=head1 NAME
-perlfaq8 - System Interaction ($Revision: 1.6 $, $Date: 2002/01/28 04:17:27 $)
+perlfaq8 - System Interaction
=head1 DESCRIPTION
=head2 How do I find out which operating system I'm running under?
-The $^O variable ($OSNAME if you use English) contains an indication of
-the name of the operating system (not its release number) that your perl
-binary was built for.
+The C<$^O> variable (C<$OSNAME> if you use C<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?
+X<exec> X<system> X<fork> X<open> X<pipe>
-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 system()
-instead.
+(contributed by brian d foy)
+
+The C<exec> function's job is to turn your process into another
+command and never to return. If that's not what you want to do, don't
+use C<exec>. :)
+
+If you want to run an external command and still keep your Perl process
+going, look at a piped C<open>, C<fork>, or C<system>.
=head2 How do I do fancy stuff with the keyboard/screen/mouse?
=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 below.
+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:
+color, you can use the C<Term::ANSIColor> module from CPAN:
- use Term::ANSIColor;
- print color("red"), "Stop!\n", color("reset");
- print color("green"), "Go!\n", color("reset");
+ 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;
+ 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.
+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";
+ 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
+The C<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');
+ 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
+using the standard C<POSIX> module, which is already on your system
(assuming your system supports POSIX).
- use HotKey;
- $key = readkey();
+ use HotKey;
+ $key = readkey();
-And here's the HotKey module, which hides the somewhat mystifying calls
+And here's the C<HotKey> module, which hides the somewhat mystifying calls
to manipulate the POSIX termios structures.
- # HotKey.pm
- package HotKey;
+ # HotKey.pm
+ package HotKey;
- @ISA = qw(Exporter);
- @EXPORT = qw(cbreak cooked readkey);
+ @ISA = qw(Exporter);
+ @EXPORT = qw(cbreak cooked readkey);
- use strict;
- use POSIX qw(:termios_h);
- my ($term, $oterm, $echo, $noecho, $fd_stdin);
+ use strict;
+ use POSIX qw(:termios_h);
+ my ($term, $oterm, $echo, $noecho, $fd_stdin);
- $fd_stdin = fileno(STDIN);
- $term = POSIX::Termios->new();
- $term->getattr($fd_stdin);
- $oterm = $term->getlflag();
+ $fd_stdin = fileno(STDIN);
+ $term = POSIX::Termios->new();
+ $term->getattr($fd_stdin);
+ $oterm = $term->getlflag();
- $echo = ECHO | ECHOK | ICANON;
- $noecho = $oterm & ~$echo;
+ $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 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 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;
- }
+ sub readkey {
+ my $key = '';
+ cbreak();
+ sysread(STDIN, $key, 1);
+ cooked();
+ return $key;
+ }
- END { cooked() }
+ END { cooked() }
- 1;
+ 1;
=head2 How do I check whether input is ready on the keyboard?
The easiest way to do this is to read a key in nonblocking mode with the
-Term::ReadKey module from CPAN, passing it an argument of -1 to indicate
+C<Term::ReadKey> module from CPAN, passing it an argument of -1 to indicate
not to block:
- use Term::ReadKey;
+ use Term::ReadKey;
- ReadMode('cbreak');
+ ReadMode('cbreak');
- if (defined ($char = ReadKey(-1)) ) {
- # input was waiting and it was $char
- } else {
- # no input was waiting
- }
+ if (defined ($char = ReadKey(-1)) ) {
+ # input was waiting and it was $char
+ } else {
+ # no input was waiting
+ }
- ReadMode('normal'); # restore normal tty settings
+ ReadMode('normal'); # restore normal tty settings
=head2 How do I clear the screen?
-If you only have do so infrequently, use C<system>:
+(contributed by brian d foy)
+
+To clear the screen, you just have to print the special sequence
+that tells the terminal to clear the screen. Once you have that
+sequence, output it when you want to clear the screen.
+
+You can use the C<Term::ANSIScreen> module to get the special
+sequence. Import the C<cls> function (or the C<:screen> tag):
+
+ use Term::ANSIScreen qw(cls);
+ my $clear_screen = cls();
+
+ print $clear_screen;
+
+The C<Term::Cap> module can also get the special sequence if you want
+to deal with the low-level details of terminal control. The C<Tputs>
+method returns the string for the given capability:
+
+ use Term::Cap;
+
+ $terminal = Term::Cap->Tgetent( { OSPEED => 9600 } );
+ $clear_string = $terminal->Tputs('cl');
+
+ print $clear_screen;
+
+On Windows, you can use the C<Win32::Console> module. After creating
+an object for the output filehandle you want to affect, call the
+C<Cls> method:
- system("clear");
+ Win32::Console;
-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:
+ $OUT = Win32::Console->new(STD_OUTPUT_HANDLE);
+ my $clear_string = $OUT->Cls;
- $clear_string = `clear`;
- print $clear_string;
+ print $clear_screen;
-If you're planning on doing other screen manipulations, like cursor
-positions, etc, you might wish to use Term::Cap module:
+If you have a command-line program that does the job, you can call
+it in backticks to capture whatever it outputs so you can use it
+later:
- use Term::Cap;
- $terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
- $clear_string = $terminal->Tputs('cl');
+ $clear_string = `clear`;
+
+ print $clear_string;
=head2 How do I get the screen size?
-If you have Term::ReadKey module installed from CPAN,
+If you have C<Term::ReadKey> module installed from CPAN,
you can use it to fetch the width and height in characters
and in pixels:
- use Term::ReadKey;
- ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
+ use Term::ReadKey;
+ ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
-This is more portable than the raw C<ioctl>, but not as
+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";
+ 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?
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
+You may do this with an old-style C<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
+You can also do this for most systems using the C<Term::ReadKey> module
from CPAN, which is easier to use and in theory more portable.
- use Term::ReadKey;
+ use Term::ReadKey;
- ReadMode('noecho');
- $password = ReadLine(0);
+ ReadMode('noecho');
+ $password = ReadLine(0);
=head2 How do I read and write the serial port?
If you expect to use both read and write operations on the device,
you'll have to open it for update (see L<perlfunc/"open"> for
details). You may wish to open it without running the risk of
-blocking by using sysopen() and C<O_RDWR|O_NDELAY|O_NOCTTY> from the
-Fcntl module (part of the standard perl distribution). See
+blocking by using C<sysopen()> and C<O_RDWR|O_NDELAY|O_NOCTTY> from the
+C<Fcntl> module (part of the standard perl distribution). See
L<perlfunc/"sysopen"> for more on this approach.
=item end of line
give the numeric values you want directly, using octal ("\015"), hex
("0x0D"), or as a control-character specification ("\cM").
- print DEV "atv1\012"; # wrong, for some devices
- print DEV "atv1\015"; # right, for some devices
+ print DEV "atv1\012"; # wrong, for some devices
+ print DEV "atv1\015"; # right, for some devices
Even though with normal text files a "\n" will do the trick, there is
still no unified scheme for terminating a line that is portable
=item flushing output
-If you expect characters to get to your device when you print() them,
-you'll want to autoflush that filehandle. You can use select()
-and the C<$|> variable to control autoflushing (see L<perlvar/$|>
-and L<perlfunc/select>, or L<perlfaq5>, ``How do I flush/unbuffer an
-output filehandle? Why must I do this?''):
+If you expect characters to get to your device when you C<print()> them,
+you'll want to autoflush that filehandle. You can use C<select()>
+and the C<$|> 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?"):
- $oldh = select(DEV);
- $| = 1;
- select($oldh);
+ $oldh = select(DEV);
+ $| = 1;
+ select($oldh);
You'll also see code that does this without a temporary variable, as in
- select((select(DEV), $| = 1)[0]);
+ 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 $| variable:
+of code just because you're afraid of a little C<$|> variable:
- use IO::Handle;
- DEV->autoflush(1);
+ use IO::Handle;
+ DEV->autoflush(1);
As mentioned in the previous item, this still doesn't work when using
socket I/O between Unix and Macintosh. You'll need to hard code your
=item non-blocking input
-If you are doing a blocking read() or sysread(), you'll have to
+If you are doing a blocking C<read()> or C<sysread()>, you'll have to
arrange for an alarm handler to provide a timeout (see
L<perlfunc/alarm>). If you have a non-blocking open, you'll likely
have a non-blocking read, which means you may have to use a 4-arg
-select() to determine whether I/O is ready on that device (see
+C<select()> to determine whether I/O is ready on that device (see
L<perlfunc/"select">.
=back
-While trying to read from his caller-id box, the notorious Jamie Zawinski
-<jwz@netscape.com>, after much gnashing of teeth and fighting with sysread,
-sysopen, POSIX's tcgetattr business, and various other functions that
-go bump in the night, finally came up with this:
-
- sub open_modem {
- use IPC::Open2;
- my $stty = `/bin/stty -g`;
- open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
- # starting cu hoses /dev/tty's stty settings, even when it has
- # been opened on a pipe...
- system("/bin/stty $stty");
- $_ = <MODEM_IN>;
- chomp;
- if ( !m/^Connected/ ) {
- print STDERR "$0: cu printed `$_' instead of `Connected'\n";
+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 C<sysread>, C<sysopen>, POSIX's C<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?
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.
-Programs like Crack
-can forcibly (and intelligently) try to guess passwords, but don't
-(can't) guarantee quick success.
+password system employs one-way encryption. It's more like hashing
+than encryption. The best you can do is check whether something else
+hashes to the same string. You can't turn a hash back into the
+original string. Programs like Crack can forcibly (and intelligently)
+try to guess passwords, but don't (can't) guarantee quick success.
If you're worried about users selecting bad passwords, you should
proactively check when they try to change their password (by modifying
=head2 How do I start a process in the background?
-You could use
+(contributed by brian d foy)
+
+There's not a single way to run code in the background so you don't
+have to wait for it to finish before your program moves on to other
+tasks. Process management depends on your particular operating system,
+and many of the techniques are in L<perlipc>.
+
+Several CPAN modules may be able to help, including C<IPC::Open2> or
+C<IPC::Open3>, C<IPC::Run>, C<Parallel::Jobs>,
+C<Parallel::ForkManager>, C<POE>, C<Proc::Background>, and
+C<Win32::Process>. There are many other modules you might use, so
+check those namespaces for other options too.
- system("cmd &")
+If you are on a Unix-like system, you might be able to get away with a
+system call where you put an C<&> on the end of the command:
-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
-on a Unix-like system:
+ system("cmd &")
+
+You can also try using C<fork>, as described in L<perlfunc> (although
+this is the same thing that many of the modules will do for you).
=over 4
=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} = 'IGNORE';
- $SIG{CHLD} = sub { wait };
+You can also use a double fork. You immediately C<wait()> for your
+first child, and the init daemon will C<wait()> for your grandchild once
+it exits.
+
+ unless ($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 &")>.
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.
-
-Be warned that very few C libraries are re-entrant. 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().
-
-Unless you're exceedingly careful, the only safe things to do inside a
-signal handler are (1) set a variable and (2) exit. 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).
-
-For example:
-
- $Interrupted = 0; # to ensure it has a value
- $SIG{INT} = sub {
- $Interrupted++;
- syswrite(STDERR, "ouch\n", 5);
- }
-
-However, because syscalls restart by default, you'll find that if
-you're in a "slow" call, such as <FH>, read(), connect(), or
-wait(), that the only way to terminate them is by "longjumping" out;
-that is, by raising an exception. See the time-out handler for a
-blocking flock() in L<perlipc/"Signals"> or the section on ``Signals''
-in the Camel book.
+section on "Signals" in the Camel.
+
+You can set the values of the C<%SIG> hash to be the functions you want
+to handle the signal. After perl catches the signal, it looks in C<%SIG>
+for a key with the same name as the signal, then calls the subroutine
+value for that key.
+
+ # as an anonymous subroutine
+
+ $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) };
+
+ # or a reference to a function
+
+ $SIG{INT} = \&ouch;
+
+ # or the name of the function as a string
+
+ $SIG{INT} = "ouch";
+
+Perl versions before 5.8 had in its C source code signal handlers which
+would catch the signal and possibly run a Perl function that you had set
+in C<%SIG>. This violated the rules of signal handling at that level
+causing perl to dump core. Since version 5.8.0, perl looks at C<%SIG>
+B<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 and your shadow library was written
-properly, the
getpw*() functions described in L<perlfunc> should in
+properly, the
C<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(5)> for specifics) and use
-pwd_mkdb(8) to install it (see L<pwd_mkdb(8)> for more details).
+varies from system to system--see L<passwd> for specifics) and use
+C<pwd_mkdb(8)> to install it (see L<pwd_mkdb> for more details).
=head2 How do I set the time and date?
Assuming you're running under sufficient permissions, you should be
-able to set the system-wide date and time by running the date(1)
+able to set the system-wide date and time by running the C<date(1)>
program. (There is no way to set the time and date on a per-process
basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT;
the VMS equivalent is C<set time>.
However, if all you want to do is change your time zone, you can
probably get away with setting an environment variable:
- $ENV{TZ} = "MST7MDT"; # unixish
- $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
- system "trn comp.lang.perl.misc";
+ $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?
+X<Time::HiRes> X<BSD::Itimer> X<sleep> X<select>
-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">. 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).
+If you want finer granularity than the 1 second that the C<sleep()>
+function provides, the easiest way is to use the C<select()> function as
+documented in L<perlfunc/"select">. Try the C<Time::HiRes> and
+the C<BSD::Itimer> modules (available from CPAN, and starting from
+Perl 5.8 C<Time::HiRes> is part of the standard distribution).
=head2 How can I measure time under a second?
+X<Time::HiRes> X<BSD::Itimer> X<sleep> X<select>
-In general, you may not be able to. The Time::HiRes module (available
-from CPAN, and starting from Perl 5.8 part of the standard distribution)
-provides this functionality for some systems.
-
-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';
-
- $TIMEVAL_T = "LL";
-
- $done = $start = pack($TIMEVAL_T, ());
+(contributed by brian d foy)
- 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, $done);
-
- # fix microseconds
- for ($done[1], $start[1]) { $_ /= 1_000_000 }
-
- $delta_time = sprintf "%.4f", ($done[0] + $done[1] )
- -
- ($start[0] + $start[1] );
+The C<Time::HiRes> module (part of the standard distribution as of
+Perl 5.8) measures time with the C<gettimeofday()> system call, which
+returns the time in microseconds since the epoch. If you can't install
+C<Time::HiRes> for older Perls and you are on a Unixish system, you
+may be able to call C<gettimeofday(2)> directly. See
+L<perlfunc/syscall>.
=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).
+C<atexit()>. Each package's END block is called when the program or
+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: $!";
- }
+ 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
+The C<END> block isn't called when untrapped signals kill the program,
+though, so if you use C<END> blocks you should also use
use sigtrap qw(die normal-signals);
-Perl's exception-handling mechanism is its eval() operator. You can
-use eval() as setjmp and die() as longjmp. For details of this, see
-the section on signals, especially the time-out handler for a blocking
-flock() in L<perlipc/"Signals"> or the section on ``Signals'' in
-the Camel Book.
+Perl's exception-handling mechanism is its C<eval()> operator. You
+can use C<eval()> as C<setjmp> and C<die()> as C<longjmp>. For
+details of this, see the section on signals, especially the time-out
+handler for a blocking C<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).
+C<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.
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
+C<syscall()>, you can use the C<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 C<Win32::API>. On Macs, try C<Mac::Carbon>. If no module
+has an interface to the C function, you can inline a bit of C in your
+Perl source with C<Inline::C>.
=head2 Where do I get the include files to do ioctl() or syscall()?
-Historically, these would be generated by the h2ph tool, part of the
-standard perl distribution. This program converts cpp(1) directives
+Historically, these would be generated by the C<h2ph> tool, part of the
+standard perl distribution. This program converts C<cpp(1)> directives
in C header files to files containing subroutine definitions, like
-&SYS_getitimer, which you can use as arguments to your functions.
+C<&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 F<errno.h>, F<syscall.h>, and F<socket.h> were fine,
-but the hard ones like F<ioctl.h> nearly always need to hand-edited.
+but the hard ones like F<ioctl.h> nearly always need to be hand-edited.
Here's how to install the *.ph files:
- 1. become super-user
- 2. cd /usr/include
- 3. h2ph *.h */*.h
+ 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
+sanity you probably ought to use C<h2xs> (also part of the standard perl
distribution). This tool converts C header files to Perl extensions.
-See L<perlxstut> for how to get started with h2xs.
+See L<perlxstut> for how to get started with C<h2xs>.
If your system doesn't support dynamic loading, you still probably
-ought to use
h2xs. See L<perlxstut> and L<ExtUtils::MakeMaker> for
+ought to use
C<h2xs>. See L<perlxstut> and L<ExtUtils::MakeMaker> for
more information (in brief, just use B<make perl> instead of a plain
B<make> to rebuild perl with a new static extension).
=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>). See
-L<perlipc/"Bidirectional Communication with Another Process"> and
+The C<IPC::Open2> module (part of the standard perl distribution) is
+an easy-to-use approach that internally uses C<pipe()>, C<fork()>, and
+C<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
+You may also use the C<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>).
+arguments from C<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()
+You're confusing the purpose of C<system()> and backticks (``). C<system()>
runs a command and returns exit status information (as a 16 bit value:
the low 7 bits are the signal the process died from, if any, and
the high 8 bits are the actual exit value). Backticks (``) run a
command and return what it sent to STDOUT.
- $exit_status = system("mail-users");
- $output_string = `ls`;
+ $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()
-
-With system(), both STDOUT and STDERR will go the same place as the
-script's STDOUT and STDERR, unless the system() command redirects them.
-Backticks and open() read B<only> the STDOUT of your command.
+ system $cmd; # using system()
+ $output = `$cmd`; # using backticks (``)
+ open (PIPE, "cmd |"); # using open()
+
+With C<system()>, both STDOUT and STDERR will go the same place as the
+script's STDOUT and STDERR, unless the C<system()> command redirects them.
+Backticks and C<open()> read B<only> the STDOUT of your command.
+
+You can also use the C<open3()> function from C<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 B<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
+This fails because the C<open()> makes STDERR go to where STDOUT was
+going at the time of the C<open()>. The backticks then make STDOUT go to
a string, but don't change STDERR (which still goes to the old
STDOUT).
-Note that you I<must> use Bourne shell (sh(1)) redirection syntax in
-backticks, not csh(1)! Details on why Perl's system() and backtick
+Note that you I<must> use Bourne shell (C<sh(1)>) redirection syntax in
+backticks, not C<csh(1)>! Details on why Perl's C<system()> and backtick
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/olddoc/FMTEYEWTK.tgz . To
+Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . To
capture a command's STDERR and STDOUT together:
- $output = `cmd 2>&1`; # either with backticks
- $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe
- while (<PH>) { } # plus a read
+ $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
+ $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
+ $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
+ $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
-and safest to redirect them separately to files, and then read from those
-files when the program is done:
+to redirect them separately to files, and then read from those files
+when the program is done:
- system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
+ 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");
+ 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
=head2 Why doesn't open() return an error when a pipe open fails?
-If the second argument to a piped C<open> contains shell
-metacharacters, perl fork()s, then exec()s a shell to decode the
+If the second argument to a piped C<open()> contains shell
+metacharacters, perl C<fork()>s, then C<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
-L<IPC::Open3> module.
+C<IPC::Open3> module.
-If there are no shell metacharacters in the argument of C<open>, Perl
+If there are no shell metacharacters in the argument of C<open()>, Perl
runs the command directly, without using the shell, and can correctly
report whether the command started.
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.
+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.
Consider this line:
- `cat /etc/termcap`;
+ `cat /etc/termcap`;
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`;
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 it is generated, instead of only
-at the end) and also check the return value.
+which will echo the cat command's output as it is generated, instead
+of waiting until the program has completed to print it out. It also
+checks the return value.
-system() also provides direct control over whether shell wildcard
+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 C<open()> with multiple arguments.
+Just like the list forms of C<system()> and C<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;
- }
-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">.
+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 C<system()>, no shell escapes happen when you C<exec()> a
+list. Further examples of this can be found in L<perlipc/"Safe Pipe
+Opens">.
-Note that if you're stuck on Microsoft, no solution to this vexing issue
-is even possible. Even if Perl were to emulate fork(), you'd still
-be hosed, because Microsoft gives no argc/argv-style API. Their API
-always reparses from a single string, which is fundamentally wrong,
-but you're not likely to get the Gods of Redmond to acknowledge this
-and fix it for you.
+Note that if you're using Windows, no solution to this vexing issue is
+even possible. Even if Perl were to emulate C<fork()>, you'd still be
+stuck, because Windows does not have an 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)?
-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:
+This happens only if your perl is compiled to use stdio instead of
+perlio, which is the default. Some (maybe all?) stdios set error and
+eof flags that you may need to clear. The C<POSIX> module defines
+C<clearerr()> that you can use. That is the technically correct way to
+do it. Here are some less reliable workarounds:
=over 4
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
=head2 Can I use perl to run a telnet or ftp session?
-Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
-CPAN). http://www.cpan.org/scripts/netstuff/telnet.emul.shar
-will also help for emulating the telnet protocol, but Net::Telnet is
-quite probably easier to use..
+Try the C<Net::FTP>, C<TCP::Client>, and C<Net::Telnet> modules
+(available from CPAN).
+http://www.cpan.org/scripts/netstuff/telnet.emul.shar will also help
+for emulating the telnet protocol, but C<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')
- || 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;
+ 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
+Once upon a time, there was a library called L<chat2.pl> (part of the
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.
+other modules from CPAN, C<IO::Pty> and C<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?
different process from the shell it was started from. Changes to a
process are not reflected in its parent--only in any children
created after the change. There is shell magic that may allow you to
-fake it by eval()ing the script's output in your shell; check out the
-comp.unix.questions FAQ for details.
+fake it by C<eval()>ing the script's output in your shell; check out the
+comp.unix.questions FAQ for details.
=back
=item *
-Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty(4)>
-for details. Or better yet, you can just use the POSIX::setsid()
+Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty>
+for details. Or better yet, you can just use the C<POSIX::setsid()>
function, so you don't have to worry about process groups.
=item *
Background yourself like this:
- fork && exit;
+ fork && exit;
=back
-The Proc::Daemon module, available from CPAN, provides a function to
+The C<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,
-sometimes not.
+(contributed by brian d foy)
+
+This is a difficult question to answer, and the best answer is
+only a guess.
+
+What do you really want to know? If you merely want to know if one of
+your filehandles is connected to a terminal, you can try the C<-t>
+file test:
+
+ if( -t STDOUT ) {
+ print "I'm connected to a terminal!\n";
+ }
+
+However, you might be out of luck if you expect that means there is a
+real person on the other side. With the C<Expect> module, another
+program can pretend to be a person. The program might even come close
+to passing the Turing test.
+
+The C<IO::Interactive> module does the best it can to give you an
+answer. Its C<is_interactive> function returns an output filehandle;
+that filehandle points to standard output if the module thinks the
+session is interactive. Otherwise, the filehandle is a null handle
+that simply discards the output:
- if (-t STDIN && -t STDOUT) {
- print "Now what? ";
- }
+ use IO::Interactive;
+
+ print { is_interactive } "I might go to standard output!\n";
-On POSIX systems, you can test whether your own process group matches
-the current process group of your controlling terminal as follows:
+This still doesn't guarantee that a real person is answering your
+prompts or reading your output.
- use POSIX qw/getpgrp tcgetpgrp/;
- open(TTY, "/dev/tty") or die $!;
- $tpgrp = tcgetpgrp(fileno(*TTY));
- $pgrp = getpgrp();
- if ($tpgrp == $pgrp) {
- print "foreground\n";
- } else {
- print "background\n";
- }
+If you want to know how to handle automated testing for your
+distribution, you can check the environment. The CPAN
+Testers, for instance, set the value of C<AUTOMATED_TESTING>:
+
+ unless( $ENV{AUTOMATED_TESTING} ) {
+ print "Hello interactive tester!\n";
+ }
=head2 How do I timeout a slow event?
-Use the alarm() function, probably in conjunction with a signal
+Use the C<alarm()> function, probably in conjunction with a signal
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.
+"Signals" in the Camel. You may instead use the more flexible
+C<Sys::AlarmCall> module available from CPAN.
+
+The C<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?
+X<BSD::Resource> X<limit> X<CPU>
+
+(contributed by Xho)
+
+Use the C<BSD::Resource> module from CPAN. As an example:
-Use the BSD::Resource module from CPAN.
+ use BSD::Resource;
+ setrlimit(RLIMIT_CPU,10,20) or die $!;
+
+This sets the soft and hard limits to 10 and 20 seconds, respectively.
+After 10 seconds of time spent running on the CPU (not "wall" time),
+the process will be sent a signal (XCPU on some systems) which, if not
+trapped, will cause the process to terminate. If that signal is
+trapped, then after 10 more seconds (20 seconds in total) the process
+will be killed with a non-trappable signal.
+
+See the C<BSD::Resource> and your systems documentation for the gory
+details.
=head2 How do I avoid zombies on a Unix system?
-Use the reaper code from L<perlipc/"Signals"> to call wait() when a
+Use the reaper code from L<perlipc/"Signals"> to call C<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.cpan.org/modules/DBD .
-A lot of information on this can be found at http://dbi.perl.org/
+The C<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: C<Win32::ODBC>, C<Alzabo>,
+C<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
+You can't. You need to imitate the
C<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. Or you can check for it:
- $rc = system($cmd);
- if ($rc & 127) { die "signal death" }
+ $rc = system($cmd);
+ if ($rc & 127) { die "signal death" }
=head2 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():
+C<O_NDELAY> or C<O_NONBLOCK> flag from the C<Fcntl> module in conjunction with
+C<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 install a module from CPAN?
+=head2 How do I tell the difference between errors from the shell and perl?
-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. To manually install
-the CPAN module, or any well-behaved CPAN module for that matter, follow
-these steps:
+(answer contributed by brian d foy)
-=over 4
+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.
-=item 1
+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.
-Unpack the source into a temporary area.
+Consider this script, which has an error you may not notice immediately.
-=item 2
+ #!/usr/locl/bin/perl
- perl Makefile.PL
+ print "Hello World\n";
-=item 3
+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 C<print()> function,
+but my shebang line is not the path to perl, so the shell runs the
+script, and I get the error.
- make
+ $ ./test
+ ./test: line 3: print: command not found
-=item 4
+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
- make test
+ BEGIN {
+ $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; };
+ $SIG{__DIE__} = sub{ print STDERR "Perl: ", @_; exit 1};
+ }
-=item 5
+ $a = 1 + undef;
+ $x / 0;
+ __END__
- make install
+The perl message comes out with "Perl" in front. The C<BEGIN> block
+works at compile time so all of the compilation errors and warnings
+get the "Perl:" prefix too.
-=back
+ 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 L<perldiag> manpage. If you don't find the error in
+there, it probably isn't a perl error.
+
+Looking up every message is not the easiest way, so let perl to do it
+for you. Use the diagnostics pragma with turns perl's normal messages
+into longer discussions on the topic.
+
+ use diagnostics;
+
+If you don't get a paragraph or two of expanded discussion, it
+might not be perl's message.
-If your version of perl is compiled without dynamic loading, then you
-just need to replace step 3 (B<make>) with B<make perl> and you will
-get a new F<perl> binary with your extension linked in.
+=head2 How do I install a module from CPAN?
+
+(contributed by brian d foy)
+
+The easiest way is to have a module also named CPAN do it for you by using
+the C<cpan> command the comes with Perl. You can give it a list of modules
+to install:
+
+ $ cpan IO::Interactive Getopt::Whatever
+
+If you prefer C<CPANPLUS>, it's just as easy:
+
+ $ cpanp i IO::Interactive Getopt::Whatever
+
+If you want to install a distribution from the current directory, you can
+tell C<CPAN.pm> to install C<.> (the full stop):
+
+ $ cpan .
-See L<ExtUtils::MakeMaker> for more details on building extensions.
-See also the next question, ``What's the difference between require
-and use?''.
+See the documentation for either of those commands to see what else
+you can do.
+
+If you want to try to install a distribution by yourself, resolving
+all dependencies on your own, you follow one of two possible build
+paths.
+
+For distributions that use I<Makefile.PL>:
+
+ $ perl Makefile.PL
+ $ make test install
+
+For distributions that use I<Build.PL>:
+
+ $ perl Build.PL
+ $ ./Build test
+ $ ./Build install
+
+Some distributions may need to link to libraries or other third-party
+code and their build and installation sequences may be more complicated.
+Check any I<README> or I<INSTALL> files that you may find.
=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:
+(contributed by brian d foy)
+
+Perl runs C<require> statement at run-time. Once Perl loads, compiles,
+and runs the file, it doesn't do anything else. The C<use> statement
+is the same as a C<require> run at compile-time, but Perl also calls the
+C<import> method for the loaded package. These two are the same:
+
+ use MODULE qw(import list);
- 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.
+ BEGIN {
+ require MODULE;
+ MODULE->import(import list);
+ }
- 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 $file.
+However, you can suppress the C<import> by using an explicit, empty
+import list. Both of these still happen at compile-time:
- 3) require Module is like require "Module.pm", except the former
- 3.1: translates each "::" into your system's directory separator.
- 3.2: primes the parser to disambiguate class Module as an indirect object.
+ use MODULE ();
- 4) use Module is like require Module, except the former
- 4.1: loads the module at compile time, not run-time.
- 4.2: imports symbols and semantics from that package to the current one.
+ BEGIN {
+ require MODULE;
+ }
-In general, you usually want C<use> and a proper Perl module.
+Since C<use> will also call the C<import> method, the actual value
+for C<MODULE> must be a bareword. That is, C<use> cannot load files
+by name, although C<require> can:
+
+ require "$ENV{HOME}/lib/Foo.pm"; # no @INC searching!
+
+See the entry for C<use> in L<perlfunc> for more details.
=head2 How do I keep my own module/library directory?
-When you build modules, use the PREFIX option when generating
-Makefiles:
+When you build modules, tell Perl where to install the modules.
+
+For C<Makefile.PL>-based distributions, use the INSTALL_BASE option
+when generating Makefiles:
+
+ perl Makefile.PL INSTALL_BASE=/mydir/perl
- perl Makefile.PL PREFIX=/u/mydir/perl
+You can set this in your C<CPAN.pm> configuration so modules
+automatically install in your private library directory when you use
+the CPAN.pm shell:
-then either set the PERL5LIB environment variable before you run
-scripts that use the modules/libraries (see L<perlrun>) or say
+ % cpan
+ cpan> o conf makepl_arg INSTALL_BASE=/mydir/perl
+ cpan> o conf commit
- use lib '/u/mydir/perl';
+For C<Build.PL>-based distributions, use the --install_base option:
-This is almost the same as
+ perl Build.PL --install_base /mydir/perl
- BEGIN {
- unshift(@INC, '/u/mydir/perl');
- }
+You can configure C<CPAN.pm> to automatically use this option too:
-except that the lib module checks for machine-dependent subdirectories.
-See Perl's L<lib> for more information.
+ % cpan
+ cpan> o conf mbuild_arg "--install_base /mydir/perl"
+ cpan> o conf commit
+
+INSTALL_BASE tells these tools to put your modules into
+F</mydir/perl/lib/perl5>. See L<How do I add a directory to my
+include path (@INC) at runtime?> for details on how to run your newly
+installed modules.
+
+There is one caveat with INSTALL_BASE, though, since it acts
+differently than the PREFIX and LIB settings that older versions of
+C<ExtUtils::MakeMaker> advocated. INSTALL_BASE does not support
+installing modules for multiple versions of Perl or different
+architectures under the same directory. You should consider if you
+really want that , and if you do, use the older PREFIX and LIB
+settings. See the C<ExtUtils::Makemaker> documentation for more details.
=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;
+(contributed by brian d foy)
+
+If you know the directory already, you can add it to C<@INC> as you would
+for any other directory. You might <use lib> if you know the directory
+at compile time:
+
+ use lib $directory;
+
+The trick in this task is to find the directory. Before your script does
+anything else (such as a C<chdir>), you can get the current working
+directory with the C<Cwd> module, which comes with Perl:
+
+ BEGIN {
+ use Cwd;
+ our $directory = cwd;
+ }
+
+ use lib $directory;
+
+You can do a similar thing with the value of C<$0>, which holds the
+script name. That might hold a relative path, but C<rel2abs> can turn
+it into an absolute path. Once you have the
+
+ BEGIN {
+ use File::Spec::Functions qw(rel2abs);
+ use File::Basename qw(dirname);
+
+ my $path = rel2abs( $0 );
+ our $directory = dirname( $path );
+ }
-=head2 How do I add a directory to my include path at runtime?
+ use lib $directory;
-Here are the suggested ways of modifying your include path:
+The C<FindBin> module, which comes with Perl, might work. It finds the
+directory of the currently running script and puts it in C<$Bin>, which
+you can then use to construct the right library path:
- the PERLLIB environment variable
- the PERL5LIB environment variable
- the perl -Idir command line flag
- the use lib pragma, as in
- use lib "$ENV{HOME}/myown_perllib";
+ use FindBin qw($Bin);
+
+=head2 How do I add a directory to my include path (@INC) at runtime?
+
+Here are the suggested ways of modifying your include path, including
+environment variables, run-time switches, and in-code statements:
+
+=over 4
+
+=item the PERLLIB environment variable
+
+ $ export PERLLIB=/path/to/my/dir
+ $ perl program.pl
+
+=item the PERL5LIB environment variable
+
+ $ export PERL5LIB=/path/to/my/dir
+ $ perl program.pl
+
+=item the perl -Idir command line flag
+
+ $ perl -I/path/to/my/dir program.pl
+
+=item the use lib pragma:
+
+ use lib "$ENV{HOME}/myown_perllib";
+
+=back
-The latter is particularly useful because it knows about machine
-dependent architectures. The lib.pm pragmatic module was first
+The last is particularly useful because it knows about machine
+dependent architectures. The C<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,
+It's a Perl 4 style file defining values for system networking
+constants. Sometimes it is built using C<h2ph> when Perl is installed,
but other times it is not. Modern programs C<use Socket;> instead.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
-All rights reserved.
+Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and
+other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
https://perl5.git.perl.org / perl5.git / blobdiff