=head1 NAME
-perlfaq8 - System Interaction ($Revision: 1.17 $, $Date: 2003/01/26 17:44:04 $)
+perlfaq8 - System Interaction ($Revision: 3606 $)
=head1 DESCRIPTION
=item Keyboard
- Term::Cap Standard perl distribution
- Term::ReadKey CPAN
- Term::ReadLine::Gnu CPAN
- Term::ReadLine::Perl CPAN
- Term::Screen CPAN
+ Term::Cap Standard perl distribution
+ Term::ReadKey CPAN
+ Term::ReadLine::Gnu CPAN
+ Term::ReadLine::Perl CPAN
+ Term::Screen CPAN
=item Screen
- Term::Cap Standard perl distribution
- Curses CPAN
- Term::ANSIColor CPAN
+ Term::Cap Standard perl distribution
+ Curses CPAN
+ Term::ANSIColor CPAN
=item Mouse
- Tk CPAN
+ Tk CPAN
=back
-Some of these specific cases are shown below.
+Some of these specific cases are shown as examples in other answers
+in this section of the perlfaq.
=head2 How do I print something out in color?
If you expect characters to get to your device when you print() them,
you'll want to autoflush that filehandle. You can use select()
and the C<$|> variable to control autoflushing (see L<perlvar/$E<verbar>>
-and L<perlfunc/select>, or L<perlfaq5>, ``How do I flush/unbuffer an
-output filehandle? Why must I do this?''):
+and L<perlfunc/select>, or L<perlfaq5>, "How do I flush/unbuffer an
+output filehandle? Why must I do this?"):
$oldh = select(DEV);
$| = 1;
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.
+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().
+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.
-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).
+ # as an anonymous subroutine
-For example:
+ $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) };
- $Interrupted = 0; # to ensure it has a value
- $SIG{INT} = sub {
- $Interrupted++;
- syswrite(STDERR, "ouch\n", 5);
- }
+ # or a reference to a function
+
+ $SIG{INT} = \&ouch;
+
+ # or the name of the function as a string
+
+ $SIG{INT} = "ouch";
+
+Perl versions before 5.8 had in its C source code signal handlers which
+would catch the signal and possibly run a Perl function that you had set
+in %SIG. This violated the rules of signal handling at that level
+causing perl to dump core. Since version 5.8.0, perl looks at %SIG
+*after* the signal has been caught, rather than while it is being caught.
+Previous versions of this answer were incorrect.
-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.
=head2 How do I modify the shadow password file on a Unix system?
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
+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
use IPC::Open3;
use Symbol qw(gensym);
use IO::File;
- local *CATCHOUT = IO::File->new_tempfile;
- local *CATCHERR = IO::File->new_tempfile;
+ 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;
use IPC::Open3;
use Symbol qw(gensym);
use IO::File;
- local *CATCHERR = IO::File->new_tempfile;
+ local *CATCHERR = IO::File->new_tmpfile;
my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd");
while( <CATCHOUT> ) {}
waitpid($pid, 0);
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.
the current process group of your controlling terminal as follows:
use POSIX qw/getpgrp tcgetpgrp/;
- open(TTY, "/dev/tty") or die $!;
- $tpgrp = tcgetpgrp(fileno(*TTY));
- $pgrp = getpgrp();
- if ($tpgrp == $pgrp) {
- print "foreground\n";
+
+ # 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 {
- print "background\n";
+ $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 in L<perlipc/"Signals"> and the section on
-``Signals'' in the Camel. You may instead use the more flexible
+"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.
sysopen():
use Fcntl;
- sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
- or die "can't open /tmp/somefile: $!":
+ 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;
+
+If you don't get a paragraph or two of expanded discussion, it
+might not be perl's message.
=head2 How do I install a module from CPAN?
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?''.
+See also the next question, "What's the difference between require
+and use?".
=head2 What's the difference between require and use?
=head2 How do I keep my own module/library directory?
-When you build modules, use the PREFIX option when generating
+When you build modules, use the PREFIX and LIB options when generating
Makefiles:
- perl Makefile.PL PREFIX=/u/mydir/perl
+ 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 '/u/mydir/perl';
+ use lib '/mydir/perl/lib';
This is almost the same as
BEGIN {
- unshift(@INC, '/u/mydir/perl');
+ unshift(@INC, '/mydir/perl/lib');
}
except that the lib module checks for machine-dependent subdirectories.
use lib "$FindBin::Bin";
use your_own_modules;
-=head2 How do I add a directory to my include path at runtime?
+=head2 How do I add a directory to my include path (@INC) at runtime?
Here are the suggested ways of modifying your include path:
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: 3606 $
+
+Date: $Date: 2006-03-06 12:05:47 +0100 (lun, 06 mar 2006) $
+
+See L<perlfaq> for source control details and availability.
+
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2003 Tom Christiansen and Nathan Torkington.
-All rights reserved.
+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.