perlfaq8.1

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.\" ========================================================================
.\"
.IX Title "PERLFAQ8 1"
.TH PERLFAQ8 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.SH "NAME"
perlfaq8 \- System Interaction ($Revision: 10183 $)
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This section of the Perl \s-1FAQ\s0 covers questions involving operating
system interaction.  Topics include interprocess communication (\s-1IPC\s0),
control over the user-interface (keyboard, screen and pointing
devices), and most anything else not related to data manipulation.
.PP
Read the FAQs and documentation specific to the port of perl to your
operating system (eg, perlvms, perlplan9, ...).  These should
contain more detailed information on the vagaries of your perl.
.Sh "How do I find out which operating system I'm running under?"
.IX Subsection "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.
.Sh "How come \fIexec()\fP doesn't return?"
.IX Subsection "How come exec() doesn't return?"
Because that's what it does: it replaces your currently running
program with a different one.  If you want to keep going (as is
probably the case if you're asking this question) use \fIsystem()\fR
instead.
.Sh "How do I do fancy stuff with the keyboard/screen/mouse?"
.IX Subsection "How do I do fancy stuff with the keyboard/screen/mouse?"
How you access/control keyboards, screens, and pointing devices
(\*(L"mice\*(R") is system-dependent.  Try the following modules:
.IP "Keyboard" 4
.IX Item "Keyboard"
.Vb 5
\&        Term::Cap               Standard perl distribution
\&        Term::ReadKey           CPAN
\&        Term::ReadLine::Gnu     CPAN
\&        Term::ReadLine::Perl    CPAN
\&        Term::Screen            CPAN
.Ve
.IP "Screen" 4
.IX Item "Screen"
.Vb 3
\&        Term::Cap               Standard perl distribution
\&        Curses                  CPAN
\&        Term::ANSIColor         CPAN
.Ve
.IP "Mouse" 4
.IX Item "Mouse"
.Vb 1
\&        Tk                      CPAN
.Ve
.PP
Some of these specific cases are shown as examples in other answers
in this section of the perlfaq.
.Sh "How do I print something out in color?"
.IX Subsection "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 \s-1ANSI\s0 terminal that understands
color, you can use the Term::ANSIColor module from \s-1CPAN:\s0
.PP
.Vb 3
\&        use Term::ANSIColor;
\&        print color("red"), "Stop!\en", color("reset");
\&        print color("green"), "Go!\en", color("reset");
.Ve
.PP
Or like this:
.PP
.Vb 3
\&        use Term::ANSIColor qw(:constants);
\&        print RED, "Stop!\en", RESET;
\&        print GREEN, "Go!\en", RESET;
.Ve
.Sh "How do I read just one key without waiting for a return key?"
.IX Subsection "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 \fBstty\fR command as shown in
\&\*(L"getc\*(R" in perlfunc, but as you see, that's already getting you into
portability snags.
.PP
.Vb 6
\&        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";
.Ve
.PP
The Term::ReadKey module from \s-1CPAN\s0 offers an easy-to-use interface that
should be more efficient than shelling out to \fBstty\fR for each key.
It even includes limited support for Windows.
.PP
.Vb 4
\&        use Term::ReadKey;
\&        ReadMode(\*(Aqcbreak\*(Aq);
\&        $key = ReadKey(0);
\&        ReadMode(\*(Aqnormal\*(Aq);
.Ve
.PP
However, using the code requires that you have a working C compiler
and can use it to build and install a \s-1CPAN\s0 module.  Here's a solution
using the standard \s-1POSIX\s0 module, which is already on your systems
(assuming your system supports \s-1POSIX\s0).
.PP
.Vb 2
\&        use HotKey;
\&        $key = readkey();
.Ve
.PP
And here's the HotKey module, which hides the somewhat mystifying calls
to manipulate the \s-1POSIX\s0 termios structures.
.PP
.Vb 2
\&        # HotKey.pm
\&        package HotKey;
\&
\&        @ISA = qw(Exporter);
\&        @EXPORT = qw(cbreak cooked readkey);
\&
\&        use strict;
\&        use POSIX qw(:termios_h);
\&        my ($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\*(Aqt 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 = \*(Aq\*(Aq;
\&                cbreak();
\&                sysread(STDIN, $key, 1);
\&                cooked();
\&                return $key;
\&        }
\&
\&        END { cooked() }
\&
\&        1;
.Ve
.Sh "How do I check whether input is ready on the keyboard?"
.IX Subsection "How do I check whether input is ready on the keyboard?"
The easiest way to do this is to read a key in nonblocking mode with the
Term::ReadKey module from \s-1CPAN\s0, passing it an argument of \-1 to indicate
not to block:
.PP
.Vb 1
\&        use Term::ReadKey;
\&
\&        ReadMode(\*(Aqcbreak\*(Aq);
\&
\&        if (defined ($char = ReadKey(\-1)) ) {
\&                # input was waiting and it was $char
\&        } else {
\&                # no input was waiting
\&        }
\&
\&        ReadMode(\*(Aqnormal\*(Aq);                  # restore normal tty settings
.Ve
.Sh "How do I clear the screen?"
.IX Subsection "How do I clear the screen?"
If you only have do so infrequently, use \f(CW\*(C`system\*(C'\fR:
.PP
.Vb 1
\&        system("clear");
.Ve
.PP
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:
.PP
.Vb 2
\&        $clear_string = \`clear\`;
\&        print $clear_string;
.Ve
.PP
If you're planning on doing other screen manipulations, like cursor
positions, etc, you might wish to use Term::Cap module:
.PP
.Vb 3
\&        use Term::Cap;
\&        $terminal = Term::Cap\->Tgetent( {OSPEED => 9600} );
\&        $clear_string = $terminal\->Tputs(\*(Aqcl\*(Aq);
.Ve
.Sh "How do I get the screen size?"
.IX Subsection "How do I get the screen size?"
If you have Term::ReadKey module installed from \s-1CPAN\s0,
you can use it to fetch the width and height in characters
and in pixels:
.PP
.Vb 2
\&        use Term::ReadKey;
\&        ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
.Ve
.PP
This is more portable than the raw \f(CW\*(C`ioctl\*(C'\fR, but not as
illustrative:
.PP
.Vb 10
\&        require \*(Aqsys/ioctl.ph\*(Aq;
\&        die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
\&        open(TTY, "+</dev/tty")                     or die "No tty: $!";
\&        unless (ioctl(TTY, &TIOCGWINSZ, $winsize=\*(Aq\*(Aq)) {
\&                die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\en", &TIOCGWINSZ;
\&        }
\&        ($row, $col, $xpixel, $ypixel) = unpack(\*(AqS4\*(Aq, $winsize);
\&        print "(row,col) = ($row,$col)";
\&        print "  (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
\&        print "\en";
.Ve
.Sh "How do I ask the user for a password?"
.IX Subsection "How do I ask the user for a password?"
(This question has nothing to do with the web.  See a different
\&\s-1FAQ\s0 for that.)
.PP
There's an example of this in \*(L"crypt\*(R" in perlfunc).  First, you put the
terminal into \*(L"no echo\*(R" mode, then just read the password normally.
You may do this with an old-style \fIioctl()\fR function, \s-1POSIX\s0 terminal
control (see \s-1POSIX\s0 or its documentation the Camel Book), or a call
to the \fBstty\fR program, with varying degrees of portability.
.PP
You can also do this for most systems using the Term::ReadKey module
from \s-1CPAN\s0, which is easier to use and in theory more portable.
.PP
.Vb 1
\&        use Term::ReadKey;