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NAME

perlfaq8 - System Interaction ($Revision: 1.21 $, $Date: 1997/04/24 22:44:19 $)

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, 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 $^O variable ($OSTYPE if you use English) contains the operating system 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

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, 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.

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 "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, as in the older

use FileHandle;
DEV->autoflush(1);

and the newer

use IO::Handle;
DEV->autoflush(1);

You can use select() and the $| variable to control autoflushing (see perlvar/$ and "select" in perlfunc):

$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]);

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 "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.

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. 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 "fork" in perlfunc, with further examples in perlipc. 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 opening 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 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 "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 you then trap. Signals are documented in "Signals" in perlipc 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 "Signals" in perlipc or chapter 6 of the Camel.

How do I modify the shadow password file on a Unix system?

If perl was installed correctly, the getpw*() functions described in perlfunc provide (read-only) access to 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(5) 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";

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. 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.

In general, you may not be able to. But if you 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 perlmod manpage for more details). It 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 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 perlfunc).

Remember to check the modules that came with your distribution, and CPAN as well - someone may already have written a module to do it.

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 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 insecure. Perl gives you a number of options (described in perlsec) 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 IPC::Open2).

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 8 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 .

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 doesn't open() return an error when a pipe open fails?

It does, but probably not how you expect it to. On systems that follow the standard fork()/exec() paradigm (eg, Unix), it works like this: open() causes a fork(). In the parent, open() returns with the process ID of the child. The child exec()s the command to be piped to/from. The parent can't know whether the exec() was successful or not - all it can return is whether the fork() succeeded or not. To find out if the command 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 perlipc.

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 humungous) 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 its 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.

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. These days, your best bet is to look at the Comm.pl library available from CPAN.

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 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 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.

VMS

Change to %ENV persist after Perl exits, but directory changes do not.

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.

  • Open /dev/tty and use the the TIOCNOTTY ioctl on it. See tty(4) for details.

  • Change directory to /

  • Reopen STDIN, STDOUT, and STDERR so they're not connected to the old tty.

  • Background yourself like this:

    fork && exit;

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(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 "Signals" in perlipc 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 "Signals" in perlipc to call wait() when a SIGCHLD is received, or else use the double-fork technique described in "fork" in perlfunc.

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/dbperl/DBD .

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.

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 CPAN module?

The easiest way is to have the CPAN module 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 ExtUtils::MakeMaker for more details on building extensions, the question "How do I keep my own module/library directory?"

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 perlrun) or say

use lib '/u/mydir/perl';

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 commpand 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.

How do I get one key from the terminal at a time, under POSIX?

#!/usr/bin/perl -w
use strict;
$| = 1;
for (1..4) {
    my $got;
    print "gimme: ";
    $got = getone();
    print "--> $got\n";
}
exit;

BEGIN {
    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);
        $term->setcc(VTIME, 1);
        $term->setattr($fd_stdin, TCSANOW);
    }

    sub cooked {
        $term->setlflag($oterm);
        $term->setcc(VTIME, 0);
        $term->setattr($fd_stdin, TCSANOW);
    }

    sub getone {
        my $key = '';
        cbreak();
        sysread(STDIN, $key, 1);
        cooked();
        return $key;
    }

}
END { cooked() }

AUTHOR AND COPYRIGHT

Copyright (c) 1997 Tom Christiansen and Nathan Torkington. All rights reserved. See perlfaq for distribution information.

1 POD Error

The following errors were encountered while parsing the POD:

Around line 125:

alternative text 'perlvar/$' contains non-escaped | or /