=head1 NAME
perlfaq8 - System Interaction
=head1 DESCRIPTION
This section of the Perl FAQ covers questions involving operating
system interaction. Topics include interprocess communication (IPC),
control over the user-interface (keyboard, screen and pointing
devices), and most anything else not related to data manipulation.
Read the FAQs and documentation specific to the port of perl to your
operating system (eg, L, L, ...). These should
contain more detailed information on the vagaries of your perl.
=head2 How do I find out which operating system I'm running under?
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?
X X X X X
(contributed by brian d foy)
The C function's job is to turn your process into another
command and never to return. If that's not what you want to do, don't
use C. :)
If you want to run an external command and still keep your Perl process
going, look at a piped C, C, or C.
=head2 How do I do fancy stuff with the keyboard/screen/mouse?
How you access/control keyboards, screens, and pointing devices
("mice") is system-dependent. Try the following modules:
=over 4
=item Keyboard
Term::Cap Standard perl distribution
Term::ReadKey CPAN
Term::ReadLine::Gnu CPAN
Term::ReadLine::Perl CPAN
Term::Screen CPAN
=item Screen
Term::Cap Standard perl distribution
Curses CPAN
Term::ANSIColor CPAN
=item Mouse
Tk CPAN
=back
Some of these specific cases are shown as examples in other answers
in this section of the perlfaq.
=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 command as shown in
L, but as you see, that's already getting you into
portability snags.
open(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 2>&1";
The Term::ReadKey module from CPAN offers an easy-to-use interface that
should be more efficient than shelling out to B for each key.
It even includes limited support for Windows.
use Term::ReadKey;
ReadMode('cbreak');
$key = ReadKey(0);
ReadMode('normal');
However, using the code 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?
(contributed by brian d foy)
To clear the screen, you just have to print the special sequence
that tells the terminal to clear the screen. Once you have that
sequence, output it when you want to clear the screen.
You can use the C module to get the special
sequence. Import the C function (or the C<:screen> tag):
use Term::ANSIScreen qw(cls);
my $clear_screen = cls();
print $clear_screen;
The C module can also get the special sequence if you want
to deal with the low-level details of terminal control. The C
method returns the string for the given capability:
use Term::Cap;
$terminal = Term::Cap->Tgetent( { OSPEED => 9600 } );
$clear_string = $terminal->Tputs('cl');
print $clear_screen;
On Windows, you can use the C module. After creating
an object for the output filehandle you want to affect, call the
C method:
Win32::Console;
$OUT = Win32::Console->new(STD_OUTPUT_HANDLE);
my $clear_string = $OUT->Cls;
print $clear_screen;
If you have a command-line program that does the job, you can call
it in backticks to capture whatever it outputs so you can use it
later:
$clear_string = `clear`;
print $clear_string;
=head2 How do I get the screen size?
If you have Term::ReadKey module installed from CPAN,
you can use it to fetch the width and height in characters
and in pixels:
use Term::ReadKey;
($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
This is more portable than the raw C, but not as
illustrative:
require 'sys/ioctl.ph';
die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
open(TTY, "+). 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 L or its documentation the Camel Book), or a call
to the B 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.
use Term::ReadKey;
ReadMode('noecho');
$password = ReadLine(0);
=head2 How do I read and write the serial port?
This depends on which operating system your program is running on. In
the case of Unix, the serial ports will be accessible through files in
/dev; on other systems, device names will doubtless differ.
Several problem areas common to all device interaction are the
following:
=over 4
=item lockfiles
Your system may use lockfiles to control multiple access. Make sure
you follow the correct protocol. Unpredictable behavior can result
from multiple processes reading from one device.
=item open mode
If you expect to use both read and write operations on the device,
you'll have to open it for update (see L for
details). You may wish to open it without running the risk of
blocking by using sysopen() and C from the
Fcntl module (part of the standard perl distribution). See
L for more on this approach.
=item end of line
Some devices will be expecting a "\r" at the end of each line rather
than a "\n". In some ports of perl, "\r" and "\n" are different from
their usual (Unix) ASCII values of "\012" and "\015". You may have to
give the numeric values you want directly, using octal ("\015"), hex
("0x0D"), or as a control-character specification ("\cM").
print DEV "atv1\012"; # wrong, for some devices
print DEV "atv1\015"; # right, for some devices
Even though with normal text files a "\n" will do the trick, there is
still no unified scheme for terminating a line that is portable
between Unix, DOS/Win, and Macintosh, except to terminate I line
ends with "\015\012", and strip what you don't need from the output.
This applies especially to socket I/O and autoflushing, discussed
next.
=item flushing output
If you expect characters to get to your device when you print() them,
you'll want to autoflush that filehandle. You can use select()
and the C<$|> variable to control autoflushing (see L>
and L, or L, "How do I flush/unbuffer an
output filehandle? Why must I do this?"):
$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]);
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 hard code your
line terminators, in that case.
=item non-blocking input
If you are doing a blocking read() or sysread(), you'll have to
arrange for an alarm handler to provide a timeout (see
L). 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
L.
=back
While trying to read from his caller-id box, the notorious Jamie Zawinski
C<< >>, 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");
$_ = ;
chomp;
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. It's more like hashing
than encryption. The best you can do is check 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
passwd(1), for example).
=head2 How do I start a process in the background?
(contributed by brian d foy)
There's not a single way to run code in the background so you don't
have to wait for it to finish before your program moves on to other
tasks. Process management depends on your particular operating system,
and many of the techniques are in L.
Several CPAN modules may be able to help, including IPC::Open2 or
IPC::Open3, IPC::Run, Parallel::Jobs, Parallel::ForkManager, POE,
Proc::Background, and Win32::Process. There are many other modules you
might use, so check those namespaces for other options too.
If you are on a unix-like system, you might be able to get away with a
system call where you put an C<&> on the end of the command:
system("cmd &")
You can also try using C, as described in L (although
this is the same thing that many of the modules will do for you).
=over 4
=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
access them at once, strange things can happen. You may want to close
or reopen these for the child. You can get around this with
Cing a pipe (see L) but on some systems this
means that the child process cannot outlive the parent.
=item Signals
You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is
sent when you write to a filehandle whose child process has closed (an
untrapped SIGPIPE can cause your program to silently die). This is
not an issue with C.
=item Zombies
You have to be prepared to "reap" the child process when it finishes.
$SIG{CHLD} = sub { wait };
$SIG{CHLD} = 'IGNORE';
You can also use a double fork. You immediately wait() for your
first child, and the init daemon will wait() for your grandchild once
it exits.
unless ($pid = fork) {
unless (fork) {
exec "what you really wanna do";
die "exec failed!";
}
exit 0;
}
waitpid($pid, 0);
See L for other examples of code to do this.
Zombies are not an issue with C.
=back
=head2 How do I trap control characters/signals?
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 and the
section on "Signals" in the Camel.
You can set the values of the %SIG hash to be the functions you want
to handle the signal. After perl catches the signal, it looks in %SIG
for a key with the same name as the signal, then calls the subroutine
value for that key.
# as an anonymous subroutine
$SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) };
# or a reference to a function
$SIG{INT} = \&ouch;
# or the name of the function as a string
$SIG{INT} = "ouch";
Perl versions before 5.8 had in its C source code signal handlers which
would catch the signal and possibly run a Perl function that you had set
in %SIG. This violated the rules of signal handling at that level
causing perl to dump core. Since version 5.8.0, perl looks at %SIG
*after* the signal has been caught, rather than while it is being caught.
Previous versions of this answer were incorrect.
=head2 How do I modify the shadow password file on a Unix system?
If perl was installed correctly and your shadow library was written
properly, the getpw*() functions described in L 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 for specifics) and use
pwd_mkdb(8) to install it (see L for more details).
=head2 How do I set the time and date?
Assuming you're running under sufficient permissions, you should be
able to set the system-wide date and time by running the date(1)
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 C.
However, if all you want to do is change your time zone, 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.misc";
=head2 How can I sleep() or alarm() for under a second?
X X X X