PERLFAQ8(1perl)

📦

version 5.44.0 · 2 versions

5.44.0 · arch rolling (latest)
5.42.3 · arch rolling

💖 perlfaq8 — System Interaction (yes bestie, we're going that deep) 💖

okay so this is part of the Perl FAQ, specifically the system interaction chapter, and it's giving very much "talking to your operating system" energy. 💅

✨ VERSION ✨

version 5.20250619 — that's the one, no cap 🫶

🌸 DESCRIPTION 🌸

This lil slice of the Perl FAQ is all about questions where you're interacting with the operating system. Topics it covers: interprocess communication (IPC), controlling the user interface (keyboard, screen, pointing devices), and basically everything else that isn't about wrangling data. 🎀

Also, fr fr, go read the FAQs and docs that are specific to the port of perl you're running on your OS (like perlvms, perlplan9, ...). Those have the detailed tea on all the weird little quirks of your particular perl. 👑

🦋 How do I find out which operating system I'm running under?

The $^O variable — or $OSNAME if you use English — holds an indication of the name of the operating system that your perl binary was built for. Note: the name, not the release number. iykyk ⭐

💀 Why does `exec()` not return?

(contributed by brian d foy)

Because that's literally its whole job, bestie. The exec function turns your process into another command and never comes back. If that's not the vibe you're going for, don't use exec. :) 🫠

If you want to run an external command and keep your Perl process alive and thriving, look at a piped open, fork, or system instead. ✨

👑 How do I do fancy stuff with the keyboard/screen/mouse?

How you access and control keyboards, screens, and pointing devices ("mice") is fully system-dependent. Try these 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
Wx — CPAN
Gtk2 — CPAN
Qt4 — kdebindings4 package

Some of these specific cases show up as examples in the other answers in this section of the perlfaq. 🌈

🎀 How do I print something out in color?

In general? You don't. Because you genuinely don't know whether the person on the other end has a color-aware display device. Rude to assume, honestly. 🙈

But if you do know 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 remarkably system-dependent, like it's honestly a mess. On many systems you can just use the stty command as shown in "getc" in perlfunc, but as you can see, that's already dragging 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 every single key. It even includes limited support for Windows. ✨

use Term::ReadKey;
ReadMode('cbreak');
$key = ReadKey(0);
ReadMode('normal');

However — and this is the catch — using that code requires you to have a working C compiler and to be able to use it to build and install a CPAN module. So 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 needed to manipulate the POSIX termios structures:

# HotKey.pm
package HotKey;

use strict;
use warnings;

use parent 'Exporter';
our @EXPORT = qw(cbreak cooked readkey);

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?

Easiest way: read a key in nonblocking mode with the Term::ReadKey module from CPAN, and pass it an argument of -1 to say "please do not block, I'm busy":

use Term::ReadKey;

ReadMode('cbreak');

if (defined (my $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?

(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 whenever you want the screen cleared. Simple as. ✨

You can use the Term::ANSIScreen module to get that 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;

my $terminal = Term::Cap->Tgetent( { OSPEED => 9600 } );
my $clear_screen = $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;

my $OUT = Win32::Console->new(STD_OUTPUT_HANDLE);
$OUT->Cls;

And 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:

my $clear_string = `clear`;

print $clear_string;

🍓 How do I get the screen size?

If you have the Term::ReadKey module installed from CPAN, you can use it to fetch the width and height in characters and in pixels:

use Term::ReadKey;
my ($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(my $tty_fh, "+</dev/tty")                     or die "No tty: $!";
unless (ioctl($tty_fh, &TIOCGWINSZ, $winsize='')) {
    die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ;
}
my ($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, bestie. Different FAQ for that one.)

There's an example of this in "crypt" in perlfunc. First you put the terminal into "no echo" mode, then you 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 on most systems using the Term::ReadKey module from CPAN, which is easier to use and in theory more portable:

use Term::ReadKey;

ReadMode('noecho');
my $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 are common to all device interaction: ⬇️

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 "\015" and "\012". 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's portable between Unix, DOS/Win, and Macintosh — except to terminate ALL line ends with "\015\012", and then strip whatever you don't need from the output. This applies especially to socket I/O and autoflushing, which we're talking about next. 👀

flushing output 🚿

If you expect characters to actually reach 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?"):

my $old_handle = select($dev_fh);
$| = 1;
select($old_handle);

You'll also see code that does this without a temporary variable, as in

select((select($deb_handle), $| = 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_fh->autoflush(1);

...and yeah, like we said up there ^^, this still doesn't work when you're doing socket I/O between Unix and Macintosh. In that case you gotta hard-code your line terminators. No wiggle room, bestie. 🙈

non-blocking input

If you're doing a blocking read() or sysread(), you're gonna have to set up an alarm handler to give you a timeout (peep "alarm" in perlfunc). 🫶 But if your open is non-blocking, your read probably is too, which means you might have to bust out the 4-arg select() to figure out whether I/O is actually 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?

The iconic move here is: you spend lots and lots of money on dedicated hardware. But bestie, that's bound to get you talked about. 💅

Okay no fr fr: you can't, not if they're Unix password files. The Unix password system uses one-way encryption — it's way more like hashing than encryption. The most you can do is check whether something else hashes to the same string. You literally cannot turn a hash back into the original string. 🙈 Programs like Crack can forcibly (and intelligently) try to guess passwords, but they don't (and can't) guarantee quick success.

If you're stressed about your users picking terrible passwords, you should proactively check when they try to change their password (by modifying passwd(1), for example). Prevention queen behavior. 👑

🌈 How do I start a process in the background?

(contributed by brian d foy)

There's no 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 a bunch of these techniques are covered in perlipc.

Several CPAN modules might help you out, 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're on a Unix-like system, you might be able to get away with a system call where you slap a "&" on the end of the command:

system("cmd &")

You can also try using fork, as described in perlfunc (although this is the same thing many of the modules will do for you).

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 (and 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?

Plot twist: you don't actually "trap" a control character. ✨ Instead, that character generates a signal, which gets sent to your terminal's currently foregrounded process group, and that's what you 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";

Now for the lore 📜: Perl versions before 5.8 had, in their C source code, signal handlers that would catch the signal and possibly run a Perl function you'd 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's being caught. Previous versions of this answer were incorrect. We said what we said. 🙈

🔐 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, give you (read-only) access to entries in the shadow password file. To actually change the file, you make a brand new shadow password file (the format varies from system to system — see passwd(1) 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 with 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 wanna 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). Bestie that's a glow-up fr. ✨

🦋 How can I measure time under a second?

(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're on a Unixish system, you may be able to call gettimeofday(2) directly. See "syscall" in perlfunc.

💖 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)

You can use the "END" block to simulate atexit(). Each package's "END" block is called when the program or thread ends. See the 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: $!";
}

Heads up bestie: 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. 🫶

💀 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 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. Slay. 💅

🧚 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 be hand-edited. ✍️

Here's how to install the *.ph files:

1. Become the 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.

okay so if your system literally cannot do dynamic loading, you should still probably be using h2xs. like, don't skip it. perlxstut and ExtUtils::MakeMaker have the full tea, but the tl;dr: run make perl instead of a plain make when you wanna rebuild perl with your new static extension. that's it, that's the whole vibe 💅

✨ Why do setuid perl scripts complain about kernel problems? 💀

bestie, some operating systems have actual bugs in the kernel that make setuid scripts inherently insecure — and no amount of clever scripting fixes that. perl's got your back though: it gives you a whole menu of options (all written up in perlsec) so you can work around those cursed systems. go read up. 🫶

🦋 How can I open a pipe both to and from a command?

IPC::Open2! it ships with standard perl, it's easy to use, and under the hood it does the whole pipe(), fork(), and exec() dance for you. 😌

but fr fr — read the deadlock warnings in its docs first. like, actually read them. see IPC::Open2.

also check out "Bidirectional Communication with Another Process" in perlipc, and "Bidirectional Communication with Yourself" in perlipc. yes, both. 💖

you can also use IPC::Open3 (also standard perl), but ⚠️ heads up: its argument order is different from IPC::Open2 (see IPC::Open3). do not get it twisted.

🌸 Why can't I get the output of a command with system()?

oh no bestie, you're mixing up what system() and backticks (``) are for. they're two different girls doing two different jobs:

system() runs a command and hands you back exit status info — 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 hand you back what it sent to STDOUT.

    my $exit_status   = system("mail-users");
    my $output_string = `ls`;

🌈 How can I capture STDERR from an external command?

okay so there are three basic ways of running external commands:

    system $cmd;        # using system()
    my $output = `$cmd`;        # using backticks (``)
    open (my $pipe_fh, "$cmd |");    # using open()

with system(), both STDOUT and STDERR go wherever the script's STDOUT and STDERR go — unless, of course, the system() command redirects them somewhere else. backticks and open() only read STDOUT of your command. that's the whole ballgame. 🎀

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;
    my $in = '';
    open(NULL, ">", File::Spec->devnull);
    my $pid = open3($in, \*PH, ">&NULL", "cmd");
    while( <PH> ) { }
    waitpid($pid, 0);

To capture a program's STDERR, but discard its STDOUT:

    use IPC::Open3;
    use File::Spec;
    my $in = '';
    open(NULL, ">", File::Spec->devnull);
    my $pid = open3($in, ">&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;
    my $in = '';
    my $pid = open3($in, ">&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 IO::File;
    my $in = '';
    local *CATCHOUT = IO::File->new_tmpfile;
    local *CATCHERR = IO::File->new_tmpfile;
    my $pid = open3($in, ">&CATCHOUT", ">&CATCHERR", "cmd");
    waitpid($pid, 0);
    seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
    while( <CATCHOUT> ) {}
    while( <CATCHERR> ) {}

but lowkey there's no real need for both of them to be tempfiles... the following should work just as well, and without deadlocking:

    use IPC::Open3;
    my $in = '';
    use IO::File;
    local *CATCHERR = IO::File->new_tmpfile;
    my $pid = open3($in, \*CATCHOUT, ">&CATCHERR", "cmd");
    while( <CATCHOUT> ) {}
    waitpid($pid, 0);
    seek CATCHERR, 0, 0;
    while( <CATCHERR> ) {}

and it'll be faster, too, since we can start processing the program's stdout immediately instead of sitting around waiting for the program to finish. ⚡

with any of these, you can switch up 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 does not work:

    open(STDERR, ">&STDOUT");
    $alloutput = `cmd args`;  # stderr still escapes

why does it flop? because the open() makes STDERR go to where STDOUT was going at the time of the open(). then the backticks make STDOUT go to a string, but they don't change STDERR (which is still going to the old STDOUT). messy. 😭

✨ also note: 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. yeah. the order is the whole point, iykyk. 👑

🧚 Why doesn't open() return an error when a pipe open fails?

okay so if the second argument to a piped open() has shell metacharacters in it, perl fork()s and then exec()s a shell to decode those metacharacters and eventually run the program you actually wanted. so if the program couldn't be run, it's the shell that gets the message — not Perl. the only thing 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 though. see "How can I capture STDERR from an external command?" elsewhere in this document, or just use the IPC::Open3 module. 💖

and if there are no shell metacharacters in the argument of open(), perl runs the command directly, no shell involved, and can correctly report whether the command started. periodt.

🍓 What's wrong with using backticks in a void context?

strictly speaking? nothing. 😌

stylistically speaking? bestie... it's just 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 so you can use it in your program. system is another — and it does not do that.

writing backticks in your program sends a very clear message to whoever's reading your code that you wanted to collect the output of the command. so why would you 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. and 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 completely finished to print it out. plus it checks the return value. two birds, one slay. 🐦🔥

also! system gives you direct control over whether shell wildcard processing may take place, whereas backticks do not.

🎀 How can I call backticks without shell processing?

ngl this one's a bit tricky. you can't just casually 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 do it like 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. more examples of this live in "Safe Pipe Opens" in perlipc.

💀 and a heads-up for the Windows girlies: if you're using Windows, no solution to this vexing issue is even possible. even though Perl emulates fork(), you'll still be stuck, because Windows does not have an argc/argv-style API. that's just the way it is. 😭

🦋 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?

(Ctrl+D on Unix, Ctrl+Z on MS-DOS — that's your ^D and ^Z babes 💖)

okay so this only happens if your perl is compiled to use stdio instead of perlio — and perlio is the default, so if you're hitting this, something's up. some (maybe all?) stdios set error and eof flags that you may need to clear. the POSIX module defines clearerr() that you can use for exactly this. that is the technically correct way to do it. ✨

here are some less reliable workarounds, in order of "eh, maybe":

try keeping around the seekpointer and go there, like this:

    my $where = tell($log_fh);
    seek($log_fh, $where, 0);

if that doesn't work, try seeking to a different part of the file and then back.
if that doesn't work, try seeking to a different part of the file, reading something, and then seeking back.
if that doesn't work, give up on your stdio package and use sysread. 🏳️

How can I convert my shell script to perl? 🐚➡️🐪

Bestie. Learn Perl and rewrite it. Seriously — there's no simple converter, and I need you to accept that. 😤

Here's the tea: 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 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
my $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;

How can I write expect in Perl? 🧚

Story time, babes. 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. iconic trio honestly 💅

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. No cap.

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]";

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 "kill" in perlfunc). It's common to first send a TERM signal, wait a little bit, and then send a KILL signal to finish it off. polite, then unhinged ✨

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::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. life saver fr fr 🫶

How do I find out if I'm running interactively or not? 🤔

(contributed by brian d foy)

This is a difficult question to answer, and the best answer is only a guess. 💀

What do you really want to know? If you merely want to know if one of your filehandles is connected to a terminal, you can try the "-t" file test:

if( -t STDOUT ) {
    print "I'm connected to a terminal!\n";
}

However, you might be out of luck if you expect that means there is a real person on the other side. With the Expect module, another program can pretend to be a person. The program might even come close to passing the Turing test. 👀

The IO::Interactive module does the best it can to give you an answer. Its "is_interactive" function returns an output filehandle; that filehandle points to standard output if the module thinks the session is interactive. Otherwise, the filehandle is a null handle that simply discards the output:

use IO::Interactive;

print { is_interactive } "I might go to standard output!\n";

This still doesn't guarantee that a real person is answering your prompts or reading your output. 😭

If you want to know how to handle automated testing for your distribution, you can check the environment. The CPAN Testers, for instance, set the value of "AUTOMATED_TESTING":

unless( $ENV{AUTOMATED_TESTING} ) {
    print "Hello interactive tester!\n";
}

How do I timeout a slow event? ⏰

Use the alarm() function, probably in conjunction with a signal handler, as documented in "Signals" in perlipc and the section on "Signals" in the Camel. You may instead use the more flexible Sys::AlarmCall module available from CPAN. ✨

Heads up: the alarm() function is not implemented on all versions of Windows. Check the documentation for your specific version of Perl.

How do I set CPU limits? 🖥️

(contributed by Xho)

Use the BSD::Resource module from CPAN. As an example:

use BSD::Resource;
setrlimit(RLIMIT_CPU,10,20) or die $!;

This sets the soft and hard limits to 10 and 20 seconds, respectively. After 10 seconds of time spent running on the CPU (not "wall" time), the process will be sent a signal (XCPU on some systems) which, if not trapped, will cause the process to terminate. If that signal is trapped, then after 10 more seconds (20 seconds in total) the process will be killed with a non-trappable signal. 💀

See the BSD::Resource and your systems documentation for the gory details. 🩸

How do I avoid zombies on a Unix system? 🧟

Use the reaper code from "Signals" in perlipc to call wait() when a SIGCHLD is received, or else use the double-fork technique described in "How do I start a process in the background?" in perlfaq8. slay the undead 👑

How do I use an SQL database? 🗄️

The DBI module provides an abstract interface to most database servers and types, including Oracle, DB2, Sybase, mysql, Postgresql, ODBC, and flat files. The DBI module accesses each database type through a database driver, or DBD. You can see a complete list of available drivers on CPAN: http://www.cpan.org/modules/by-module/DBD/ . You can read more about DBI on http://dbi.perl.org/ . 🌸

Other modules provide more specific access: Win32::ODBC, Alzabo, "iodbc", and others found on CPAN Search: https://metacpan.org/ . ✨

How do I make a system() exit on control-C? 😱

You can't. 💅 You need to imitate the system() call (see perlipc 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(my $fh, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
    or die "can't open /foo/somefile: $!";

How do I tell the difference between errors from the shell and perl? 🤨

(answer contributed by brian d foy)

When you run a Perl script, something else is running the script for you, and that something else may output error messages. The script might emit its own warnings and error messages. Most of the time you cannot tell who said what. mystery! 🔍

You probably cannot fix the thing that runs perl, but you can change how perl outputs its warnings by defining a custom warning and die functions.

Consider this script, which has an error you may not notice immediately. 🙈

#!/usr/locl/bin/perl

print "Hello World\n";

I get an error when I run this from my shell (which happens to be bash). That may look like perl forgot it has a print() function, but my shebang line is not the path to perl, so the shell runs the script, and I get the error.

$ ./test
./test: line 3: print: command not found

A quick and dirty fix involves a little bit of code, but this may be all you need to figure out the problem. ✨

#!/usr/bin/perl -w

BEGIN {
    $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; };
    $SIG{__DIE__}  = sub{ print STDERR "Perl: ", @_; exit 1};
}

$a = 1 + undef;
$x / 0;
__END__

The perl message comes out with "Perl" in front. The "BEGIN" block works at compile time so all of the compilation errors and warnings get the "Perl:" prefix too. 🎀

Perl: Useless use of division (/) in void context at ./test line 9.
Perl: Name "main::a" used only once: possible typo at ./test line 8.
Perl: Name "main::x" used only once: possible typo at ./test line 9.
Perl: Use of uninitialized value in addition (+) at ./test line 8.
Perl: Use of uninitialized value in division (/) at ./test line 9.
Perl: Illegal division by zero at ./test line 9.
Perl: Illegal division by zero at -e line 3.

If I don't see that "Perl:", it's not from perl. 💅

You could also just know all the perl errors, and although there are some people who may know all of them, you probably don't. However, they all should be in the perldiag manpage. If you don't find the error in there, it probably isn't a perl error. 🔥

Looking up every message is not the easiest way, so let perl do it for you. Use the diagnostics pragma, which turns perl's normal messages into longer discussions on the topic.

use diagnostics;

If you don't get a paragraph or two of expanded discussion, it might not be perl's message. 👑

How do I install a module from CPAN? 📦

(contributed by brian d foy)

The easiest way is to have a module also named CPAN do it for you by using the "cpan" command that comes with Perl. You can give it a list of modules to install:

$ cpan IO::Interactive Getopt::Whatever

If you prefer "CPANPLUS", it's just as easy:

$ cpanp i IO::Interactive Getopt::Whatever

so like if you're trying to install a distribution from the current directory, you can tell CPAN.pm to install "." (the full stop):

$ cpan .

go read the docs for either of those commands to see what else you can do, bestie 💖

If you wanna try to install a distribution by yourself, resolving all dependencies on your own, you follow one of two possible build paths.

For distributions that use Makefile.PL:

$ perl Makefile.PL
$ make test install

For distributions that use Build.PL:

$ perl Build.PL
$ ./Build test
$ ./Build install

Some distributions may need to link to libraries or other third-party code and their build and installation sequences may be more complicated. Check any README or INSTALL files that you may find. 👀

💖 What's the difference between `require` and `use`? 💖

(contributed by brian d foy)

Perl runs require statement at run-time. Once Perl loads, compiles, and runs the file, it doesn't do anything else. The use statement is the same as a require run at compile-time, but Perl also calls the import method for the loaded package. These two are the same:

use MODULE qw(import list);

BEGIN {
    require MODULE;
    MODULE->import(import list);
}

However, you can suppress the import by using an explicit, empty import list. Both of these still happen at compile-time:

use MODULE ();

BEGIN {
    require MODULE;
}

Since use will also call the import method, the actual value for MODULE must be a bareword. That is, use cannot load files by name, although require can:

require "$ENV{HOME}/lib/Foo.pm"; # no @INC searching!

See the entry for use in perlfunc for more details.

🌸 How do I keep my own module/library directory? 🌸

When you build modules, tell Perl where to install the modules.

If you want to install modules for your own use, the easiest way might be local::lib, which you can download from CPAN. It sets various installation settings for you, and uses those same settings within your programs. 🫶

If you want more flexibility, you need to configure your CPAN client for your particular situation.

For Makefile.PL-based distributions, use the INSTALL_BASE option when generating Makefiles:

perl Makefile.PL INSTALL_BASE=/mydir/perl

You can set this in your CPAN.pm configuration so modules automatically install in your private library directory when you use the CPAN.pm shell:

% cpan
cpan> o conf makepl_arg INSTALL_BASE=/mydir/perl
cpan> o conf commit

For Build.PL-based distributions, use the --install_base option:

perl Build.PL --install_base /mydir/perl

You can configure CPAN.pm to automatically use this option too:

% cpan
cpan> o conf mbuild_arg "--install_base /mydir/perl"
cpan> o conf commit

INSTALL_BASE tells these tools to put your modules into /mydir/perl/lib/perl5. See "How do I add a directory to my include path (@INC) at runtime?" for details on how to run your newly installed modules.

There is one caveat with INSTALL_BASE, though, since it acts differently from the PREFIX and LIB settings that older versions of ExtUtils::MakeMaker advocated. INSTALL_BASE does not support installing modules for multiple versions of Perl or different architectures under the same directory. You should consider whether you really want that and, if you do, use the older PREFIX and LIB settings. See the ExtUtils::MakeMaker documentation for more details.

bestie, that caveat is giving "read it twice" energy 💅

🦋 How do I add the directory my program lives in to the module/library search path? 🦋

(contributed by brian d foy)

If you know the directory already, you can add it to @INC as you would for any other directory. You might use lib if you know the directory at compile time:

use lib $directory;

The trick in this task is to find the directory. Before your script does anything else (such as a chdir), you can get the current working directory with the Cwd module, which comes with Perl:

BEGIN {
    use Cwd;
    our $directory = cwd;
}

use lib $directory;

You can do a similar thing with the value of $0, which holds the script name. That might hold a relative path, but rel2abs can turn it into an absolute path. Once you have the:

BEGIN {
    use File::Spec::Functions qw(rel2abs);
    use File::Basename qw(dirname);

    my $path   = rel2abs( $0 );
    our $directory = dirname( $path );
}

use lib $directory;

The FindBin module, which comes with Perl, might work. It finds the directory of the currently running script and puts it in $Bin, which you can then use to construct the right library path:

use FindBin qw($Bin);

You can also use local::lib to do much of the same thing. Install modules using local::lib's settings then use the module in your program:

 use local::lib; # sets up a local lib at ~/perl5

See the local::lib documentation for more details.

🌈 How do I add a directory to my include path (`@INC`) at runtime? 🌈

Here are the suggested ways of modifying your include path, including environment variables, run-time switches, and in-code statements:

the PERLLIB environment variable:

$ export PERLLIB=/path/to/my/dir
$ perl program.pl

the PERL5LIB environment variable:

$ export PERL5LIB=/path/to/my/dir
$ perl program.pl

the perl -Idir command line flag:

$ perl -I/path/to/my/dir program.pl

the lib pragma:

use lib "$ENV{HOME}/myown_perllib";

the local::lib module:

use local::lib;

use local::lib "~/myown_perllib";

⭐ Where are modules installed? ⭐

Modules are installed on a case-by-case basis (as provided by the methods described in the previous section), and in the operating system. All of these paths are stored in @INC, which you can display with the one-liner

perl -e 'print join("\n",@INC,"")'

The same information is displayed at the end of the output from the command

perl -V

To find out where a module's source code is located, use

perldoc -l Encode

to display the path to the module. In some cases (for example, the AutoLoader module), this command will show the path to a separate pod file; the module itself should be in the same directory, with a 'pm' file extension.

💀 What is `socket.ph` and where do I get it? 💀

It's a Perl 4 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 should use use Socket; instead.

👑 AUTHOR AND COPYRIGHT 👑

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.

so yeah, that's the whole vibe: this page is from perl v5.44.0, dated 2026-10-17, and it's PERLFAQ8(1perl) — stay iconic, bestie 💅✨

📦 where this page comes from

arch rolling: perl 5.44.0-1 · Artistic-1.0-Perl AND GPL-1.0-or-later
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms

PERLFAQ8(1perl)

💖 perlfaq8 — System Interaction (yes bestie, we're going that deep) 💖

✨ VERSION ✨

🌸 DESCRIPTION 🌸

🦋 How do I find out which operating system I'm running under?

💀 Why does exec() not return?

👑 How do I do fancy stuff with the keyboard/screen/mouse?

🎀 How do I print something out in color?

🧚 How do I read just one key without waiting for a return key?

💅 How do I check whether input is ready on the keyboard?

⭐ How do I clear the screen?

🍓 How do I get the screen size?

🧁 How do I ask the user for a password?

🦄 How do I read and write the serial port?

non-blocking input

💀 How do I decode encrypted password files?

🌈 How do I start a process in the background?

STDIN, STDOUT, and STDERR are shared

Signals

Zombies 🧟

🎀 How do I trap control characters/signals?

🔐 How do I modify the shadow password file on a Unix system?

⏰ How do I set the time and date?

🌸 How can I sleep() or alarm() for under a second?

🦋 How can I measure time under a second?

💖 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)

💀 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?

👑 How can I call my system's unique C functions from Perl?

🧚 Where do I get the include files to do ioctl() or syscall()?

✨ Why do setuid perl scripts complain about kernel problems? 💀

🦋 How can I open a pipe both to and from a command?

🌸 Why can't I get the output of a command with system()?

🌈 How can I capture STDERR from an external command?

🧚 Why doesn't open() return an error when a pipe open fails?

🍓 What's wrong with using backticks in a void context?

🎀 How can I call backticks without shell processing?

🦋 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?

How can I convert my shell script to perl? 🐚➡️🐪

Can I use perl to run a telnet or ftp session? 📡

How can I write expect in Perl? 🧚

Is there a way to hide perl's command line from programs such as "ps"? 🕵️‍♀️

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

How do I close a process's filehandle without waiting for it to complete? 🔪

How do I fork a daemon process? 👻

How do I find out if I'm running interactively or not? 🤔

How do I timeout a slow event? ⏰

How do I set CPU limits? 🖥️

How do I avoid zombies on a Unix system? 🧟

How do I use an SQL database? 🗄️

How do I make a system() exit on control-C? 😱

How do I open a file without blocking? 🚪

How do I tell the difference between errors from the shell and perl? 🤨

How do I install a module from CPAN? 📦

💖 What's the difference between require and use? 💖

🌸 How do I keep my own module/library directory? 🌸

🦋 How do I add the directory my program lives in to the module/library search path? 🦋

🌈 How do I add a directory to my include path (@INC) at runtime? 🌈

⭐ Where are modules installed? ⭐

💀 What is socket.ph and where do I get it? 💀

👑 AUTHOR AND COPYRIGHT 👑

💀 Why does `exec()` not return?

💖 What's the difference between `require` and `use`? 💖

🌈 How do I add a directory to my include path (`@INC`) at runtime? 🌈

💀 What is `socket.ph` and where do I get it? 💀