need to use one of the experimental Regexp extension modules from CPAN (which might well be overkill for your purposes), or else put together something like this, inspired from a routine in Jeffrey Friedl's book:
sub _bm_build {
my $condition = shift;
my @regexp = @_; # this MUST not be local(); need my()
my $expr = join $condition => map { "m/\$regexp[$_]/o" } (0..$#regexp);
my $match_func = eval "sub { $expr }";
die if $@; # propagate $@; this shouldn't happen!
return $match_func;
}
sub bm_and { _bm_build('&&', @_) }
sub bm_or { _bm_build('||', @_) }
$f1 = bm_and qw{
xterm
(?i)window
};
$f2 = bm_or qw{
\b[Ff]ree\b
\bBSD\B
(?i)sys(tem)?\s*[V5]\b
};
# feed me /etc/termcap, prolly
while ( <> ) {
print "1: $_" if &$f1;
print "2: $_" if &$f2;
}
\b work for me?
\b is a synonym for \s+, and that it's the edge between whitespace characters and non-whitespace
characters. Neither is correct. \b is the place between a \w
character and a \W character (that is, \b is the edge of a ``word''). It's a zero-width assertion, just like ^, $, and all the other anchors, so it doesn't consume any characters. the perlre manpage
describes the behaviour of all the regexp metacharacters.
Here are examples of the incorrect application of \b, with fixes:
"two words" =~ /(\w+)\b(\w+)/; # WRONG
"two words" =~ /(\w+)\s+(\w+)/; # right
" =matchless= text" =~ /\b=(\w+)=\b/; # WRONG
" =matchless= text" =~ /=(\w+)=/; # right
Although they may not do what you thought they did, \b and \B
can still be quite useful. For an example of the correct use of
\b, see the example of matching duplicate words over multiple lines.
An example of using \B is the pattern \Bis\B. This will find occurrences of ``is'' on the insides of words only, as in
``thistle'', but not ``this'' or ``island''.
\G in a regular expression?
\G is used in a match or substitution in conjunction the
/g modifier (and ignored if there's no /g) to anchor the regular expression to the point just past where the last
match occurred, i.e. the pos point.
For example, suppose you had a line of text quoted in standard mail and
Usenet notation, (that is, with leading > characters), and you want change each leading > into a corresponding :. You could do so in this way:
s/^(>+)/':' x length($1)/gem;
Or, using \G, the much simpler (and faster):
s/\G>/:/g;
A more sophisticated use might involve a tokenizer. The following lex-like example is courtesy of Jeffrey Friedl. It did not work in 5.003 due to bugs in that release, but does work in 5.004 or better:
while (<>) {
chomp;
PARSER: {
m/ \G( \d+\b )/gx && do { print "number: $1\n"; redo; };
m/ \G( \w+ )/gx && do { print "word: $1\n"; redo; };
m/ \G( \s+ )/gx && do { print "space: $1\n"; redo; };
m/ \G( [^\w\d]+ )/gx && do { print "other: $1\n"; redo; };
}
}
Of course, that could have been written as
while (<>) {
chomp;
PARSER: {
if ( /\G( \d+\b )/gx {
print "number: $1\n";
redo PARSER;
}
if ( /\G( \w+ )/gx {
print "word: $1\n";
redo PARSER;
}
if ( /\G( \s+ )/gx {
print "space: $1\n";
redo PARSER;
}
if ( /\G( [^\w\d]+ )/gx {
print "other: $1\n";
redo PARSER;
}
}
}
But then you lose the vertical alignment of the regular expressions.
egrep program, they are
in fact implemented as NFAs (non-deterministic finite automata) to allow
backtracking and backreferencing. And they aren't POSIX-style either,
because those guarantee worst-case behavior for all cases. (It seems that
some people prefer guarantees of consistency, even when what's guaranteed
is slowness.) See the book ``Mastering Regular Expressions'' (from
O'Reilly) by Jeffrey Friedl for all the details you could ever hope to know
on these matters (a full citation appears in
the perlfaq2 manpage).
grep that's not better
written as a for (well, foreach, technically) loop.
Let's suppose you have some weird Martian encoding where pairs of ASCII uppercase letters encode single Martian letters (i.e. the two bytes ``CV'' make a single Martian letter, as do the two bytes ``SG'', ``VS'', ``XX'', etc.). Other bytes represent single characters, just like ASCII.
So, the string of Martian ``I am CVSGXX!'' uses 12 bytes to encode the nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'.
Now, say you want to search for the single character /GX/. Perl doesn't know about Martian, so it'll find the two bytes ``GX'' in
the ``I am CVSGXX!'' string, even though that character isn't there: it
just looks like it is because ``SG'' is next to ``XX'', but there's no real
``GX''. This is a big problem.
Here are a few ways, all painful, to deal with it:
$martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``maritan'' bytes
# are no longer adjacent.
print "found GX!\n" if $martian =~ /GX/;
Or like this:
@chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g;
# above is conceptually similar to: @chars = $text =~ m/(.)/g;
#
foreach $char (@chars) {
print "found GX!\n", last if $char eq 'GX';
}
Or like this:
while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded
print "found GX!\n", last if $1 eq 'GX';
}
Or like this:
die "sorry, Perl doesn't (yet) have Martian support )-:\n";
In addition, a sample program which converts half-width to full-width katakana (in Shift-JIS or EUC encoding) is available from CPAN as
There are many double- (and multi-) byte encodings commonly used these days. Some versions of these have 1-, 2-, 3-, and 4-byte characters, all mixed.
$ for scalar values (number, string or reference)
@ for arrays
% for hashes (associative arrays)
* for all types of that symbol name. In version 4 you used them like
pointers, but in modern perls you can just use references.
While there are a few places where you don't actually need these type specifiers, you should always use them.
A couple of others that you're likely to encounter that aren't really type specifiers are:
<> are used for inputting a record from a filehandle.
\ takes a reference to something.
Note that <FILE> is neither the type specifier for files nor the name of the handle. It is the <> operator applied to the handle FILE. It reads one line (well, record - see
$/) from the handle FILE in scalar context, or all lines in list context. When performing open, close, or any other operation
besides <> on files, or even talking about the handle, do
not use the brackets. These are correct: eof, seek and ``copying from STDIN to FILE''.
use strict). But a hash key consisting of a simple word (that isn't the name of a
defined subroutine) and the left-hand operand to the => operator both count as though they were quoted:
This is like this
------------ ---------------
$foo{line} $foo{"line"}
bar => stuff "bar" => stuff
The final semicolon in a block is optional, as is the final comma in a list. Good style (see the perlstyle manpage) says to put them in except for one-liners:
if ($whoops) { exit 1 }
@nums = (1, 2, 3);
if ($whoops) {
exit 1;
}
@lines = (
"There Beren came from mountains cold",
"And lost he wandered under leaves",
);
$dir = (getpwnam($user))[7];
Another way is to use undef as an element on the left-hand-side:
($dev, $ino, undef, undef, $uid, $gid) = stat($file);
$^W variable (documented in the perlvar manpage) controls runtime warnings for a block:
{
local $^W = 0; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
}
Note that like all the punctuation variables, you cannot currently use
my on $^W, only local.
A new use warnings pragma is in the works to provide finer control over all this. The curious
should check the perl5-porters mailing list archives for details.
A common mistake is to write:
unlink $file || die "snafu";
This gets interpreted as:
unlink ($file || die "snafu");
To avoid this problem, either put in extra parentheses or use the super low
precedence or operator:
(unlink $file) || die "snafu";
unlink $file or die "snafu";
The ``English'' operators (and, or, xor, and not) deliberately have precedence lower than that of list operators for just
such situations as the one above.
Another operator with surprising precedence is exponentiation. It binds
more tightly even than unary minus, making -2**2 product a negative not a positive four. It is also right-associating,
meaning that 2**3**2 is two raised to the ninth power, not eight squared.
$person = {}; # new anonymous hash
$person->{AGE} = 24; # set field AGE to 24
$person->{NAME} = "Nat"; # set field NAME to "Nat"
If you're looking for something a bit more rigorous, try the perltoot manpage.
Here's a convenient template you might wish you use when starting your own module. Make sure to change the names appropriately.
package Some::Module; # assumes Some/Module.pm
use strict;
BEGIN {
use Exporter ();
use vars qw($VERSION @ISA @EXPORT @EXPORT_OK %EXPORT_TAGS);
## set the version for version checking; uncomment to use ## $VERSION = 1.00;
# if using RCS/CVS, this next line may be preferred,
# but beware two-digit versions.
$VERSION = do{my@r=q$Revision: 1.15 $=~/\d+/g;sprintf '%d.'.'%02d'x$#r,@r};
@ISA = qw(Exporter); @EXPORT = qw(&func1 &func2 &func3); %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
# your exported package globals go here,
# as well as any optionally exported functions
@EXPORT_OK = qw($Var1 %Hashit);
}
use vars @EXPORT_OK;
# non-exported package globals go here
use vars qw( @more $stuff );
# initialize package globals, first exported ones
$Var1 = '';
%Hashit = ();
# then the others (which are still accessible as $Some::Module::stuff)
$stuff = '';
@more = ();
# all file-scoped lexicals must be created before
# the functions below that use them.
# file-private lexicals go here
my $priv_var = '';
my %secret_hash = ();
# here's a file-private function as a closure,
# callable as &$priv_func; it cannot be prototyped.
my $priv_func = sub {
# stuff goes here.
};
# make all your functions, whether exported or not;
# remember to put something interesting in the {} stubs
sub func1 {} # no prototype
sub func2() {} # proto'd void
sub func3($$) {} # proto'd to 2 scalars
# this one isn't exported, but could be called!
sub func4(\%) {} # proto'd to 1 hash ref
END { } # module clean-up code here (global destructor)
1; # modules must return true
kill is given no processes to signal):
sub is_tainted {
return ! eval { join('',@_), kill 0; 1; };
}
This is not -w clean, however. There is no -w clean way to detect taintedness - take this as a hint that you should
untaint all possibly-tainted data.
Closure is a computer science term with a precise but hard-to-explain meaning. Closures are implemented in Perl as anonymous subroutines with lasting references to lexical variables outside their own scopes. These lexicals magically refer to the variables that were around when the subroutine was defined (deep binding).
Closures make sense in any programming language where you can have the return value of a function be itself a function, as you can in Perl. Note that some languages provide anonymous functions but are not capable of providing proper closures; the Python language, for example. For more information on closures, check out any textbook on functional programming. Scheme is a language that not only supports but encourages closures.
Here's a classic function-generating function:
sub add_function_generator {
return sub { shift + shift };
}
$add_sub = add_function_generator();
$sum = &$add_sub(4,5); # $sum is 9 now.
The closure works as a function template with some customization slots left out to be filled later. The anonymous
subroutine returned by add_function_generator isn't
technically a closure because it refers to no lexicals outside its own
scope.
Contrast this with the following make_adder function, in which
the returned anonymous function contains a reference to a lexical variable
outside the scope of that function itself. Such a reference requires that
Perl return a proper closure, thus locking in for all time the value that
the lexical had when the function was created.
sub make_adder {
my $addpiece = shift;
return sub { shift + $addpiece };
}
$f1 = make_adder(20);
$f2 = make_adder(555);
Now &$f1 is always 20 plus whatever $n you pass in, whereas
&$f2 is always 555 plus whatever $n you pass in. The
$addpiece in the closure sticks around.
Closures are often used for less esoteric purposes. For example, when you want to pass in a bit of code into a function:
my $line;
timeout( 30, sub { $line = <STDIN> } );
If the code to execute had been passed in as a string, '$line =
<STDIN>', there would have been no way for the hypothetical timeout
function to access the lexical variable $line back in its
caller's scope.
func( \$some_scalar );
func( \$some_array );
func( [ 1 .. 10 ] );
func( \%some_hash );
func( { this => 10, that => 20 } );
func( \&some_func );
func( sub { $_[0] ** $_[1] } );
*FH
or \*FH notation (``typeglobs'' - see the perldata manpage for more information), or create filehandles dynamically using the old
FileHandle or the new IO::File modules, both part of the standard Perl
distribution.
use Fcntl;
use IO::File;
my $fh = new IO::File $filename, O_WRONLY|O_APPEND;
or die "Can't append to $filename: $!";
func($fh);
sub compare($$) {
my ($val1, $regexp) = @_;
my $retval = eval { $val =~ /$regexp/ };
die if $@;
return $retval;
}
$match = compare("old McDonald", q/d.*D/);
Make sure you never say something like this:
return eval "\$val =~ /$regexp/"; # WRONG
or someone can sneak shell escapes into the regexp due to the double interpolation of the eval and the double-quoted string. For example:
$pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
eval "\$string =~ /$pattern_of_evil/";
Those preferring to be very, very clever might see the O'Reilly book,
Mastering Regular Expressions, by Jeffrey Friedl. Page 273's Build_MatchMany_Function is
particularly interesting. A complete citation of this book is given in the perlfaq2 manpage.
call_a_lot(10, $some_obj, "methname")
sub call_a_lot {
my ($count, $widget, $trick) = @_;
for (my $i = 0; $i < $count; $i++) {
$widget->$trick();
}
}
or you can use a closure to bundle up the object and its method call and arguments:
my $whatnot = sub { $some_obj->obfuscate(@args) };
func($whatnot);
sub func {
my $code = shift;
&$code();
}
You could also investigate the can method in the UNIVERSAL
class (part of the standard perl distribution).
Here's code to implement a function-private variable:
BEGIN {
my $counter = 42;
sub prev_counter { return --$counter }
sub next_counter { return $counter++ }
}
Now prev_counter and next_counter share a private
variable $counter that was initialized at compile time.
To declare a file-private variable, you'll still use a my,
putting it at the outer scope level at the top of the file. Assume this is
in file Pax.pm:
package Pax;
my $started = scalar(localtime(time()));
sub begun { return $started }
When use Pax or require Pax loads this module, the variable will be initialized. It won't get
garbage-collected the way most variables going out of scope do, because the
begun function cares about it, but no one else can get it. It
is not called $Pax::started because its scope is unrelated to the package.
It's scoped to the file. You could conceivably have several packages in
that same file all accessing the same private variable, but another file
with the same package couldn't get to it.
$x, and assigns a new value for the duration of the subroutine, which is
visible in other functions called from that subroutine. This is done at run-time, so is called dynamic scoping.
local always affects global variables, also called package
variables or dynamic variables.
my creates a new variable that is only visible in the current subroutine. This
is done at compile-time, so is called lexical or static scoping.
my always affects private variables, also called lexical
variables or (improperly) static(ly scoped) variables.
For instance:
sub visible {
print "var has value $var\n";
}
sub dynamic {
local $var = 'local'; # new temporary value for the still-global
visible(); # variable called $var
}
sub lexical {
my $var = 'private'; # new private variable, $var
visible(); # (invisible outside of sub scope)
}
$var = 'global';
visible(); # prints global
dynamic(); # prints local
lexical(); # prints global
Notice how at no point does the value ``private'' get printed. That's
because $var only has that value within the block of the
lexical function, and it is hidden from called subroutine.
In summary, local doesn't make what you think of as private,
local variables. It gives a global variable a temporary value.
my is what you're looking for if you want private variables.
See also the perlsub manpage, which explains this all in more detail.
use strict "refs". So instead of $var, use ${'var'}.
local $var = "global";
my $var = "lexical";
print "lexical is $var\n";
no strict 'refs';
print "global is ${'var'}\n";
If you know your package, you can just mention it explicitly, as in
$Some_Pack::var. Note that the notation $::var is not the dynamic $var in the current package, but rather the one in
the main
package, as though you had written $main::var. Specifying the package
directly makes you hard-code its name, but it executes faster and avoids
running afoul of use strict "refs".
my). However,
dynamic variables (aka global, local, or package variables) are effectively
shallowly bound. Consider this just one more reason not to use them. See
the answer to What's a closure?.
=. The
<FH> read operation, like so many of Perl's functions and operators, can tell
which context it was called in and behaves appropriately. In general, the
scalar function can help. This function does nothing to the
data itself (contrary to popular myth) but rather tells its argument to
behave in whatever its scalar fashion is. If that function doesn't have a
defined scalar behavior, this of course doesn't help you (such as with
sort).
To enforce scalar context in this particular case, however, you need merely omit the parentheses:
local($foo) = <FILE>; # WRONG
local($foo) = scalar(<FILE>); # ok
local $foo = <FILE>; # right
You should probably be using lexical variables anyway, although the issue is the same here:
my($foo) = <FILE>; # WRONG
my $foo = <FILE>; # right
If you want to override a predefined function, such as open,
then you'll have to import the new definition from a different module. See Overriding Builtin Functions. There's also an example in Class/Template.
If you want to overload a Perl operator, such as + or **, then you'll want to use the use overload pragma, documented in the overload manpage.
If you're talking about obscuring method calls in parent classes, see Overridden Methods.
&foo, you allow that function access to your current @_ values,
and you by-pass prototypes. That means that the function doesn't get an
empty @_, it gets yours! While not strictly speaking a bug (it's documented
that way in the perlsub manpage), it would be hard to consider this a feature in most cases.
When you call your function as &foo, then you do get a new @_, but prototyping is still circumvented.
Normally, you want to call a function using foo. You may only omit the parentheses if the function is already known to the
compiler because it already saw the definition (use but not require), or via a forward reference or use subs declaration. Even in this case, you get a clean @_ without any
of the old values leaking through where they don't belong.
Here's a simple example of a switch based on pattern matching. We'll do a multi-way conditional based on the type of reference stored in $whatchamacallit:
SWITCH:
for (ref $whatchamacallit) {
/^$/ && die "not a reference";
/SCALAR/ && do {
print_scalar($$ref);
last SWITCH;
};
/ARRAY/ && do {
print_array(@$ref);
last SWITCH;
};
/HASH/ && do {
print_hash(%$ref);
last SWITCH;
};
/CODE/ && do {
warn "can't print function ref";
last SWITCH;
};
# DEFAULT
warn "User defined type skipped";
}
When it comes to undefined variables that would trigger a warning under -w, you can use a handler to trap the pseudo-signal
__WARN__ like this:
$SIG{__WARN__} = sub {
for ( $_[0] ) {
/Use of uninitialized value/ && do {
# promote warning to a fatal
die $_;
};
# other warning cases to catch could go here;
warn $_; }
};
print
ref to find out the class $object was blessed into.
Another possible reason for problems is because you've used the indirect
object syntax (eg, find Guru "Samy") on a class name before Perl has seen that such a package exists. It's
wisest to make sure your packages are all defined before you start using
them, which will be taken care of if you use the use statement instead of
require. If not, make sure to use arrow notation (eg,
Guru-find>) instead. Object notation is explained in
the perlobj manpage.
my $packname = ref bless [];
But if you're a method and you want to print an error message that includes the kind of object you were called on (which is not necessarily the same as the one in which you were compiled):
sub amethod {
my $self = shift;
my $class = ref($self) || $self;
warn "called me from a $class object";
}
Read the FAQs and documentation specific to the port of perl to your operating system (eg, the perlvms manpage, the perlplan9 manpage, ...). These should contain more detailed information on the vagaries of your perl.
system instead.
Term::Cap Standard perl distribution
Term::ReadKey CPAN
Term::ReadLine::Gnu CPAN
Term::ReadLine::Perl CPAN
Term::Screen CPAN
Term::Cap Standard perl distribution
Curses CPAN
Term::ANSIColor CPAN
Tk CPAN
There's an example of this in crypt). First, you put the terminal into ``no echo'' mode, then just read the
password normally. You may do this with an old-style ioctl
function, POSIX terminal control (see the POSIX manpage, and Chapter 7 of the Camel), or a call to the stty program, with varying degrees of portability.
You can also do this for most systems using the Term::ReadKey module from CPAN, which is easier to use and in theory more portable.
sysopen and O_RDWR|O_NDELAY|O_NOCTTY from the Fcntl module (part of the standard perl distribution). See
sysopen for more on this approach.
print DEV "atv1\012"; # wrong, for some devices
print DEV "atv1\015"; # right, for some devices
Even though with normal text files, a ``\n'' will do the trick, there is still no unified scheme for terminating a line that is portable between Unix, DOS/Win, and Macintosh, except to terminate ALL line ends with ``\015\012'', and strip what you don't need from the output. This applies especially to socket I/O and autoflushing, discussed next.
print
them, you'll want to autoflush that filehandle, as in the older
use FileHandle;
DEV->autoflush(1);
and the newer
use IO::Handle;
DEV->autoflush(1);
You can use select and the $| variable to control autoflushing (see $| and select):
$oldh = select(DEV);
$| = 1;
select($oldh);
You'll also see code that does this without a temporary variable, as in
select((select(DEV), $| = 1)[0]);
As mentioned in the previous item, this still doesn't work when using socket I/O between Unix and Macintosh. You'll need to hardcode your line terminators, in that case.
read or sysread,
you'll have to arrange for an alarm handler to provide a timeout (see
alarm). If you have a non-blocking open, you'll likely have a non-blocking read,
which means you may have to use a 4-arg select to determine
whether I/O is ready on that device (see
select.
Seriously, you can't if they are Unix password files - the Unix password system employs one-way encryption. Programs like Crack can forcibly (and intelligently) try to guess passwords, but don't (can't) guarantee quick success.
If you're worried about users selecting bad passwords, you should
proactively check when they try to change their password (by modifying
passwd, for example).
system("cmd &")
or you could use fork as documented in fork, with further examples in the perlipc manpage. Some things to be aware of, if you're on a Unix-like system:
$SIG{CHLD} = sub { wait };
See Signals for other examples of code to do this. Zombies are not an issue with system.
Be warned that very few C libraries are re-entrant. Therefore, if you
attempt to print in a handler that got invoked during another
stdio operation your internal structures will likely be in an inconsistent
state, and your program will dump core. You can sometimes avoid this by
using syswrite instead of print.
Unless you're exceedingly careful, the only safe things to do inside a
signal handler are: set a variable and exit. And in the first case, you
should only set a variable in such a way that malloc is not
called (eg, by setting a variable that already has a value).
For example:
$Interrupted = 0; # to ensure it has a value
$SIG{INT} = sub {
$Interrupted++;
syswrite(STDERR, "ouch\n", 5);
}
However, because syscalls restart by default, you'll find that if you're in
a ``slow'' call, such as <FH>, read, connect, or wait, that the
only way to terminate them is by ``longjumping'' out; that is, by raising
an exception. See the time-out handler for a blocking flock in Signals or chapter 6 of the Camel.
pwd_mkdb to install it (see pwd_mkdb(5) for more details).
date program.
(There is no way to set the time and date on a per-process basis.) This
mechanism will work for Unix, MS-DOS, Windows, and NT; the VMS equivalent
is set time.
However, if all you want to do is change your timezone, you can probably get away with setting an environment variable:
$ENV{TZ} = "MST7MDT"; # unixish
$ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
system "trn comp.lang.perl";
sleep
function provides, the easiest way is to use the select
function as documented in select. If your system has itimers and syscall support, you can
check out the old example in http://www.perl.com/CPAN/doc/misc/ancient/tutorial/eg/itimers.pl
.
In general, you may not be able to. But if you system supports both the
syscall function in Perl as well as a system call like
gettimeofday, then you may be able to do something like this:
require 'sys/syscall.ph';
$TIMEVAL_T = "LL";
$done = $start = pack($TIMEVAL_T, ());
syscall( &SYS_gettimeofday, $start, 0)) != -1
or die "gettimeofday: $!";
##########################
# DO YOUR OPERATION HERE #
##########################
syscall( &SYS_gettimeofday, $done, 0) != -1
or die "gettimeofday: $!";
@start = unpack($TIMEVAL_T, $start);
@done = unpack($TIMEVAL_T, $done);
# fix microseconds
for ($done[1], $start[1]) { $_ /= 1_000_000 }
$delta_time = sprintf "%.4f", ($done[0] + $done[1] )
-
($start[0] + $start[1] );
atexit. Each package's END block is called when the program or
thread ends (see the perlmod manpage manpage for more details). It isn't called when untrapped signals kill the
program, though, so if you use END blocks you should also use
use sigtrap qw(die normal-signals);
Perl's exception-handling mechanism is its eval operator. You
can use eval as setjmp and die as longjmp. For
details of this, see the section on signals, especially the time-out
handler for a blocking flock in Signals and chapter 6 of the Camel.
If exception handling is all you're interested in, try the exceptions.pl library (part of the standard perl distribution).
If you want the atexit syntax (and an rmexit as
well), try the AtExit module available from CPAN.
Note that even though SunOS and Solaris are binary compatible, these values are different. Go figure.
syscall, you can use the syscall function (documented in
the perlfunc manpage).
Remember to check the modules that came with your distribution, and CPAN as well - someone may already have written a module to do it.
cpp
directives in C header files to files containing subroutine definitions,
like &SYS_getitimer, which you can use as arguments to your functions.
It doesn't work perfectly, but it usually gets most of the job done. Simple
files like errno.h, syscall.h, and socket.h were fine, but the hard ones like ioctl.h nearly always need to hand-edited. Here's how to install the *.ph files:
1. become super-user
2. cd /usr/include
3. h2ph *.h */*.h
If your system supports dynamic loading, for reasons of portability and sanity you probably ought to use h2xs (also part of the standard perl distribution). This tool converts C header files to Perl extensions. See the perlxstut manpage for how to get started with h2xs.
If your system doesn't support dynamic loading, you still probably ought to use h2xs. See the perlxstut manpage and MakeMaker for more information (in brief, just use make perl instead of a plain make to rebuild perl with a new static extension).
pipe,
fork, and exec to do the job. Make sure you read
the deadlock warnings in its documentation, though (see Open2).
system $cmd; # using system()
$output = `$cmd`; # using backticks (``)
open (PIPE, "cmd |"); # using open()
With system, both STDOUT and STDERR will go the same place as
the script's versions of these, unless the command redirects them.
Backticks and open read only the STDOUT of your command.
With any of these, you can change file descriptors before the call:
open(STDOUT, ">logfile");
system("ls");
or you can use Bourne shell file-descriptor redirection:
$output = `$cmd 2>some_file`;
open (PIPE, "cmd 2>some_file |");
You can also use file-descriptor redirection to make STDERR a duplicate of STDOUT:
$output = `$cmd 2>&1`;
open (PIPE, "cmd 2>&1 |");
Note that you cannot simply open STDERR to be a dup of STDOUT in your Perl program and avoid calling the shell to do the redirection. This doesn't work:
open(STDERR, ">&STDOUT");
$alloutput = `cmd args`; # stderr still escapes
This fails because the open makes STDERR go to where STDOUT
was going at the time of the open. The backticks then make
STDOUT go to a string, but don't change STDERR (which still goes to the old
STDOUT).
Note that you must use Bourne shell (sh(1)) redirection syntax in backticks, not
csh! Details on why Perl's system and backtick
and pipe opens all use the Bourne shell are in http://www.perl.com/CPAN/doc/FMTEYEWTK/versus/csh.whynot
.
You may also use the IPC::Open3 module (part of the standard perl distribution), but be warned that it has a different order of arguments from IPC::Open2 (see Open3).
fork/exec paradigm (eg, Unix), it works like this:
open causes a fork. In the parent,
open returns with the process ID of the child. The child
execs the command to be piped to/from. The parent can't know
whether the exec was successful or not - all it can return is
whether the fork succeeded or not. To find out if the command
succeeded, you have to catch SIGCHLD and wait to get the exit
status.
On systems that follow the spawn paradigm, open might do what you expect - unless perl uses a shell to start your command. In
this case the fork/exec description still applies.
`cp file file.bak`;
And now they think ``Hey, I'll just always use backticks to run programs.''
Bad idea: backticks are for capturing a program's output; the
system function is for running programs.
Consider this line:
`cat /etc/termcap`;
You haven't assigned the output anywhere, so it just wastes memory (for a
little while). Plus you forgot to check $? to see whether the program even ran correctly. Even if you wrote
print `cat /etc/termcap`;
In most cases, this could and probably should be written as
system("cat /etc/termcap") == 0
or die "cat program failed!";
Which will get the output quickly (as its generated, instead of only at the end ) and also check the return value.
system also provides direct control over whether shell
wildcard processing may take place, whereas backticks do not.
@ok = `grep @opts '$search_string' @filenames`;
You have to do this:
my @ok = ();
if (open(GREP, "-|")) {
while (<GREP>) {
chomp;
push(@ok, $_);
}
close GREP;
} else {
exec 'grep', @opts, $search_string, @filenames;
}
Just as with system, no shell escapes happen when you
exec a list.
clearerr that you can use. That is the
technically correct way to do it. Here are some less reliable workarounds:
$where = tell(LOG);
seek(LOG, $where, 0);
To actually alter the visible command line, you can assign to the variable
$0 as documented in the perlvar manpage. This won't work on all operating systems, though. Daemon programs like
sendmail place their state there, as in:
$0 = "orcus [accepting connections]";
evaling the script's output in your shell; check out the
comp.unix.questions FAQ for details.
%ENV persist after Perl exits, but directory changes
do not.
fork && exit;
perl Makefile.PL PREFIX=/u/mydir/perl
then either set the PERL5LIB environment variable before you run scripts that use the modules/libraries (see the perlrun manpage) or say
use lib '/u/mydir/perl';
See Perl's the lib manpage for more information.
-t STDIN and -t STDOUT can give clues, sometimes not.
if (-t STDIN && -t STDOUT) {
print "Now what? ";
}
On POSIX systems, you can test whether your own process group matches the current process group of your controlling terminal as follows:
use POSIX qw/getpgrp tcgetpgrp/;
open(TTY, "/dev/tty") or die $!;
$tpgrp = tcgetpgrp(TTY);
$pgrp = getpgrp();
if ($tpgrp == $pgrp) {
print "foreground\n";
} else {
print "background\n";
}
alarm function, probably in conjunction with a signal
handler, as documented Signals and chapter 6 of the Camel. You may instead use the more flexible
Sys::AlarmCall module available from CPAN.
wait when a SIGCHLD is received, or else use the
double-fork technique described in fork.
system call (see the perlipc manpage for sample code) and then have a signal handler for the INT signal that
passes the signal on to the subprocess.
sysopen:
use Fcntl;
sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
or die "can't open /tmp/somefile: $!":
If your version of perl is compiled without dynamic loading, then you just need to replace step 3 (make) with make perl and you will get a new perl binary with your extension linked in.
See MakeMaker for more details on building extensions.
Seriously, if you can demonstrate that you've read the following FAQs and that your problem isn't something simple that can be easily answered, you'll probably receive a courteous and useful reply to your question if you post it on comp.infosystems.www.authoring.cgi (if it's something to do with HTTP, HTML, or the CGI protocols). Questions that appear to be Perl questions but are really CGI ones that are posted to comp.lang.perl.misc may not be so well received.
The useful FAQs are:
http://www.perl.com/perl/faq/idiots-guide.html
http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml
http://www.perl.com/perl/faq/perl-cgi-faq.html
http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html
http://www.boutell.com/faq/
Many folks attempt a simple-minded regular expression approach, like
s/<.*?>//g, but that fails in many cases because the tags may continue over line
breaks, they may contain quoted angle-brackets, or HTML comment may be
present. Plus folks forget to convert entities, like < for example.
Here's one ``simple-minded'' approach, that works for most files:
#!/usr/bin/perl -p0777
s/<(?:[^>'"]*|(['"]).*?\1)*>//gs
If you want a more complete solution, see the 3-stage striphtml program in http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/striphtml.gz .
#!/usr/bin/perl -n00
# qxurl - tchrist@perl.com
print "$2\n" while m{
< \s*
A \s+ HREF \s* = \s* (["']) (.*?) \1
\s* >
}gsix;
This version does not adjust relative URLs, understand alternate bases, deal with HTML comments, or accept URLs themselves as arguments. It also runs about 100x faster than a more ``complete'' solution using the LWP suite of modules, such as the http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/xurl.gz program.
start_multipart_form method, which isn't the same as the
startform method.
$html_code = `lynx -source $url`;
$text_data = `lynx -dump $url`;
$string = "http://altavista.digital.com/cgi-bin/query?pg=q&;what=news&fmt=.&q=%2Bcgi-bin+%2Bperl.exe";
$string =~ s/%([a-fA-F0-9]{2})/chr(hex($1))/ge;
Encoding is a bit harder, because you can't just blindly change all the
non-alphanumunder character (\W) into their hex escapes. It's important that characters with special
meaning like / and ?
not be translated. Probably the easiest way to get this right is to avoid
reinventing the wheel and just use the URI::Escape module, which is part of
the libwww-perl package (LWP) available from CPAN.
Content-Type as the headers of your reply, send back a Location: header. Officially this should be a
URI: header, so the CGI.pm module (available from CPAN) sends back both:
Location: http://www.domain.com/newpage
URI: http://www.domain.com/newpage
Note that relative URLs in these headers can cause strange effects because of ``optimizations'' that servers do.
use HTTPD::UserAdmin ();
HTTPD::UserAdmin
->new(DB => "/foo/.htpasswd")
->add($username => $password);
$/ = '';
$header = <MSG>;
$header =~ s/\n\s+/ /g; # merge continuation lines
%head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );
That solution doesn't do well if, for example, you're trying to maintain all the Received lines. A more complete approach is to use the Mail::Header module from CPAN (part of the MailTools package).
$ENV{CONTENT_LENGTH} and
$ENV{QUERY_STRING}. It's true that this can work, but there are also a lot of versions of
this floating around that are quite simply broken!
Please do not be tempted to reinvent the wheel. Instead, use the CGI.pm or CGI_Lite.pm (available from CPAN), or if you're trapped in the module-free land of perl1 .. perl4, you might look into cgi-lib.pl (available from http://www.bio.cam.ac.uk/web/form.html).
Without sending mail to the address and seeing whether it bounces (and even then you face the halting problem), you cannot determine whether an email address is valid. Even if you apply the email header standard, you can have problems, because there are deliverable addresses that aren't RFC-822 (the mail header standard) compliant, and addresses that aren't deliverable which are compliant.
Many are tempted to try to eliminate many frequently-invalid email
addresses with a simple regexp, such as
/^[\w.-]+\@+\w+$/. However, this also throws out many valid ones, and says nothing about
potential deliverability, so is not suggested. Instead, see http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/ckaddr.gz
, which actually checks against the full RFC spec (except for nested
comments), looks for addresses you may not wish to accept email to (say,
Bill Clinton or your postmaster), and then makes sure that the hostname
given can be looked up in DNS. It's not fast, but it works.
use MIME::base64;
$decoded = decode_base64($encoded);
A more direct approach is to use the unpack function's ``u''
format after minor transliterations:
tr#A-Za-z0-9+/##cd; # remove non-base64 chars
tr#A-Za-z0-9+/# -_#; # convert to uuencoded format
$len = pack("c", 32 + 0.75*length); # compute length byte
print unpack("u", $len . $_); # uudecode and print
use Sys::Hostname;
$address = sprintf('%s@%s', getpwuid($<), hostname);
Company policies on email address can mean that this generates addresses that the company's email system will not accept, so you should ask for users' email addresses when this matters. Furthermore, not all systems on which Perl runs are so forthcoming with this information as is Unix.
The Mail::Util module from CPAN (part of the MailTools package) provides a
mailaddress function that tries to guess the mail address of
the user. It makes a more intelligent guess than the code above, using
information given when the module was installed, but it could still be
incorrect. Again, the best way is often just to ask the user.
`hostname`
program. While sometimes expedient, this isn't very portable. It's one of
those tradeoffs of convenience versus portability.
The Sys::Hostname module (part of the standard perl distribution) will give
you the hostname after which you can find out the IP address (assuming you
have working DNS) with a gethostbyname call.
use Socket;
use Sys::Hostname;
my $host = hostname();
my $addr = inet_ntoa(scalar(gethostbyname($name)) || 'localhost');
Probably the simplest way to learn your DNS domain name is to grok it out of /etc/resolv.conf, at least under Unix. Of course, this assumes several things about your resolv.conf configuration, including that it exists.
(We still need a good DNS domain name-learning method for non-Unix systems.)
perl -MNews::NNTPClient
-e 'print News::NNTPClient->;new->list("newsgroups")'