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PERLFAQ8(1)
NAME
perlfaq8 - System Interaction ($Revision: 1.39 $, $Date: 1999/05/23
18:37:57 $)
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, the perlvms manpage, the perlplan9 manpage, ...).
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 below.
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 the getc entry
in the perlfunc manpage, 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 the crypt entry in the perlfunc manpage).
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 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');
$password = ReadLine(0);
How do I read and write the serial port?
This depends on which operating system your program is running on. In the
case of Unix, the serial ports will be accessible through files in /dev; on
other systems, device names will doubtless differ. Several problem areas
common to all device interaction are the following:
lockfiles
Your system may use lockfiles to control multiple access. Make sure
you follow the correct protocol. Unpredictable behavior can result
from multiple processes reading from one device.
open mode
If you expect to use both read and write operations on the device,
you'll have to open it for update (see the open entry in the perlfunc
manpage for details). You may wish to open it without running the risk
of blocking by using sysopen() and "O_RDWR|O_NDELAY|O_NOCTTY" from the
Fcntl module (part of the standard perl distribution). See the sysopen
entry in the perlfunc manpage for more on this approach.
end of line
Some devices will be expecting a "\r" at the end of each line rather
than a "\n". In some ports of perl, "\r" and "\n" are different from
their usual (Unix) ASCII values of "\012" and "\015". You may have to
give the numeric values you want directly, using octal ("\015"), hex
("0x0D"), or as a control-character specification ("\cM").
print DEV "atv1\012"; # wrong, for some devices
print DEV "atv1\015"; # right, for some devices
Even though with normal text files a "\n" will do the trick, there is
still no unified scheme for terminating a line that is portable between
Unix, DOS/Win, and Macintosh, except to terminate ALL line ends with
"\015\012", and strip what you don't need from the output. This
applies especially to socket I/O and autoflushing, discussed next.
flushing output
If you expect characters to get to your device when you print() them,
you'll want to autoflush that filehandle. You can use select() and the
"$|" variable to control autoflushing (see perlvar/$ and the select
entry in the perlfunc manpage, or the perlfaq5 manpage, ``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 hardcode 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 the alarm entry in the
perlfunc manpage). 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 the select entry
in the perlfunc manpage.
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?
You could use
system("cmd &")
or you could use fork as documented in the fork entry in the perlfunc
manpage, with further examples in the perlipc manpage. Some things to be
aware of, if you're on a Unix-like 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 the open entry in the perlfunc manpage) 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 };
See the Signals entry in the perlipc manpage 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 the Signals entry in the perlipc manpage and the section on ``Signals''
in 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
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 (1) set a variable and (2) exit. 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 the Signals
entry in the perlipc manpage or the section on ``Signals'' in the Camel
book.
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 the perlfunc manpage 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(5) for specifics) and use
pwd_mkdb(8) to install it (see pwd_mkdb(8) 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 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.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
the select entry in the perlfunc manpage. Try the Time::HiRes and the
BSD::Itimer modules (available from CPAN).
How can I measure time under a second?
In general, you may not be able to. The Time::HiRes module (available from
CPAN) 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] + $done[1] )
-
($start[0] + $start[1] );
How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
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 the perlmod manpage 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);
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 the Signals entry in the perlipc manpage or the section on ``Signals''
in the Camel Book.
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.
Why doesn't my sockets program work under System V (Solaris)? What does
the error message "Protocol not supported" mean?
Some Sys-V based systems, notably Solaris 2.X, redefined some of the
standard socket constants. Since these were constant across all
architectures, they were often hardwired into perl code. The proper way to
deal with this is to "use Socket" to get the correct values.
Note that even though SunOS and Solaris are binary compatible, these values
are different. Go figure.
How can I call my system's unique C functions from Perl?
In most cases, you write an external module to do it--see the answer to
"Where can I learn about linking C with Perl? [h2xs, xsubpp]". However, if
the function is a system call, and your system supports 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.
Where do I get the include files to do ioctl() or syscall()?
Historically, these would be generated by the h2ph tool, part of the
standard perl distribution. This program converts cpp(1) 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 the ExtUtils::MakeMaker manpage
for more information (in brief, just use make perl instead of a plain make
to rebuild perl with a new static extension).
Why do setuid perl scripts complain about kernel problems?
Some operating systems have bugs in the kernel that make setuid scripts
inherently insecure. Perl gives you a number of options (described in the
perlsec manpage) to work around such systems.
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 the IPC::Open2 manpage). See the Bidirectional Communication with
Another Process entry in the perlipc manpage and the Bidirectional
Communication with Yourself entry in the perlipc manpage
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 the IPC::Open3 manpage).
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 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.
$exit_status = system("mail-users");
$output_string = `ls`;
How can I capture STDERR from an external command?
There are three basic ways of running external commands:
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 STDOUT and STDERR, unless the system() 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(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:
$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.
Why doesn't open() return an error when a pipe open fails?
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 the perlipc manpage.
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 shell could be
successfully started...it's best to avoid shell metacharacters.
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.
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 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:
`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). You forgot to check "$?" to see whether the program even
ran correctly, too. Even if you wrote
print `cat /etc/termcap`;
this code could and probably should be written as
system("cat /etc/termcap") == 0
or die "cat program failed!";
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.
How can I call backticks without shell processing?
This is a bit tricky. Instead of writing
@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.
Further examples of this can be found in the Safe Pipe Opens entry in the
perlipc manpage.
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.
Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on
MS-DOS)?
Some stdio's set error and eof flags that need clearing. The POSIX module
defines clearerr() that you can use. That is the technically correct way
to do it. Here are some less reliable workarounds:
1 Try keeping around the seekpointer and go there, like this:
$where = tell(LOG);
seek(LOG, $where, 0);
2 If that doesn't work, try seeking to a different part of the file and
then back.
3 If that doesn't work, try seeking to a different part of the file,
reading something, and then seeking back.
4 If that doesn't work, give up on your stdio package and use sysread.
How can I convert my shell script to perl?
Learn Perl and rewrite it. Seriously, there's no simple converter. Things
that are awkward to do in the shell are easy to do in Perl, and this very
awkwardness is what would make a shell->perl converter nigh-on impossible
to write. By rewriting it, you'll think about what you're really trying to
do, and hopefully will escape the shell's pipeline datastream paradigm,
which while convenient for some matters, causes many inefficiencies.
Can I use perl to run a telnet or ftp session?
Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
CPAN). http://www.perl.com/CPAN/scripts/netstuff/telnet.emul.shar will
also help for emulating the telnet protocol, but Net::Telnet is quite
probably easier to use..
If all you want to do is pretend to be telnet but don't need the initial
telnet handshaking, then the standard dual-process approach will suffice:
use IO::Socket; # new in 5.004
$handle = IO::Socket::INET->new('www.perl.com:80')
|| die "can't connect to port 80 on www.perl.com: $!";
$handle->autoflush(1);
if (fork()) { # XXX: undef means failure
select($handle);
print while <STDIN>; # everything from stdin to socket
} else {
print while <$handle>; # everything from socket to stdout
}
close $handle;
exit;
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. If you find it
somewhere, 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.
Is there a way to hide perl's command line from programs such as "ps"?
First of all note that if you're doing this for security reasons (to avoid
people seeing passwords, for example) then you should rewrite your program
so that critical information is never given as an argument. Hiding the
arguments won't make your program completely secure.
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]";
I {changed directory, modified my environment} in a perl script. How come
the change disappeared when I exited the script? How do I get my changes
to be visible?
Unix
In the strictest sense, it can't be done--the script executes as a
different process from the shell it was started from. Changes to a
process are not reflected in its parent--only in any 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.
How do I close a process's filehandle without waiting for it to complete?
Assuming your system supports such things, just send an appropriate signal
to the process (see the kill entry in the perlfunc manpage). It's common
to first send a TERM signal, wait a little bit, and then send a KILL signal
to finish it off.
How do I fork a daemon process?
If by daemon process you mean one that's detached (disassociated from its
tty), then the following process is reported to work on most Unixish
systems. Non-Unix users should check their Your_OS::Process module for
other solutions.
· Open /dev/tty and use the TIOCNOTTY ioctl on it. See tty(4) for
details. Or better yet, you can just use the POSIX:\fIs0:setsid()
function, so you don't have to worry about process groups.
· Change directory to /
· Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
tty.
· Background yourself like this:
fork && exit;
The Proc::Daemon module, available from CPAN, provides a function to
perform these actions for you.
How do I find out if I'm running interactively or not?
Good question. Sometimes "-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(fileno(*TTY));
$pgrp = getpgrp();
if ($tpgrp == $pgrp) {
print "foreground\n";
} else {
print "background\n";
}
How do I timeout a slow event?
Use the alarm() function, probably in conjunction with a signal handler, as
documented in the Signals entry in the perlipc manpage and the section on
``Signals'' in the Camel. You may instead use the more flexible
Sys::AlarmCall module available from CPAN.
How do I set CPU limits?
Use the BSD::Resource module from CPAN.
How do I avoid zombies on a Unix system?
Use the reaper code from the Signals entry in the perlipc manpage to call
wait() when a SIGCHLD is received, or else use the double-fork technique
described in the fork entry in the perlfunc manpage.
How do I use an SQL database?
There are a number of excellent interfaces to SQL databases. See the
DBD::* modules available from http://www.perl.com/CPAN/modules/DBD . A lot
of information on this can be found at
http://www.symbolstone.org/technology/perl/DBI/
How do I make a system() exit on control-C?
You can't. You need to imitate the 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. Or you can check for it:
$rc = system($cmd);
if ($rc & 127) { die "signal death" }
How do I open a file without blocking?
If you're lucky enough to be using a system that supports non-blocking
reads (most Unixish systems do), you need only to use the O_NDELAY or
O_NONBLOCK flag from the Fcntl module in conjunction with sysopen():
use Fcntl;
sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
or die "can't open /tmp/somefile: $!":
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:
1 Unpack the source into a temporary area.
2
perl Makefile.PL
3
make
4
make test
5
make install
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 the ExtUtils::MakeMaker manpage for more details on building
extensions. See also the next question, ``What's the difference between
require and use?''.
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 $file.
3) require Module is like require "Module.pm", except the former
3.1: translates each "::" into your system's directory separator.
3.2: primes the parser to disambiguate class Module as an indirect object.
4) use Module is like require Module, except the former
4.1: loads the module at compile time, not run-time.
4.2: imports symbols and semantics from that package to the current one.
In general, you usually want "use" and a proper Perl module.
How do I keep my own module/library directory?
When you build modules, use the PREFIX option when generating Makefiles:
perl Makefile.PL PREFIX=/u/mydir/perl
then either set the PERL5LIB environment variable before you run scripts
that use the modules/libraries (see the perlrun manpage) or say
use lib '/u/mydir/perl';
This is almost the same as
BEGIN {
unshift(@INC, '/u/mydir/perl');
}
except that the lib module checks for machine-dependent subdirectories.
See Perl's the lib manpage for more information.
How do I add the directory my program lives in to the module/library search
path?
use FindBin;
use lib "$FindBin::Bin";
use your_own_modules;
How do I add a directory to my include path at runtime?
Here are the suggested ways of modifying your include path:
the PERLLIB environment variable
the PERL5LIB environment variable
the perl -Idir command line flag
the use lib pragma, as in
use lib "$ENV{HOME}/myown_perllib";
The latter is particularly useful because it knows about machine dependent
architectures. The lib.pm pragmatic module was first included with the
5.002 release of Perl.
What is socket.ph and where do I get it?
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 "use Socket;" instead.
AUTHOR AND COPYRIGHT
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 its
complete documentation whether printed or otherwise, this work may be
distributed only under the terms of Perl's Artistic License. Any
distribution of this file or derivatives thereof outside of that package
require that special arrangements be made with copyright holder.
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.
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