PERLFAQ8(1) PERLFAQ8(1)
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, 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 $^O variable ($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
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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.
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 section on open 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 section on sysopen 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.
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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
the section on $| in the perlvar manpage 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]);
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 section on select in the perlfunc manpage.
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).
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How do I start a process in the background?
You could use
system("cmd &")
or you could use fork as documented in the section on fork 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
opening a pipe (see the section on open 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 section on Signals 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 you then trap. Signals are documented in the
section on Signals 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).
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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
section on Signals 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, the getpw*() functions described in the
perlfunc manpage 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 the passwd(5) manpage for specifics) and use
pwd_mkdb(8) to install it (see the pwd_mkdb(5) manpage 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 the section on select 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.
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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 the perlmod manpage 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 the section on Signals 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).
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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).
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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).
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:
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$output = `$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 the IPC::Open3 manpage).
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 the perlipc manpage.
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."
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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:
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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?
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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 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.
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 the section on kill 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.
⊕ Open /dev/tty and use the the TIOCNOTTY ioctl on it. See the tty(4)
manpage for details.
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⊕ 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 the section on Signals 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?
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Use the reaper code from the section on Signals 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/dbperl/DBD
.
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.
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: $!":
How do I install a CPAN module?
The easiest way is to have the CPAN module do it for you. This module
comes with perl version 5.004 and later. To manually install the CPAN
module, or any well-behaved CPAN module for that matter, follow these
steps:
1 Unpack the source into a temporary area.
2
perl Makefile.PL
3
make
4
make test
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5
make install
If your version of perl is compiled without dynamic loading, then you
just need to replace step 3 (make) with make perl and you will get a new
perl binary with your extension linked in.
See the ExtUtils::MakeMaker manpage for more details on building
extensions, the question "How do I keep my own module/library directory?"
How do I keep my own module/library directory?
When you build modules, use the PREFIX option when generating Makefiles:
perl Makefile.PL PREFIX=/u/mydir/perl
then either set the PERL5LIB environment variable before you run scripts
that use the modules/libraries (see the perlrun manpage) or say
use lib '/u/mydir/perl';
See Perl's the lib manpage for more information.
How do I add the directory my program lives in to the module/library
search path?
use FindBin;
use lib "$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 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?
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#!/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() }
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PERLFAQ8(1) PERLFAQ8(1)
AUTHOR AND COPYRIGHT
Copyright (c) 1997 Tom Christiansen and Nathan Torkington. All rights
reserved. See the perlfaq manpage for distribution information.
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PERLFAQ8(1) PERLFAQ8(1)
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