=head1 NAME
-perlfaq8 - System Interaction ($Revision: 1.17 $, $Date: 2003/01/26 17:44:04 $)
+perlfaq8 - System Interaction ($Revision: 6628 $)
=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?
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");
+ 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?
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";
+ 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');
+ 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();
+ use HotKey;
+ $key = readkey();
And here's the HotKey module, which hides the somewhat mystifying calls
to manipulate the POSIX termios structures.
- # HotKey.pm
- package HotKey;
+ # HotKey.pm
+ package HotKey;
- @ISA = qw(Exporter);
- @EXPORT = qw(cbreak cooked readkey);
+ @ISA = qw(Exporter);
+ @EXPORT = qw(cbreak cooked readkey);
- use strict;
- use POSIX qw(:termios_h);
- my ($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 $key;
+ }
- END { cooked() }
+ END { cooked() }
- 1;
+ 1;
=head2 How do I check whether input is ready on the keyboard?
Term::ReadKey module from CPAN, passing it an argument of -1 to indicate
not to block:
- use Term::ReadKey;
+ use Term::ReadKey;
- ReadMode('cbreak');
+ ReadMode('cbreak');
- if (defined ($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>:
- system("clear");
+ 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;
+ $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');
+ use Term::Cap;
+ $terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
+ $clear_string = $terminal->Tputs('cl');
=head2 How do I get the screen size?
you can use it to fetch the width and height in characters
and in pixels:
- use Term::ReadKey;
- ($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
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?
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;
+ use Term::ReadKey;
- ReadMode('noecho');
- $password = ReadLine(0);
+ ReadMode('noecho');
+ $password = ReadLine(0);
=head2 How do I read and write the serial port?
give the numeric values you want directly, using octal ("\015"), hex
("0x0D"), or as a control-character specification ("\cM").
- print DEV "atv1\012"; # wrong, for some devices
- print DEV "atv1\015"; # right, for some devices
+ print DEV "atv1\012"; # wrong, for some devices
+ print DEV "atv1\015"; # right, for some devices
Even though with normal text files a "\n" will do the trick, there is
still no unified scheme for terminating a line that is portable
If you expect characters to get to your device when you print() them,
you'll want to autoflush that filehandle. You can use select()
and the C<$|> 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);
- $| = 1;
- select($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:
- 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
=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,
+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";
+ 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 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
You have to be prepared to "reap" the child process when it finishes.
- $SIG{CHLD} = sub { wait };
+ $SIG{CHLD} = sub { wait };
- $SIG{CHLD} = 'IGNORE';
+ $SIG{CHLD} = 'IGNORE';
You can also use a double fork. You immediately wait() for your
first child, and the init daemon will wait() for your grandchild once
it exits.
unless ($pid = fork) {
- unless (fork) {
- exec "what you really wanna do";
- die "exec failed!";
- }
- exit 0;
- }
- waitpid($pid,0);
-
+ unless (fork) {
+ exec "what you really wanna do";
+ die "exec failed!";
+ }
+ exit 0;
+ }
+ waitpid($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 %SIG hash to be the functions you want
+to handle the signal. After perl catches the signal, it looks in %SIG
+for a key with the same name as the signal, then calls the subroutine
+value for that key.
+
+ # as an anonymous subroutine
+
+ $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) };
+
+ # or a reference to a function
+
+ $SIG{INT} = \&ouch;
+
+ # or the name of the function as a string
+
+ $SIG{INT} = "ouch";
+
+Perl versions before 5.8 had in its C source code signal handlers which
+would catch the signal and possibly run a Perl function that you had set
+in %SIG. This violated the rules of signal handling at that level
+causing perl to dump core. Since version 5.8.0, perl looks at %SIG
+*after* the signal has been caught, rather than while it is being caught.
+Previous versions of this answer were incorrect.
=head2 How do I modify the shadow password file on a Unix system?
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?
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)
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
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
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. On
+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.
but the hard ones like F<ioctl.h> nearly always need to 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
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()
+ 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.
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);
+ use IPC::Open3;
+ use File::Spec;
+ use Symbol qw(gensym);
+ open(NULL, ">", File::Spec->devnull);
+ my $pid = open3(gensym, \*PH, ">&NULL", "cmd");
+ waitpid($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);
+ use IPC::Open3;
+ use File::Spec;
+ use Symbol qw(gensym);
+ open(NULL, ">", File::Spec->devnull);
+ my $pid = open3(gensym, ">&NULL", \*PH, "cmd");
+ waitpid($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);
+ use IPC::Open3;
+ use Symbol qw(gensym);
+ my $pid = open3(gensym, ">&STDERR", \*PH, "cmd");
+ waitpid($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_tempfile;
- local *CATCHERR = IO::File->new_tempfile;
- my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd");
- waitpid($pid, 0);
- seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
- while( <CATCHOUT> ) {}
- while( <CATCHERR> ) {}
+ 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_tempfile;
- my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd");
- while( <CATCHOUT> ) {}
- waitpid($pid, 0);
- seek CATCHERR, 0, 0;
- while( <CATCHERR> ) {}
+ 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
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
+ $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
+
while (<PH>) { } # plus a read
To read both a command's STDOUT and its STDERR separately, it's easiest
to redirect them separately to files, and then read from those files
when the program is done:
- system("program args 1>program.stdout 2>program.stderr");
+ system("program args 1>program.stdout 2>program.stderr");
Ordering is important in all these examples. That's because the shell
processes file descriptor redirections in strictly left to right order.
- system("prog args 1>tmpfile 2>&1");
- system("prog args 2>&1 1>tmpfile");
+ system("prog args 1>tmpfile 2>&1");
+ system("prog args 2>&1 1>tmpfile");
The first command sends both standard out and standard error to the
temporary file. The second command sends only the old standard output
Consider this line:
- `cat /etc/termcap`;
+ `cat /etc/termcap`;
You forgot to check C<$?> 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.
-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. You can't simply write the command
like this:
- @ok = `grep @opts '$search_string' @filenames`;
+ @ok = `grep @opts '$search_string' @filenames`;
As of Perl 5.8.0, you can use open() with multiple arguments.
Just like the list forms of system() and exec(), no shell
escapes happen.
- open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
- chomp(@ok = <GREP>);
- close GREP;
+ open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
+ chomp(@ok = <GREP>);
+ close GREP;
You can also:
- my @ok = ();
- if (open(GREP, "-|")) {
- while (<GREP>) {
- chomp;
- push(@ok, $_);
- }
- close GREP;
- } else {
- exec 'grep', @opts, $search_string, @filenames;
- }
+ 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">.
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
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?
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?
Background yourself like this:
- fork && exit;
+ fork && exit;
=back
=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(fileno(*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 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.
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?
O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with
sysopen():
- use Fcntl;
- sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
- or die "can't open /foo/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;
+
+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?
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
+ $ perl -MCPAN -e shell
- cpan shell -- CPAN exploration and modules installation (v1.59_54)
- ReadLine support enabled
+ cpan shell -- CPAN exploration and modules installation (v1.59_54)
+ ReadLine support enabled
- cpan> install Some::Module
+ cpan> install Some::Module
To manually install the CPAN module, or any well-behaved CPAN module
for that matter, follow these steps:
=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?''.
+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) 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) 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 $file.
- 3) require Module is like require "Module.pm", except the former
+ 3) require Module is like require "Module.pm", except the former
3.1: translates each "::" into your system's directory separator.
3.2: primes the parser to disambiguate class Module as an indirect object.
- 4) use Module is like require Module, except the former
+ 4) use Module is like require Module, except the former
4.1: loads the module at compile time, not run-time.
4.2: imports symbols and semantics from that package to the current one.
When you build modules, use the PREFIX and LIB options when generating
Makefiles:
- perl Makefile.PL PREFIX=/mydir/perl LIB=/mydir/perl/lib
+ perl Makefile.PL PREFIX=/mydir/perl LIB=/mydir/perl/lib
then either set the PERL5LIB environment variable before you run
scripts that use the modules/libraries (see L<perlrun>) or say
- use lib '/mydir/perl/lib';
+ use lib '/mydir/perl/lib';
This is almost the same as
- BEGIN {
+ BEGIN {
unshift(@INC, '/mydir/perl/lib');
- }
+ }
except that the lib module checks for machine-dependent subdirectories.
See Perl's L<lib> for more information.
=head2 How do I add the directory my program lives in to the module/library search path?
- use FindBin;
- use lib "$FindBin::Bin";
- use your_own_modules;
+ 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 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
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) 1997-2003 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.
https://perl5.git.perl.org / perl5.git / blobdiff