perlfaq8(1)
NAME
perlfaq8 - System Interaction ($Revision: 1.27 $, $Date: 2005/12/31
00:54:37 $)
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, perlvms, perlplan9, ...). These should contain
more detailed information on the vagaries of your perl.
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.
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 system() instead.
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:
- Keyboard
- Term::Cap Standard perl distribution
Term::ReadKey CPAN
Term::ReadLine::Gnu CPAN
Term::ReadLine::Perl CPAN
Term::Screen CPAN - Screen
- Term::Cap Standard perl distribution
Curses CPAN
Term::ANSIColor CPAN - Mouse
- Tk CPAN
- Some of these specific cases are shown as examples in other answers in this section of the perlfaq.
- 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;- 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 stty command as shown in "getc" in perlfunc, 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 stty 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 $key; - }
- END { cooked() }
- 1;
- How do I check whether input is ready on the keyboard?
- The easiest way to do this is to read a key in nonblocking mode with
the Term::ReadKey module from CPAN, passing it an argument of -1 to
indicate not to block:
use Term::ReadKey;- ReadMode('cbreak');
- if (defined ($char = ReadKey(-1)) ) {
# input was waiting and it was $char
- } else {
# no input was waiting
- }
- ReadMode('normal'); # restore normal tty settings
- How do I clear the screen?
- If you only have do so infrequently, use "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 = $terminal->Tputs('cl');- How do I get the screen size?
- If you have Term::ReadKey module installed from CPAN, you can use it to fetch the width and height in characters and in pixels:
use Term::ReadKey;
($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();- This is more portable than the raw "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"; - How do I ask the user for a password?
- (This question has nothing to do with the web. See a different FAQ for that.)
- There's an example of this in "crypt" in perlfunc). 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 POSIX or its documentation the Camel Book), 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.
use Term::ReadKey;- ReadMode('noecho');
$password = ReadLine(0); - 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: - 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. - open mode
- If you expect to use both read and write operations on the device,
you'll have to open it for update (see "open" in perlfunc for
details). You may wish to open it without running the risk of
blocking by using sysopen() and "O_RDWR|O_NDELAY|O_NOCTTY" from the Fcntl module (part of the standard perl distribution). See
"sysopen" in perlfunc for more on this approach. - 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 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.
- 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 $| variable to control autoflushing (see "$|" in perlvar and "select" in perlfunc, or perlfaq5, "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: - 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. - 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 "alarm" in
perlfunc). 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" in
perlfunc. - 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>;
chomp;
if ( !m/^Connected/ ) {print STDERR "$0: cu printed `$_' instead of `Connected'\n";}- }
- 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 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 passwd(1), for example). - How do I start a process in the background?
- Several modules can start other processes that do not block your Perl
program. You can use IPC::Open3, Parallel::Jobs, IPC::Run, and some of the POE modules. See CPAN for more details. - You could also use
system("cmd &")- or you could use fork as documented in "fork" in perlfunc, with further examples in perlipc. Some things to be aware of, if you're on a Unixlike system:
- 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 "open"ing a pipe (see "open" in perlfunc) but on
some systems this means that the child process cannot outlive the
parent. - 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 "system("cmd&")". - 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 "Signals" in perlipc for other examples of code to do this.
Zombies are not an issue with "system("prog &")". - 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 "Signals" in perlipc 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. - 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 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 passwd for specifics) and use pwd_mkdb(8) to install it (see pwd_mkdb for more details). - 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 "set time". - 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";- How can I sleep() or alarm() for under a second?
- If you want finer granularity than the 1 second that the sleep() function provides, the easiest way is to use the select() function as documented in "select" in perlfunc. Try the Time::HiRes and the
BSD::Itimer modules (available from CPAN, and starting from Perl 5.8
Time::HiRes is part of the standard distribution). - How can I measure time under a second?
- In general, you may not be able to. The Time::HiRes module (available from CPAN, and starting from Perl 5.8 part of the standard distribution) provides this functionality for some systems.
- 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';- $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] )
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and other
authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
- Irrespective of its distribution, all code examples in this file are
hereby placed into the public domain. You are permitted and encouraged to use this code in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit would be courteous but is not required.