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perlfaq8 ()
  • >> perlfaq8 (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • perlfaq8 (1) ( Разные man: Команды и прикладные программы пользовательского уровня )
  • 
    
    

    NAME

         perlfaq8 - System Interaction ($Revision: 1.39 $, $Date:
         1999/05/23 18:37:57 $)
    
    
    

    DESCRIPTION

         This section of the Perl FAQ covers questions involving
         operating system interaction.  This involves interprocess
         communication (IPC), control over the user-interface
         (keyboard, screen and pointing devices), and most anything
         else not related to data manipulation.
    
         Read the FAQs and documentation specific to the port of perl
         to your operating system (eg, the perlvms manpage, the
         perlplan9 manpage, ...).  These should contain more detailed
         information on the vagaries of your perl.
    
         How do I find out which operating system I'm running under?
    
         The $^O variable ($OSNAME if you use English) contains an
         indication of the name of the operating system (not its
         release number) that your perl binary was built for.
    
         How come exec() doesn't return?
    
         Because that's what it does: it replaces your currently
         running program with a different one.  If you want to keep
         going (as is probably the case if you're asking this
         question) use system() instead.
    
         How do I do fancy stuff with the keyboard/screen/mouse?
    
         How you access/control keyboards, screens, and pointing
         devices ("mice") is system-dependent.  Try the following
         modules:
    
         Keyboard
                 Term::Cap                   Standard perl distribution
                 Term::ReadKey               CPAN
                 Term::ReadLine::Gnu         CPAN
                 Term::ReadLine::Perl        CPAN
                 Term::Screen                CPAN
    
    
         Screen
                 Term::Cap                   Standard perl distribution
                 Curses                      CPAN
                 Term::ANSIColor             CPAN
    
    
         Mouse
                 Tk                          CPAN
    
         Some of these specific cases are shown below.
    
         How do I print something out in color?
    
         In general, you don't, because you don't know whether the
         recipient has a color-aware display device.  If you know
         that they have an ANSI terminal that understands color, you
         can use the Term::ANSIColor module from CPAN:
    
             use Term::ANSIColor;
             print color("red"), "Stop!\n", color("reset");
             print color("green"), "Go!\n", color("reset");
    
         Or like this:
    
             use Term::ANSIColor qw(:constants);
             print RED, "Stop!\n", RESET;
             print GREEN, "Go!\n", RESET;
    
    
         How do I read just one key without waiting for a return key?
    
         Controlling input buffering is a remarkably system-dependent
         matter.  On many systems, you can just use the stty command
         as shown in the getc entry in the perlfunc manpage, but as
         you see, that's already getting you into portability snags.
    
             open(TTY, "+</dev/tty") or die "no tty: $!";
             system "stty  cbreak </dev/tty >/dev/tty 2>&1";
             $key = getc(TTY);           # perhaps this works
             # OR ELSE
             sysread(TTY, $key, 1);      # probably this does
             system "stty -cbreak </dev/tty >/dev/tty 2>&1";
    
         The Term::ReadKey module from CPAN offers an easy-to-use
         interface that should be more efficient than shelling out to
         stty for each key.  It even includes limited support for
         Windows.
    
             use Term::ReadKey;
             ReadMode('cbreak');
             $key = ReadKey(0);
             ReadMode('normal');
    
         However, that requires that you have a working C compiler
         and can use it to build and install a CPAN module.  Here's a
         solution using the standard POSIX module, which is already
         on your systems (assuming your system supports POSIX).
    
             use HotKey;
             $key = readkey();
    
         And here's the HotKey module, which hides the somewhat
         mystifying calls to manipulate the POSIX termios structures.
    
             # HotKey.pm
             package HotKey;
    
             @ISA = qw(Exporter);
             @EXPORT = qw(cbreak cooked readkey);
    
             use strict;
             use POSIX qw(:termios_h);
             my ($term, $oterm, $echo, $noecho, $fd_stdin);
    
             $fd_stdin = fileno(STDIN);
             $term     = POSIX::Termios->new();
             $term->getattr($fd_stdin);
             $oterm     = $term->getlflag();
    
             $echo     = ECHO | ECHOK | ICANON;
             $noecho   = $oterm & ~$echo;
    
             sub cbreak {
                 $term->setlflag($noecho);  # ok, so i don't want echo either
                 $term->setcc(VTIME, 1);
                 $term->setattr($fd_stdin, TCSANOW);
             }
    
             sub cooked {
                 $term->setlflag($oterm);
                 $term->setcc(VTIME, 0);
                 $term->setattr($fd_stdin, TCSANOW);
             }
    
             sub readkey {
                 my $key = '';
                 cbreak();
                 sysread(STDIN, $key, 1);
                 cooked();
                 return $key;
             }
    
             END { cooked() }
    
             1;
    
    
         How do I check whether input is ready on the keyboard?
    
         The easiest way to do this is to read a key in nonblocking
         mode with the Term::ReadKey module from CPAN, passing it an
         argument of -1 to indicate not to block:
    
             use Term::ReadKey;
    
             ReadMode('cbreak');
    
             if (defined ($char = ReadKey(-1)) ) {
                 # input was waiting and it was $char
             } else {
                 # no input was waiting
             }
    
             ReadMode('normal');                  # restore normal tty settings
    
    
         How do I clear the screen?
    
         If you only have do so infrequently, use `system':
    
             system("clear");
    
         If you have to do this a lot, save the clear string so you
         can print it 100 times without calling a program 100 times:
    
             $clear_string = `clear`;
             print $clear_string;
    
         If you're planning on doing other screen manipulations, like
         cursor positions, etc, you might wish to use Term::Cap
         module:
    
             use Term::Cap;
             $terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
             $clear_string = $terminal->Tputs('cl');
    
    
         How do I get the screen size?
    
         If you have Term::ReadKey module installed from CPAN, you
         can use it to fetch the width and height in characters and
         in pixels:
    
             use Term::ReadKey;
             ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
    
         This is more portable than the raw `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";
    
    
         How do I ask the user for a password?
    
         (This question has nothing to do with the web.  See a
         different FAQ for that.)
    
         There's an example of this in the crypt entry in the
         perlfunc manpage).  First, you put the terminal into "no
         echo" mode, then just read the password normally.  You may
         do this with an old-style ioctl() function, POSIX terminal
         control (see the POSIX manpage, and Chapter 7 of the Camel),
         or a call to the stty program, with varying degrees of
         portability.
    
         You can also do this for most systems using the
         Term::ReadKey module from CPAN, which is easier to use and
         in theory more portable.
    
             use Term::ReadKey;
    
             ReadMode('noecho');
             $password = ReadLine(0);
    
    
         How do I read and write the serial port?
    
         This depends on which operating system your program is
         running on.  In the case of Unix, the serial ports will be
         accessible through files in /dev; on other systems, the
         devices names will doubtless differ.  Several problem areas
         common to all device interaction are the following
    
         lockfiles
             Your system may use lockfiles to control multiple
             access.  Make sure you follow the correct protocol.
             Unpredictable behaviour can result from multiple
             processes reading from one device.
    
         open mode
             If you expect to use both read and write operations on
             the device, you'll have to open it for update (see the
             open entry in the perlfunc manpage for details).  You
             may wish to open it without running the risk of blocking
             by using sysopen() and `O_RDWR|O_NDELAY|O_NOCTTY' from
             the Fcntl module (part of the standard perl
             distribution).  See the sysopen entry in the perlfunc
             manpage for more on this approach.
    
         end of line
             Some devices will be expecting a "\r" at the end of each
             line rather than a "\n".  In some ports of perl, "\r"
             and "\n" are different from their usual (Unix) ASCII
             values of "\012" and "\015".  You may have to 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
    
             Even though with normal text files, a "\n" will do the
             trick, there is still no unified scheme for terminating
             a line that is portable between Unix, DOS/Win, and
             Macintosh, except to terminate ALL line ends with
             "\015\012", and strip what you don't need from the
             output.  This applies especially to socket I/O and
             autoflushing, discussed next.
    
         flushing output
             If you expect characters to get to your device when you
             print() them, you'll want to autoflush that filehandle.
             You can use select() and the `$|' variable to control
             autoflushing (see perlvar/$ and the select entry in the
             perlfunc manpage):
    
                 $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]);
    
             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);
    
             As mentioned in the previous item, this still doesn't
             work when using socket I/O between Unix and Macintosh.
             You'll need to hardcode your line terminators, in that
             case.
    
         non-blocking input
             If you are doing a blocking read() or sysread(), you'll
             have to arrange for an alarm handler to provide a
             timeout (see the alarm entry in the perlfunc manpage).
             If you have a non-blocking open, you'll likely have a
             non-blocking read, which means you may have to use a
             4-arg select() to determine whether I/O is ready on that
             device (see the select entry in the perlfunc manpage.
    
         While trying to read from his caller-id box, the notorious
         Jamie Zawinski <jwz@netscape.com>, after much gnashing of
         teeth and fighting with sysread, sysopen, POSIX's tcgetattr
         business, and various other functions that go bump in the
         night, finally came up with this:
    
             sub open_modem {
                 use IPC::Open2;
                 my $stty = `/bin/stty -g`;
                 open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
                 # starting cu hoses /dev/tty's stty settings, even when it has
                 # been opened on a pipe...
                 system("/bin/stty $stty");
                 $_ = <MODEM_IN>;
                 chop;
                 if ( !m/^Connected/ ) {
                     print STDERR "$0: cu printed `$_' instead of `Connected'\n";
                 }
             }
    
    
         How do I decode encrypted password files?
    
         You spend lots and lots of money on dedicated hardware, but
         this is bound to get you talked about.
    
         Seriously, you can't if they are Unix password files - the
         Unix password system employs one-way encryption.  It's more
         like hashing than encryption.  The best you can check is
         whether something else hashes to the same string.  You can't
         turn a hash back into the original string.  Programs like
         Crack can forcibly (and intelligently) try to guess
         passwords, but don't (can't) guarantee quick success.
    
         If you're worried about users selecting bad passwords, you
         should proactively check when they try to change their
         password (by modifying passwd(1), for example).
    
    
    
         How do I start a process in the background?
    
         You could use
    
             system("cmd &")
    
         or you could use fork as documented in the fork entry in the
         perlfunc manpage, with further examples in the perlipc
         manpage.  Some things to be aware of, if you're on a Unix-
         like system:
    
         STDIN, STDOUT, and STDERR are shared
             Both the main process and the backgrounded one (the
             "child" process) share the same STDIN, STDOUT and STDERR
             filehandles.  If both try to access them at once,
             strange things can happen.  You may want to close or
             reopen these for the child.  You can get around this
             with `open'ing a pipe (see the open entry in the
             perlfunc manpage) but on some systems this means that
             the child process cannot outlive the parent.
    
         Signals
             You'll have to catch the SIGCHLD signal, and possibly
             SIGPIPE too.  SIGCHLD is sent when the backgrounded
             process finishes.  SIGPIPE is sent when you write to a
             filehandle whose child process has closed (an untrapped
             SIGPIPE can cause your program to silently die).  This
             is not an issue with `system("cmd&")'.
    
         Zombies
             You have to be prepared to "reap" the child process when
             it finishes
    
                 $SIG{CHLD} = sub { wait };
    
             See the Signals entry in the perlipc manpage for other
             examples of code to do this.  Zombies are not an issue
             with `system("prog &")'.
    
         How do I trap control characters/signals?
    
         You don't actually "trap" a control character.  Instead,
         that character generates a signal which is sent to your
         terminal's currently foregrounded process group, which you
         then trap in your process.  Signals are documented in the
         Signals entry in the perlipc manpage and chapter 6 of 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: set a variable and exit.
         And in the first case, you should only set a variable in
         such a way that malloc() is not called (eg, by setting a
         variable that already has a value).
    
         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 the Signals
         entry in the perlipc manpage or chapter 6 of the Camel.
    
         How do I modify the shadow password file on a Unix system?
    
         If perl was installed correctly, and your shadow library was
         written properly, the getpw*() functions described in the
         perlfunc manpage should in theory provide (read-only) access
         to entries in the shadow password file.  To change the file,
         make a new shadow password file (the format varies from
         system to system - see passwd(5) for specifics) and use
         pwd_mkdb(8) to install it (see pwd_mkdb(8) for more
         details).
    
         How do I set the time and date?
    
         Assuming you're running under sufficient permissions, you
         should be able to set the system-wide date and time by
         running the date(1) program.  (There is no way to set the
         time and date on a per-process basis.)  This mechanism will
         work for Unix, MS-DOS, Windows, and NT; the VMS equivalent
         is `set time'.
    
         However, if all you want to do is change your timezone, 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";
    
    
         How can I sleep() or alarm() for under a second?
    
         If you want finer granularity than the 1 second that the
         sleep() function provides, the easiest way is to use the
         select() function as documented in the select entry in the
         perlfunc manpage.  If your system has itimers and syscall()
         support, you can check out the old example in
         http://www.perl.com/CPAN/doc/misc/ancient/tutorial/eg/itimers.pl
         .
    
         How can I measure time under a second?
    
         In general, you may not be able to.  The Time::HiRes module
         (available from CPAN) provides this functionality for some
         systems.
    
         If your system supports both the syscall() function in Perl
         as well as a system call like gettimeofday(2), then you may
         be able to do something like this:
    
             require 'sys/syscall.ph';
    
             $TIMEVAL_T = "LL";
    
             $done = $start = pack($TIMEVAL_T, ());
    
             syscall(&SYS_gettimeofday, $start, 0) != -1
                        or die "gettimeofday: $!";
    
                ##########################
                # DO YOUR OPERATION HERE #
                ##########################
    
             syscall( &SYS_gettimeofday, $done, 0) != -1
                    or die "gettimeofday: $!";
    
             @start = unpack($TIMEVAL_T, $start);
             @done  = unpack($TIMEVAL_T, $done);
    
             # fix microseconds
             for ($done[1], $start[1]) { $_ /= 1_000_000 }
    
             $delta_time = sprintf "%.4f", ($done[0]  + $done[1]  )
                                                     -
                                          ($start[0] + $start[1] );
    
    
         How can I do an atexit() or setjmp()/longjmp()? (Exception
         handling)
    
         Release 5 of Perl added the END block, which can be used to
         simulate atexit().  Each package's END block is called when
         the program or thread ends (see the perlmod manpage manpage
         for more details).
    
         For example, you can use this to make sure your filter
         program managed to finish its output without filling up the
         disk:
    
             END {
                 close(STDOUT) || die "stdout close failed: $!";
             }
    
         The END block isn't called when untrapped signals kill the
         program, though, so if you use END blocks you should also
         use
    
                 use sigtrap qw(die normal-signals);
    
         Perl's exception-handling mechanism is its eval() operator.
         You can use eval() as setjmp and die() as longjmp.  For
         details of this, see the section on signals, especially the
         time-out handler for a blocking flock() in the Signals entry
         in the perlipc manpage and chapter 6 of the Camel.
    
         If exception handling is all you're interested in, try the
         exceptions.pl library (part of the standard perl
         distribution).
    
         If you want the atexit() syntax (and an rmexit() as well),
         try the AtExit module available from CPAN.
    
         Why doesn't my sockets program work under System V
         (Solaris)? What does the error message "Protocol not
         supported" mean?
    
         Some Sys-V based systems, notably Solaris 2.X, redefined
         some of the standard socket constants.  Since these were
         constant across all architectures, they were often hardwired
         into perl code.  The proper way to deal with this is to "use
         Socket" to get the correct values.
    
         Note that even though SunOS and Solaris are binary
         compatible, these values are different.  Go figure.
    
         How can I call my system's unique C functions from Perl?
    
         In most cases, you write an external module to do it - see
         the answer to "Where can I learn about linking C with Perl?
         [h2xs, xsubpp]".  However, if the function is a system call,
         and your system supports syscall(), you can use the syscall
         function (documented in the perlfunc manpage).
    
    
         Remember to check the modules that came with your
         distribution, and CPAN as well - someone may already have
         written a module to do it.
    
         Where do I get the include files to do ioctl() or syscall()?
    
         Historically, these would be generated by the h2ph tool,
         part of the standard perl distribution.  This program
         converts cpp(1) directives in C header files to files
         containing subroutine definitions, like &SYS_getitimer,
         which you can use as arguments to your functions.  It
         doesn't work perfectly, but it usually gets most of the job
         done.  Simple files like errno.h, syscall.h, and socket.h
         were fine, but the hard ones like 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
    
         If your system supports dynamic loading, for reasons of
         portability and sanity you probably ought to use h2xs (also
         part of the standard perl distribution).  This tool converts
         C header files to Perl extensions.  See the perlxstut
         manpage for how to get started with h2xs.
    
         If your system doesn't support dynamic loading, you still
         probably ought to use h2xs.  See the perlxstut manpage and
         the ExtUtils::MakeMaker manpage for more information (in
         brief, just use make perl instead of a plain make to rebuild
         perl with a new static extension).
    
         Why do setuid perl scripts complain about kernel problems?
    
         Some operating systems have bugs in the kernel that make
         setuid scripts inherently insecure.  Perl gives you a number
         of options (described in the perlsec manpage) to work around
         such systems.
    
         How can I open a pipe both to and from a command?
    
         The IPC::Open2 module (part of the standard perl
         distribution) is an easy-to-use approach that internally
         uses pipe(), fork(), and exec() to do the job.  Make sure
         you read the deadlock warnings in its documentation, though
         (see the IPC::Open2 manpage).  See the Bidirectional
         Communication with Another Process entry in the perlipc
         manpage and the Bidirectional Communication with Yourself
         entry in the perlipc manpage
    
         You may also use the IPC::Open3 module (part of the standard
         perl distribution), but be warned that it has a different
         order of arguments from IPC::Open2 (see the IPC::Open3
         manpage).
    
         Why can't I get the output of a command with system()?
    
         You're confusing the purpose of system() and backticks (``).
         system() runs a command and returns exit status information
         (as a 16 bit value:  the low 7 bits are the signal the
         process died from, if any, and the high 8 bits are the
         actual exit value).  Backticks (``) run a command and return
         what it sent to STDOUT.
    
             $exit_status   = system("mail-users");
             $output_string = `ls`;
    
    
         How can I capture STDERR from an external command?
    
         There are three basic ways of running external commands:
    
             system $cmd;                # using system()
             $output = `$cmd`;           # using backticks (``)
             open (PIPE, "cmd |");       # using open()
    
         With system(), both STDOUT and STDERR will go the same place
         as the script's versions of these, unless the command
         redirects them.  Backticks and open() read only the STDOUT
         of your command.
    
         With any of these, you can change file descriptors before
         the call:
    
             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 |");
    
         You can also use file-descriptor redirection to make STDERR
         a duplicate of STDOUT:
    
             $output = `$cmd 2>&1`;
             open (PIPE, "cmd 2>&1 |");
    
         Note that you 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
    
         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 a string, but don't change STDERR
         (which still goes to the old STDOUT).
    
         Note that you must use Bourne shell (sh(1)) redirection
         syntax in backticks, not csh(1)!  Details on why Perl's
         system() and backtick and pipe opens all use the Bourne
         shell are in
         http://www.perl.com/CPAN/doc/FMTEYEWTK/versus/csh.whynot .
         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
    
         To capture a command's STDOUT but discard its STDERR:
    
             $output = `cmd 2>/dev/null`;                # either with backticks
             $pid = open(PH, "cmd 2>/dev/null |");       # or with an open pipe
             while (<PH>) { }                            #    plus a read
    
         To capture a command's STDERR but discard its STDOUT:
    
             $output = `cmd 2>&1 1>/dev/null`;           # either with backticks
             $pid = open(PH, "cmd 2>&1 1>/dev/null |");  # or with an open pipe
             while (<PH>) { }                            #    plus a read
    
         To exchange a command's STDOUT and STDERR in order to
         capture the STDERR but leave its STDOUT to come out our old
         STDERR:
    
             $output = `cmd 3>&1 1>&2 2>&3 3>&-`;        # either with backticks
             $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
             while (<PH>) { }                            #    plus a read
    
         To read both a command's STDOUT and its STDERR separately,
         it's easiest and safest to redirect them separately to
         files, and then read from those files when the program is
         done:
    
             system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
    
         Ordering is important in all these examples.  That's because
         the shell processes file descriptor redirections in strictly
         left to right order.
    
             system("prog args 1>tmpfile 2>&1");
             system("prog args 2>&1 1>tmpfile");
    
         The first command sends both standard out and standard error
         to the temporary file.  The second command sends only the
         old standard output there, and the old standard error shows
         up on the old standard out.
    
         Why doesn't open() return an error when a pipe open fails?
    
         Because the pipe open takes place in two steps: first Perl
         calls fork() to start a new process, then this new process
         calls exec() to run the program you really wanted to open.
         The first step reports success or failure to your process,
         so open() can only tell you whether the fork() succeeded or
         not.
    
         To find out if the exec() step succeeded, you have to catch
         SIGCHLD and wait() to get the exit status.  You should also
         catch SIGPIPE if you're writing to the child--you may not
         have found out the exec() failed by the time you write.
         This is documented in the perlipc manpage.
    
         In some cases, even this won't work.  If the second argument
         to a piped open() contains shell metacharacters, perl
         fork()s, then exec()s a shell to decode the metacharacters
         and eventually run the desired program.  Now when you call
         wait(), you only learn whether or not the shell could be
         successfully started.  Best to avoid shell metacharacters.
    
         On systems that follow the spawn() paradigm, open() might do
         what you expect--unless perl uses a shell to start your
         command. In this case the fork()/exec() description still
         applies.
    
         What's wrong with using backticks in a void context?
    
         Strictly speaking, nothing.  Stylistically speaking, it's
         not a good way to write maintainable code because backticks
         have a (potentially humongous) return value, and you're
         ignoring it.  It's may also not be very efficient, because
         you have to read in all the lines of output, allocate memory
         for them, and then throw it away.  Too often people are
         lulled to writing:
    
             `cp file file.bak`;
    
         And now they think "Hey, I'll just always use backticks to
         run programs."  Bad idea: backticks are for capturing a
         program's output; the system() function is for running
         programs.
    
         Consider this line:
    
             `cat /etc/termcap`;
    
         You haven't assigned the output anywhere, so it just wastes
         memory (for a little while).  Plus you forgot to check `$?'
         to see whether the program even ran correctly.  Even if you
         wrote
    
             print `cat /etc/termcap`;
    
         In most cases, this could and probably should be written as
    
             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 processing may take place, whereas backticks do
         not.
    
         How can I call backticks without shell processing?
    
         This is a bit tricky.  Instead of writing
    
             @ok = `grep @opts '$search_string' @filenames`;
    
         You have to do this:
    
             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 the
         Safe Pipe Opens entry in the perlipc manpage.
    
         Note that if you're stuck on Microsoft, no solution to this
         vexing issue is even possible.  Even if Perl were to emulate
         fork(), you'd still be hosed, because Microsoft gives no
         argc/argv-style API.  Their API always reparses from a
         single string, which is fundamentally wrong, but you're not
         likely to get the Gods of Redmond to acknowledge this and
         fix it for you.
    
    
    
         Why can't my script read from STDIN after I gave it EOF (^D
         on Unix, ^Z on MS-DOS)?
    
         Because some stdio's set error and eof flags that need
         clearing.  The POSIX module defines clearerr() that you can
         use.  That is the technically correct way to do it.  Here
         are some less reliable workarounds:
    
         1   Try keeping around the seekpointer and go there, like
             this:
    
                 $where = tell(LOG);
                 seek(LOG, $where, 0);
    
    
         2   If that doesn't work, try seeking to a different part of
             the file and then back.
    
         3   If that doesn't work, try seeking to a different part of
             the file, reading something, and then seeking back.
    
         4   If that doesn't work, give up on your stdio package and
             use sysread.
    
         How can I convert my shell script to perl?
    
         Learn Perl and rewrite it.  Seriously, there's no simple
         converter.  Things that are awkward to do in the shell are
         easy to do in Perl, and this very awkwardness is what would
         make a shell->perl converter nigh-on impossible to write.
         By rewriting it, you'll think about what you're really
         trying to do, and hopefully will escape the shell's pipeline
         datastream paradigm, which while convenient for some
         matters, causes many inefficiencies.
    
         Can I use perl to run a telnet or ftp session?
    
         Try the Net::FTP, TCP::Client, and Net::Telnet modules
         (available from CPAN).
         http://www.perl.com/CPAN/scripts/netstuff/telnet.emul.shar
         will also help for emulating the telnet protocol, but
         Net::Telnet is quite probably easier to use..
    
         If all you want to do is pretend to be telnet but don't need
         the initial telnet handshaking, then the standard dual-
         process approach will suffice:
    
    
    
             use IO::Socket;             # new in 5.004
             $handle = IO::Socket::INET->new('www.perl.com:80')
                     || 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;
    
    
         How can I write expect in Perl?
    
         Once upon a time, there was a library called chat2.pl (part
         of the standard perl distribution), which never really got
         finished.  If you find it somewhere, don't use it.  These
         days, your best bet is to look at the Expect module
         available from CPAN, which also requires two other modules
         from CPAN, IO::Pty and IO::Stty.
    
         Is there a way to hide perl's command line from programs
         such as "ps"?
    
         First of all note that if you're doing this for security
         reasons (to avoid people seeing passwords, for example) then
         you should rewrite your program so that critical information
         is never given as an argument.  Hiding the arguments won't
         make your program completely secure.
    
         To actually alter the visible command line, you can assign
         to the variable $0 as documented in the perlvar manpage.
         This won't work on all operating systems, though.  Daemon
         programs like sendmail place their state there, as in:
    
             $0 = "orcus [accepting connections]";
    
    
         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?
    
         Unix
             In the strictest sense, it can't be done -- the script
             executes as a different process from the shell it was
             started from.  Changes to a process are not reflected in
             its parent, only in its own children created after the
             change.  There is shell magic that may allow you to fake
             it by eval()ing the script's output in your shell; check
             out the comp.unix.questions FAQ for details.
    
         How do I close a process's filehandle without waiting for it
         to complete?
    
         Assuming your system supports such things, just send an
         appropriate signal to the process (see the kill entry in the
         perlfunc manpage.  It's common to first send a TERM signal,
         wait a little bit, and then send a KILL signal to finish it
         off.
    
         How do I fork a daemon process?
    
         If by daemon process you mean one that's detached
         (disassociated from its tty), then the following process is
         reported to work on most Unixish systems.  Non-Unix users
         should check their Your_OS::Process module for other
         solutions.
    
         o   Open /dev/tty and use the TIOCNOTTY ioctl on it.  See
             tty(4) for details.  Or better yet, you can just use the
             POSIX:\fIs0:setsid() function, so you don't have to
             worry about process groups.
    
         o   Change directory to /
    
         o   Reopen STDIN, STDOUT, and STDERR so they're not
             connected to the old tty.
    
         o   Background yourself like this:
    
                 fork && exit;
    
    
         The Proc::Daemon module, available from CPAN, provides a
         function to perform these actions for you.
    
         How do I make my program run with sh and csh?
    
         See the eg/nih script (part of the perl source
         distribution).
    
         How do I find out if I'm running interactively or not?
    
         Good question.  Sometimes `-t STDIN' and `-t STDOUT' can
         give clues, sometimes not.
    
             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";
             }
    
    
         How do I timeout a slow event?
    
         Use the alarm() function, probably in conjunction with a
         signal handler, as documented in the Signals entry in the
         perlipc manpage and chapter 6 of the Camel.  You may instead
         use the more flexible Sys::AlarmCall module available from
         CPAN.
    
         How do I set CPU limits?
    
         Use the BSD::Resource module from CPAN.
    
         How do I avoid zombies on a Unix system?
    
         Use the reaper code from the Signals entry in the perlipc
         manpage to call wait() when a SIGCHLD is received, or else
         use the double-fork technique described in the fork entry in
         the perlfunc manpage.
    
         How do I use an SQL database?
    
         There are a number of excellent interfaces to SQL databases.
         See the DBD::* modules available from
         http://www.perl.com/CPAN/modules/DBD .  A lot of information
         on this can be found at
         http://www.symbolstone.org/technology/perl/DBI/
    
         How do I make a system() exit on control-C?
    
         You can't.  You need to imitate the system() call (see the
         perlipc manpage for sample code) and then have a signal
         handler for the INT signal that passes the signal on to the
         subprocess.  Or you can check for it:
    
             $rc = system($cmd);
             if ($rc & 127) { die "signal death" }
    
    
    
         How do I open a file without blocking?
    
         If you're lucky enough to be using a system that supports
         non-blocking reads (most Unixish systems do), you need only
         to use the O_NDELAY or O_NONBLOCK flag from the Fcntl module
         in conjunction with sysopen():
    
             use Fcntl;
             sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
                 or die "can't open /tmp/somefile: $!":
    
    
         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.  To manually install the CPAN module, or any well-
         behaved CPAN module for that matter, follow these steps:
    
         1   Unpack the source into a temporary area.
    
         2
                 perl Makefile.PL
    
    
         3
                 make
    
    
         4
                 make test
    
    
         5
                 make install
    
    
         If your version of perl is compiled without dynamic loading,
         then you just need to replace step 3 (make) with make perl
         and you will get a new perl binary with your extension
         linked in.
    
         See the ExtUtils::MakeMaker manpage for more details on
         building extensions.  See also the next question.
    
         What's the difference between require and use?
    
         Perl offers several different ways to include code from one
         file into another.  Here are the deltas between the various
         inclusion constructs:
    
    
             1)  do $file is like eval `cat $file`, except the former:
                 1.1: searches @INC and updates %INC.
                 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
    
             2)  require $file is like do $file, except the former:
                 2.1: checks for redundant loading, skipping already loaded files.
                 2.2: raises an exception on failure to find, compile, or execute $file.
    
             3)  require Module is like require "Module.pm", except the former:
                 3.1: translates each "::" into your system's directory separator.
                 3.2: primes the parser to disambiguate class Module as an indirect object.
    
             4)  use Module is like require Module, except the former:
                 4.1: loads the module at compile time, not run-time.
                 4.2: imports symbols and semantics from that package to the current one.
    
         In general, you usually want `use' and a proper Perl module.
    
         How do I keep my own module/library directory?
    
         When you build modules, use the PREFIX option when
         generating Makefiles:
    
             perl Makefile.PL PREFIX=/u/mydir/perl
    
         then either set the PERL5LIB environment variable before you
         run scripts that use the modules/libraries (see the perlrun
         manpage) or say
    
             use lib '/u/mydir/perl';
    
         This is almost the same as:
    
             BEGIN {
                 unshift(@INC, '/u/mydir/perl');
             }
    
         except that the lib module checks for machine-dependent
         subdirectories.  See Perl's the lib manpage for more
         information.
    
         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;
    
    
    
         How do I add a directory to my include path at runtime?
    
         Here are the suggested ways of modifying your include path:
    
             the PERLLIB environment variable
             the PERL5LIB environment variable
             the perl -Idir command line flag
             the use lib pragma, as in
                 use lib "$ENV{HOME}/myown_perllib";
    
         The latter is particularly useful because it knows about
         machine dependent architectures.  The lib.pm pragmatic
         module was first included with the 5.002 release of Perl.
    
         What is socket.ph and where do I get it?
    
         It's a perl4-style file defining values for system
         networking constants.  Sometimes it is built using h2ph when
         Perl is installed, but other times it is not.  Modern
         programs `use Socket;' instead.
    
    
    

    AUTHOR AND COPYRIGHT

         Copyright (c) 1997-1999 Tom Christiansen and Nathan
         Torkington.  All rights reserved.
    
         When included as part of the Standard Version of Perl, or as
         part of its complete documentation whether printed or
         otherwise, this work may be distributed only under the terms
         of Perl's Artistic License.  Any distribution of this file
         or derivatives thereof outside of that package require that
         special arrangements be made with copyright holder.
    
         Irrespective of its distribution, all code examples in this
         file are hereby placed into the public domain.  You are
         permitted and encouraged to use this code in your own
         programs for fun or for profit as you see fit.  A simple
         comment in the code giving credit would be courteous but is
         not required.
    
    
    
    


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