=head1 NAME
-perlfaq8 - System Interaction ($Revision: 1.21 $, $Date: 1997/04/24 22:44:19 $)
+perlfaq8 - System Interaction ($Revision: 1.39 $, $Date: 1999/05/23 18:37:57 $)
=head1 DESCRIPTION
=head2 How do I find out which operating system I'm running under?
-The $^O variable ($OSTYPE if you use English) contains the operating
-system that your perl binary was built for.
+The $^O variable ($OSNAME if you use English) contains an indication of
+the name of the operating system (not its release number) that your perl
+binary was built for.
=head2 How come exec() doesn't return?
=back
+Some of these specific cases are shown below.
+
+=head2 How do I print something out in color?
+
+In general, you don't, because you don't know whether
+the recipient has a color-aware display device. If you
+know that they have an ANSI terminal that understands
+color, you can use the Term::ANSIColor module from CPAN:
+
+ use Term::ANSIColor;
+ print color("red"), "Stop!\n", color("reset");
+ print color("green"), "Go!\n", color("reset");
+
+Or like this:
+
+ use Term::ANSIColor qw(:constants);
+ print RED, "Stop!\n", RESET;
+ print GREEN, "Go!\n", RESET;
+
+=head2 How do I read just one key without waiting for a return key?
+
+Controlling input buffering is a remarkably system-dependent matter.
+On many systems, you can just use the B<stty> command as shown in
+L<perlfunc/getc>, but as you see, that's already getting you into
+portability snags.
+
+ open(TTY, "+</dev/tty") or die "no tty: $!";
+ system "stty cbreak </dev/tty >/dev/tty 2>&1";
+ $key = getc(TTY); # perhaps this works
+ # OR ELSE
+ sysread(TTY, $key, 1); # probably this does
+ system "stty -cbreak </dev/tty >/dev/tty 2>&1";
+
+The Term::ReadKey module from CPAN offers an easy-to-use interface that
+should be more efficient than shelling out to B<stty> for each key.
+It even includes limited support for Windows.
+
+ use Term::ReadKey;
+ ReadMode('cbreak');
+ $key = ReadKey(0);
+ ReadMode('normal');
+
+However, that requires that you have a working C compiler and can use it
+to build and install a CPAN module. Here's a solution using
+the standard POSIX module, which is already on your systems (assuming
+your system supports POSIX).
+
+ use HotKey;
+ $key = readkey();
+
+And here's the HotKey module, which hides the somewhat mystifying calls
+to manipulate the POSIX termios structures.
+
+ # HotKey.pm
+ package HotKey;
+
+ @ISA = qw(Exporter);
+ @EXPORT = qw(cbreak cooked readkey);
+
+ use strict;
+ use POSIX qw(:termios_h);
+ my ($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); # ok, so i don't want echo either
+ $term->setcc(VTIME, 1);
+ $term->setattr($fd_stdin, TCSANOW);
+ }
+
+ sub cooked {
+ $term->setlflag($oterm);
+ $term->setcc(VTIME, 0);
+ $term->setattr($fd_stdin, TCSANOW);
+ }
+
+ sub readkey {
+ my $key = '';
+ cbreak();
+ sysread(STDIN, $key, 1);
+ cooked();
+ return $key;
+ }
+
+ END { cooked() }
+
+ 1;
+
+=head2 How do I check whether input is ready on the keyboard?
+
+The easiest way to do this is to read a key in nonblocking mode with the
+Term::ReadKey module from CPAN, passing it an argument of -1 to indicate
+not to block:
+
+ use Term::ReadKey;
+
+ ReadMode('cbreak');
+
+ if (defined ($char = ReadKey(-1)) ) {
+ # input was waiting and it was $char
+ } else {
+ # no input was waiting
+ }
+
+ ReadMode('normal'); # restore normal tty settings
+
+=head2 How do I clear the screen?
+
+If you only have do so infrequently, use C<system>:
+
+ system("clear");
+
+If you have to do this a lot, save the clear string
+so you can print it 100 times without calling a program
+100 times:
+
+ $clear_string = `clear`;
+ print $clear_string;
+
+If you're planning on doing other screen manipulations, like cursor
+positions, etc, you might wish to use Term::Cap module:
+
+ use Term::Cap;
+ $terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
+ $clear_string = $terminal->Tputs('cl');
+
+=head2 How do I get the screen size?
+
+If you have Term::ReadKey module installed from CPAN,
+you can use it to fetch the width and height in characters
+and in pixels:
+
+ use Term::ReadKey;
+ ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
+
+This is more portable than the raw C<ioctl>, but not as
+illustrative:
+
+ require 'sys/ioctl.ph';
+ die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
+ open(TTY, "+</dev/tty") or die "No tty: $!";
+ unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) {
+ die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ;
+ }
+ ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
+ print "(row,col) = ($row,$col)";
+ print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
+ print "\n";
+
=head2 How do I ask the user for a password?
(This question has nothing to do with the web. See a different
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.
+ use Term::ReadKey;
+
+ ReadMode('noecho');
+ $password = ReadLine(0);
+
=head2 How do I read and write the serial port?
This depends on which operating system your program is running on. In
=item flushing output
If you expect characters to get to your device when you print() them,
-you'll want to autoflush that filehandle, as in the older
-
- use FileHandle;
- DEV->autoflush(1);
-
-and the newer
-
- use IO::Handle;
- DEV->autoflush(1);
-
-You can use select() and the C<$|> variable to control autoflushing
-(see L<perlvar/$|> and L<perlfunc/select>):
+you'll want to autoflush that filehandle. You can use select()
+and the C<$|> variable to control autoflushing (see L<perlvar/$|>
+and L<perlfunc/select>):
$oldh = select(DEV);
$| = 1;
select((select(DEV), $| = 1)[0]);
+Or if you don't mind pulling in a few thousand lines
+of code just because you're afraid of a little $| variable:
+
+ use IO::Handle;
+ DEV->autoflush(1);
+
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.
=back
+While trying to read from his caller-id box, the notorious Jamie Zawinski
+<jwz@netscape.com>, after much gnashing of teeth and fighting with sysread,
+sysopen, POSIX's tcgetattr business, and various other functions that
+go bump in the night, finally came up with this:
+
+ sub open_modem {
+ use IPC::Open2;
+ my $stty = `/bin/stty -g`;
+ open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
+ # starting cu hoses /dev/tty's stty settings, even when it has
+ # been opened on a pipe...
+ system("/bin/stty $stty");
+ $_ = <MODEM_IN>;
+ chop;
+ if ( !m/^Connected/ ) {
+ print STDERR "$0: cu printed `$_' instead of `Connected'\n";
+ }
+ }
+
=head2 How do I decode encrypted password files?
You spend lots and lots of money on dedicated hardware, but this is
bound to get you talked about.
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.
+password system employs one-way encryption. It's more like hashing than
+encryption. The best you can check is whether something else hashes to
+the same string. You can't turn a hash back into the original string.
+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
=over 4
-=item STDIN, STDOUT and STDERR are shared
+=item STDIN, STDOUT, and STDERR are shared
Both the main process and the backgrounded one (the "child" process)
share the same STDIN, STDOUT and STDERR filehandles. If both try to
=head2 How do I trap control characters/signals?
-You don't actually "trap" a control character. Instead, that
-character generates a signal, which you then trap. Signals are
-documented in L<perlipc/"Signals"> and chapter 6 of the Camel.
+You don't actually "trap" a control character. Instead, that character
+generates a signal which is sent to your terminal's currently
+foregrounded process group, which you then trap in your process.
+Signals are documented in L<perlipc/"Signals"> and chapter 6 of the Camel.
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
=head2 How do I modify the shadow password file on a Unix system?
-If perl was installed correctly, the getpw*() functions described in
-L<perlfunc> provide (read-only) access to the shadow password file.
-To change the file, make a new shadow password file (the format varies
-from system to system - see L<passwd(5)> for specifics) and use
-pwd_mkdb(8) to install it (see L<pwd_mkdb(5)> for more details).
+If perl was installed correctly, and your shadow library was written
+properly, the getpw*() functions described in L<perlfunc> should in
+theory provide (read-only) access to entries in the shadow password
+file. To change the file, make a new shadow password file (the format
+varies from system to system - see L<passwd(5)> for specifics) and use
+pwd_mkdb(8) to install it (see L<pwd_mkdb(8)> for more details).
=head2 How do I set the time and date?
$ENV{TZ} = "MST7MDT"; # unixish
$ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
- system "trn comp.lang.perl";
+ system "trn comp.lang.perl.misc";
=head2 How can I sleep() or alarm() for under a second?
In general, you may not be able to. The Time::HiRes module (available
from CPAN) provides this functionality for some systems.
-In general, you may not be able to. But if your system supports both the
-syscall() function in Perl as well as a system call like gettimeofday(2),
-then you may be able to do something like this:
+If your system supports both the syscall() function in Perl as well as
+a system call like gettimeofday(2), then you may be able to do
+something like this:
require 'sys/syscall.ph';
$done = $start = pack($TIMEVAL_T, ());
- syscall( &SYS_gettimeofday, $start, 0)) != -1
+ syscall(&SYS_gettimeofday, $start, 0) != -1
or die "gettimeofday: $!";
##########################
Release 5 of Perl added the END block, which can be used to simulate
atexit(). Each package's END block is called when the program or
-thread ends (see L<perlmod> 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
+thread ends (see L<perlmod> manpage for more details).
+
+For example, you can use this to make sure your filter program
+managed to finish its output without filling up the disk:
+
+ END {
+ close(STDOUT) || die "stdout close failed: $!";
+ }
+
+The END block 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);
=head2 How can I open a pipe both to and from a command?
The IPC::Open2 module (part of the standard perl distribution) is an
-easy-to-use approach that internally uses pipe(), fork(), and exec()
-to do the job. Make sure you read the deadlock warnings in its
-documentation, though (see L<IPC::Open2>).
+easy-to-use approach that internally uses pipe(), fork(), and exec() to do
+the job. Make sure you read the deadlock warnings in its documentation,
+though (see L<IPC::Open2>). See L<perlipc/"Bidirectional Communication
+with Another Process"> and L<perlipc/"Bidirectional Communication with
+Yourself">
+
+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 L<IPC::Open3>).
=head2 Why can't I get the output of a command with system()?
You're confusing the purpose of system() and backticks (``). system()
runs a command and returns exit status information (as a 16 bit value:
-the low 8 bits are the signal the process died from, if any, and
+the low 7 bits are the signal the process died from, if any, and
the high 8 bits are the actual exit value). Backticks (``) run a
command and return what it sent to STDOUT.
backticks, not csh(1)! 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 .
+To capture a command's STDERR and STDOUT together:
-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 L<IPC::Open3>).
+ $output = `cmd 2>&1`; # either with backticks
+ $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe
+ while (<PH>) { } # plus a read
+
+To capture a command's STDOUT but discard its STDERR:
+
+ $output = `cmd 2>/dev/null`; # either with backticks
+ $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe
+ while (<PH>) { } # plus a read
+
+To capture a command's STDERR but discard its STDOUT:
+
+ $output = `cmd 2>&1 1>/dev/null`; # either with backticks
+ $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe
+ while (<PH>) { } # plus a read
+
+To exchange a command's STDOUT and STDERR in order to capture the STDERR
+but leave its STDOUT to come out our old STDERR:
+
+ $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks
+ $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
+ while (<PH>) { } # plus a read
+
+To read both a command's STDOUT and its STDERR separately, it's easiest
+and safest to redirect them separately to files, and then read from those
+files when the program is done:
+
+ system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
+
+Ordering is important in all these examples. That's because the shell
+processes file descriptor redirections in strictly left to right order.
+
+ system("prog args 1>tmpfile 2>&1");
+ system("prog args 2>&1 1>tmpfile");
+
+The first command sends both standard out and standard error to the
+temporary file. The second command sends only the old standard output
+there, and the old standard error shows up on the old standard out.
=head2 Why doesn't open() return an error when a pipe open fails?
-It does, but probably not how you expect it to. On systems that
-follow the standard 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 exec()s 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. You should also catch SIGPIPE if
-you're writing to the child -- you may not have found out the exec()
+Because the pipe open takes place in two steps: first Perl calls
+fork() to start a new process, then this new process calls exec() to
+run the program you really wanted to open. The first step reports
+success or failure to your process, so open() can only tell you
+whether the fork() succeeded or not.
+
+To find out if the exec() step succeeded, you have to catch SIGCHLD
+and wait() to get the exit status. You should also catch SIGPIPE if
+you're writing to the child--you may not have found out the exec()
failed by the time you write. This is documented in L<perlipc>.
+In some cases, even this won't work. If the second argument to a
+piped open() contains shell metacharacters, perl fork()s, then exec()s
+a shell to decode the metacharacters and eventually run the desired
+program. Now when you call wait(), you only learn whether or not the
+I<shell> could be successfully started. Best to avoid shell
+metacharacters.
+
On systems that follow the spawn() paradigm, open() I<might> do what
-you expect - unless perl uses a shell to start your command. In this
+you expect--unless perl uses a shell to start your command. In this
case the fork()/exec() description still applies.
=head2 What's wrong with using backticks in a void context?
Strictly speaking, nothing. Stylistically speaking, it's not a good
way to write maintainable code because backticks have a (potentially
-humungous) return value, and you're ignoring it. It's may also not be very
+humongous) return value, and you're ignoring it. It's may also not be very
efficient, because you have to read in all the lines of output, allocate
memory for them, and then throw it away. Too often people are lulled
to writing:
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.
+Which will get the output quickly (as it is 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.
}
Just as with system(), no shell escapes happen when you exec() a list.
+Further examples of this can be found in L<perlipc/"Safe Pipe Opens">.
+
+Note that if you're stuck on Microsoft, no solution to this vexing issue
+is even possible. Even if Perl were to emulate fork(), you'd still
+be hosed, because Microsoft gives no argc/argv-style API. Their API
+always reparses from a single string, which is fundamentally wrong,
+but you're not likely to get the Gods of Redmond to acknowledge this
+and fix it for you.
=head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
=head2 How can I write expect in Perl?
Once upon a time, there was a library called chat2.pl (part of the
-standard perl distribution), which never really got finished. These
-days, your best bet is to look at the Comm.pl library available from
-CPAN.
+standard perl distribution), which never really got finished. If you
+find it somewhere, I<don't use it>. These days, your best bet is to
+look at the Expect module available from CPAN, which also requires two
+other modules from CPAN, IO::Pty and IO::Stty.
=head2 Is there a way to hide perl's command line from programs such as "ps"?
process are not reflected in its parent, only in its own children
created after the change. There is shell magic that may allow you to
fake it by eval()ing the script's output in your shell; check out the
-comp.unix.questions FAQ for details.
-
-=item VMS
-
-Change to %ENV persist after Perl exits, but directory changes do not.
+comp.unix.questions FAQ for details.
=back
=item *
-Open /dev/tty and use the the TIOCNOTTY ioctl on it. See L<tty(4)>
-for details.
+Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty(4)>
+for details. Or better yet, you can just use the POSIX::setsid()
+function, so you don't have to worry about process groups.
=item *
=back
+The Proc::Daemon module, available from CPAN, provides a function to
+perform these actions for you.
+
=head2 How do I make my program run with sh and csh?
See the F<eg/nih> script (part of the perl source distribution).
use POSIX qw/getpgrp tcgetpgrp/;
open(TTY, "/dev/tty") or die $!;
- $tpgrp = tcgetpgrp(TTY);
+ $tpgrp = tcgetpgrp(fileno(*TTY));
$pgrp = getpgrp();
if ($tpgrp == $pgrp) {
print "foreground\n";
There are a number of excellent interfaces to SQL databases. See the
DBD::* modules available from
http://www.perl.com/CPAN/modules/dbperl/DBD .
+A lot of information on this can be found at
+http://www.hermetica.com/technologia/perl/DBI/index.html .
=head2 How do I make a system() exit on control-C?
You can't. You need to imitate the system() call (see L<perlipc> for
sample code) and then have a signal handler for the INT signal that
-passes the signal on to the subprocess.
+passes the signal on to the subprocess. Or you can check for it:
+
+ $rc = system($cmd);
+ if ($rc & 127) { die "signal death" }
=head2 How do I open a file without blocking?
sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
or die "can't open /tmp/somefile: $!":
-=head2 How do I install a CPAN module?
+
+
-The easiest way is to have the CPAN module do it for you. This module
-comes with perl version 5.004 and later. To manually install the CPAN
-module, or any well-behaved CPAN module for that matter, follow these
-steps:
+=head2 How do I install a module from CPAN?
+
+The easiest way is to have a module also named CPAN do it for you.
+This module comes with perl version 5.004 and later. To manually install
+the CPAN module, or any well-behaved CPAN module for that matter, follow
+these steps:
=over 4
just need to replace step 3 (B<make>) with B<make perl> and you will
get a new F<perl> binary with your extension linked in.
-See L<ExtUtils::MakeMaker> for more details on building extensions
-and an answer to the question "How do I keep my own module/library
-directory?"
+See L<ExtUtils::MakeMaker> for more details on building extensions.
+See also the next question.
+
+=head2 What's the difference between require and use?
+
+Perl offers several different ways to include code from one file into
+another. Here are the deltas between the various inclusion constructs:
+
+ 1) do $file is like eval `cat $file`, except the former:
+ 1.1: searches @INC and updates %INC.
+ 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
+
+ 2) require $file is like do $file, except the former:
+ 2.1: checks for redundant loading, skipping already loaded files.
+ 2.2: raises an exception on failure to find, compile, or execute $file.
+
+ 3) require Module is like require "Module.pm", except the former:
+ 3.1: translates each "::" into your system's directory separator.
+ 3.2: primes the parser to disambiguate class Module as an indirect object.
+
+ 4) use Module is like require Module, except the former:
+ 4.1: loads the module at compile time, not run-time.
+ 4.2: imports symbols and semantics from that package to the current one.
+
+In general, you usually want C<use> and a proper Perl module.
=head2 How do I keep my own module/library directory?
use lib '/u/mydir/perl';
+This is almost the same as:
+
+ BEGIN {
+ unshift(@INC, '/u/mydir/perl');
+ }
+
+except that the lib module checks for machine-dependent subdirectories.
See Perl's L<lib> for more information.
=head2 How do I add the directory my program lives in to the module/library search path?
the PERLLIB environment variable
the PERL5LIB environment variable
- the perl -Idir commpand line flag
+ the perl -Idir command line flag
the use lib pragma, as in
use lib "$ENV{HOME}/myown_perllib";
dependent architectures. The lib.pm pragmatic module was first
included with the 5.002 release of Perl.
-=head1 How do I get one key from the terminal at a time, under POSIX?
+=head2 What is socket.ph and where do I get it?
- #!/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() }
+It's a perl4-style file defining values for system networking
+constants. Sometimes it is built using h2ph when Perl is installed,
+but other times it is not. Modern programs C<use Socket;> instead.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.
+Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
All rights reserved.
When included as part of the Standard Version of Perl, or as part of
projects / p5sagit/p5-mst-13.2.git / blobdiff