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.TH PERLFAQ8 1 "2006-01-07" "perl v5.8.8" "Perl Programmers Reference Guide"
perlfaq8 \- System Interaction ($Revision: 1.27 $, $Date: 2005/12/31 00:54:37 $)
This section of the Perl \s-1FAQ\s0 covers questions involving operating
system interaction. Topics include interprocess communication (\s-1IPC\s0),
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.
.Sh "How do I find out which operating system I'm running under?"
.IX Subsection "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
.Sh "How come \fIexec()\fP doesn't return?"
.IX Subsection "How come exec() doesn't return?"
Because that's what it does: it replaces your currently running
program with a different one. If you want to keep going (as is
probably the case if you're asking this question) use \fIsystem()\fR
.Sh "How do I do fancy stuff with the keyboard/screen/mouse?"
.IX Subsection "How do I do fancy stuff with the keyboard/screen/mouse?"
How you access/control keyboards, screens, and pointing devices
(\*(L"mice\*(R") is system\-dependent. Try the following modules:
\& Term::Cap Standard perl distribution
\& Term::ReadLine::Gnu CPAN
\& Term::ReadLine::Perl CPAN
\& Term::Cap Standard perl distribution
Some of these specific cases are shown as examples in other answers
in this section of the perlfaq.
.Sh "How do I print something out in color?"
.IX Subsection "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 \s-1ANSI\s0 terminal that understands
color, you can use the Term::ANSIColor module from \s-1CPAN:\s0
\& print color("red"), "Stop!\en", color("reset");
\& print color("green"), "Go!\en", color("reset");
\& use Term::ANSIColor qw(:constants);
\& print RED, "Stop!\en", RESET;
\& print GREEN, "Go!\en", RESET;
.Sh "How do I read just one key without waiting for a return key?"
.IX Subsection "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 \fBstty\fR command as shown in
\&\*(L"getc\*(R" in perlfunc, but as you see, that's already getting you into
\& open(TTY, "+</dev/tty") or die "no tty: $!";
\& system "stty cbreak </dev/tty >/dev/tty 2>&1";
\& $key = getc(TTY); # perhaps this works
\& sysread(TTY, $key, 1); # probably this does
\& system "stty -cbreak </dev/tty >/dev/tty 2>&1";
The Term::ReadKey module from \s-1CPAN\s0 offers an easy-to-use interface that
should be more efficient than shelling out to \fBstty\fR for each key.
It even includes limited support for Windows.
However, using the code requires that you have a working C compiler
and can use it to build and install a \s-1CPAN\s0 module. Here's a solution
using the standard \s-1POSIX\s0 module, which is already on your systems
(assuming your system supports \s-1POSIX\s0).
And here's the HotKey module, which hides the somewhat mystifying calls
to manipulate the \s-1POSIX\s0 termios structures.
\& @EXPORT = qw(cbreak cooked readkey);
\& use POSIX qw(:termios_h);
\& my ($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;
\& $term->setlflag($noecho); # ok, so i don't want echo either
\& $term->setcc(VTIME, 1);
\& $term->setattr($fd_stdin, TCSANOW);
\& $term->setlflag($oterm);
\& $term->setcc(VTIME, 0);
\& $term->setattr($fd_stdin, TCSANOW);
\& sysread(STDIN, $key, 1);
.Sh "How do I check whether input is ready on the keyboard?"
.IX Subsection "How do I check whether input is ready on the keyboard?"
The easiest way to do this is to read a key in nonblocking mode with the
Term::ReadKey module from \s-1CPAN\s0, passing it an argument of \-1 to indicate
\& if (defined ($char = ReadKey(-1)) ) {
\& # input was waiting and it was $char
\& # no input was waiting
\& ReadMode('normal'); # restore normal tty settings
.Sh "How do I clear the screen?"
.IX Subsection "How do I clear the screen?"
If you only have do so infrequently, use \f(CW\*(C`system\*(C'\fR:
If you have to do this a lot, save the clear string
so you can print it 100 times without calling a program
\& $clear_string = `clear`;
If you're planning on doing other screen manipulations, like cursor
positions, etc, you might wish to use Term::Cap module:
\& $terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
\& $clear_string = $terminal->Tputs('cl');
.Sh "How do I get the screen size?"
.IX Subsection "How do I get the screen size?"
If you have Term::ReadKey module installed from \s-1CPAN\s0,
you can use it to fetch the width and height in characters
\& ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
This is more portable than the raw \f(CW\*(C`ioctl\*(C'\fR, but not as
\& require 'sys/ioctl.ph';
\& die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
\& open(TTY, "+</dev/tty") or die "No tty: $!";
\& unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) {
\& die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\en", &TIOCGWINSZ;
\& ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
\& print "(row,col) = ($row,$col)";
\& print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
.Sh "How do I ask the user for a password?"
.IX Subsection "How do I ask the user for a password?"
(This question has nothing to do with the web. See a different
There's an example of this in \*(L"crypt\*(R" in perlfunc). First, you put the
terminal into \*(L"no echo\*(R" mode, then just read the password normally.
You may do this with an old-style \fIioctl()\fR function, \s-1POSIX\s0 terminal
control (see \s-1POSIX\s0 or its documentation the Camel Book), or a call
to the \fBstty\fR program, with varying degrees of portability.
You can also do this for most systems using the Term::ReadKey module
from \s-1CPAN\s0, which is easier to use and in theory more portable.
\& $password = ReadLine(0);
.Sh "How do I read and write the serial port?"
.IX Subsection "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
Your system may use lockfiles to control multiple access. Make sure
you follow the correct protocol. Unpredictable behavior can result
from multiple processes reading from one device.
If you expect to use both read and write operations on the device,
you'll have to open it for update (see \*(L"open\*(R" in perlfunc for
details). You may wish to open it without running the risk of
blocking by using \fIsysopen()\fR and \f(CW\*(C`O_RDWR|O_NDELAY|O_NOCTTY\*(C'\fR from the
Fcntl module (part of the standard perl distribution). See
\&\*(L"sysopen\*(R" in perlfunc for more on this approach.
Some devices will be expecting a \*(L"\er\*(R" at the end of each line rather
than a \*(L"\en\*(R". In some ports of perl, \*(L"\er\*(R" and \*(L"\en\*(R" are different from
their usual (Unix) \s-1ASCII\s0 values of \*(L"\e012\*(R" and \*(L"\e015\*(R". You may have to
give the numeric values you want directly, using octal (\*(L"\e015\*(R"), hex
(\*(L"0x0D\*(R"), or as a control-character specification (\*(L"\ecM\*(R").
\& print DEV "atv1\e012"; # wrong, for some devices
\& print DEV "atv1\e015"; # right, for some devices
Even though with normal text files a \*(L"\en\*(R" 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 \fI\s-1ALL\s0\fR line
ends with \*(L"\e015\e012\*(R", and strip what you don't need from the output.
This applies especially to socket I/O and autoflushing, discussed
.IX Item "flushing output"
If you expect characters to get to your device when you \fIprint()\fR them,
you'll want to autoflush that filehandle. You can use \fIselect()\fR
and the \f(CW$|\fR variable to control autoflushing (see "$|" in perlvar
and \*(L"select\*(R" in perlfunc, or perlfaq5, \*(L"How do I flush/unbuffer an
output filehandle? Why must I do this?\*(R"):
You'll also see code that does this without a temporary variable, as in
\& select((select(DEV), $| = 1)[0]);
Or if you don't mind pulling in a few thousand lines
of code just because you're afraid of a little $| variable:
As mentioned in the previous item, this still doesn't work when using
socket I/O between Unix and Macintosh. You'll need to hard code your
line terminators, in that case.
.IP "non-blocking input" 4
.IX Item "non-blocking input"
If you are doing a blocking \fIread()\fR or \fIsysread()\fR, you'll have to
arrange for an alarm handler to provide a timeout (see
\&\*(L"alarm\*(R" 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
\&\fIselect()\fR to determine whether I/O is ready on that device (see
\&\*(L"select\*(R" 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, \s-1POSIX\s0's tcgetattr business, and various other functions that
go bump in the night, finally came up with this:
\& my $stty = `/bin/stty -g`;
\& open2( \e*MODEM_IN, \e*MODEM_OUT, "cu -l$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");
\& if ( !m/^Connected/ ) {
\& print STDERR "$0: cu printed `$_' instead of `Connected'\en";
.Sh "How do I decode encrypted password files?"
.IX Subsection "How do I decode encrypted password files?"
You spend lots and lots of money on dedicated hardware, but this is
bound to get you talked about.
Seriously, you can't if they are Unix password files\*(--the Unix
password system employs one-way encryption. It's more like hashing than
encryption. The best you can check is whether something else hashes to
the same string. You can't turn a hash back into the original string.
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
\&\fIpasswd\fR\|(1), for example).
.Sh "How do I start a process in the background?"
.IX Subsection "How do I start a process in the background?"
Several modules can start other processes that do not block
your Perl program. You can use IPC::Open3, Parallel::Jobs,
IPC::Run, and some of the \s-1POE\s0 modules. See \s-1CPAN\s0 for more
or you could use fork as documented in \*(L"fork\*(R" in perlfunc, with
further examples in perlipc. Some things to be aware of, if you're
.IP "\s-1STDIN\s0, \s-1STDOUT\s0, and \s-1STDERR\s0 are shared" 4
.IX Item "STDIN, STDOUT, and STDERR are shared"
Both the main process and the backgrounded one (the \*(L"child\*(R" process)
share the same \s-1STDIN\s0, \s-1STDOUT\s0 and \s-1STDERR\s0 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
\&\f(CW\*(C`open\*(C'\fRing a pipe (see \*(L"open\*(R" in perlfunc) but on some systems this
means that the child process cannot outlive the parent.
You'll have to catch the \s-1SIGCHLD\s0 signal, and possibly \s-1SIGPIPE\s0 too.
\&\s-1SIGCHLD\s0 is sent when the backgrounded process finishes. \s-1SIGPIPE\s0 is
sent when you write to a filehandle whose child process has closed (an
untrapped \s-1SIGPIPE\s0 can cause your program to silently die). This is
not an issue with \f(CW\*(C`system("cmd&")\*(C'\fR.
You have to be prepared to \*(L"reap\*(R" the child process when it finishes.
\& $SIG{CHLD} = sub { wait };
\& $SIG{CHLD} = 'IGNORE';
You can also use a double fork. You immediately \fIwait()\fR for your
first child, and the init daemon will \fIwait()\fR for your grandchild once
\& unless ($pid = fork) {
\& exec "what you really wanna do";
See \*(L"Signals\*(R" in perlipc for other examples of code to do this.
Zombies are not an issue with \f(CW\*(C`system("prog &")\*(C'\fR.
.Sh "How do I trap control characters/signals?"
.IX Subsection "How do I trap control characters/signals?"
You don't actually \*(L"trap\*(R" 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 \*(L"Signals\*(R" in perlipc and the
section on \*(L"Signals\*(R" in the Camel.
You can set the values of the \f(CW%SIG\fR hash to be the functions you want
to handle the signal. After perl catches the signal, it looks in \f(CW%SIG\fR
for a key with the same name as the signal, then calls the subroutine
\& # as an anonymous subroutine
\& $SIG{INT} = sub { syswrite(STDERR, "ouch\en", 5 ) };
\& # or a reference to a function
\& # or the name of the function as a string
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 \f(CW%SIG\fR. This violated the rules of signal handling at that level
causing perl to dump core. Since version 5.8.0, perl looks at \f(CW%SIG\fR
*after* the signal has been caught, rather than while it is being caught.
Previous versions of this answer were incorrect.
.Sh "How do I modify the shadow password file on a Unix system?"
.IX Subsection "How do I modify the shadow password file on a Unix system?"
If perl was installed correctly and your shadow library was written
properly, the getpw*() functions described in perlfunc should in
theory provide (read\-only) access to entries in the shadow password
file. To change the file, make a new shadow password file (the format
varies from system to system\*(--see passwd for specifics) and use
\&\fIpwd_mkdb\fR\|(8) to install it (see pwd_mkdb for more details).
.Sh "How do I set the time and date?"
.IX Subsection "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 \fIdate\fR\|(1)
program. (There is no way to set the time and date on a per-process
basis.) This mechanism will work for Unix, \s-1MS\-DOS\s0, Windows, and \s-1NT\s0;
the \s-1VMS\s0 equivalent is \f(CW\*(C`set time\*(C'\fR.
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";
.Sh "How can I \fIsleep()\fP or \fIalarm()\fP for under a second?"
.IX Subsection "How can I sleep() or alarm() for under a second?"
If you want finer granularity than the 1 second that the \fIsleep()\fR
function provides, the easiest way is to use the \fIselect()\fR function as
documented in \*(L"select\*(R" in perlfunc. Try the Time::HiRes and
the BSD::Itimer modules (available from \s-1CPAN\s0, and starting from
Perl 5.8 Time::HiRes is part of the standard distribution).
.Sh "How can I measure time under a second?"
.IX Subsection "How can I measure time under a second?"
In general, you may not be able to. The Time::HiRes module (available
from \s-1CPAN\s0, and starting from Perl 5.8 part of the standard distribution)
provides this functionality for some systems.
If your system supports both the \fIsyscall()\fR function in Perl as well as
a system call like \fIgettimeofday\fR\|(2), then you may be able to do
\& require 'sys/syscall.ph';
\& $done = $start = pack($TIMEVAL_T, ());
\& syscall(&SYS_gettimeofday, $start, 0) != -1
\& or die "gettimeofday: $!";
\& ##########################
\& # DO YOUR OPERATION HERE #
\& ##########################
\& syscall( &SYS_gettimeofday, $done, 0) != -1
\& or die "gettimeofday: $!";
\& @start = unpack($TIMEVAL_T, $start);
\& @done = unpack($TIMEVAL_T, $done);
\& for ($done[1], $start[1]) { $_ /= 1_000_000 }
\& $delta_time = sprintf "%.4f", ($done[0] + $done[1] )
\& ($start[0] + $start[1] );
.Sh "How can I do an \fIatexit()\fP or \fIsetjmp()\fP/\fIlongjmp()\fP? (Exception handling)"
.IX Subsection "How can I do an atexit() or setjmp()/longjmp()? (Exception handling)"
Release 5 of Perl added the \s-1END\s0 block, which can be used to simulate
\&\fIatexit()\fR. Each package's \s-1END\s0 block is called when the program or
thread ends (see perlmod manpage for more details).
For example, you can use this to make sure your filter program
managed to finish its output without filling up the disk:
\& close(STDOUT) || die "stdout close failed: $!";
The \s-1END\s0 block isn't called when untrapped signals kill the program,
though, so if you use \s-1END\s0 blocks you should also use
\& use sigtrap qw(die normal-signals);
Perl's exception-handling mechanism is its \fIeval()\fR operator. You can
use \fIeval()\fR as setjmp and \fIdie()\fR as longjmp. For details of this, see
the section on signals, especially the time-out handler for a blocking
\&\fIflock()\fR in \*(L"Signals\*(R" in perlipc or the section on \*(L"Signals\*(R" in
If exception handling is all you're interested in, try the
exceptions.pl library (part of the standard perl distribution).
If you want the \fIatexit()\fR syntax (and an \fIrmexit()\fR as well), try the
AtExit module available from \s-1CPAN\s0.
.ie n .Sh "Why doesn't my sockets program work under System V (Solaris)? What does the error message ""Protocol not supported"" mean?"
.el .Sh "Why doesn't my sockets program work under System V (Solaris)? What does the error message ``Protocol not supported'' mean?"
.IX Subsection "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 \*(L"use Socket\*(R" to get the correct values.
Note that even though SunOS and Solaris are binary compatible, these
values are different. Go figure.
.Sh "How can I call my system's unique C functions from Perl?"
.IX Subsection "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 \*(L"Where can I learn about linking C with Perl? [h2xs, xsubpp]\*(R".
However, if the function is a system call, and your system supports
\&\fIsyscall()\fR, you can use the syscall function (documented in
Remember to check the modules that came with your distribution, and
\&\s-1CPAN\s0 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.
.Sh "Where do I get the include files to do \fIioctl()\fP or \fIsyscall()\fP?"
.IX Subsection "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 \fIcpp\fR\|(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 \fIerrno.h\fR, \fIsyscall.h\fR, and \fIsocket.h\fR were fine,
but the hard ones like \fIioctl.h\fR nearly always need to hand\-edited.
Here's how to install the *.ph files:
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 \fBmake perl\fR instead of a plain
\&\fBmake\fR to rebuild perl with a new static extension).
.Sh "Why do setuid perl scripts complain about kernel problems?"
.IX Subsection "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.
.Sh "How can I open a pipe both to and from a command?"
.IX Subsection "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 \fIpipe()\fR, \fIfork()\fR, and \fIexec()\fR to do
the job. Make sure you read the deadlock warnings in its documentation,
though (see IPC::Open2). See
\&\*(L"Bidirectional Communication with Another Process\*(R" in perlipc and
\&\*(L"Bidirectional Communication with Yourself\*(R" 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).
.Sh "Why can't I get the output of a command with \fIsystem()\fP?"
.IX Subsection "Why can't I get the output of a command with system()?"
You're confusing the purpose of \fIsystem()\fR and backticks (``). \fIsystem()\fR
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 \s-1STDOUT\s0.
\& $exit_status = system("mail-users");
\& $output_string = `ls`;
.Sh "How can I capture \s-1STDERR\s0 from an external command?"
.IX Subsection "How can I capture STDERR from an external command?"
There are three basic ways of running external commands:
\& system $cmd; # using system()
\& $output = `$cmd`; # using backticks (``)
\& open (PIPE, "cmd |"); # using open()
With \fIsystem()\fR, both \s-1STDOUT\s0 and \s-1STDERR\s0 will go the same place as the
script's \s-1STDOUT\s0 and \s-1STDERR\s0, unless the \fIsystem()\fR command redirects them.
Backticks and \fIopen()\fR read \fBonly\fR the \s-1STDOUT\s0 of your command.
You can also use the \fIopen3()\fR function from IPC::Open3. Benjamin
Goldberg provides some sample code:
To capture a program's \s-1STDOUT\s0, but discard its \s-1STDERR:\s0
\& use Symbol qw(gensym);
\& open(NULL, ">", File::Spec->devnull);
\& my $pid = open3(gensym, \e*PH, ">&NULL", "cmd");
To capture a program's \s-1STDERR\s0, but discard its \s-1STDOUT:\s0
\& use Symbol qw(gensym);
\& open(NULL, ">", File::Spec->devnull);
\& my $pid = open3(gensym, ">&NULL", \e*PH, "cmd");
To capture a program's \s-1STDERR\s0, and let its \s-1STDOUT\s0 go to our own \s-1STDERR:\s0
\& use Symbol qw(gensym);
\& my $pid = open3(gensym, ">&STDERR", \e*PH, "cmd");
To read both a command's \s-1STDOUT\s0 and its \s-1STDERR\s0 separately, you can
redirect them to temp files, let the command run, then read the temp
\& use Symbol qw(gensym);
\& local *CATCHOUT = IO::File->new_tmpfile;
\& local *CATCHERR = IO::File->new_tmpfile;
\& my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd");
\& seek $_, 0, 0 for \e*CATCHOUT, \e*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 Symbol qw(gensym);
\& local *CATCHERR = IO::File->new_tmpfile;
\& my $pid = open3(gensym, \e*CATCHOUT, ">&CATCHERR", "cmd");
\& while( <CATCHOUT> ) {}
\& 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");
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 \s-1STDERR\s0 a
duplicate of \s-1STDOUT:\s0
\& $output = `$cmd 2>&1`;
\& open (PIPE, "cmd 2>&1 |");
Note that you \fIcannot\fR simply open \s-1STDERR\s0 to be a dup of \s-1STDOUT\s0
in your Perl program and avoid calling the shell to do the redirection.
\& open(STDERR, ">&STDOUT");
\& $alloutput = `cmd args`; # stderr still escapes
This fails because the \fIopen()\fR makes \s-1STDERR\s0 go to where \s-1STDOUT\s0 was
going at the time of the \fIopen()\fR. The backticks then make \s-1STDOUT\s0 go to
a string, but don't change \s-1STDERR\s0 (which still goes to the old
Note that you \fImust\fR use Bourne shell (\fIsh\fR\|(1)) redirection syntax in
backticks, not \fIcsh\fR\|(1)! Details on why Perl's \fIsystem()\fR and backtick
and pipe opens all use the Bourne shell are in the
\&\fIversus/csh.whynot\fR article in the \*(L"Far More Than You Ever Wanted To
Know\*(R" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . To
capture a command's \s-1STDERR\s0 and \s-1STDOUT\s0 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 \s-1STDOUT\s0 but discard its \s-1STDERR:\s0
\& $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 \s-1STDERR\s0 but discard its \s-1STDOUT:\s0
\& $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 \s-1STDOUT\s0 and \s-1STDERR\s0 in order to capture the \s-1STDERR\s0
but leave its \s-1STDOUT\s0 to come out our old \s-1STDERR:\s0
\& $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 \s-1STDOUT\s0 and its \s-1STDERR\s0 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.
.Sh "Why doesn't \fIopen()\fP return an error when a pipe open fails?"
.IX Subsection "Why doesn't open() return an error when a pipe open fails?"
If the second argument to a piped \fIopen()\fR contains shell
metacharacters, perl \fIfork()\fRs, then \fIexec()\fRs 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 \s-1STDERR\s0 and
check it for error messages. See \*(L"How can I capture \s-1STDERR\s0 from an external command?\*(R" elsewhere in this document, or use the
If there are no shell metacharacters in the argument of \fIopen()\fR, Perl
runs the command directly, without using the shell, and can correctly
report whether the command started.
.Sh "What's wrong with using backticks in a void context?"
.IX Subsection "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 \f(CW\*(C`system\*(C'\fR 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?
You forgot to check \f(CW$?\fR 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 get the output quickly (as it is generated, instead of only
at the end) and also check the return value.
\&\fIsystem()\fR also provides direct control over whether shell wildcard
processing may take place, whereas backticks do not.
.Sh "How can I call backticks without shell processing?"
.IX Subsection "How can I call backticks without shell processing?"
This is a bit tricky. You can't simply write the command
\& @ok = `grep @opts '$search_string' @filenames`;
As of Perl 5.8.0, you can use \fIopen()\fR with multiple arguments.
Just like the list forms of \fIsystem()\fR and \fIexec()\fR, no shell
\& open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
\& if (open(GREP, "-|")) {
\& exec 'grep', @opts, $search_string, @filenames;
Just as with \fIsystem()\fR, no shell escapes happen when you \fIexec()\fR a list.
Further examples of this can be found in \*(L"Safe Pipe Opens\*(R" in perlipc.
Note that if you're use Microsoft, no solution to this vexing issue
is even possible. Even if Perl were to emulate \fIfork()\fR, you'd still
be stuck, because Microsoft does not have a argc/argv\-style \s-1API\s0.
.Sh "Why can't my script read from \s-1STDIN\s0 after I gave it \s-1EOF\s0 (^D on Unix, ^Z on \s-1MS\-DOS\s0)?"
.IX Subsection "Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?"
Some stdio's set error and eof flags that need clearing. The
\&\s-1POSIX\s0 module defines \fIclearerr()\fR that you can use. That is the
technically correct way to do it. Here are some less reliable
Try keeping around the seekpointer and go there, like this:
If that doesn't work, try seeking to a different part of the file and
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.
.Sh "How can I convert my shell script to perl?"
.IX Subsection "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.
.Sh "Can I use perl to run a telnet or ftp session?"
.IX Subsection "Can I use perl to run a telnet or ftp session?"
Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
\&\s-1CPAN\s0). 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
\& use IO::Socket; # new in 5.004
\& $handle = IO::Socket::INET->new('www.perl.com:80')
\& || die "can't connect to port 80 on www.perl.com: $!";
\& $handle->autoflush(1);
\& if (fork()) { # XXX: undef means failure
\& print while <STDIN>; # everything from stdin to socket
\& print while <$handle>; # everything from socket to stdout
.Sh "How can I write expect in Perl?"
.IX Subsection "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, \fIdon't use it\fR. These days, your best bet is to
look at the Expect module available from \s-1CPAN\s0, which also requires two
other modules from \s-1CPAN\s0, IO::Pty and IO::Stty.
.ie n .Sh "Is there a way to hide perl's command line from programs such as ""ps""?"
.el .Sh "Is there a way to hide perl's command line from programs such as ``ps''?"
.IX Subsection "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
To actually alter the visible command line, you can assign to the
variable \f(CW$0\fR as documented in perlvar. This won't work on all
operating systems, though. Daemon programs like sendmail place their
\& $0 = "orcus [accepting connections]";
.Sh "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?"
.IX Subsection "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?"
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 \fIeval()\fRing the script's output in your shell; check out the
comp.unix.questions \s-1FAQ\s0 for details.
.Sh "How do I close a process's filehandle without waiting for it to complete?"
.IX Subsection "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 \*(L"kill\*(R" in perlfunc). It's common to first send a \s-1TERM\s0
signal, wait a little bit, and then send a \s-1KILL\s0 signal to finish it off.
.Sh "How do I fork a daemon process?"
.IX Subsection "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 \s-1TIOCNOTTY\s0 ioctl on it. See tty
for details. Or better yet, you can just use the \fIPOSIX::setsid()\fR
function, so you don't have to worry about process groups.
Reopen \s-1STDIN\s0, \s-1STDOUT\s0, and \s-1STDERR\s0 so they're not connected to the old
Background yourself like this:
The Proc::Daemon module, available from \s-1CPAN\s0, provides a function to
perform these actions for you.
.Sh "How do I find out if I'm running interactively or not?"
.IX Subsection "How do I find out if I'm running interactively or not?"
Good question. Sometimes \f(CW\*(C`\-t STDIN\*(C'\fR and \f(CW\*(C`\-t STDOUT\*(C'\fR can give clues,
\& if (-t STDIN && -t STDOUT) {
On \s-1POSIX\s0 systems, you can test whether your own process group matches
the current process group of your controlling terminal as follows:
\& use POSIX qw/getpgrp tcgetpgrp/;
\& open(TTY, "/dev/tty") or die $!;
\& $tpgrp = tcgetpgrp(fileno(*TTY));
\& if ($tpgrp == $pgrp) {
\& print "foreground\en";
\& print "background\en";
.Sh "How do I timeout a slow event?"
.IX Subsection "How do I timeout a slow event?"
Use the \fIalarm()\fR function, probably in conjunction with a signal
handler, as documented in \*(L"Signals\*(R" in perlipc and the section on
\&\*(L"Signals\*(R" in the Camel. You may instead use the more flexible
Sys::AlarmCall module available from \s-1CPAN\s0.
The \fIalarm()\fR function is not implemented on all versions of Windows.
Check the documentation for your specific version of Perl.
.Sh "How do I set \s-1CPU\s0 limits?"
.IX Subsection "How do I set CPU limits?"
Use the BSD::Resource module from \s-1CPAN\s0.
.Sh "How do I avoid zombies on a Unix system?"
.IX Subsection "How do I avoid zombies on a Unix system?"
Use the reaper code from \*(L"Signals\*(R" in perlipc to call \fIwait()\fR when a
\&\s-1SIGCHLD\s0 is received, or else use the double-fork technique described
in \*(L"How do I start a process in the background?\*(R" in perlfaq8.
.Sh "How do I use an \s-1SQL\s0 database?"
.IX Subsection "How do I use an SQL database?"
The \s-1DBI\s0 module provides an abstract interface to most database
servers and types, including Oracle, \s-1DB2\s0, Sybase, mysql, Postgresql,
\&\s-1ODBC\s0, and flat files. The \s-1DBI\s0 module accesses each database type
through a database driver, or \s-1DBD\s0. You can see a complete list of
available drivers on \s-1CPAN:\s0 http://www.cpan.org/modules/by\-module/DBD/ .
You can read more about \s-1DBI\s0 on http://dbi.perl.org .
Other modules provide more specific access: Win32::ODBC, Alzabo, iodbc,
and others found on \s-1CPAN\s0 Search: http://search.cpan.org .
.Sh "How do I make a \fIsystem()\fP exit on control\-C?"
.IX Subsection "How do I make a system() exit on control-C?"
You can't. You need to imitate the \fIsystem()\fR call (see perlipc for
sample code) and then have a signal handler for the \s-1INT\s0 signal that
passes the signal on to the subprocess. Or you can check for it:
\& if ($rc & 127) { die "signal death" }
.Sh "How do I open a file without blocking?"
.IX Subsection "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(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
\& or die "can't open /foo/somefile: $!":
.Sh "How do I tell the difference between errors from the shell and perl?"
.IX Subsection "How do I tell the difference between errors from the shell and perl?"
(answer contributed by brian d foy, \f(CW\*(C`<bdfoy@cpan.org>\*(C'\fR
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
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.
\& print "Hello World\en";
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 \fIprint()\fR function,
but my shebang line is not the path to perl, so the shell runs the
script, and I get the error.
\& ./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.
\& $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; };
\& $SIG{__DIE__} = sub{ print STDERR "Perl: ", @_; exit 1};
The perl message comes out with \*(L"Perl\*(R" in front. The \s-1BEGIN\s0 block
works at compile time so all of the compilation errors and warnings
get the \*(L"Perl:\*(R" 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 \*(L"Perl:\*(R", 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.
If you don't get a paragraph or two of expanded discussion, it
might not be perl's message.
.Sh "How do I install a module from \s-1CPAN\s0?"
.IX Subsection "How do I install a module from CPAN?"
The easiest way is to have a module also named \s-1CPAN\s0 do it for you.
This module comes with perl version 5.004 and later.
\& $ perl -MCPAN -e shell
\& cpan shell -- CPAN exploration and modules installation (v1.59_54)
\& ReadLine support enabled
\& cpan> install Some::Module
To manually install the \s-1CPAN\s0 module, or any well-behaved \s-1CPAN\s0 module
for that matter, follow these steps:
Unpack the source into a temporary area.
If your version of perl is compiled without dynamic loading, then you
just need to replace step 3 (\fBmake\fR) with \fBmake perl\fR and you will
get a new \fIperl\fR binary with your extension linked in.
See ExtUtils::MakeMaker for more details on building extensions.
See also the next question, \*(L"What's the difference between require
.Sh "What's the difference between require and use?"
.IX Subsection "What's the difference between require and use?"
Perl offers several different ways to include code from one file into
another. Here are the deltas between the various inclusion constructs:
\& 1) do $file is like eval `cat $file`, except the former
\& 1.1: searches @INC and updates %INC.
\& 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
\& 2) require $file is like do $file, except the former
\& 2.1: checks for redundant loading, skipping already loaded files.
\& 2.2: raises an exception on failure to find, compile, or execute $file.
\& 3) require Module is like require "Module.pm", except the former
\& 3.1: translates each "::" into your system's directory separator.
\& 3.2: primes the parser to disambiguate class Module as an indirect object.
\& 4) use Module is like require Module, except the former
\& 4.1: loads the module at compile time, not run-time.
\& 4.2: imports symbols and semantics from that package to the current one.
In general, you usually want \f(CW\*(C`use\*(C'\fR and a proper Perl module.
.Sh "How do I keep my own module/library directory?"
.IX Subsection "How do I keep my own module/library directory?"
When you build modules, use the \s-1PREFIX\s0 and \s-1LIB\s0 options when generating
\& perl Makefile.PL PREFIX=/mydir/perl LIB=/mydir/perl/lib
then either set the \s-1PERL5LIB\s0 environment variable before you run
scripts that use the modules/libraries (see perlrun) or say
\& use lib '/mydir/perl/lib';
This is almost the same as
\& unshift(@INC, '/mydir/perl/lib');
except that the lib module checks for machine-dependent subdirectories.
See Perl's lib for more information.
.Sh "How do I add the directory my program lives in to the module/library search path?"
.IX Subsection "How do I add the directory my program lives in to the module/library search path?"
\& use lib "$FindBin::Bin";
.Sh "How do I add a directory to my include path (@INC) at runtime?"
.IX Subsection "How do I add a directory to my include path (@INC) at runtime?"
Here are the suggested ways of modifying your include path:
\& the PERLLIB environment variable
\& the PERL5LIB environment variable
\& the perl -Idir command line flag
\& the use lib pragma, as in
\& use lib "$ENV{HOME}/myown_perllib";
The latter is particularly useful because it knows about machine
dependent architectures. The lib.pm pragmatic module was first
included with the 5.002 release of Perl.
.Sh "What is socket.ph and where do I get it?"
.IX Subsection "What is socket.ph and where do I get it?"
It's a perl4\-style file defining values for system networking
constants. Sometimes it is built using h2ph when Perl is installed,
but other times it is not. Modern programs \f(CW\*(C`use Socket;\*(C'\fR instead.
.SH "AUTHOR AND COPYRIGHT"
.IX Header "AUTHOR AND COPYRIGHT"
Copyright (c) 1997\-2006 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain. You are permitted and
encouraged to use this code in your own programs for fun
or for profit as you see fit. A simple comment in the code giving
credit would be courteous but is not required.
projects / OpenSPARC-T2-SAM / blob