PERLFAQ8(1) Perl Programmers Reference Guide PERLFAQ8(1)
NAME
perlfaq8 - System Interaction
VERSION
version 5.20240218
DESCRIPTION
This section of the Perl FAQ covers questions involving operating system
interaction. Topics include interprocess communication (IPC), control
over the user-interface (keyboard, screen and pointing devices), and
most anything else not related to data manipulation.
Read the FAQs and documentation specific to the port of perl to your
operating system (eg, perlvms, perlplan9, ...). These should contain
more detailed information on the vagaries of your perl.
How do I find out which operating system I'm running under?
The $^O variable ($OSNAME if you use "English") contains an indication
of the name of the operating system (not its release number) that your
perl binary was built for.
Why does exec() not return?
(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 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
Wx CPAN
Gtk2 CPAN
Qt4 kdebindings4 package
Some of these specific cases are shown as examples in other answers in
this section of the perlfaq.
How do I print something out in color?
In general, you don't, because you don't know whether the recipient has
a color-aware display device. If you know that they have an ANSI
terminal that understands color, you can use the Term::ANSIColor module
from CPAN:
use Term::ANSIColor;
print color("red"), "Stop!\n", color("reset");
print color("green"), "Go!\n", color("reset");
Or like this:
use Term::ANSIColor qw(:constants);
print RED, "Stop!\n", RESET;
print GREEN, "Go!\n", RESET;
How do I read just one key without waiting for a return key?
Controlling input buffering is a remarkably system-dependent matter. On
many systems, you can just use the stty command as shown in "getc" in
perlfunc, but as you see, that's already getting you into portability
snags.
open(TTY, "+</dev/tty") or die "no tty: $!";
system "stty cbreak </dev/tty >/dev/tty 2>&1";
$key = getc(TTY); # perhaps this works
# OR ELSE
sysread(TTY, $key, 1); # probably this does
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
The Term::ReadKey module from CPAN offers an easy-to-use interface that
should be more efficient than shelling out to stty for each key. It
even includes limited support for Windows.
use Term::ReadKey;
ReadMode('cbreak');
$key = ReadKey(0);
ReadMode('normal');
However, using the code requires that you have a working C compiler and
can use it to build and install a CPAN module. Here's a solution using
the standard POSIX module, which is already on your system (assuming
your system supports POSIX).
use HotKey;
$key = readkey();
And here's the "HotKey" module, which hides the somewhat mystifying
calls to manipulate the POSIX termios structures.
# HotKey.pm
package HotKey;
use strict;
use warnings;
use parent 'Exporter';
our @EXPORT = qw(cbreak cooked readkey);
use POSIX qw(:termios_h);
my ($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?
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 (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 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;
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);
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:
my $clear_string = `clear`;
print $clear_string;
How do I get the screen size?
If you have Term::ReadKey module installed from CPAN, you can use it to
fetch the width and height in characters and in pixels:
use Term::ReadKey;
my ($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. 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');
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 common to all device interaction are the following:
lockfiles
Your system may use lockfiles to control multiple access. Make sure
you follow the correct protocol. Unpredictable behavior can result
from multiple processes reading from one device.
open mode
If you expect to use both read and write operations on the device,
you'll have to open it for update (see "open" in perlfunc for
details). You may wish to open it without running the risk of
blocking by using sysopen() and "O_RDWR|O_NDELAY|O_NOCTTY" from the
Fcntl module (part of the standard perl distribution). See "sysopen"
in perlfunc for more on this approach.
end of line
Some devices will be expecting a "\r" at the end of each line rather
than a "\n". In some ports of perl, "\r" and "\n" are different from
their usual (Unix) ASCII values of "\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 is portable
between Unix, DOS/Win, and Macintosh, except to terminate ALL line
ends with "\015\012", and strip what you don't need from the output.
This applies especially to socket I/O and autoflushing, discussed
next.
flushing output
If you expect characters to get to your device when you print()
them, you'll want to autoflush that filehandle. You can use select()
and the $| variable to control autoflushing (see "$|" in perlvar and
"select" in perlfunc, or perlfaq5, "How do I flush/unbuffer an
output filehandle? Why must I do this?"):
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);
As mentioned in the previous item, this still doesn't work when
using socket I/O between Unix and Macintosh. You'll need to hard
code your line terminators, in that case.
non-blocking input
If you are doing a blocking read() or sysread(), you'll have to
arrange for an alarm handler to provide a timeout (see "alarm" in
perlfunc). If you have a non-blocking open, you'll likely have a
non-blocking read, which means you may have to use a 4-arg select()
to determine whether I/O is ready on that device (see "select" in
perlfunc.
While trying to read from his caller-id box, the notorious Jamie
Zawinski "<jwz@netscape.com>", after much gnashing of teeth and fighting
with "sysread", "sysopen", POSIX's "tcgetattr" business, and various
other functions that go bump in the night, finally came up with this:
sub open_modem {
use IPC::Open2;
my $stty = `/bin/stty -g`;
open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
# starting cu hoses /dev/tty's stty settings, even when it has
# been opened on a pipe...
system("/bin/stty $stty");
$_ = <MODEM_IN>;
chomp;
if ( !m/^Connected/ ) {
print STDERR "$0: cu printed `$_' instead of `Connected'\n";
}
}
How do I decode encrypted password files?
You spend lots and lots of money on dedicated hardware, but this is
bound to get you talked about.
Seriously, you can't if they are Unix password files--the Unix password
system employs one-way encryption. It's more like hashing than
encryption. The best you can 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).
How do I start a process in the background?
(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 covered 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).
STDIN, STDOUT, and STDERR are shared
Both the main process and the backgrounded one (the "child" process)
share the same STDIN, STDOUT and STDERR filehandles. If both try to
access them at once, strange things can happen. You may want to
close or reopen these for the child. You can get around this with
"open"ing a pipe (see "open" in perlfunc) but on some systems this
means that the child process cannot outlive the parent.
Signals
You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is
sent when you write to a filehandle whose child process has closed
(an untrapped SIGPIPE can cause your program to silently die). This
is not an issue with system("cmd&").
Zombies
You have to be prepared to "reap" the child process when it
finishes.
$SIG{CHLD} = sub { wait };
$SIG{CHLD} = 'IGNORE';
You can also use a double fork. You immediately wait() for your
first child, and the init daemon will wait() for your grandchild
once it exits.
unless ($pid = fork) {
unless (fork) {
exec "what you really wanna do";
die "exec failed!";
}
exit 0;
}
waitpid($pid, 0);
See "Signals" in perlipc for other examples of code to do this.
Zombies are not an issue with "system("prog &")".
How do I trap control characters/signals?
You don't actually "trap" a control character. Instead, that character
generates a signal which is sent to your terminal's currently
foregrounded process group, which you then trap in your process.
Signals are documented in "Signals" in perlipc and the section on
"Signals" in the Camel.
You can set the values of the %SIG hash to be the functions you want to
handle the signal. After perl catches the signal, it looks in %SIG for a
key with the same name as the signal, then calls the subroutine value
for that key.
# as an anonymous subroutine
$SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) };
# or a reference to a function
$SIG{INT} = \&ouch;
# or the name of the function as a string
$SIG{INT} = "ouch";
Perl versions before 5.8 had in its C source code signal handlers which
would catch the signal and possibly run a Perl function that you had set
in %SIG. This violated the rules of signal handling at that level
causing perl to dump core. Since version 5.8.0, perl looks at %SIG after
the signal has been caught, rather than while it is being caught.
Previous versions of this answer were incorrect.
How do I modify the shadow password file on a Unix system?
If perl was installed correctly and your shadow library was written
properly, the "getpw*()" functions described in perlfunc should in
theory provide (read-only) access to entries in the shadow password
file. To change the file, make a new shadow password file (the format
varies from system to system--see passwd(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 under sufficient permissions, you should be able
to set the system-wide date and time by running the date(1) program.
(There is no way to set the time and date on a per-process basis.) This
mechanism will work for Unix, MS-DOS, Windows, and NT; the VMS
equivalent is "set time".
However, if all you want to do is change your time zone, you can
probably get away with setting an environment variable:
$ENV{TZ} = "MST7MDT"; # Unixish
$ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
system('trn', 'comp.lang.perl.misc');
How can I sleep() or alarm() for under a second?
If you want finer granularity than the 1 second that the sleep()
function provides, the easiest way is to use the select() function as
documented in "select" in perlfunc. Try the Time::HiRes and the
BSD::Itimer modules (available from CPAN, and starting from Perl 5.8
Time::HiRes is part of the standard distribution).
How can I measure time under a second?
(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.
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: $!";
}
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.
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.
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).
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
perlsec) 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 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).
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.
my $exit_status = system("mail-users");
my $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()
my $output = `$cmd`; # using backticks (``)
open (my $pipe_fh, "$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;
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 there's no real need for both to be tempfiles... the following
should work just as well, 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 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.
Why doesn't open() return an error when a pipe open fails?
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.
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. 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 $? to see whether the program even ran correctly.
Even if you wrote
print `cat /etc/termcap`;
this code could and probably should be written as
system("cat /etc/termcap") == 0
or die "cat program failed!";
which will echo the cat command's output as it is generated, instead of
waiting until the program has completed to print it out. It also checks
the return value.
"system" also provides direct control over whether shell wildcard
processing may take place, whereas backticks do not.
How can I call backticks without shell processing?
This is a bit tricky. You can't simply write the command like this:
@ok = `grep @opts '$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 though Perl emulates fork(), you'll still be stuck,
because Windows does not have an argc/argv-style API.
Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on
MS-DOS)?
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:
1. Try keeping around the seekpointer and go there, like this:
my $where = tell($log_fh);
seek($log_fh, $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.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?
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 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.
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(1) 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.
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.
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.
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.
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 to do it
for you. Use the diagnostics pragma with 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
If you want to install a distribution from the current directory, you
can tell "CPAN.pm" to install "." (the full stop):
$ cpan .
See the documentation for either of those commands to see what else you
can do.
If you want to 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.
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
Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and other
authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file are
hereby placed into the public domain. You are permitted and encouraged
to use this code in your own programs for fun or for profit as you see
fit. A simple comment in the code giving credit would be courteous but
is not required.
perl v5.40.1 2026-08-30 PERLFAQ8(1)
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