perlfaq8 - System Interaction
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.
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.
(contributed by brian d foy)
The exec 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 exec.
:)
If you want to run an external command and still keep your Perl process going,
look at a piped open,
fork,
or system.
How you access/control keyboards, screens, and pointing devices ("mice") is system-dependent. Try the following modules:
Term::Cap Standard perl distribution
Term::ReadKey CPAN
Term::ReadLine::Gnu CPAN
Term::ReadLine::Perl CPAN
Term::Screen CPAN
Term::Cap Standard perl distribution
Curses CPAN
Term::ANSIColor CPAN
Tk CPAN
Some of these specific cases are shown as examples in other answers in this section of the perlfaq.
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;
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 system (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 $key;
}
END { cooked() }
1;
The easiest way to do this is to read a key in nonblocking mode with the Term::ReadKey module from CPAN, passing it an argument of -1 to indicate not to block:
use Term::ReadKey;
ReadMode('cbreak');
if (defined ($char = ReadKey(-1)) ) {
# input was waiting and it was $char
} else {
# no input was waiting
}
ReadMode('normal'); # restore normal tty settings
(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 Term::ANSIScreen module to get the special sequence. Import the cls function (or the :screen tag):
use Term::ANSIScreen qw(cls);
my $clear_screen = cls();
print $clear_screen;
The Term::Cap module can also get the special sequence if you want to deal with the low-level details of terminal control. The Tputs 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 Win32::Console module. After creating an object for the output filehandle you want to affect, call the Cls 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 $clear_string;
If you have Term::ReadKey module installed from CPAN, you can use it to fetch the width and height in characters and in pixels:
use Term::ReadKey;
($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";
(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);
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:
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.
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.
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.
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:
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.
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";
}
}
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).
(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 perlipc.
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 & on the end of the command:
system("cmd &")
You can also try using fork, as described in perlfunc (although this is the same thing that many of the modules will do for you).
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 opening a pipe (see "open" in perlfunc) but on some systems this means that the child process cannot outlive the parent.
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&").
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 &").
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.
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).
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";
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).
(contributed by brian d foy)
The Time::HiRes module (part of the standard distribution as of Perl 5.8) measures time with the gettimeofday() system call, which returns the time in microseconds since the epoch. If you can't install Time::HiRes for older Perls and you are on a Unixish system, you may be able to call gettimeofday(2) directly. See "syscall" in perlfunc.
You can use the END block to simulate atexit(). Each package's END block is called when the program or thread ends See perlmod manpage for more details about END blocks.
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 "Signals" in perlipc or the section on "Signals" in Programming Perl.
If exception handling is all you're interested in, use one of the many CPAN modules that handle exceptions, such as Try::Tiny.
If you want the atexit() syntax (and an rmexit() as well), try the AtExit module available from CPAN.
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.
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 perlfunc).
Remember to check the modules that came with your distribution, and CPAN as well--someone may already have written a module to do it. On Windows, try Win32::API. On Macs, try Mac::Carbon. If no module has an interface to the C function, you can inline a bit of C in your Perl source with Inline::C.
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 be 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 perlxstut for how to get started with h2xs.
If your system doesn't support dynamic loading, you still probably ought to use h2xs. See perlxstut and ExtUtils::MakeMaker for more information (in brief, just use make perl instead of a plain make to rebuild perl with a new static extension).
Some operating systems have bugs in the kernel that make setuid scripts inherently insecure. Perl gives you a number of options (described in perlsec) to work around such systems.
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 IPC::Open2). See "Bidirectional Communication with Another Process" in perlipc and "Bidirectional Communication with Yourself" in perlipc
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 IPC::Open3).
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`;
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.
You can also use the open3() function from IPC::Open3. Benjamin Goldberg provides some sample code:
To capture a program's STDOUT, but discard its STDERR:
use IPC::Open3;
use File::Spec;
use Symbol qw(gensym);
open(NULL, ">", File::Spec->devnull);
my $pid = open3(gensym, \*PH, ">&NULL", "cmd");
while( <PH> ) { }
waitpid($pid, 0);
To capture a program's STDERR, but discard its STDOUT:
use IPC::Open3;
use File::Spec;
use Symbol qw(gensym);
open(NULL, ">", File::Spec->devnull);
my $pid = open3(gensym, ">&NULL", \*PH, "cmd");
while( <PH> ) { }
waitpid($pid, 0);
To capture a program's STDERR, and let its STDOUT go to our own STDERR:
use IPC::Open3;
use Symbol qw(gensym);
my $pid = open3(gensym, ">&STDERR", \*PH, "cmd");
while( <PH> ) { }
waitpid($pid, 0);
To read both a command's STDOUT and its STDERR separately, you can redirect them to temp files, let the command run, then read the temp files:
use IPC::Open3;
use Symbol qw(gensym);
use IO::File;
local *CATCHOUT = IO::File->new_tmpfile;
local *CATCHERR = IO::File->new_tmpfile;
my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd");
waitpid($pid, 0);
seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
while( <CATCHOUT> ) {}
while( <CATCHERR> ) {}
But there's no real need for both to be tempfiles... the following should work just as well, without deadlocking:
use IPC::Open3;
use Symbol qw(gensym);
use IO::File;
local *CATCHERR = IO::File->new_tmpfile;
my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd");
while( <CATCHOUT> ) {}
waitpid($pid, 0);
seek CATCHERR, 0, 0;
while( <CATCHERR> ) {}
And it'll be faster, too, since we can begin processing the program's stdout immediately, rather than waiting for the program to finish.
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 the versus/csh.whynot article in the "Far More Than You Ever Wanted To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . 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 to redirect them separately to files, and then read from those files when the program is done:
system("program args 1>program.stdout 2>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.
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. If the program couldn't be run, it's the shell that gets the message, not Perl. All your Perl program can find out is whether the shell itself could be successfully started. You can still capture the shell's STDERR and check it for error messages. See "How can I capture STDERR from an external command?" elsewhere in this document, or use the IPC::Open3 module.
If there are no shell metacharacters in the argument of open(), Perl runs the command directly, without using the shell, and can correctly report whether the command started.
Strictly speaking, nothing. Stylistically speaking, it's not a good way to write maintainable code. Perl has several operators for running external commands. Backticks are one; they collect the output from the command for use in your program. The system function is another; it doesn't do this.
Writing backticks in your program sends a clear message to the readers of your code that you wanted to collect the output of the command. Why send a clear message that isn't true?
Consider this line:
`cat /etc/termcap`;
You forgot to check $? to see whether the program even ran correctly. Even if you wrote
print `cat /etc/termcap`;
this code could and probably should be written as
system("cat /etc/termcap") == 0
or die "cat program failed!";
which will echo the cat command's output as it is generated, instead of waiting until the program has completed to print it out. It also checks the return value.
system also provides direct control over whether shell wildcard processing may take place, whereas backticks do not.
This is a bit tricky. You can't simply write the command like this:
@ok = `grep @opts '$search_string' @filenames`;
As of Perl 5.8.0, you can use open() with multiple arguments. Just like the list forms of system() and exec(), no shell escapes happen.
open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
chomp(@ok = <GREP>);
close GREP;
You can also:
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 "Safe Pipe Opens" in perlipc.
Note that if you're using Windows, no solution to this vexing issue is even possible. Even if Perl were to emulate fork(), you'd still be stuck, because Windows does not have an argc/argv-style API.
This happens only if your perl is compiled to use stdio instead of perlio, which is the default. Some (maybe all?) stdios set error and eof flags that you may need to clear. The POSIX module defines clearerr() that you can use. That is the technically correct way to do it. Here are some less reliable workarounds:
$where = tell(LOG);
seek(LOG, $where, 0);
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.
Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from CPAN). http://www.cpan.org/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')
or 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;
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.
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 perlvar. This won't work on all operating systems, though. Daemon programs like sendmail place their state there, as in:
$0 = "orcus [accepting connections]";
