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PERLFAQ8(1)			 Perl Programmers Reference Guide		      PERLFAQ8(1)

NAME
       perlfaq8 - System Interaction ($Revision: 1.14 $, $Date: 2002/11/10 17:35:47 $)

DESCRIPTION
       This section of the Perl FAQ covers questions involving operating system interaction.
       Topics include interprocess communication (IPC), control over the user-interface (key-
       board, screen and pointing devices), and most anything else not related to data manipula-
       tion.

       Read the FAQs and documentation specific to the port of perl to your operating system (eg,
       perlvms, perlplan9, ...).  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-depen-
       dent.  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 dis-
       play 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 "getc" in perlfunc, 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, using the code requires that you have a working C compiler and can use it to
       build and install a CPAN module.  Here's a solution using the standard POSIX module, which
       is already on your systems (assuming your system supports POSIX).

	   use HotKey;
	   $key = readkey();

       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 "crypt" in perlfunc).  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 POSIX or its documentation the Camel Book),
       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, device 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 behavior 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 "open" in perlfunc 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 "sysopen" in perlfunc
	   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 $| variable to control aut-
	   oflushing (see perlvar/$ and "select" in perlfunc, or perlfaq5, ``How do I
	   flush/unbuffer an output filehandle?  Why must I do this?''):

	       $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 hard code 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 "alarm" in perlfunc).  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 "select" in perlfunc.

       While trying to read from his caller-id box, the notorious Jamie Zawinski <jwz@net-
       scape.com>, after much gnashing of teeth and fighting with sysread, sysopen, POSIX's tcge-
       tattr business, and various other functions that go bump in the night, finally came up
       with this:

	   sub open_modem {
	       use IPC::Open2;
	       my $stty = `/bin/stty -g`;
	       open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
	       # starting cu hoses /dev/tty's stty settings, even when it has
	       # been opened on a pipe...
	       system("/bin/stty $stty");
	       $_ = <MODEM_IN>;
	       chomp;
	       if ( !m/^Connected/ ) {
		   print STDERR "$0: cu printed `$_' instead of `Connected'\n";
	       }
	   }

       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 pass-
       words, 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?

       Several modules can start other processes that do not block your Perl program.  You can
       use IPC::Open3, Parallel::Jobs, IPC::Run, and some of the POE modules.  See CPAN for more
       details.

       You could also use

	   system("cmd &")

       or you could use fork as documented in "fork" in perlfunc, with further examples in per-
       lipc.  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 "open" in perlfunc) 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 filehan-
	   dle 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 };

	       $SIG{CHLD} = 'IGNORE';

	   You can also use a double fork. You immediately wait() for your first child, and the
	   init daemon will wait() for your grandchild once it exits.

		   unless ($pid = fork) {
			   unless (fork) {
		       exec "what you really wanna do";
		       die "exec failed!";
			   }
		   exit 0;
		   }
	       waitpid($pid,0);

	   See "Signals" in perlipc 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 "Signals" in perlipc and the section on ``Sig-
       nals'' in the Camel.

       Be warned that very few C libraries are re-entrant.  Therefore, if you attempt to print()
       in a handler that got invoked during another stdio operation your internal structures will
       likely be in an inconsistent state, and your program will dump core.  You can sometimes
       avoid this by using syswrite() instead of print().

       Unless you're exceedingly careful, the only safe things to do inside a signal handler are
       (1) set a variable and (2) exit.  In the first case, you should only set a variable in
       such a way that malloc() is not called (eg, by setting a variable that already has a
       value).

       For example:

	   $Interrupted = 0;   # to ensure it has a value
	   $SIG{INT} = sub {
	       $Interrupted++;
	       syswrite(STDERR, "ouch\n", 5);
	   }

       However, because syscalls restart by default, you'll find that if you're in a "slow" call,
       such as <FH>, read(), connect(), or wait(), that the only way to terminate them is by
       "longjumping" out; that is, by raising an exception.  See the time-out handler for a
       blocking flock() in "Signals" in perlipc or the section on ``Signals'' in the Camel book.

       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 perlfunc should in theory provide (read-only) access to entries in
       the shadow password file.  To change the file, make a new shadow password file (the format
       varies from system to system--see 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 sys-
       tem-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 time zone, you can probably get away with
       setting an environment variable:

	   $ENV{TZ} = "MST7MDT";		  # unixish
	   $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
	   system "trn comp.lang.perl.misc";

       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 "select" in perlfunc.  Try
       the Time::HiRes and the BSD::Itimer modules (available from CPAN, and starting from Perl
       5.8 Time::HiRes is part of the standard distribution).

       How can I measure time under a second?

       In general, you may not be able to.  The Time::HiRes module (available from CPAN, and
       starting from Perl 5.8 part of the standard distribution) provides this functionality for
       some systems.

       If your system supports both the syscall() function in Perl as well as a system call like
       gettimeofday(2), then you may be able to do something like this:

	   require 'sys/syscall.ph';

	   $TIMEVAL_T = "LL";

	   $done = $start = pack($TIMEVAL_T, ());

	   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 pack-
       age's END block is called when the program or thread ends (see perlmod manpage for more
       details).

       For example, you can use this to make sure your filter program managed to finish its out-
       put 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 "Signals" in perlipc or the section on ``Sig-
       nals'' in the Camel Book.

       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 con-
       stants.	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 perl-
       func).

       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 distri-
       bution.	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 func-
       tions.  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 proba-
       bly ought to use h2xs (also part of the standard perl distribution).  This tool converts C
       header files to Perl extensions.  See perlxstut for how to get started with h2xs.

       If your system doesn't support dynamic loading, you still probably ought to use h2xs.  See
       perlxstut and ExtUtils::MakeMaker 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 inse-
       cure.  Perl gives you a number of options (described in perlsec) to work around such sys-
       tems.

       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 IPC::Open2).  See "Bidirectional Com-
       munication with Another Process" in perlipc and "Bidirectional Communication with Your-
       self" in perlipc

       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 IPC::Open3).

       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 STDOUT and
       STDERR, unless the system() command redirects them.  Backticks and open() read only the
       STDOUT of your command.

       You can also use the open3() function from IPC::Open3.  Benjamin Goldberg provides some
       sample code:

       To capture a program's STDOUT, but discard its STDERR:

	   use IPC::Open3;
	   use File::Spec;
	   use Symbol qw(gensym);
	   open(NULL, ">", File::Spec->devnull);
	   my $pid = open3(gensym, \*PH, ">&NULL", "cmd");
	   while( <PH> ) { }
	   waitpid($pid, 0);

       To capture a program's STDERR, but discard its STDOUT:

	   use IPC::Open3;
	   use File::Spec;
	   use Symbol qw(gensym);
	   open(NULL, ">", File::Spec->devnull);
	   my $pid = open3(gensym, ">&NULL", \*PH, "cmd");
	   while( <PH> ) { }
	   waitpid($pid, 0);

       To capture a program's STDERR, and let its STDOUT go to our own STDERR:

	   use IPC::Open3;
	   use Symbol qw(gensym);
	   my $pid = open3(gensym, ">&STDERR", \*PH, "cmd");
	   while( <PH> ) { }
	   waitpid($pid, 0);

       To read both a command's STDOUT and its STDERR separately, you can redirect them to temp
       files, let the command run, then read the temp files:

	   use IPC::Open3;
	   use Symbol qw(gensym);
	   use IO::File;
	   local *CATCHOUT = IO::File->new_tempfile;
	   local *CATCHERR = IO::File->new_tempfile;
	   my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd");
	   waitpid($pid, 0);
	   seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
	   while( <CATCHOUT> ) {}
	   while( <CATCHERR> ) {}

       But there's no real need for *both* to be tempfiles... the following should work just as
       well, without deadlocking:

	   use IPC::Open3;
	   use Symbol qw(gensym);
	   use IO::File;
	   local *CATCHERR = IO::File->new_tempfile;
	   my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd");
	   while( <CATCHOUT> ) {}
	   waitpid($pid, 0);
	   seek CATCHERR, 0, 0;
	   while( <CATCHERR> ) {}

       And it'll be faster, too, since we can begin processing the program's stdout immediately,
       rather than waiting for the program to finish.

       With any of these, you can change file descriptors before the call:

	   open(STDOUT, ">logfile");
	   system("ls");

       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
       the versus/csh.whynot article in the "Far More Than You Ever Wanted To Know" collection in
       http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .	To capture a command's STDERR and STDOUT
       together:

	   $output = `cmd 2>&1`;		       # either with backticks
	   $pid = open(PH, "cmd 2>&1 |");	       # or with an open pipe
	   while (<PH>) { }			       #    plus a read

       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 re-
       direct 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?

       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.  If
       the program couldn't be run, it's the shell that gets the message, not Perl. All your Perl
       program can find out is whether the shell itself could be successfully started.	You can
       still capture the shell's STDERR and check it for error messages.  See "How can I capture
       STDERR from an external command?" elsewhere in this document, or use the IPC::Open3 mod-
       ule.

       If there are no shell metacharacters in the argument of open(), Perl runs the command
       directly, without using the shell, and can correctly report whether the command started.

       What's wrong with using backticks in a void context?

       Strictly speaking, nothing.  Stylistically speaking, it's not a good way to write main-
       tainable code.  Perl has several operators for running external commands.  Backticks are
       one; they collect the output from the command for use in your program.  The "system" func-
       tion is another; it doesn't do this.

       Writing backticks in your program sends a clear message to the readers of your code that
       you wanted to collect the output of the command.  Why send a clear message that isn't
       true?

       Consider this line:

	   `cat /etc/termcap`;

       You forgot to check $? to see whether the program even ran correctly.  Even if you wrote

	   print `cat /etc/termcap`;

       this code 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.  You can't simply write the command like this:

	   @ok = `grep @opts '$search_string' @filenames`;

       As of Perl 5.8.0, you can use open() with multiple arguments.  Just like the list forms of
       system() and exec(), no shell escapes happen.

	  open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
	  chomp(@ok = <GREP>);
	  close GREP;

       You can also:

	   my @ok = ();
	   if (open(GREP, "-|")) {
	       while (<GREP>) {
		   chomp;
		   push(@ok, $_);
	       }
	       close GREP;
	   } else {
	       exec 'grep', @opts, $search_string, @filenames;
	   }

       Just as with system(), no shell escapes happen when you exec() a list.  Further examples
       of this can be found in "Safe Pipe Opens" in perlipc.

       Note that if you're use Microsoft, no solution to this vexing issue is even possible.
       Even if Perl were to emulate fork(), you'd still be stuck, because Microsoft does not have
       a argc/argv-style API.

       Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?

       Some stdio's set error and eof flags that need clearing.  The POSIX module defines clear-
       err() 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 awk-
       ward 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.cpan.org/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 handshak-
       ing, 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 distribu-
       tion), 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 docu-
       mented in perlvar.  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 dis-
       appeared 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 any children created after the change.  There is shell magic that may
	   allow you to fake it by eval()ing the script's output in your shell; check out the
	   comp.unix.questions FAQ for details.

       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 "kill" in perlfunc).  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::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 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
       "Signals" in perlipc and the section on ``Signals'' in the Camel.  You may instead use the
       more flexible Sys::AlarmCall module available from CPAN.

       The alarm() function is not implemented on all versions of Windows.  Check the documenta-
       tion for your specific version of Perl.

       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 "Signals" in perlipc to call wait() when a SIGCHLD is received,
       or else use the double-fork technique described in "How do I start a process in the back-
       ground?" in perlfaq8.

       How do I use an SQL database?

       There are a number of excellent interfaces to SQL databases.  See the DBD::* modules
       available from http://www.cpan.org/modules/by-module/DBD/ .  A lot of information on this
       can be found at http://dbi.perl.org/

       How do I make a system() exit on control-C?

       You can't.  You need to imitate the system() call (see perlipc for sample code) and then
       have a signal handler for the INT signal that passes the signal on to the subprocess.  Or
       you can check for it:

	   $rc = system($cmd);
	   if ($rc & 127) { die "signal death" }

       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.

	   $ perl -MCPAN -e shell

	   cpan shell -- CPAN exploration and modules installation (v1.59_54)
	   ReadLine support enabled

	   cpan> install Some::Module

       To manually install the CPAN module, or any well-behaved CPAN module for that matter, fol-
       low 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 ExtUtils::MakeMaker for more details on building extensions.  See also the next ques-
       tion, ``What's the difference between require and use?''.

       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 mod-
       ules/libraries (see perlrun) 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 lib
       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) 1997-2002 Tom Christiansen and Nathan Torkington.	All rights reserved.

       This documentation is free; you can redistribute it and/or modify it under the same terms
       as Perl itself.

       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.

perl v5.8.0				    2003-02-18				      PERLFAQ8(1)
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