=head1 NAME
-perlfaq8 - System Interaction ($Revision: 1.17 $, $Date: 1997/03/25 18:17:12 $)
+perlfaq8 - System Interaction ($Revision: 6628 $)
=head1 DESCRIPTION
This section of the Perl FAQ covers questions involving operating
-system interaction. This involves interprocess communication (IPC),
+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.
=head2 How do I find out which operating system I'm running under?
-The $^O variable ($OSTYPE if you use English) contains the operating
-system that your perl binary was built for.
+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.
=head2 How come exec() doesn't return?
=item Keyboard
- Term::Cap Standard perl distribution
- Term::ReadKey CPAN
- Term::ReadLine::Gnu CPAN
- Term::ReadLine::Perl CPAN
- Term::Screen CPAN
+ Term::Cap Standard perl distribution
+ Term::ReadKey CPAN
+ Term::ReadLine::Gnu CPAN
+ Term::ReadLine::Perl CPAN
+ Term::Screen CPAN
=item Screen
- Term::Cap Standard perl distribution
- Curses CPAN
- Term::ANSIColor CPAN
+ Term::Cap Standard perl distribution
+ Curses CPAN
+ Term::ANSIColor CPAN
=item Mouse
- Tk CPAN
+ Tk CPAN
=back
+Some of these specific cases are shown as examples in other answers
+in this section of the perlfaq.
+
+=head2 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;
+
+=head2 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 B<stty> command as shown in
+L<perlfunc/getc>, 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 B<stty> for each key.
+It even includes limited support for Windows.
+
+ use Term::ReadKey;
+ ReadMode('cbreak');
+ $key = ReadKey(0);
+ ReadMode('normal');
+
+However, using the code requires that you have a working C compiler
+and can use it to build and install a CPAN module. Here's a solution
+using the standard POSIX module, which is already on your systems
+(assuming your system supports POSIX).
+
+ use HotKey;
+ $key = readkey();
+
+And here's the HotKey module, which hides the somewhat mystifying calls
+to manipulate the POSIX termios structures.
+
+ # HotKey.pm
+ package HotKey;
+
+ @ISA = qw(Exporter);
+ @EXPORT = qw(cbreak cooked readkey);
+
+ use strict;
+ use POSIX qw(:termios_h);
+ my ($term, $oterm, $echo, $noecho, $fd_stdin);
+
+ $fd_stdin = fileno(STDIN);
+ $term = POSIX::Termios->new();
+ $term->getattr($fd_stdin);
+ $oterm = $term->getlflag();
+
+ $echo = ECHO | ECHOK | ICANON;
+ $noecho = $oterm & ~$echo;
+
+ sub cbreak {
+ $term->setlflag($noecho); # ok, so i don't want echo either
+ $term->setcc(VTIME, 1);
+ $term->setattr($fd_stdin, TCSANOW);
+ }
+
+ sub cooked {
+ $term->setlflag($oterm);
+ $term->setcc(VTIME, 0);
+ $term->setattr($fd_stdin, TCSANOW);
+ }
+
+ sub readkey {
+ my $key = '';
+ cbreak();
+ sysread(STDIN, $key, 1);
+ cooked();
+ return $key;
+ }
+
+ END { cooked() }
+
+ 1;
+
+=head2 How do I check whether input is ready on the keyboard?
+
+The easiest way to do this is to read a key in nonblocking mode with the
+Term::ReadKey module from CPAN, passing it an argument of -1 to indicate
+not to block:
+
+ use Term::ReadKey;
+
+ ReadMode('cbreak');
+
+ if (defined ($char = ReadKey(-1)) ) {
+ # input was waiting and it was $char
+ } else {
+ # no input was waiting
+ }
+
+ ReadMode('normal'); # restore normal tty settings
+
+=head2 How do I clear the screen?
+
+If you only have do so infrequently, use C<system>:
+
+ system("clear");
+
+If you have to do this a lot, save the clear string
+so you can print it 100 times without calling a program
+100 times:
+
+ $clear_string = `clear`;
+ print $clear_string;
+
+If you're planning on doing other screen manipulations, like cursor
+positions, etc, you might wish to use Term::Cap module:
+
+ use Term::Cap;
+ $terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
+ $clear_string = $terminal->Tputs('cl');
+
+=head2 How do I get the screen size?
+
+If you have Term::ReadKey module installed from CPAN,
+you can use it to fetch the width and height in characters
+and in pixels:
+
+ use Term::ReadKey;
+ ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
+
+This is more portable than the raw C<ioctl>, but not as
+illustrative:
+
+ require 'sys/ioctl.ph';
+ die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
+ open(TTY, "+</dev/tty") or die "No tty: $!";
+ unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) {
+ die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ;
+ }
+ ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
+ print "(row,col) = ($row,$col)";
+ print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
+ print "\n";
+
=head2 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 L<perlfunc/crypt>). 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 L<POSIX>, and Chapter 7 of the Camel), or a call
+There's an example of this in L<perlfunc/crypt>). 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 L<POSIX> or its documentation the Camel Book), or a call
to the B<stty> program, with varying degrees of portability.
You can also do this for most systems using the Term::ReadKey module
from CPAN, which is easier to use and in theory more portable.
+ use Term::ReadKey;
+
+ ReadMode('noecho');
+ $password = ReadLine(0);
+
=head2 How do I read and write the serial port?
This depends on which operating system your program is running on. In
the case of Unix, the serial ports will be accessible through files in
-/dev; on other systems, the devices names will doubtless differ.
+/dev; on other systems, device names will doubtless differ.
Several problem areas common to all device interaction are the
-following
+following:
=over 4
=item lockfiles
Your system may use lockfiles to control multiple access. Make sure
-you follow the correct protocol. Unpredictable behaviour can result
+you follow the correct protocol. Unpredictable behavior can result
from multiple processes reading from one device.
=item open mode
give the numeric values you want directly, using octal ("\015"), hex
("0x0D"), or as a control-character specification ("\cM").
- print DEV "atv1\012"; # wrong, for some devices
- print DEV "atv1\015"; # right, for some devices
+ print DEV "atv1\012"; # wrong, for some devices
+ print DEV "atv1\015"; # right, for some devices
-Even though with normal text files, a "\n" will do the trick, there is
+Even though with normal text files a "\n" will do the trick, there is
still no unified scheme for terminating a line that is portable
-between Unix,
MS-DOS/Windows, and Macintosh, except to terminate I<ALL> line
+between Unix,
DOS/Win, and Macintosh, except to terminate I<ALL> line
ends with "\015\012", and strip what you don't need from the output.
This applies especially to socket I/O and autoflushing, discussed
next.
=item flushing output
If you expect characters to get to your device when you print() them,
-you'll want to autoflush that filehandle, as in the older
-
- use FileHandle;
- DEV->autoflush(1);
+you'll want to autoflush that filehandle. You can use select()
+and the C<$|> variable to control autoflushing (see L<perlvar/$E<verbar>>
+and L<perlfunc/select>, or L<perlfaq5>, "How do I flush/unbuffer an
+output filehandle? Why must I do this?"):
-and the newer
+ $oldh = select(DEV);
+ $| = 1;
+ select($oldh);
- use IO::Handle;
- DEV->autoflush(1);
-
-You can use select() and the C<$|> variable to control autoflushing
-(see L<perlvar/$|> and L<perlfunc/select>):
+You'll also see code that does this without a temporary variable, as in
- $oldh = select(DEV);
- $| = 1;
- select($oldh);
+ select((select(DEV), $| = 1)[0]);
-You'll also see code that does this without a temporary variable, as in
+Or if you don't mind pulling in a few thousand lines
+of code just because you're afraid of a little $| variable:
- select((select(DEV), $| = 1)[0]);
+ use IO::Handle;
+ DEV->autoflush(1);
As mentioned in the previous item, this still doesn't work when using
-socket I/O between Unix and Macintosh. You'll need to hardcode your
+socket I/O between Unix and Macintosh. You'll need to hard code your
line terminators, in that case.
=item non-blocking input
=back
+While trying to read from his caller-id box, the notorious Jamie Zawinski
+C<< <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";
+ }
+ }
+
=head2 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. Programs like Crack can
-forcibly (and intelligently) try to guess passwords, but don't (can't)
-guarantee quick success.
+Seriously, you can't if they are Unix password files--the Unix
+password system employs one-way encryption. It's more like hashing than
+encryption. The best you can check is whether something else hashes to
+the same string. You can't turn a hash back into the original string.
+Programs like Crack
+can forcibly (and intelligently) try to guess passwords, but don't
+(can't) guarantee quick success.
If you're worried about users selecting bad passwords, you should
proactively check when they try to change their password (by modifying
=head2 How do I start a process in the background?
-You could use
+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 POE modules. See CPAN for more
+details.
+
+You could also use
- system("cmd &")
+ system("cmd &")
or you could use fork as documented in L<perlfunc/"fork">, with
further examples in L<perlipc>. Some things to be aware of, if you're
=over 4
-=item STDIN, STDOUT and STDERR are shared
+=item 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
=item Zombies
-You have to be prepared to "reap" the child process when it finishes
+You have to be prepared to "reap" the child process when it finishes.
- $SIG{CHLD} = sub { wait };
+ $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 L<perlipc/"Signals"> for other examples of code to do this.
Zombies are not an issue with C<system("prog &")>.
=head2 How do I trap control characters/signals?
-You don't actually "trap" a control character. Instead, that
-character generates a signal, which you then trap. Signals are
-documented in L<perlipc/"Signals"> and chapter 6 of the Camel.
+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 L<perlipc/"Signals"> 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
-Be warned that very few C libraries are reentrant. Therefore, if you
-attempt to print() in a handler that got invoked during another stdio
-operation your internal structures will likely be in an
-inconsistent state, and your program will dump core. You can
-sometimes avoid this by using syswrite() instead of print().
+ $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) };
-Unless you're exceedingly careful, the only safe things to do inside a
-signal handler are: set a variable and exit. And in the first case,
-you should only set a variable in such a way that malloc() is not
-called (eg, by setting a variable that already has a value).
+ # or a reference to a function
-For example:
+ $SIG{INT} = \&ouch;
- $Interrupted = 0; # to ensure it has a value
- $SIG{INT} = sub {
- $Interrupted++;
- syswrite(STDERR, "ouch\n", 5);
- }
+ # or the name of the function as a string
-However, because syscalls restart by default, you'll find that if
-you're in a "slow" call, such as E<lt>FHE<gt>, read(), connect(), or
-wait(), that the only way to terminate them is by "longjumping" out;
-that is, by raising an exception. See the timeout handler for a
-blocking flock() in L<perlipc/"Signals"> or chapter 6 of the Camel.
+ $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.
=head2 How do I modify the shadow password file on a Unix system?
-If perl was installed correctly, the getpw*() functions described in
-L<perlfunc> provide (read-only) access to the shadow password file.
-To change the file, make a new shadow password file (the format varies
-from system to system - see L<passwd(5)> for specifics) and use
-pwd_mkdb(8) to install it (see L<pwd_mkdb(5)> for more details).
+If perl was installed correctly and your shadow library was written
+properly, the getpw*() functions described in L<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 L<passwd> for specifics) and use
+pwd_mkdb(8) to install it (see L<pwd_mkdb> for more details).
=head2 How do I set the time and date?
basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT;
the VMS equivalent is C<set time>.
-However, if all you want to do is change your timezone, you can
+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";
+ $ENV{TZ} = "MST7MDT"; # unixish
+ $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
+ system "trn comp.lang.perl.misc";
=head2 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 L<perlfunc/"select">. If your system has itimers and
-syscall() support, you can check out the old example in
-http://www.perl.com/CPAN/doc/misc/ancient/tutorial/eg/itimers.pl .
+documented in L<perlfunc/"select">. 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).
=head2 How can I measure time under a second?
In general, you may not be able to. The Time::HiRes module (available
-from CPAN) provides this functionality for some systems.
+from CPAN, and starting from Perl 5.8 part of the standard distribution)
+provides this functionality for some systems.
-In general, you may not be able to. But if you system supports both the
-syscall() function in Perl as well as a system call like gettimeofday(2),
-then you may be able to do something like this:
+If your system supports both the syscall() function in Perl as well as
+a system call like gettimeofday(2), then you may be able to do
+something like this:
- require 'sys/syscall.ph';
+ require 'sys/syscall.ph';
- $TIMEVAL_T = "LL";
+ $TIMEVAL_T = "LL";
- $done = $start = pack($TIMEVAL_T, ());
+ $done = $start = pack($TIMEVAL_T, ());
- syscall( &SYS_gettimeofday, $start, 0)) != -1
- or die "gettimeofday: $!";
+ syscall(&SYS_gettimeofday, $start, 0) != -1
+ or die "gettimeofday: $!";
- ##########################
- # DO YOUR OPERATION HERE #
- ##########################
+ ##########################
+ # DO YOUR OPERATION HERE #
+ ##########################
- syscall( &SYS_gettimeofday, $done, 0) != -1
- or die "gettimeofday: $!";
+ syscall( &SYS_gettimeofday, $done, 0) != -1
+ or die "gettimeofday: $!";
- @start = unpack($TIMEVAL_T, $start);
- @done = unpack($TIMEVAL_T, $done);
+ @start = unpack($TIMEVAL_T, $start);
+ @done = unpack($TIMEVAL_T, $done);
- # fix microseconds
- for ($done[1], $start[1]) { $_ /= 1_000_000 }
+ # fix microseconds
+ for ($done[1], $start[1]) { $_ /= 1_000_000 }
- $delta_time = sprintf "%.4f", ($done[0] + $done[1] )
- -
- ($start[0] + $start[1] );
+ $delta_time = sprintf "%.4f", ($done[0] + $done[1] )
+ -
+ ($start[0] + $start[1] );
=head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
Release 5 of Perl added the END block, which can be used to simulate
atexit(). Each package's END block is called when the program or
-thread ends (see L<perlmod> manpage for more details). It isn't
-called when untrapped signals kill the program, though, so if you use
-END blocks you should also use
+thread ends (see L<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:
+
+ 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 timeout handler for a blocking
-flock() in L<perlipc/"Signals"> and chapter 6 of the Camel.
+the section on signals, especially the time-out handler for a blocking
+flock() in L<perlipc/"Signals"> or the section on "Signals" in
+the Camel Book.
If exception handling is all you're interested in, try the
exceptions.pl library (part of the standard perl distribution).
If you want the atexit() syntax (and an rmexit() as well), try the
AtExit module available from CPAN.
-=head2 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
+=head2 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
=head2 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
+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
L<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.
+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.
=head2 Where do I get the include files to do ioctl() or syscall()?
but the hard ones like F<ioctl.h> nearly always need to hand-edited.
Here's how to install the *.ph files:
- 1. become superuser
- 2. cd /usr/include
- 3. h2ph *.h */*.h
+ 1. become super-user
+ 2. cd /usr/include
+ 3. h2ph *.h */*.h
If your system supports dynamic loading, for reasons of portability and
sanity you probably ought to use h2xs (also part of the standard perl
=head2 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 L<IPC::Open2>).
+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 L<IPC::Open2>). See
+L<perlipc/"Bidirectional Communication with Another Process"> and
+L<perlipc/"Bidirectional Communication with Yourself">
+
+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 L<IPC::Open3>).
=head2 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 8 bits are the signal the process died from, if any, and
-the high 8 bits are the actual exit value). Backticks ('') run a
+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.
- $status = system("mail-users");
- $output = `ls`;
+ $exit_status = system("mail-users");
+ $output_string = `ls`;
=head2 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()
+ system $cmd; # using system()
+ $output = `$cmd`; # using backticks (``)
+ open (PIPE, "cmd |"); # using open()
With system(), both STDOUT and STDERR will go the same place as the
-script's versions of these, unless the command redirects them.
+script's STDOUT and STDERR, unless the system() command redirects them.
Backticks and open() read B<only> the STDOUT of your command.
+You can also use the open3() function from IPC::Open3. Benjamin
+Goldberg provides some sample code:
+
+To capture a program's STDOUT, but discard its STDERR:
+
+ use IPC::Open3;
+ use File::Spec;
+ use Symbol qw(gensym);
+ open(NULL, ">", File::Spec->devnull);
+ my $pid = open3(gensym, \*PH, ">&NULL", "cmd");
+ while( <PH> ) { }
+ waitpid($pid, 0);
+
+To capture a program's STDERR, but discard its STDOUT:
+
+ use IPC::Open3;
+ use File::Spec;
+ use Symbol qw(gensym);
+ open(NULL, ">", File::Spec->devnull);
+ my $pid = open3(gensym, ">&NULL", \*PH, "cmd");
+ while( <PH> ) { }
+ waitpid($pid, 0);
+
+To capture a program's STDERR, and let its STDOUT go to our own STDERR:
+
+ use IPC::Open3;
+ use Symbol qw(gensym);
+ my $pid = open3(gensym, ">&STDERR", \*PH, "cmd");
+ while( <PH> ) { }
+ waitpid($pid, 0);
+
+To read both a command's STDOUT and its STDERR separately, you can
+redirect them to temp files, let the command run, then read the temp
+files:
+
+ use IPC::Open3;
+ use Symbol qw(gensym);
+ use IO::File;
+ local *CATCHOUT = IO::File->new_tmpfile;
+ local *CATCHERR = IO::File->new_tmpfile;
+ my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd");
+ waitpid($pid, 0);
+ seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
+ while( <CATCHOUT> ) {}
+ while( <CATCHERR> ) {}
+
+But there's no real need for *both* to be tempfiles... the following
+should work just as well, without deadlocking:
+
+ use IPC::Open3;
+ use Symbol qw(gensym);
+ use IO::File;
+ local *CATCHERR = IO::File->new_tmpfile;
+ my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd");
+ while( <CATCHOUT> ) {}
+ waitpid($pid, 0);
+ seek CATCHERR, 0, 0;
+ while( <CATCHERR> ) {}
+
+And it'll be faster, too, since we can begin processing the program's
+stdout immediately, rather than waiting for the program to finish.
+
With any of these, you can change file descriptors before the call:
- open(STDOUT, ">logfile");
- system("ls");
+ 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 |");
+ $output = `$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 |");
+ $output = `$cmd 2>&1`;
+ open (PIPE, "cmd 2>&1 |");
Note that you I<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
+ 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
Note that you I<must> use Bourne shell (sh(1)) redirection syntax in
backticks, not csh(1)! Details on why Perl's system() and backtick
-and pipe opens all use the Bourne shell are in
-http://www.perl.com/CPAN/doc/FMTEYEWTK/versus/csh.whynot .
+and pipe opens all use the Bourne shell are in the
+F<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:
-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 L<IPC::Open3>).
+ $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.
=head2 Why doesn't open() return an error when a pipe open fails?
-It does, but probably not how you expect it to. On systems that
-follow the standard fork()/exec() paradigm (eg, Unix), it works like
-this: open() causes a fork(). In the parent, open() returns with the
-process ID of the child. The child exec()s the command to be piped
-to/from. The parent can't know whether the exec() was successful or
-not - all it can return is whether the fork() succeeded or not. To
-find out if the command succeeded, you have to catch SIGCHLD and
-wait() to get the exit status. You should also catch SIGPIPE if
-you're writing to the child -- you may not have found out the exec()
-failed by the time you write. This is documented in L<perlipc>.
-
-On systems that follow the spawn() paradigm, open() I<might> do what
-you expect - unless perl uses a shell to start your command. In this
-case the fork()/exec() description still applies.
+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 L<"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.
=head2 What's wrong with using backticks in a void context?
Strictly speaking, nothing. Stylistically speaking, it's not a good
-way to write maintainable code because backticks have a (potentially
-humungous) return value, and you're ignoring it. It's may also not be very
-efficient, because you have to read in all the lines of output, allocate
-memory for them, and then throw it away. Too often people are lulled
-to writing:
+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 C<system> function is
+another; it doesn't do this.
- `cp file file.bak`;
-
-And now they think "Hey, I'll just always use backticks to run programs."
-Bad idea: backticks are for capturing a program's output; the system()
-function is for running programs.
+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`;
+ `cat /etc/termcap`;
-You haven't assigned the output anywhere, so it just wastes memory
-(for a little while). Plus you forgot to check C<$?> to see whether
-the program even ran correctly. Even if you wrote
+You forgot to check C<$?> to see whether the program even ran
+correctly. Even if you wrote
- print `cat /etc/termcap`;
+ print `cat /etc/termcap`;
-In most cases, this could and probably should be written as
+this code could and probably should be written as
- system("cat /etc/termcap") == 0
+ system("cat /etc/termcap") == 0
or die "cat program failed!";
-Which will get the output quickly (as its generated, instead of only
-at the end ) and also check the return value.
+which will get the output quickly (as it is generated, instead of only
+at the end) and also check the return value.
-system() also provides direct control over whether shell wildcard
+C<system> also provides direct control over whether shell wildcard
processing may take place, whereas backticks do not.
=head2 How can I call backticks without shell processing?
-This is a bit tricky. Instead of writing
+This is a bit tricky. You can't simply write the command
+like this:
- @ok = `grep @opts '$search_string' @filenames`;
+ @ok = `grep @opts '$search_string' @filenames`;
-You have to do this:
+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.
- my @ok = ();
- if (open(GREP, "-|")) {
- while (<GREP>) {
- chomp;
- push(@ok, $_);
- }
+ open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
+ chomp(@ok = <GREP>);
close GREP;
- } else {
- exec 'grep', @opts, $search_string, @filenames;
- }
+
+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 L<perlipc/"Safe Pipe Opens">.
+
+Note that if you're use Microsoft, no solution to this vexing issue
+is even possible. Even if Perl were to emulate fork(), you'd still
+be stuck, because Microsoft does not have a argc/argv-style API.
=head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
-Because some stdio's set error and eof flags that need clearing. The
+Some stdio's set error and eof flags that need clearing. The
POSIX module defines clearerr() that you can use. That is the
technically correct way to do it. Here are some less reliable
workarounds:
Try keeping around the seekpointer and go there, like this:
- $where = tell(LOG);
- seek(LOG, $where, 0);
+ $where = tell(LOG);
+ seek(LOG, $where, 0);
=item 2
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 data stream paradigm, which while convenient for some matters,
+pipeline datastream paradigm, which while convenient for some matters,
causes many inefficiencies.
=head2 Can I use perl to run a telnet or ftp session?
-Try the Net::FTP and TCP::Client modules (available from CPAN).
-http://www.perl.com/CPAN/scripts/netstuff/telnet.emul.shar will also
-help for emulating the telnet protocol.
+Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
+CPAN). http://www.cpan.org/scripts/netstuff/telnet.emul.shar
+will also help for emulating the telnet protocol, but Net::Telnet is
+quite probably easier to use..
+
+If all you want to do is pretend to be telnet but don't need
+the initial telnet handshaking, then the standard dual-process
+approach will suffice:
+
+ use IO::Socket; # new in 5.004
+ $handle = IO::Socket::INET->new('www.perl.com:80')
+ or die "can't connect to port 80 on www.perl.com: $!";
+ $handle->autoflush(1);
+ if (fork()) { # XXX: undef means failure
+ select($handle);
+ print while <STDIN>; # everything from stdin to socket
+ } else {
+ print while <$handle>; # everything from socket to stdout
+ }
+ close $handle;
+ exit;
=head2 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. These
-days, your best bet is to look at the Comm.pl library available from
-CPAN.
+standard perl distribution), which never really got finished. If you
+find it somewhere, I<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.
=head2 Is there a way to hide perl's command line from programs such as "ps"?
operating systems, though. Daemon programs like sendmail place their
state there, as in:
- $0 = "orcus [accepting connections]";
+ $0 = "orcus [accepting connections]";
=head2 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?
=item Unix
-In the strictest sense, it can't be done -- the script executes as a
+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 its own children
+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.
-=item VMS
-
-Change to %ENV persist after Perl exits, but directory changes do not.
-
=back
=head2 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<perlfunc/"kill">. It's common to first send a TERM
+to the process (see L<perlfunc/"kill">). It's common to first send a TERM
signal, wait a little bit, and then send a KILL signal to finish it off.
=head2 How do I fork a daemon process?
=item *
-Open /dev/tty and use the the TIOCNOTTY ioctl on it. See L<tty(4)>
-for details.
+Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty>
+for details. Or better yet, you can just use the POSIX::setsid()
+function, so you don't have to worry about process groups.
=item *
Background yourself like this:
- fork && exit;
+ fork && exit;
=back
-=head2 How do I make my program run with sh and csh?
-
-See the F<eg/nih> script (part of the perl source distribution).
-
-=head2 How do I keep my own module/library directory?
-
-When you build modules, use the PREFIX option when generating
-Makefiles:
-
- perl Makefile.PL PREFIX=/u/mydir/perl
-
-then either set the PERL5LIB environment variable before you run
-scripts that use the modules/libraries (see L<perlrun>) or say
-
- use lib '/u/mydir/perl';
-
-See Perl's L<lib> for more information.
+The Proc::Daemon module, available from CPAN, provides a function to
+perform these actions for you.
=head2 How do I find out if I'm running interactively or not?
-Good question. Sometimes C<-t STDIN> and C<-t STDOUT> can give clues,
+Good question. Sometimes C<-t STDIN> and C<-t STDOUT> can give clues,
sometimes not.
- if (-t STDIN && -t STDOUT) {
- print "Now what? ";
- }
+ if (-t STDIN && -t STDOUT) {
+ print "Now what? ";
+ }
On POSIX systems, you can test whether your own process group matches
the current process group of your controlling terminal as follows:
- use POSIX qw/getpgrp tcgetpgrp/;
- open(TTY, "/dev/tty") or die $!;
- $tpgrp = tcgetpgrp(TTY);
- $pgrp = getpgrp();
- if ($tpgrp == $pgrp) {
- print "foreground\n";
- } else {
- print "background\n";
- }
+ use POSIX qw/getpgrp tcgetpgrp/;
+
+ # Some POSIX systems, such as Linux, can be
+ # without a /dev/tty at boot time.
+ if (!open(TTY, "/dev/tty")) {
+ print "no tty\n";
+ } else {
+ $tpgrp = tcgetpgrp(fileno(*TTY));
+ $pgrp = getpgrp();
+ if ($tpgrp == $pgrp) {
+ print "foreground\n";
+ } else {
+ print "background\n";
+ }
+ }
=head2 How do I timeout a slow event?
Use the alarm() function, probably in conjunction with a signal
-handler, as documented L<perlipc/"Signals"> and chapter 6 of the
-Camel. You may instead use the more flexible Sys::AlarmCall module
-available from CPAN.
+handler, as documented in L<perlipc/"Signals"> 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.
=head2 How do I set CPU limits?
Use the reaper code from L<perlipc/"Signals"> to call wait() when a
SIGCHLD is received, or else use the double-fork technique described
-in L<perlfunc/fork>.
+in L<perlfaq8/"How do I start a process in the background?">.
=head2 How do I use an SQL database?
-There are a number of excellent interfaces to SQL databases. See the
-DBD::* modules available from
-http://www.perl.com/CPAN/modules/dbperl/DBD .
+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: http://search.cpan.org .
=head2 How do I make a system() exit on control-C?
You can't. You need to imitate the system() call (see L<perlipc> for
sample code) and then have a signal handler for the INT signal that
-passes the signal on to the subprocess.
+passes the signal on to the subprocess. Or you can check for it:
+
+ $rc = system($cmd);
+ if ($rc & 127) { die "signal death" }
=head2 How do I open a file without blocking?
O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with
sysopen():
- use Fcntl;
- sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
- or die "can't open /tmp/somefile: $!":
+ use Fcntl;
+ sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
+ or die "can't open /foo/somefile: $!":
+
+=head2 How do I tell the difference between errors from the shell and perl?
+
+(answer contributed by brian d foy, C<< <bdfoy@cpan.org> >>
+
+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;
-=head2 How do I install a CPAN module?
+If you don't get a paragraph or two of expanded discussion, it
+might not be perl's message.
-The easiest way is to have the CPAN module do it for you. This module
-comes with perl version 5.004 and later. To manually install the CPAN
-module, or any well-behaved CPAN module for that matter, follow these
-steps:
+=head2 How do I install a module from CPAN?
+
+The easiest way is to have a module also named CPAN do it for you.
+This module comes with perl version 5.004 and later.
+
+ $ 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 CPAN module, or any well-behaved CPAN module
+for that matter, follow these steps:
=over 4
=item 2
- perl Makefile.PL
+ perl Makefile.PL
=item 3
- make
+ make
=item 4
- make test
+ make test
=item 5
- make install
+ make install
=back
get a new F<perl> binary with your extension linked in.
See L<ExtUtils::MakeMaker> for more details on building extensions.
+See also the next question, "What's the difference between require
+and use?".
+
+=head2 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 $file.
+
+ 3) require Module is like require "Module.pm", except the former
+ 3.1: translates each "::" into your system's directory separator.
+ 3.2: primes the parser to disambiguate class Module as an indirect object.
+
+ 4) use Module is like require Module, except the former
+ 4.1: loads the module at compile time, not run-time.
+ 4.2: imports symbols and semantics from that package to the current one.
+
+In general, you usually want C<use> and a proper Perl module.
+
+=head2 How do I keep my own module/library directory?
+
+When you build modules, use the PREFIX and LIB options when generating
+Makefiles:
+
+ perl Makefile.PL PREFIX=/mydir/perl LIB=/mydir/perl/lib
+
+then either set the PERL5LIB environment variable before you run
+scripts that use the modules/libraries (see L<perlrun>) or say
+
+ use lib '/mydir/perl/lib';
+
+This is almost the same as
+
+ BEGIN {
+ unshift(@INC, '/mydir/perl/lib');
+ }
+
+except that the lib module checks for machine-dependent subdirectories.
+See Perl's L<lib> for more information.
+
+=head2 How do I add the directory my program lives in to the module/library search path?
+
+ use FindBin;
+ use lib "$FindBin::Bin";
+ use your_own_modules;
+
+=head2 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.
+
+=head2 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 C<use Socket;> instead.
+
+=head1 REVISION
+
+Revision: $Revision: 6628 $
+
+Date: $Date: 2006-07-09 14:46:14 +0200 (dim, 09 jui 2006) $
+
+See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
-All rights reserved. See L<perlfaq> for distribution information.
+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.
https://perl5.git.perl.org / perl5.git / blobdiff