[14489] | 1 | =head1 NAME
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| 2 |
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| 3 | perlfaq8 - System Interaction ($Revision: 1.27 $, $Date: 2005/12/31 00:54:37 $)
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| 4 |
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| 5 | =head1 DESCRIPTION
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| 6 |
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| 7 | This section of the Perl FAQ covers questions involving operating
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| 8 | system interaction. Topics include interprocess communication (IPC),
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| 9 | control over the user-interface (keyboard, screen and pointing
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| 10 | devices), and most anything else not related to data manipulation.
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| 11 |
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| 12 | Read the FAQs and documentation specific to the port of perl to your
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| 13 | operating system (eg, L<perlvms>, L<perlplan9>, ...). These should
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| 14 | contain more detailed information on the vagaries of your perl.
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| 15 |
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| 16 | =head2 How do I find out which operating system I'm running under?
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| 17 |
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| 18 | The $^O variable ($OSNAME if you use English) contains an indication of
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| 19 | the name of the operating system (not its release number) that your perl
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| 20 | binary was built for.
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| 21 |
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| 22 | =head2 How come exec() doesn't return?
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| 23 |
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| 24 | Because that's what it does: it replaces your currently running
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| 25 | program with a different one. If you want to keep going (as is
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| 26 | probably the case if you're asking this question) use system()
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| 27 | instead.
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| 28 |
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| 29 | =head2 How do I do fancy stuff with the keyboard/screen/mouse?
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| 30 |
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| 31 | How you access/control keyboards, screens, and pointing devices
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| 32 | ("mice") is system-dependent. Try the following modules:
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| 33 |
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| 34 | =over 4
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| 35 |
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| 36 | =item Keyboard
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| 37 |
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| 38 | Term::Cap Standard perl distribution
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| 39 | Term::ReadKey CPAN
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| 40 | Term::ReadLine::Gnu CPAN
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| 41 | Term::ReadLine::Perl CPAN
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| 42 | Term::Screen CPAN
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| 43 |
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| 44 | =item Screen
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| 45 |
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| 46 | Term::Cap Standard perl distribution
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| 47 | Curses CPAN
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| 48 | Term::ANSIColor CPAN
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| 49 |
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| 50 | =item Mouse
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| 51 |
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| 52 | Tk CPAN
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| 53 |
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| 54 | =back
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| 55 |
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| 56 | Some of these specific cases are shown as examples in other answers
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| 57 | in this section of the perlfaq.
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| 58 |
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| 59 | =head2 How do I print something out in color?
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| 60 |
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| 61 | In general, you don't, because you don't know whether
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| 62 | the recipient has a color-aware display device. If you
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| 63 | know that they have an ANSI terminal that understands
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| 64 | color, you can use the Term::ANSIColor module from CPAN:
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| 65 |
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| 66 | use Term::ANSIColor;
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| 67 | print color("red"), "Stop!\n", color("reset");
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| 68 | print color("green"), "Go!\n", color("reset");
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| 69 |
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| 70 | Or like this:
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| 71 |
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| 72 | use Term::ANSIColor qw(:constants);
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| 73 | print RED, "Stop!\n", RESET;
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| 74 | print GREEN, "Go!\n", RESET;
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| 75 |
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| 76 | =head2 How do I read just one key without waiting for a return key?
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| 77 |
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| 78 | Controlling input buffering is a remarkably system-dependent matter.
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| 79 | On many systems, you can just use the B<stty> command as shown in
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| 80 | L<perlfunc/getc>, but as you see, that's already getting you into
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| 81 | portability snags.
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| 82 |
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| 83 | open(TTY, "+</dev/tty") or die "no tty: $!";
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| 84 | system "stty cbreak </dev/tty >/dev/tty 2>&1";
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| 85 | $key = getc(TTY); # perhaps this works
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| 86 | # OR ELSE
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| 87 | sysread(TTY, $key, 1); # probably this does
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| 88 | system "stty -cbreak </dev/tty >/dev/tty 2>&1";
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| 89 |
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| 90 | The Term::ReadKey module from CPAN offers an easy-to-use interface that
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| 91 | should be more efficient than shelling out to B<stty> for each key.
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| 92 | It even includes limited support for Windows.
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| 93 |
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| 94 | use Term::ReadKey;
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| 95 | ReadMode('cbreak');
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| 96 | $key = ReadKey(0);
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| 97 | ReadMode('normal');
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| 98 |
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| 99 | However, using the code requires that you have a working C compiler
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| 100 | and can use it to build and install a CPAN module. Here's a solution
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| 101 | using the standard POSIX module, which is already on your systems
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| 102 | (assuming your system supports POSIX).
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| 103 |
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| 104 | use HotKey;
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| 105 | $key = readkey();
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| 106 |
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| 107 | And here's the HotKey module, which hides the somewhat mystifying calls
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| 108 | to manipulate the POSIX termios structures.
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| 109 |
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| 110 | # HotKey.pm
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| 111 | package HotKey;
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| 112 |
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| 113 | @ISA = qw(Exporter);
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| 114 | @EXPORT = qw(cbreak cooked readkey);
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| 115 |
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| 116 | use strict;
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| 117 | use POSIX qw(:termios_h);
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| 118 | my ($term, $oterm, $echo, $noecho, $fd_stdin);
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| 119 |
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| 120 | $fd_stdin = fileno(STDIN);
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| 121 | $term = POSIX::Termios->new();
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| 122 | $term->getattr($fd_stdin);
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| 123 | $oterm = $term->getlflag();
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| 124 |
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| 125 | $echo = ECHO | ECHOK | ICANON;
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| 126 | $noecho = $oterm & ~$echo;
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| 127 |
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| 128 | sub cbreak {
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| 129 | $term->setlflag($noecho); # ok, so i don't want echo either
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| 130 | $term->setcc(VTIME, 1);
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| 131 | $term->setattr($fd_stdin, TCSANOW);
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| 132 | }
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| 133 |
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| 134 | sub cooked {
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| 135 | $term->setlflag($oterm);
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| 136 | $term->setcc(VTIME, 0);
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| 137 | $term->setattr($fd_stdin, TCSANOW);
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| 138 | }
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| 139 |
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| 140 | sub readkey {
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| 141 | my $key = '';
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| 142 | cbreak();
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| 143 | sysread(STDIN, $key, 1);
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| 144 | cooked();
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| 145 | return $key;
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| 146 | }
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| 147 |
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| 148 | END { cooked() }
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| 149 |
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| 150 | 1;
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| 151 |
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| 152 | =head2 How do I check whether input is ready on the keyboard?
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| 153 |
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| 154 | The easiest way to do this is to read a key in nonblocking mode with the
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| 155 | Term::ReadKey module from CPAN, passing it an argument of -1 to indicate
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| 156 | not to block:
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| 157 |
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| 158 | use Term::ReadKey;
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| 159 |
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| 160 | ReadMode('cbreak');
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| 161 |
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| 162 | if (defined ($char = ReadKey(-1)) ) {
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| 163 | # input was waiting and it was $char
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| 164 | } else {
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| 165 | # no input was waiting
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| 166 | }
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| 167 |
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| 168 | ReadMode('normal'); # restore normal tty settings
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| 169 |
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| 170 | =head2 How do I clear the screen?
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| 171 |
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| 172 | If you only have do so infrequently, use C<system>:
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| 173 |
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| 174 | system("clear");
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| 175 |
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| 176 | If you have to do this a lot, save the clear string
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| 177 | so you can print it 100 times without calling a program
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| 178 | 100 times:
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| 179 |
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| 180 | $clear_string = `clear`;
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| 181 | print $clear_string;
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| 182 |
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| 183 | If you're planning on doing other screen manipulations, like cursor
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| 184 | positions, etc, you might wish to use Term::Cap module:
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| 185 |
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| 186 | use Term::Cap;
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| 187 | $terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
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| 188 | $clear_string = $terminal->Tputs('cl');
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| 189 |
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| 190 | =head2 How do I get the screen size?
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| 191 |
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| 192 | If you have Term::ReadKey module installed from CPAN,
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| 193 | you can use it to fetch the width and height in characters
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| 194 | and in pixels:
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| 195 |
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| 196 | use Term::ReadKey;
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| 197 | ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
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| 198 |
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| 199 | This is more portable than the raw C<ioctl>, but not as
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| 200 | illustrative:
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| 201 |
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| 202 | require 'sys/ioctl.ph';
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| 203 | die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
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| 204 | open(TTY, "+</dev/tty") or die "No tty: $!";
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| 205 | unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) {
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| 206 | die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ;
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| 207 | }
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| 208 | ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
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| 209 | print "(row,col) = ($row,$col)";
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| 210 | print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
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| 211 | print "\n";
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| 212 |
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| 213 | =head2 How do I ask the user for a password?
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| 214 |
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| 215 | (This question has nothing to do with the web. See a different
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| 216 | FAQ for that.)
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| 217 |
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| 218 | There's an example of this in L<perlfunc/crypt>). First, you put the
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| 219 | terminal into "no echo" mode, then just read the password normally.
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| 220 | You may do this with an old-style ioctl() function, POSIX terminal
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| 221 | control (see L<POSIX> or its documentation the Camel Book), or a call
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| 222 | to the B<stty> program, with varying degrees of portability.
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| 223 |
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| 224 | You can also do this for most systems using the Term::ReadKey module
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| 225 | from CPAN, which is easier to use and in theory more portable.
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| 226 |
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| 227 | use Term::ReadKey;
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| 228 |
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| 229 | ReadMode('noecho');
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| 230 | $password = ReadLine(0);
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| 231 |
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| 232 | =head2 How do I read and write the serial port?
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| 233 |
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| 234 | This depends on which operating system your program is running on. In
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| 235 | the case of Unix, the serial ports will be accessible through files in
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| 236 | /dev; on other systems, device names will doubtless differ.
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| 237 | Several problem areas common to all device interaction are the
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| 238 | following:
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| 239 |
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| 240 | =over 4
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| 241 |
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| 242 | =item lockfiles
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| 243 |
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| 244 | Your system may use lockfiles to control multiple access. Make sure
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| 245 | you follow the correct protocol. Unpredictable behavior can result
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| 246 | from multiple processes reading from one device.
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| 247 |
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| 248 | =item open mode
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| 249 |
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| 250 | If you expect to use both read and write operations on the device,
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| 251 | you'll have to open it for update (see L<perlfunc/"open"> for
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| 252 | details). You may wish to open it without running the risk of
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| 253 | blocking by using sysopen() and C<O_RDWR|O_NDELAY|O_NOCTTY> from the
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| 254 | Fcntl module (part of the standard perl distribution). See
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| 255 | L<perlfunc/"sysopen"> for more on this approach.
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| 256 |
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| 257 | =item end of line
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| 258 |
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| 259 | Some devices will be expecting a "\r" at the end of each line rather
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| 260 | than a "\n". In some ports of perl, "\r" and "\n" are different from
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| 261 | their usual (Unix) ASCII values of "\012" and "\015". You may have to
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| 262 | give the numeric values you want directly, using octal ("\015"), hex
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| 263 | ("0x0D"), or as a control-character specification ("\cM").
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| 264 |
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| 265 | print DEV "atv1\012"; # wrong, for some devices
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| 266 | print DEV "atv1\015"; # right, for some devices
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| 267 |
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| 268 | Even though with normal text files a "\n" will do the trick, there is
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| 269 | still no unified scheme for terminating a line that is portable
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| 270 | between Unix, DOS/Win, and Macintosh, except to terminate I<ALL> line
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| 271 | ends with "\015\012", and strip what you don't need from the output.
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| 272 | This applies especially to socket I/O and autoflushing, discussed
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| 273 | next.
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| 274 |
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| 275 | =item flushing output
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| 276 |
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| 277 | If you expect characters to get to your device when you print() them,
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| 278 | you'll want to autoflush that filehandle. You can use select()
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| 279 | and the C<$|> variable to control autoflushing (see L<perlvar/$E<verbar>>
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| 280 | and L<perlfunc/select>, or L<perlfaq5>, "How do I flush/unbuffer an
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| 281 | output filehandle? Why must I do this?"):
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| 282 |
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| 283 | $oldh = select(DEV);
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| 284 | $| = 1;
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| 285 | select($oldh);
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| 286 |
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| 287 | You'll also see code that does this without a temporary variable, as in
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| 288 |
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| 289 | select((select(DEV), $| = 1)[0]);
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| 290 |
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| 291 | Or if you don't mind pulling in a few thousand lines
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| 292 | of code just because you're afraid of a little $| variable:
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| 293 |
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| 294 | use IO::Handle;
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| 295 | DEV->autoflush(1);
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| 296 |
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| 297 | As mentioned in the previous item, this still doesn't work when using
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| 298 | socket I/O between Unix and Macintosh. You'll need to hard code your
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| 299 | line terminators, in that case.
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| 300 |
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| 301 | =item non-blocking input
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| 302 |
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| 303 | If you are doing a blocking read() or sysread(), you'll have to
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| 304 | arrange for an alarm handler to provide a timeout (see
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| 305 | L<perlfunc/alarm>). If you have a non-blocking open, you'll likely
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| 306 | have a non-blocking read, which means you may have to use a 4-arg
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| 307 | select() to determine whether I/O is ready on that device (see
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| 308 | L<perlfunc/"select">.
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| 309 |
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| 310 | =back
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| 311 |
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| 312 | While trying to read from his caller-id box, the notorious Jamie Zawinski
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| 313 | <[email protected]>, after much gnashing of teeth and fighting with sysread,
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| 314 | sysopen, POSIX's tcgetattr business, and various other functions that
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| 315 | go bump in the night, finally came up with this:
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| 316 |
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| 317 | sub open_modem {
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| 318 | use IPC::Open2;
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| 319 | my $stty = `/bin/stty -g`;
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| 320 | open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
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| 321 | # starting cu hoses /dev/tty's stty settings, even when it has
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| 322 | # been opened on a pipe...
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| 323 | system("/bin/stty $stty");
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| 324 | $_ = <MODEM_IN>;
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| 325 | chomp;
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| 326 | if ( !m/^Connected/ ) {
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| 327 | print STDERR "$0: cu printed `$_' instead of `Connected'\n";
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| 328 | }
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| 329 | }
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| 330 |
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| 331 | =head2 How do I decode encrypted password files?
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| 332 |
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| 333 | You spend lots and lots of money on dedicated hardware, but this is
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| 334 | bound to get you talked about.
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| 335 |
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| 336 | Seriously, you can't if they are Unix password files--the Unix
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| 337 | password system employs one-way encryption. It's more like hashing than
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| 338 | encryption. The best you can check is whether something else hashes to
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| 339 | the same string. You can't turn a hash back into the original string.
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| 340 | Programs like Crack
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| 341 | can forcibly (and intelligently) try to guess passwords, but don't
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| 342 | (can't) guarantee quick success.
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| 343 |
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| 344 | If you're worried about users selecting bad passwords, you should
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| 345 | proactively check when they try to change their password (by modifying
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| 346 | passwd(1), for example).
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| 347 |
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| 348 | =head2 How do I start a process in the background?
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| 349 |
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| 350 | Several modules can start other processes that do not block
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| 351 | your Perl program. You can use IPC::Open3, Parallel::Jobs,
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| 352 | IPC::Run, and some of the POE modules. See CPAN for more
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| 353 | details.
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| 354 |
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| 355 | You could also use
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| 356 |
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| 357 | system("cmd &")
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| 358 |
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| 359 | or you could use fork as documented in L<perlfunc/"fork">, with
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| 360 | further examples in L<perlipc>. Some things to be aware of, if you're
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| 361 | on a Unix-like system:
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| 362 |
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| 363 | =over 4
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| 364 |
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| 365 | =item STDIN, STDOUT, and STDERR are shared
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| 366 |
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| 367 | Both the main process and the backgrounded one (the "child" process)
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| 368 | share the same STDIN, STDOUT and STDERR filehandles. If both try to
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| 369 | access them at once, strange things can happen. You may want to close
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| 370 | or reopen these for the child. You can get around this with
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| 371 | C<open>ing a pipe (see L<perlfunc/"open">) but on some systems this
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| 372 | means that the child process cannot outlive the parent.
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| 373 |
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| 374 | =item Signals
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| 375 |
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| 376 | You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
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| 377 | SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is
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| 378 | sent when you write to a filehandle whose child process has closed (an
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| 379 | untrapped SIGPIPE can cause your program to silently die). This is
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| 380 | not an issue with C<system("cmd&")>.
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| 381 |
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| 382 | =item Zombies
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| 383 |
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| 384 | You have to be prepared to "reap" the child process when it finishes.
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| 385 |
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| 386 | $SIG{CHLD} = sub { wait };
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| 387 |
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| 388 | $SIG{CHLD} = 'IGNORE';
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| 389 |
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| 390 | You can also use a double fork. You immediately wait() for your
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| 391 | first child, and the init daemon will wait() for your grandchild once
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| 392 | it exits.
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| 393 |
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| 394 | unless ($pid = fork) {
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| 395 | unless (fork) {
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| 396 | exec "what you really wanna do";
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| 397 | die "exec failed!";
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| 398 | }
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| 399 | exit 0;
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| 400 | }
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| 401 | waitpid($pid,0);
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| 402 |
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| 403 |
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| 404 | See L<perlipc/"Signals"> for other examples of code to do this.
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| 405 | Zombies are not an issue with C<system("prog &")>.
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| 406 |
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| 407 | =back
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| 408 |
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| 409 | =head2 How do I trap control characters/signals?
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| 410 |
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| 411 | You don't actually "trap" a control character. Instead, that character
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| 412 | generates a signal which is sent to your terminal's currently
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| 413 | foregrounded process group, which you then trap in your process.
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| 414 | Signals are documented in L<perlipc/"Signals"> and the
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| 415 | section on "Signals" in the Camel.
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| 416 |
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| 417 | You can set the values of the %SIG hash to be the functions you want
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| 418 | to handle the signal. After perl catches the signal, it looks in %SIG
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| 419 | for a key with the same name as the signal, then calls the subroutine
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| 420 | value for that key.
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| 421 |
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| 422 | # as an anonymous subroutine
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| 423 |
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| 424 | $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) };
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| 425 |
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| 426 | # or a reference to a function
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| 427 |
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| 428 | $SIG{INT} = \&ouch;
|
---|
| 429 |
|
---|
| 430 | # or the name of the function as a string
|
---|
| 431 |
|
---|
| 432 | $SIG{INT} = "ouch";
|
---|
| 433 |
|
---|
| 434 | Perl versions before 5.8 had in its C source code signal handlers which
|
---|
| 435 | would catch the signal and possibly run a Perl function that you had set
|
---|
| 436 | in %SIG. This violated the rules of signal handling at that level
|
---|
| 437 | causing perl to dump core. Since version 5.8.0, perl looks at %SIG
|
---|
| 438 | *after* the signal has been caught, rather than while it is being caught.
|
---|
| 439 | Previous versions of this answer were incorrect.
|
---|
| 440 |
|
---|
| 441 |
|
---|
| 442 | =head2 How do I modify the shadow password file on a Unix system?
|
---|
| 443 |
|
---|
| 444 | If perl was installed correctly and your shadow library was written
|
---|
| 445 | properly, the getpw*() functions described in L<perlfunc> should in
|
---|
| 446 | theory provide (read-only) access to entries in the shadow password
|
---|
| 447 | file. To change the file, make a new shadow password file (the format
|
---|
| 448 | varies from system to system--see L<passwd> for specifics) and use
|
---|
| 449 | pwd_mkdb(8) to install it (see L<pwd_mkdb> for more details).
|
---|
| 450 |
|
---|
| 451 | =head2 How do I set the time and date?
|
---|
| 452 |
|
---|
| 453 | Assuming you're running under sufficient permissions, you should be
|
---|
| 454 | able to set the system-wide date and time by running the date(1)
|
---|
| 455 | program. (There is no way to set the time and date on a per-process
|
---|
| 456 | basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT;
|
---|
| 457 | the VMS equivalent is C<set time>.
|
---|
| 458 |
|
---|
| 459 | However, if all you want to do is change your time zone, you can
|
---|
| 460 | probably get away with setting an environment variable:
|
---|
| 461 |
|
---|
| 462 | $ENV{TZ} = "MST7MDT"; # unixish
|
---|
| 463 | $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
|
---|
| 464 | system "trn comp.lang.perl.misc";
|
---|
| 465 |
|
---|
| 466 | =head2 How can I sleep() or alarm() for under a second?
|
---|
| 467 |
|
---|
| 468 | If you want finer granularity than the 1 second that the sleep()
|
---|
| 469 | function provides, the easiest way is to use the select() function as
|
---|
| 470 | documented in L<perlfunc/"select">. Try the Time::HiRes and
|
---|
| 471 | the BSD::Itimer modules (available from CPAN, and starting from
|
---|
| 472 | Perl 5.8 Time::HiRes is part of the standard distribution).
|
---|
| 473 |
|
---|
| 474 | =head2 How can I measure time under a second?
|
---|
| 475 |
|
---|
| 476 | In general, you may not be able to. The Time::HiRes module (available
|
---|
| 477 | from CPAN, and starting from Perl 5.8 part of the standard distribution)
|
---|
| 478 | provides this functionality for some systems.
|
---|
| 479 |
|
---|
| 480 | If your system supports both the syscall() function in Perl as well as
|
---|
| 481 | a system call like gettimeofday(2), then you may be able to do
|
---|
| 482 | something like this:
|
---|
| 483 |
|
---|
| 484 | require 'sys/syscall.ph';
|
---|
| 485 |
|
---|
| 486 | $TIMEVAL_T = "LL";
|
---|
| 487 |
|
---|
| 488 | $done = $start = pack($TIMEVAL_T, ());
|
---|
| 489 |
|
---|
| 490 | syscall(&SYS_gettimeofday, $start, 0) != -1
|
---|
| 491 | or die "gettimeofday: $!";
|
---|
| 492 |
|
---|
| 493 | ##########################
|
---|
| 494 | # DO YOUR OPERATION HERE #
|
---|
| 495 | ##########################
|
---|
| 496 |
|
---|
| 497 | syscall( &SYS_gettimeofday, $done, 0) != -1
|
---|
| 498 | or die "gettimeofday: $!";
|
---|
| 499 |
|
---|
| 500 | @start = unpack($TIMEVAL_T, $start);
|
---|
| 501 | @done = unpack($TIMEVAL_T, $done);
|
---|
| 502 |
|
---|
| 503 | # fix microseconds
|
---|
| 504 | for ($done[1], $start[1]) { $_ /= 1_000_000 }
|
---|
| 505 |
|
---|
| 506 | $delta_time = sprintf "%.4f", ($done[0] + $done[1] )
|
---|
| 507 | -
|
---|
| 508 | ($start[0] + $start[1] );
|
---|
| 509 |
|
---|
| 510 | =head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
|
---|
| 511 |
|
---|
| 512 | Release 5 of Perl added the END block, which can be used to simulate
|
---|
| 513 | atexit(). Each package's END block is called when the program or
|
---|
| 514 | thread ends (see L<perlmod> manpage for more details).
|
---|
| 515 |
|
---|
| 516 | For example, you can use this to make sure your filter program
|
---|
| 517 | managed to finish its output without filling up the disk:
|
---|
| 518 |
|
---|
| 519 | END {
|
---|
| 520 | close(STDOUT) || die "stdout close failed: $!";
|
---|
| 521 | }
|
---|
| 522 |
|
---|
| 523 | The END block isn't called when untrapped signals kill the program,
|
---|
| 524 | though, so if you use END blocks you should also use
|
---|
| 525 |
|
---|
| 526 | use sigtrap qw(die normal-signals);
|
---|
| 527 |
|
---|
| 528 | Perl's exception-handling mechanism is its eval() operator. You can
|
---|
| 529 | use eval() as setjmp and die() as longjmp. For details of this, see
|
---|
| 530 | the section on signals, especially the time-out handler for a blocking
|
---|
| 531 | flock() in L<perlipc/"Signals"> or the section on "Signals" in
|
---|
| 532 | the Camel Book.
|
---|
| 533 |
|
---|
| 534 | If exception handling is all you're interested in, try the
|
---|
| 535 | exceptions.pl library (part of the standard perl distribution).
|
---|
| 536 |
|
---|
| 537 | If you want the atexit() syntax (and an rmexit() as well), try the
|
---|
| 538 | AtExit module available from CPAN.
|
---|
| 539 |
|
---|
| 540 | =head2 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
|
---|
| 541 |
|
---|
| 542 | Some Sys-V based systems, notably Solaris 2.X, redefined some of the
|
---|
| 543 | standard socket constants. Since these were constant across all
|
---|
| 544 | architectures, they were often hardwired into perl code. The proper
|
---|
| 545 | way to deal with this is to "use Socket" to get the correct values.
|
---|
| 546 |
|
---|
| 547 | Note that even though SunOS and Solaris are binary compatible, these
|
---|
| 548 | values are different. Go figure.
|
---|
| 549 |
|
---|
| 550 | =head2 How can I call my system's unique C functions from Perl?
|
---|
| 551 |
|
---|
| 552 | In most cases, you write an external module to do it--see the answer
|
---|
| 553 | to "Where can I learn about linking C with Perl? [h2xs, xsubpp]".
|
---|
| 554 | However, if the function is a system call, and your system supports
|
---|
| 555 | syscall(), you can use the syscall function (documented in
|
---|
| 556 | L<perlfunc>).
|
---|
| 557 |
|
---|
| 558 | Remember to check the modules that came with your distribution, and
|
---|
| 559 | CPAN as well---someone may already have written a module to do it. On
|
---|
| 560 | Windows, try Win32::API. On Macs, try Mac::Carbon. If no module
|
---|
| 561 | has an interface to the C function, you can inline a bit of C in your
|
---|
| 562 | Perl source with Inline::C.
|
---|
| 563 |
|
---|
| 564 | =head2 Where do I get the include files to do ioctl() or syscall()?
|
---|
| 565 |
|
---|
| 566 | Historically, these would be generated by the h2ph tool, part of the
|
---|
| 567 | standard perl distribution. This program converts cpp(1) directives
|
---|
| 568 | in C header files to files containing subroutine definitions, like
|
---|
| 569 | &SYS_getitimer, which you can use as arguments to your functions.
|
---|
| 570 | It doesn't work perfectly, but it usually gets most of the job done.
|
---|
| 571 | Simple files like F<errno.h>, F<syscall.h>, and F<socket.h> were fine,
|
---|
| 572 | but the hard ones like F<ioctl.h> nearly always need to hand-edited.
|
---|
| 573 | Here's how to install the *.ph files:
|
---|
| 574 |
|
---|
| 575 | 1. become super-user
|
---|
| 576 | 2. cd /usr/include
|
---|
| 577 | 3. h2ph *.h */*.h
|
---|
| 578 |
|
---|
| 579 | If your system supports dynamic loading, for reasons of portability and
|
---|
| 580 | sanity you probably ought to use h2xs (also part of the standard perl
|
---|
| 581 | distribution). This tool converts C header files to Perl extensions.
|
---|
| 582 | See L<perlxstut> for how to get started with h2xs.
|
---|
| 583 |
|
---|
| 584 | If your system doesn't support dynamic loading, you still probably
|
---|
| 585 | ought to use h2xs. See L<perlxstut> and L<ExtUtils::MakeMaker> for
|
---|
| 586 | more information (in brief, just use B<make perl> instead of a plain
|
---|
| 587 | B<make> to rebuild perl with a new static extension).
|
---|
| 588 |
|
---|
| 589 | =head2 Why do setuid perl scripts complain about kernel problems?
|
---|
| 590 |
|
---|
| 591 | Some operating systems have bugs in the kernel that make setuid
|
---|
| 592 | scripts inherently insecure. Perl gives you a number of options
|
---|
| 593 | (described in L<perlsec>) to work around such systems.
|
---|
| 594 |
|
---|
| 595 | =head2 How can I open a pipe both to and from a command?
|
---|
| 596 |
|
---|
| 597 | The IPC::Open2 module (part of the standard perl distribution) is an
|
---|
| 598 | easy-to-use approach that internally uses pipe(), fork(), and exec() to do
|
---|
| 599 | the job. Make sure you read the deadlock warnings in its documentation,
|
---|
| 600 | though (see L<IPC::Open2>). See
|
---|
| 601 | L<perlipc/"Bidirectional Communication with Another Process"> and
|
---|
| 602 | L<perlipc/"Bidirectional Communication with Yourself">
|
---|
| 603 |
|
---|
| 604 | You may also use the IPC::Open3 module (part of the standard perl
|
---|
| 605 | distribution), but be warned that it has a different order of
|
---|
| 606 | arguments from IPC::Open2 (see L<IPC::Open3>).
|
---|
| 607 |
|
---|
| 608 | =head2 Why can't I get the output of a command with system()?
|
---|
| 609 |
|
---|
| 610 | You're confusing the purpose of system() and backticks (``). system()
|
---|
| 611 | runs a command and returns exit status information (as a 16 bit value:
|
---|
| 612 | the low 7 bits are the signal the process died from, if any, and
|
---|
| 613 | the high 8 bits are the actual exit value). Backticks (``) run a
|
---|
| 614 | command and return what it sent to STDOUT.
|
---|
| 615 |
|
---|
| 616 | $exit_status = system("mail-users");
|
---|
| 617 | $output_string = `ls`;
|
---|
| 618 |
|
---|
| 619 | =head2 How can I capture STDERR from an external command?
|
---|
| 620 |
|
---|
| 621 | There are three basic ways of running external commands:
|
---|
| 622 |
|
---|
| 623 | system $cmd; # using system()
|
---|
| 624 | $output = `$cmd`; # using backticks (``)
|
---|
| 625 | open (PIPE, "cmd |"); # using open()
|
---|
| 626 |
|
---|
| 627 | With system(), both STDOUT and STDERR will go the same place as the
|
---|
| 628 | script's STDOUT and STDERR, unless the system() command redirects them.
|
---|
| 629 | Backticks and open() read B<only> the STDOUT of your command.
|
---|
| 630 |
|
---|
| 631 | You can also use the open3() function from IPC::Open3. Benjamin
|
---|
| 632 | Goldberg provides some sample code:
|
---|
| 633 |
|
---|
| 634 | To capture a program's STDOUT, but discard its STDERR:
|
---|
| 635 |
|
---|
| 636 | use IPC::Open3;
|
---|
| 637 | use File::Spec;
|
---|
| 638 | use Symbol qw(gensym);
|
---|
| 639 | open(NULL, ">", File::Spec->devnull);
|
---|
| 640 | my $pid = open3(gensym, \*PH, ">&NULL", "cmd");
|
---|
| 641 | while( <PH> ) { }
|
---|
| 642 | waitpid($pid, 0);
|
---|
| 643 |
|
---|
| 644 | To capture a program's STDERR, but discard its STDOUT:
|
---|
| 645 |
|
---|
| 646 | use IPC::Open3;
|
---|
| 647 | use File::Spec;
|
---|
| 648 | use Symbol qw(gensym);
|
---|
| 649 | open(NULL, ">", File::Spec->devnull);
|
---|
| 650 | my $pid = open3(gensym, ">&NULL", \*PH, "cmd");
|
---|
| 651 | while( <PH> ) { }
|
---|
| 652 | waitpid($pid, 0);
|
---|
| 653 |
|
---|
| 654 | To capture a program's STDERR, and let its STDOUT go to our own STDERR:
|
---|
| 655 |
|
---|
| 656 | use IPC::Open3;
|
---|
| 657 | use Symbol qw(gensym);
|
---|
| 658 | my $pid = open3(gensym, ">&STDERR", \*PH, "cmd");
|
---|
| 659 | while( <PH> ) { }
|
---|
| 660 | waitpid($pid, 0);
|
---|
| 661 |
|
---|
| 662 | To read both a command's STDOUT and its STDERR separately, you can
|
---|
| 663 | redirect them to temp files, let the command run, then read the temp
|
---|
| 664 | files:
|
---|
| 665 |
|
---|
| 666 | use IPC::Open3;
|
---|
| 667 | use Symbol qw(gensym);
|
---|
| 668 | use IO::File;
|
---|
| 669 | local *CATCHOUT = IO::File->new_tmpfile;
|
---|
| 670 | local *CATCHERR = IO::File->new_tmpfile;
|
---|
| 671 | my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd");
|
---|
| 672 | waitpid($pid, 0);
|
---|
| 673 | seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
|
---|
| 674 | while( <CATCHOUT> ) {}
|
---|
| 675 | while( <CATCHERR> ) {}
|
---|
| 676 |
|
---|
| 677 | But there's no real need for *both* to be tempfiles... the following
|
---|
| 678 | should work just as well, without deadlocking:
|
---|
| 679 |
|
---|
| 680 | use IPC::Open3;
|
---|
| 681 | use Symbol qw(gensym);
|
---|
| 682 | use IO::File;
|
---|
| 683 | local *CATCHERR = IO::File->new_tmpfile;
|
---|
| 684 | my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd");
|
---|
| 685 | while( <CATCHOUT> ) {}
|
---|
| 686 | waitpid($pid, 0);
|
---|
| 687 | seek CATCHERR, 0, 0;
|
---|
| 688 | while( <CATCHERR> ) {}
|
---|
| 689 |
|
---|
| 690 | And it'll be faster, too, since we can begin processing the program's
|
---|
| 691 | stdout immediately, rather than waiting for the program to finish.
|
---|
| 692 |
|
---|
| 693 | With any of these, you can change file descriptors before the call:
|
---|
| 694 |
|
---|
| 695 | open(STDOUT, ">logfile");
|
---|
| 696 | system("ls");
|
---|
| 697 |
|
---|
| 698 | or you can use Bourne shell file-descriptor redirection:
|
---|
| 699 |
|
---|
| 700 | $output = `$cmd 2>some_file`;
|
---|
| 701 | open (PIPE, "cmd 2>some_file |");
|
---|
| 702 |
|
---|
| 703 | You can also use file-descriptor redirection to make STDERR a
|
---|
| 704 | duplicate of STDOUT:
|
---|
| 705 |
|
---|
| 706 | $output = `$cmd 2>&1`;
|
---|
| 707 | open (PIPE, "cmd 2>&1 |");
|
---|
| 708 |
|
---|
| 709 | Note that you I<cannot> simply open STDERR to be a dup of STDOUT
|
---|
| 710 | in your Perl program and avoid calling the shell to do the redirection.
|
---|
| 711 | This doesn't work:
|
---|
| 712 |
|
---|
| 713 | open(STDERR, ">&STDOUT");
|
---|
| 714 | $alloutput = `cmd args`; # stderr still escapes
|
---|
| 715 |
|
---|
| 716 | This fails because the open() makes STDERR go to where STDOUT was
|
---|
| 717 | going at the time of the open(). The backticks then make STDOUT go to
|
---|
| 718 | a string, but don't change STDERR (which still goes to the old
|
---|
| 719 | STDOUT).
|
---|
| 720 |
|
---|
| 721 | Note that you I<must> use Bourne shell (sh(1)) redirection syntax in
|
---|
| 722 | backticks, not csh(1)! Details on why Perl's system() and backtick
|
---|
| 723 | and pipe opens all use the Bourne shell are in the
|
---|
| 724 | F<versus/csh.whynot> article in the "Far More Than You Ever Wanted To
|
---|
| 725 | Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . To
|
---|
| 726 | capture a command's STDERR and STDOUT together:
|
---|
| 727 |
|
---|
| 728 | $output = `cmd 2>&1`; # either with backticks
|
---|
| 729 | $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe
|
---|
| 730 | while (<PH>) { } # plus a read
|
---|
| 731 |
|
---|
| 732 | To capture a command's STDOUT but discard its STDERR:
|
---|
| 733 |
|
---|
| 734 | $output = `cmd 2>/dev/null`; # either with backticks
|
---|
| 735 | $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe
|
---|
| 736 | while (<PH>) { } # plus a read
|
---|
| 737 |
|
---|
| 738 | To capture a command's STDERR but discard its STDOUT:
|
---|
| 739 |
|
---|
| 740 | $output = `cmd 2>&1 1>/dev/null`; # either with backticks
|
---|
| 741 | $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe
|
---|
| 742 | while (<PH>) { } # plus a read
|
---|
| 743 |
|
---|
| 744 | To exchange a command's STDOUT and STDERR in order to capture the STDERR
|
---|
| 745 | but leave its STDOUT to come out our old STDERR:
|
---|
| 746 |
|
---|
| 747 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks
|
---|
| 748 | $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
|
---|
| 749 | while (<PH>) { } # plus a read
|
---|
| 750 |
|
---|
| 751 | To read both a command's STDOUT and its STDERR separately, it's easiest
|
---|
| 752 | to redirect them separately to files, and then read from those files
|
---|
| 753 | when the program is done:
|
---|
| 754 |
|
---|
| 755 | system("program args 1>program.stdout 2>program.stderr");
|
---|
| 756 |
|
---|
| 757 | Ordering is important in all these examples. That's because the shell
|
---|
| 758 | processes file descriptor redirections in strictly left to right order.
|
---|
| 759 |
|
---|
| 760 | system("prog args 1>tmpfile 2>&1");
|
---|
| 761 | system("prog args 2>&1 1>tmpfile");
|
---|
| 762 |
|
---|
| 763 | The first command sends both standard out and standard error to the
|
---|
| 764 | temporary file. The second command sends only the old standard output
|
---|
| 765 | there, and the old standard error shows up on the old standard out.
|
---|
| 766 |
|
---|
| 767 | =head2 Why doesn't open() return an error when a pipe open fails?
|
---|
| 768 |
|
---|
| 769 | If the second argument to a piped open() contains shell
|
---|
| 770 | metacharacters, perl fork()s, then exec()s a shell to decode the
|
---|
| 771 | metacharacters and eventually run the desired program. If the program
|
---|
| 772 | couldn't be run, it's the shell that gets the message, not Perl. All
|
---|
| 773 | your Perl program can find out is whether the shell itself could be
|
---|
| 774 | successfully started. You can still capture the shell's STDERR and
|
---|
| 775 | check it for error messages. See L<"How can I capture STDERR from an
|
---|
| 776 | external command?"> elsewhere in this document, or use the
|
---|
| 777 | IPC::Open3 module.
|
---|
| 778 |
|
---|
| 779 | If there are no shell metacharacters in the argument of open(), Perl
|
---|
| 780 | runs the command directly, without using the shell, and can correctly
|
---|
| 781 | report whether the command started.
|
---|
| 782 |
|
---|
| 783 | =head2 What's wrong with using backticks in a void context?
|
---|
| 784 |
|
---|
| 785 | Strictly speaking, nothing. Stylistically speaking, it's not a good
|
---|
| 786 | way to write maintainable code. Perl has several operators for
|
---|
| 787 | running external commands. Backticks are one; they collect the output
|
---|
| 788 | from the command for use in your program. The C<system> function is
|
---|
| 789 | another; it doesn't do this.
|
---|
| 790 |
|
---|
| 791 | Writing backticks in your program sends a clear message to the readers
|
---|
| 792 | of your code that you wanted to collect the output of the command.
|
---|
| 793 | Why send a clear message that isn't true?
|
---|
| 794 |
|
---|
| 795 | Consider this line:
|
---|
| 796 |
|
---|
| 797 | `cat /etc/termcap`;
|
---|
| 798 |
|
---|
| 799 | You forgot to check C<$?> to see whether the program even ran
|
---|
| 800 | correctly. Even if you wrote
|
---|
| 801 |
|
---|
| 802 | print `cat /etc/termcap`;
|
---|
| 803 |
|
---|
| 804 | this code could and probably should be written as
|
---|
| 805 |
|
---|
| 806 | system("cat /etc/termcap") == 0
|
---|
| 807 | or die "cat program failed!";
|
---|
| 808 |
|
---|
| 809 | which will get the output quickly (as it is generated, instead of only
|
---|
| 810 | at the end) and also check the return value.
|
---|
| 811 |
|
---|
| 812 | system() also provides direct control over whether shell wildcard
|
---|
| 813 | processing may take place, whereas backticks do not.
|
---|
| 814 |
|
---|
| 815 | =head2 How can I call backticks without shell processing?
|
---|
| 816 |
|
---|
| 817 | This is a bit tricky. You can't simply write the command
|
---|
| 818 | like this:
|
---|
| 819 |
|
---|
| 820 | @ok = `grep @opts '$search_string' @filenames`;
|
---|
| 821 |
|
---|
| 822 | As of Perl 5.8.0, you can use open() with multiple arguments.
|
---|
| 823 | Just like the list forms of system() and exec(), no shell
|
---|
| 824 | escapes happen.
|
---|
| 825 |
|
---|
| 826 | open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
|
---|
| 827 | chomp(@ok = <GREP>);
|
---|
| 828 | close GREP;
|
---|
| 829 |
|
---|
| 830 | You can also:
|
---|
| 831 |
|
---|
| 832 | my @ok = ();
|
---|
| 833 | if (open(GREP, "-|")) {
|
---|
| 834 | while (<GREP>) {
|
---|
| 835 | chomp;
|
---|
| 836 | push(@ok, $_);
|
---|
| 837 | }
|
---|
| 838 | close GREP;
|
---|
| 839 | } else {
|
---|
| 840 | exec 'grep', @opts, $search_string, @filenames;
|
---|
| 841 | }
|
---|
| 842 |
|
---|
| 843 | Just as with system(), no shell escapes happen when you exec() a list.
|
---|
| 844 | Further examples of this can be found in L<perlipc/"Safe Pipe Opens">.
|
---|
| 845 |
|
---|
| 846 | Note that if you're use Microsoft, no solution to this vexing issue
|
---|
| 847 | is even possible. Even if Perl were to emulate fork(), you'd still
|
---|
| 848 | be stuck, because Microsoft does not have a argc/argv-style API.
|
---|
| 849 |
|
---|
| 850 | =head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
|
---|
| 851 |
|
---|
| 852 | Some stdio's set error and eof flags that need clearing. The
|
---|
| 853 | POSIX module defines clearerr() that you can use. That is the
|
---|
| 854 | technically correct way to do it. Here are some less reliable
|
---|
| 855 | workarounds:
|
---|
| 856 |
|
---|
| 857 | =over 4
|
---|
| 858 |
|
---|
| 859 | =item 1
|
---|
| 860 |
|
---|
| 861 | Try keeping around the seekpointer and go there, like this:
|
---|
| 862 |
|
---|
| 863 | $where = tell(LOG);
|
---|
| 864 | seek(LOG, $where, 0);
|
---|
| 865 |
|
---|
| 866 | =item 2
|
---|
| 867 |
|
---|
| 868 | If that doesn't work, try seeking to a different part of the file and
|
---|
| 869 | then back.
|
---|
| 870 |
|
---|
| 871 | =item 3
|
---|
| 872 |
|
---|
| 873 | If that doesn't work, try seeking to a different part of
|
---|
| 874 | the file, reading something, and then seeking back.
|
---|
| 875 |
|
---|
| 876 | =item 4
|
---|
| 877 |
|
---|
| 878 | If that doesn't work, give up on your stdio package and use sysread.
|
---|
| 879 |
|
---|
| 880 | =back
|
---|
| 881 |
|
---|
| 882 | =head2 How can I convert my shell script to perl?
|
---|
| 883 |
|
---|
| 884 | Learn Perl and rewrite it. Seriously, there's no simple converter.
|
---|
| 885 | Things that are awkward to do in the shell are easy to do in Perl, and
|
---|
| 886 | this very awkwardness is what would make a shell->perl converter
|
---|
| 887 | nigh-on impossible to write. By rewriting it, you'll think about what
|
---|
| 888 | you're really trying to do, and hopefully will escape the shell's
|
---|
| 889 | pipeline datastream paradigm, which while convenient for some matters,
|
---|
| 890 | causes many inefficiencies.
|
---|
| 891 |
|
---|
| 892 | =head2 Can I use perl to run a telnet or ftp session?
|
---|
| 893 |
|
---|
| 894 | Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
|
---|
| 895 | CPAN). http://www.cpan.org/scripts/netstuff/telnet.emul.shar
|
---|
| 896 | will also help for emulating the telnet protocol, but Net::Telnet is
|
---|
| 897 | quite probably easier to use..
|
---|
| 898 |
|
---|
| 899 | If all you want to do is pretend to be telnet but don't need
|
---|
| 900 | the initial telnet handshaking, then the standard dual-process
|
---|
| 901 | approach will suffice:
|
---|
| 902 |
|
---|
| 903 | use IO::Socket; # new in 5.004
|
---|
| 904 | $handle = IO::Socket::INET->new('www.perl.com:80')
|
---|
| 905 | || die "can't connect to port 80 on www.perl.com: $!";
|
---|
| 906 | $handle->autoflush(1);
|
---|
| 907 | if (fork()) { # XXX: undef means failure
|
---|
| 908 | select($handle);
|
---|
| 909 | print while <STDIN>; # everything from stdin to socket
|
---|
| 910 | } else {
|
---|
| 911 | print while <$handle>; # everything from socket to stdout
|
---|
| 912 | }
|
---|
| 913 | close $handle;
|
---|
| 914 | exit;
|
---|
| 915 |
|
---|
| 916 | =head2 How can I write expect in Perl?
|
---|
| 917 |
|
---|
| 918 | Once upon a time, there was a library called chat2.pl (part of the
|
---|
| 919 | standard perl distribution), which never really got finished. If you
|
---|
| 920 | find it somewhere, I<don't use it>. These days, your best bet is to
|
---|
| 921 | look at the Expect module available from CPAN, which also requires two
|
---|
| 922 | other modules from CPAN, IO::Pty and IO::Stty.
|
---|
| 923 |
|
---|
| 924 | =head2 Is there a way to hide perl's command line from programs such as "ps"?
|
---|
| 925 |
|
---|
| 926 | First of all note that if you're doing this for security reasons (to
|
---|
| 927 | avoid people seeing passwords, for example) then you should rewrite
|
---|
| 928 | your program so that critical information is never given as an
|
---|
| 929 | argument. Hiding the arguments won't make your program completely
|
---|
| 930 | secure.
|
---|
| 931 |
|
---|
| 932 | To actually alter the visible command line, you can assign to the
|
---|
| 933 | variable $0 as documented in L<perlvar>. This won't work on all
|
---|
| 934 | operating systems, though. Daemon programs like sendmail place their
|
---|
| 935 | state there, as in:
|
---|
| 936 |
|
---|
| 937 | $0 = "orcus [accepting connections]";
|
---|
| 938 |
|
---|
| 939 | =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?
|
---|
| 940 |
|
---|
| 941 | =over 4
|
---|
| 942 |
|
---|
| 943 | =item Unix
|
---|
| 944 |
|
---|
| 945 | In the strictest sense, it can't be done--the script executes as a
|
---|
| 946 | different process from the shell it was started from. Changes to a
|
---|
| 947 | process are not reflected in its parent--only in any children
|
---|
| 948 | created after the change. There is shell magic that may allow you to
|
---|
| 949 | fake it by eval()ing the script's output in your shell; check out the
|
---|
| 950 | comp.unix.questions FAQ for details.
|
---|
| 951 |
|
---|
| 952 | =back
|
---|
| 953 |
|
---|
| 954 | =head2 How do I close a process's filehandle without waiting for it to complete?
|
---|
| 955 |
|
---|
| 956 | Assuming your system supports such things, just send an appropriate signal
|
---|
| 957 | to the process (see L<perlfunc/"kill">). It's common to first send a TERM
|
---|
| 958 | signal, wait a little bit, and then send a KILL signal to finish it off.
|
---|
| 959 |
|
---|
| 960 | =head2 How do I fork a daemon process?
|
---|
| 961 |
|
---|
| 962 | If by daemon process you mean one that's detached (disassociated from
|
---|
| 963 | its tty), then the following process is reported to work on most
|
---|
| 964 | Unixish systems. Non-Unix users should check their Your_OS::Process
|
---|
| 965 | module for other solutions.
|
---|
| 966 |
|
---|
| 967 | =over 4
|
---|
| 968 |
|
---|
| 969 | =item *
|
---|
| 970 |
|
---|
| 971 | Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty>
|
---|
| 972 | for details. Or better yet, you can just use the POSIX::setsid()
|
---|
| 973 | function, so you don't have to worry about process groups.
|
---|
| 974 |
|
---|
| 975 | =item *
|
---|
| 976 |
|
---|
| 977 | Change directory to /
|
---|
| 978 |
|
---|
| 979 | =item *
|
---|
| 980 |
|
---|
| 981 | Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
|
---|
| 982 | tty.
|
---|
| 983 |
|
---|
| 984 | =item *
|
---|
| 985 |
|
---|
| 986 | Background yourself like this:
|
---|
| 987 |
|
---|
| 988 | fork && exit;
|
---|
| 989 |
|
---|
| 990 | =back
|
---|
| 991 |
|
---|
| 992 | The Proc::Daemon module, available from CPAN, provides a function to
|
---|
| 993 | perform these actions for you.
|
---|
| 994 |
|
---|
| 995 | =head2 How do I find out if I'm running interactively or not?
|
---|
| 996 |
|
---|
| 997 | Good question. Sometimes C<-t STDIN> and C<-t STDOUT> can give clues,
|
---|
| 998 | sometimes not.
|
---|
| 999 |
|
---|
| 1000 | if (-t STDIN && -t STDOUT) {
|
---|
| 1001 | print "Now what? ";
|
---|
| 1002 | }
|
---|
| 1003 |
|
---|
| 1004 | On POSIX systems, you can test whether your own process group matches
|
---|
| 1005 | the current process group of your controlling terminal as follows:
|
---|
| 1006 |
|
---|
| 1007 | use POSIX qw/getpgrp tcgetpgrp/;
|
---|
| 1008 | open(TTY, "/dev/tty") or die $!;
|
---|
| 1009 | $tpgrp = tcgetpgrp(fileno(*TTY));
|
---|
| 1010 | $pgrp = getpgrp();
|
---|
| 1011 | if ($tpgrp == $pgrp) {
|
---|
| 1012 | print "foreground\n";
|
---|
| 1013 | } else {
|
---|
| 1014 | print "background\n";
|
---|
| 1015 | }
|
---|
| 1016 |
|
---|
| 1017 | =head2 How do I timeout a slow event?
|
---|
| 1018 |
|
---|
| 1019 | Use the alarm() function, probably in conjunction with a signal
|
---|
| 1020 | handler, as documented in L<perlipc/"Signals"> and the section on
|
---|
| 1021 | "Signals" in the Camel. You may instead use the more flexible
|
---|
| 1022 | Sys::AlarmCall module available from CPAN.
|
---|
| 1023 |
|
---|
| 1024 | The alarm() function is not implemented on all versions of Windows.
|
---|
| 1025 | Check the documentation for your specific version of Perl.
|
---|
| 1026 |
|
---|
| 1027 | =head2 How do I set CPU limits?
|
---|
| 1028 |
|
---|
| 1029 | Use the BSD::Resource module from CPAN.
|
---|
| 1030 |
|
---|
| 1031 | =head2 How do I avoid zombies on a Unix system?
|
---|
| 1032 |
|
---|
| 1033 | Use the reaper code from L<perlipc/"Signals"> to call wait() when a
|
---|
| 1034 | SIGCHLD is received, or else use the double-fork technique described
|
---|
| 1035 | in L<perlfaq8/"How do I start a process in the background?">.
|
---|
| 1036 |
|
---|
| 1037 | =head2 How do I use an SQL database?
|
---|
| 1038 |
|
---|
| 1039 | The DBI module provides an abstract interface to most database
|
---|
| 1040 | servers and types, including Oracle, DB2, Sybase, mysql, Postgresql,
|
---|
| 1041 | ODBC, and flat files. The DBI module accesses each database type
|
---|
| 1042 | through a database driver, or DBD. You can see a complete list of
|
---|
| 1043 | available drivers on CPAN: http://www.cpan.org/modules/by-module/DBD/ .
|
---|
| 1044 | You can read more about DBI on http://dbi.perl.org .
|
---|
| 1045 |
|
---|
| 1046 | Other modules provide more specific access: Win32::ODBC, Alzabo, iodbc,
|
---|
| 1047 | and others found on CPAN Search: http://search.cpan.org .
|
---|
| 1048 |
|
---|
| 1049 | =head2 How do I make a system() exit on control-C?
|
---|
| 1050 |
|
---|
| 1051 | You can't. You need to imitate the system() call (see L<perlipc> for
|
---|
| 1052 | sample code) and then have a signal handler for the INT signal that
|
---|
| 1053 | passes the signal on to the subprocess. Or you can check for it:
|
---|
| 1054 |
|
---|
| 1055 | $rc = system($cmd);
|
---|
| 1056 | if ($rc & 127) { die "signal death" }
|
---|
| 1057 |
|
---|
| 1058 | =head2 How do I open a file without blocking?
|
---|
| 1059 |
|
---|
| 1060 | If you're lucky enough to be using a system that supports
|
---|
| 1061 | non-blocking reads (most Unixish systems do), you need only to use the
|
---|
| 1062 | O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with
|
---|
| 1063 | sysopen():
|
---|
| 1064 |
|
---|
| 1065 | use Fcntl;
|
---|
| 1066 | sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
|
---|
| 1067 | or die "can't open /foo/somefile: $!":
|
---|
| 1068 |
|
---|
| 1069 | =head2 How do I tell the difference between errors from the shell and perl?
|
---|
| 1070 |
|
---|
| 1071 | (answer contributed by brian d foy, C<< <[email protected]> >>
|
---|
| 1072 |
|
---|
| 1073 | When you run a Perl script, something else is running the script for you,
|
---|
| 1074 | and that something else may output error messages. The script might
|
---|
| 1075 | emit its own warnings and error messages. Most of the time you cannot
|
---|
| 1076 | tell who said what.
|
---|
| 1077 |
|
---|
| 1078 | You probably cannot fix the thing that runs perl, but you can change how
|
---|
| 1079 | perl outputs its warnings by defining a custom warning and die functions.
|
---|
| 1080 |
|
---|
| 1081 | Consider this script, which has an error you may not notice immediately.
|
---|
| 1082 |
|
---|
| 1083 | #!/usr/locl/bin/perl
|
---|
| 1084 |
|
---|
| 1085 | print "Hello World\n";
|
---|
| 1086 |
|
---|
| 1087 | I get an error when I run this from my shell (which happens to be
|
---|
| 1088 | bash). That may look like perl forgot it has a print() function,
|
---|
| 1089 | but my shebang line is not the path to perl, so the shell runs the
|
---|
| 1090 | script, and I get the error.
|
---|
| 1091 |
|
---|
| 1092 | $ ./test
|
---|
| 1093 | ./test: line 3: print: command not found
|
---|
| 1094 |
|
---|
| 1095 | A quick and dirty fix involves a little bit of code, but this may be all
|
---|
| 1096 | you need to figure out the problem.
|
---|
| 1097 |
|
---|
| 1098 | #!/usr/bin/perl -w
|
---|
| 1099 |
|
---|
| 1100 | BEGIN {
|
---|
| 1101 | $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; };
|
---|
| 1102 | $SIG{__DIE__} = sub{ print STDERR "Perl: ", @_; exit 1};
|
---|
| 1103 | }
|
---|
| 1104 |
|
---|
| 1105 | $a = 1 + undef;
|
---|
| 1106 | $x / 0;
|
---|
| 1107 | __END__
|
---|
| 1108 |
|
---|
| 1109 | The perl message comes out with "Perl" in front. The BEGIN block
|
---|
| 1110 | works at compile time so all of the compilation errors and warnings
|
---|
| 1111 | get the "Perl:" prefix too.
|
---|
| 1112 |
|
---|
| 1113 | Perl: Useless use of division (/) in void context at ./test line 9.
|
---|
| 1114 | Perl: Name "main::a" used only once: possible typo at ./test line 8.
|
---|
| 1115 | Perl: Name "main::x" used only once: possible typo at ./test line 9.
|
---|
| 1116 | Perl: Use of uninitialized value in addition (+) at ./test line 8.
|
---|
| 1117 | Perl: Use of uninitialized value in division (/) at ./test line 9.
|
---|
| 1118 | Perl: Illegal division by zero at ./test line 9.
|
---|
| 1119 | Perl: Illegal division by zero at -e line 3.
|
---|
| 1120 |
|
---|
| 1121 | If I don't see that "Perl:", it's not from perl.
|
---|
| 1122 |
|
---|
| 1123 | You could also just know all the perl errors, and although there are
|
---|
| 1124 | some people who may know all of them, you probably don't. However, they
|
---|
| 1125 | all should be in the perldiag manpage. If you don't find the error in
|
---|
| 1126 | there, it probably isn't a perl error.
|
---|
| 1127 |
|
---|
| 1128 | Looking up every message is not the easiest way, so let perl to do it
|
---|
| 1129 | for you. Use the diagnostics pragma with turns perl's normal messages
|
---|
| 1130 | into longer discussions on the topic.
|
---|
| 1131 |
|
---|
| 1132 | use diagnostics;
|
---|
| 1133 |
|
---|
| 1134 | If you don't get a paragraph or two of expanded discussion, it
|
---|
| 1135 | might not be perl's message.
|
---|
| 1136 |
|
---|
| 1137 | =head2 How do I install a module from CPAN?
|
---|
| 1138 |
|
---|
| 1139 | The easiest way is to have a module also named CPAN do it for you.
|
---|
| 1140 | This module comes with perl version 5.004 and later.
|
---|
| 1141 |
|
---|
| 1142 | $ perl -MCPAN -e shell
|
---|
| 1143 |
|
---|
| 1144 | cpan shell -- CPAN exploration and modules installation (v1.59_54)
|
---|
| 1145 | ReadLine support enabled
|
---|
| 1146 |
|
---|
| 1147 | cpan> install Some::Module
|
---|
| 1148 |
|
---|
| 1149 | To manually install the CPAN module, or any well-behaved CPAN module
|
---|
| 1150 | for that matter, follow these steps:
|
---|
| 1151 |
|
---|
| 1152 | =over 4
|
---|
| 1153 |
|
---|
| 1154 | =item 1
|
---|
| 1155 |
|
---|
| 1156 | Unpack the source into a temporary area.
|
---|
| 1157 |
|
---|
| 1158 | =item 2
|
---|
| 1159 |
|
---|
| 1160 | perl Makefile.PL
|
---|
| 1161 |
|
---|
| 1162 | =item 3
|
---|
| 1163 |
|
---|
| 1164 | make
|
---|
| 1165 |
|
---|
| 1166 | =item 4
|
---|
| 1167 |
|
---|
| 1168 | make test
|
---|
| 1169 |
|
---|
| 1170 | =item 5
|
---|
| 1171 |
|
---|
| 1172 | make install
|
---|
| 1173 |
|
---|
| 1174 | =back
|
---|
| 1175 |
|
---|
| 1176 | If your version of perl is compiled without dynamic loading, then you
|
---|
| 1177 | just need to replace step 3 (B<make>) with B<make perl> and you will
|
---|
| 1178 | get a new F<perl> binary with your extension linked in.
|
---|
| 1179 |
|
---|
| 1180 | See L<ExtUtils::MakeMaker> for more details on building extensions.
|
---|
| 1181 | See also the next question, "What's the difference between require
|
---|
| 1182 | and use?".
|
---|
| 1183 |
|
---|
| 1184 | =head2 What's the difference between require and use?
|
---|
| 1185 |
|
---|
| 1186 | Perl offers several different ways to include code from one file into
|
---|
| 1187 | another. Here are the deltas between the various inclusion constructs:
|
---|
| 1188 |
|
---|
| 1189 | 1) do $file is like eval `cat $file`, except the former
|
---|
| 1190 | 1.1: searches @INC and updates %INC.
|
---|
| 1191 | 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
|
---|
| 1192 |
|
---|
| 1193 | 2) require $file is like do $file, except the former
|
---|
| 1194 | 2.1: checks for redundant loading, skipping already loaded files.
|
---|
| 1195 | 2.2: raises an exception on failure to find, compile, or execute $file.
|
---|
| 1196 |
|
---|
| 1197 | 3) require Module is like require "Module.pm", except the former
|
---|
| 1198 | 3.1: translates each "::" into your system's directory separator.
|
---|
| 1199 | 3.2: primes the parser to disambiguate class Module as an indirect object.
|
---|
| 1200 |
|
---|
| 1201 | 4) use Module is like require Module, except the former
|
---|
| 1202 | 4.1: loads the module at compile time, not run-time.
|
---|
| 1203 | 4.2: imports symbols and semantics from that package to the current one.
|
---|
| 1204 |
|
---|
| 1205 | In general, you usually want C<use> and a proper Perl module.
|
---|
| 1206 |
|
---|
| 1207 | =head2 How do I keep my own module/library directory?
|
---|
| 1208 |
|
---|
| 1209 | When you build modules, use the PREFIX and LIB options when generating
|
---|
| 1210 | Makefiles:
|
---|
| 1211 |
|
---|
| 1212 | perl Makefile.PL PREFIX=/mydir/perl LIB=/mydir/perl/lib
|
---|
| 1213 |
|
---|
| 1214 | then either set the PERL5LIB environment variable before you run
|
---|
| 1215 | scripts that use the modules/libraries (see L<perlrun>) or say
|
---|
| 1216 |
|
---|
| 1217 | use lib '/mydir/perl/lib';
|
---|
| 1218 |
|
---|
| 1219 | This is almost the same as
|
---|
| 1220 |
|
---|
| 1221 | BEGIN {
|
---|
| 1222 | unshift(@INC, '/mydir/perl/lib');
|
---|
| 1223 | }
|
---|
| 1224 |
|
---|
| 1225 | except that the lib module checks for machine-dependent subdirectories.
|
---|
| 1226 | See Perl's L<lib> for more information.
|
---|
| 1227 |
|
---|
| 1228 | =head2 How do I add the directory my program lives in to the module/library search path?
|
---|
| 1229 |
|
---|
| 1230 | use FindBin;
|
---|
| 1231 | use lib "$FindBin::Bin";
|
---|
| 1232 | use your_own_modules;
|
---|
| 1233 |
|
---|
| 1234 | =head2 How do I add a directory to my include path (@INC) at runtime?
|
---|
| 1235 |
|
---|
| 1236 | Here are the suggested ways of modifying your include path:
|
---|
| 1237 |
|
---|
| 1238 | the PERLLIB environment variable
|
---|
| 1239 | the PERL5LIB environment variable
|
---|
| 1240 | the perl -Idir command line flag
|
---|
| 1241 | the use lib pragma, as in
|
---|
| 1242 | use lib "$ENV{HOME}/myown_perllib";
|
---|
| 1243 |
|
---|
| 1244 | The latter is particularly useful because it knows about machine
|
---|
| 1245 | dependent architectures. The lib.pm pragmatic module was first
|
---|
| 1246 | included with the 5.002 release of Perl.
|
---|
| 1247 |
|
---|
| 1248 | =head2 What is socket.ph and where do I get it?
|
---|
| 1249 |
|
---|
| 1250 | It's a perl4-style file defining values for system networking
|
---|
| 1251 | constants. Sometimes it is built using h2ph when Perl is installed,
|
---|
| 1252 | but other times it is not. Modern programs C<use Socket;> instead.
|
---|
| 1253 |
|
---|
| 1254 | =head1 AUTHOR AND COPYRIGHT
|
---|
| 1255 |
|
---|
| 1256 | Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
|
---|
| 1257 | other authors as noted. All rights reserved.
|
---|
| 1258 |
|
---|
| 1259 | This documentation is free; you can redistribute it and/or modify it
|
---|
| 1260 | under the same terms as Perl itself.
|
---|
| 1261 |
|
---|
| 1262 | Irrespective of its distribution, all code examples in this file
|
---|
| 1263 | are hereby placed into the public domain. You are permitted and
|
---|
| 1264 | encouraged to use this code in your own programs for fun
|
---|
| 1265 | or for profit as you see fit. A simple comment in the code giving
|
---|
| 1266 | credit would be courteous but is not required.
|
---|