[9768] in Perl-Users-Digest
Perl-Users Digest, Issue: 3361 Volume: 8
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Wed Aug 5 09:17:20 1998
Date: Wed, 5 Aug 98 06:08:54 -0700
From: Perl-Users Digest <Perl-Users-Request@ruby.OCE.ORST.EDU>
To: Perl-Users@ruby.OCE.ORST.EDU (Perl-Users Digest)
Perl-Users Digest Wed, 5 Aug 1998 Volume: 8 Number: 3361
Today's topics:
perlfaq8 - System Interaction (part 8 of 9) <perlfaq-suggestions@mox.perl.com>
Special: Digest Administrivia (Last modified: 12 Mar 98 (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: 5 Aug 1998 12:30:46 GMT
From: Tom Christiansen <perlfaq-suggestions@mox.perl.com>
Subject: perlfaq8 - System Interaction (part 8 of 9)
Message-Id: <6q9j9m$2ad$1@csnews.cs.colorado.edu>
+-----------------------------------------------------------------------+
| The following document was generated from its original pod using |
| the pod2text program included with the standard perl release, plus |
| small cosmetic mark-ups. The FAQ is also distributed with all Perl |
| releases as standard manpages; their latest versions can be retrieved |
| from http://language.perl.com/misc/faqs.tar.gz if you'd like. |
+-----------------------------------------------------------------------+
NAME
perlfaq8 - System Interaction ($Revision: 1.26 $, $Date: 1998/08/05
12:20:28 $)
DESCRIPTION
This section of the Perl FAQ covers questions involving operating
system interaction. This involves interprocess communication (IPC),
control over the user-interface (keyboard, screen and pointing
devices), and most anything else not related to data manipulation.
Read the FAQs and documentation specific to the port of perl to
your operating system (eg, the perlvms manpage, the perlplan9
manpage, ...). These should contain more detailed information on
the vagaries of your perl.
How do I find out which operating system I'm running under?
The $^O variable ($OSNAME if you use English) contains 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
Some of these specific cases are shown below.
How do I print something out in color?
In general, you don't, because you don't know whether the recipient
has a color-aware display device. If you know that they have an
ANSI terminal that understands color, you can use the
Term::ANSIColor module from CPAN:
use Term::ANSIColor;
print color("red"), "Stop!\n", color("reset");
print color("green"), "Go!\n", color("reset");
Or like this:
use Term::ANSIColor qw(:constants);
print RED, "Stop!\n", RESET;
print GREEN, "Go!\n", RESET;
How do I read just one key without waiting for a return key?
Controlling input buffering is a remarkably system-dependent
matter. If most systems, you can just use the stty command as shown
in the "getc" entry in the perlfunc manpage, but as you see, that's
already getting you into portability snags.
open(TTY, "+</dev/tty") or die "no tty: $!";
system "stty cbreak </dev/tty >/dev/tty 2>&1";
$key = getc(TTY); # perhaps this works
# OR ELSE
sysread(TTY, $key, 1); # probably this does
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
The Term::ReadKey module from CPAN offers an easy-to-use interface
that should be more efficient than shelling out to stty for each
key. It even includes limited support for Windows.
use Term::ReadKey;
ReadMode('cbreak');
$key = ReadKey(0);
ReadMode('normal');
However, that requires that you have a working C compiler and can
use it to build and install a CPAN module. Here's a solution using
the standard POSIX module, which is already on your systems
(assuming your system supports POSIX).
use HotKey;
$key = readkey();
And here's the HotKey module, which hides the somewhat mystifying
calls to manipulate the POSIX termios structures.
# HotKey.pm
package HotKey;
@ISA = qw(Exporter);
@EXPORT = qw(cbreak cooked readkey);
use strict;
use POSIX qw(:termios_h);
my ($term, $oterm, $echo, $noecho, $fd_stdin);
$fd_stdin = fileno(STDIN);
$term = POSIX::Termios->new();
$term->getattr($fd_stdin);
$oterm = $term->getlflag();
$echo = ECHO | ECHOK | ICANON;
$noecho = $oterm & ~$echo;
sub cbreak {
$term->setlflag($noecho); # ok, so i don't want echo either
$term->setcc(VTIME, 1);
$term->setattr($fd_stdin, TCSANOW);
}
sub cooked {
$term->setlflag($oterm);
$term->setcc(VTIME, 0);
$term->setattr($fd_stdin, TCSANOW);
}
sub readkey {
my $key = '';
cbreak();
sysread(STDIN, $key, 1);
cooked();
return $key;
}
END { cooked() }
1;
How do I check whether input is ready on the keyboard?
The easiest way to do this is to read a key in nonblocking mode
with the Term::ReadKey module from CPAN, passing it an argument of
-1 to indicate not to block:
use Term::ReadKey;
ReadMode('cbreak');
if (defined ($char = ReadKey(-1)) ) {
# input was waiting and it was $char
} else {
# no input was waiting
}
ReadMode('normal'); # restore normal tty settings
How do I clear the screen?
If you only have to so infrequently, use `system':
system("clear");
If you have to do this a lot, save the clear string so you can
print it 100 times without calling a program 100 times:
$clear_string = `clear`;
print $clear_string;
If you're planning on doing other screen manipulations, like cursor
positions, etc, you might wish to use Term::Cap module:
use Term::Cap;
$terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
$clear_string = $terminal->Tputs('cl');
How do I get the screen size?
If you have Term::ReadKey module installed from CPAN, you can use
it to fetch the width and height in characters and in pixels:
use Term::ReadKey;
($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
This is more portable than the raw `ioctl', but not as
illustrative:
require 'sys/ioctl.ph';
die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
open(TTY, "+</dev/tty") or die "No tty: $!";
unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) {
die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ;
}
($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
print "(row,col) = ($row,$col)";
print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
print "\n";
How do I ask the user for a password?
(This question has nothing to do with the web. See a different FAQ
for that.)
There's an example of this in the "crypt" entry in the perlfunc
manpage). First, you put the terminal into "no echo" mode, then
just read the password normally. You may do this with an old-style
ioctl() function, POSIX terminal control (see the POSIX manpage,
and Chapter 7 of the Camel), or a call to the stty program, with
varying degrees of portability.
You can also do this for most systems using the Term::ReadKey
module from CPAN, which is easier to use and in theory more
portable.
use Term::ReadKey;
ReadMode('noecho');
$password = ReadLine(0);
How do I read and write the serial port?
This depends on which operating system your program is running on.
In the case of Unix, the serial ports will be accessible through
files in /dev; on other systems, the devices names will doubtless
differ. Several problem areas common to all device interaction are
the following
lockfiles
Your system may use lockfiles to control multiple access. Make
sure you follow the correct protocol. Unpredictable behaviour
can result from multiple processes reading from one device.
open mode
If you expect to use both read and write operations on the
device, you'll have to open it for update (see the section on
"open" in the perlfunc manpage for details). You may wish to
open it without running the risk of blocking by using sysopen()
and `O_RDWR|O_NDELAY|O_NOCTTY' from the Fcntl module (part of
the standard perl distribution). See the section on "sysopen"
in the perlfunc manpage for more on this approach.
end of line
Some devices will be expecting a "\r" at the end of each line
rather than a "\n". In some ports of perl, "\r" and "\n" are
different from their usual (Unix) ASCII values of "\012" and
"\015". You may have to give the numeric values you want
directly, using octal ("\015"), hex ("0x0D"), or as a control-
character specification ("\cM").
print DEV "atv1\012"; # wrong, for some devices
print DEV "atv1\015"; # right, for some devices
Even though with normal text files, a "\n" will do the trick,
there is still no unified scheme for terminating a line that is
portable between Unix, DOS/Win, and Macintosh, except to
terminate *ALL* line ends with "\015\012", and strip what you
don't need from the output. This applies especially to socket
I/O and autoflushing, discussed next.
flushing output
If you expect characters to get to your device when you print()
them, you'll want to autoflush that filehandle. You can use
select() and the `$|' variable to control autoflushing (see 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]);
Or if you don't mind pulling in a few thousand lines of code
just because you're afraid of a little $| variable:
use IO::Handle;
DEV->autoflush(1);
As mentioned in the previous item, this still doesn't work when
using socket I/O between Unix and Macintosh. You'll need to
hardcode your line terminators, in that case.
non-blocking input
If you are doing a blocking read() or sysread(), you'll have to
arrange for an alarm handler to provide a timeout (see the
"alarm" entry in the perlfunc manpage). If you have a non-
blocking open, you'll likely have a non-blocking read, which
means you may have to use a 4-arg select() to determine whether
I/O is ready on that device (see the section on "select" in the
perlfunc manpage.
While trying to read from his caller-id box, the notorious Jamie
Zawinski <jwz@netscape.com>, after much gnashing of teeth and
fighting with sysread, sysopen, POSIX's tcgetattr business, and
various other functions that go bump in the night, finally came up
with this:
sub open_modem {
use IPC::Open2;
my $stty = `/bin/stty -g`;
open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
# starting cu hoses /dev/tty's stty settings, even when it has
# been opened on a pipe...
system("/bin/stty $stty");
$_ = <MODEM_IN>;
chop;
if ( !m/^Connected/ ) {
print STDERR "$0: cu printed `$_' instead of `Connected'\n";
}
}
How do I decode encrypted password files?
You spend lots and lots of money on dedicated hardware, but this is
bound to get you talked about.
Seriously, you can't if they are Unix password files - the Unix
password system employs one-way encryption. It's more like hashing
than encryption. The best you can check is whether something else
hashes to the same string. You can't turn a hash back into the
original string. Programs like Crack can forcibly (and
intelligently) try to guess passwords, but don't (can't) guarantee
quick success.
If you're worried about users selecting bad passwords, you should
proactively check when they try to change their password (by
modifying passwd(1), for example).
How do I start a process in the background?
You could use
system("cmd &")
or you could use fork as documented in the 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 `open'ing 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 is sent to your terminal's
currently foregrounded process group, which you then trap in your
process. 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).
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, and your shadow library was
written properly, the getpw*() functions described in the perlfunc
manpage should in theory provide (read-only) access to entries in
the shadow password file. To change the file, make a new shadow
password file (the format varies from system to system - see 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.misc";
How can I sleep() or alarm() for under a second?
If you want finer granularity than the 1 second that the sleep()
function provides, the easiest way is to use the select() function
as documented in the 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.
In general, you may not be able to. But if your system supports
both the syscall() function in Perl as well as a system call like
gettimeofday(2), then you may be able to do something like this:
require 'sys/syscall.ph';
$TIMEVAL_T = "LL";
$done = $start = pack($TIMEVAL_T, ());
syscall( &SYS_gettimeofday, $start, 0)) != -1
or die "gettimeofday: $!";
##########################
# DO YOUR OPERATION HERE #
##########################
syscall( &SYS_gettimeofday, $done, 0) != -1
or die "gettimeofday: $!";
@start = unpack($TIMEVAL_T, $start);
@done = unpack($TIMEVAL_T, $done);
# fix microseconds
for ($done[1], $start[1]) { $_ /= 1_000_000 }
$delta_time = sprintf "%.4f", ($done[0] + $done[1] )
-
($start[0] + $start[1] );
How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
Release 5 of Perl added the END block, which can be used to
simulate atexit(). Each package's END block is called when the
program or thread ends (see the perlmod manpage manpage for more
details).
For example, you can use this to make sure your filter program
managed to finish its output without filling up the disk:
END {
close(STDOUT) || die "stdout close failed: $!";
}
The END block isn't called when untrapped signals kill the program,
though, so if you use END blocks you should also use
use sigtrap qw(die normal-signals);
Perl's exception-handling mechanism is its eval() operator. You can
use eval() as setjmp and die() as longjmp. For details of this, see
the section on signals, especially the time-out handler for a
blocking flock() in the 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).
If you want the atexit() syntax (and an rmexit() as well), try the
AtExit module available from CPAN.
Why doesn't my sockets program work under System V (Solaris)? What
does the error message "Protocol not supported" mean?
Some Sys-V based systems, notably Solaris 2.X, redefined some of
the standard socket constants. Since these were constant across all
architectures, they were often hardwired into perl code. The proper
way to deal with this is to "use Socket" to get the correct values.
Note that even though SunOS and Solaris are binary compatible,
these values are different. Go figure.
How can I call my system's unique C functions from Perl?
In most cases, you write an external module to do it - see the
answer to "Where can I learn about linking C with Perl? [h2xs,
xsubpp]". However, if the function is a system call, and your
system supports syscall(), you can use the syscall function
(documented in the perlfunc manpage).
Remember to check the modules that came with your distribution, and
CPAN as well - someone may already have written a module to do it.
Where do I get the include files to do ioctl() or syscall()?
Historically, these would be generated by the h2ph tool, part of
the standard perl distribution. This program converts cpp(1)
directives in C header files to files containing subroutine
definitions, like &SYS_getitimer, which you can use as arguments to
your functions. It doesn't work perfectly, but it usually gets most
of the job done. Simple files like errno.h, syscall.h, and socket.h
were fine, but the hard ones like ioctl.h nearly always need to
hand-edited. Here's how to install the *.ph files:
1. become super-user
2. cd /usr/include
3. h2ph *.h */*.h
If your system supports dynamic loading, for reasons of portability
and sanity you probably ought to use h2xs (also part of the
standard perl distribution). This tool converts C header files to
Perl extensions. See the perlxstut manpage for how to get started
with h2xs.
If your system doesn't support dynamic loading, you still probably
ought to use h2xs. See the perlxstut manpage and the
ExtUtils::MakeMaker manpage for more information (in brief, just
use make perl instead of a plain make to rebuild perl with a new
static extension).
Why do setuid perl scripts complain about kernel problems?
Some operating systems have bugs in the kernel that make setuid
scripts inherently insecure. Perl gives you a number of options
(described in the perlsec manpage) to work around such systems.
How can I open a pipe both to and from a command?
The IPC::Open2 module (part of the standard perl distribution) is
an easy-to-use approach that internally uses pipe(), fork(), and
exec() to do the job. Make sure you read the deadlock warnings in
its documentation, though (see the IPC::Open2 manpage). See the
section on "Bidirectional Communication with Another Process" in
the perlipc manpage and the section on "Bidirectional Communication
with Yourself" in the perlipc manpage
You may also use the IPC::Open3 module (part of the standard perl
distribution), but be warned that it has a different order of
arguments from IPC::Open2 (see the IPC::Open3 manpage).
Why can't I get the output of a command with system()?
You're confusing the purpose of system() and backticks (``).
system() runs a command and returns exit status information (as a
16 bit value: the low 7 bits are the signal the process died from,
if any, and the high 8 bits are the actual exit value). Backticks
(``) run a command and return what it sent to STDOUT.
$exit_status = system("mail-users");
$output_string = `ls`;
How can I capture STDERR from an external command?
There are three basic ways of running external commands:
system $cmd; # using system()
$output = `$cmd`; # using backticks (``)
open (PIPE, "cmd |"); # using open()
With system(), both STDOUT and STDERR will go the same place as the
script's versions of these, unless the command redirects them.
Backticks and open() read only the STDOUT of your command.
With any of these, you can change file descriptors before the call:
open(STDOUT, ">logfile");
system("ls");
or you can use Bourne shell file-descriptor redirection:
$output = `$cmd 2>some_file`;
open (PIPE, "cmd 2>some_file |");
You can also use file-descriptor redirection to make STDERR a
duplicate of STDOUT:
$output = `$cmd 2>&1`;
open (PIPE, "cmd 2>&1 |");
Note that you *cannot* simply open STDERR to be a dup of STDOUT in
your Perl program and avoid calling the shell to do the
redirection. This doesn't work:
open(STDERR, ">&STDOUT");
$alloutput = `cmd args`; # stderr still escapes
This fails because the open() makes STDERR go to where STDOUT was
going at the time of the open(). The backticks then make STDOUT go
to a string, but don't change STDERR (which still goes to the old
STDOUT).
Note that you *must* use Bourne shell (sh(1)) redirection syntax in
backticks, not csh(1)! Details on why Perl's system() and backtick
and pipe opens all use the Bourne shell are in
http://www.perl.com/CPAN/doc/FMTEYEWTK/versus/csh.whynot . To
capture a command's STDERR and STDOUT together:
$output = `cmd 2>&1`; # either with backticks
$pid = open(PH, "cmd 2>&1 |"); # or with an open pipe
while (<PH>) { } # plus a read
To capture a command's STDOUT but discard its STDERR:
$output = `cmd 2>/dev/null`; # either with backticks
$pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe
while (<PH>) { } # plus a read
To capture a command's STDERR but discard its STDOUT:
$output = `cmd 2>&1 1>/dev/null`; # either with backticks
$pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe
while (<PH>) { } # plus a read
To exchange a command's STDOUT and STDERR in order to capture the
STDERR but leave its STDOUT to come out our old STDERR:
$output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks
$pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
while (<PH>) { } # plus a read
To read both a command's STDOUT and its STDERR separately, it's
easiest and safest to redirect them separately to files, and then
read from those files when the program is done:
system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
Ordering is important in all these examples. That's because the
shell processes file descriptor redirections in strictly left to
right order.
system("prog args 1>tmpfile 2>&1");
system("prog args 2>&1 1>tmpfile");
The first command sends both standard out and standard error to the
temporary file. The second command sends only the old standard
output there, and the old standard error shows up on the old
standard out.
Why doesn't open() return an error when a pipe open fails?
It does, but probably not how you expect it to. On systems that
follow the standard fork()/exec() paradigm (such as 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." 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.
There are more examples of this the section on "Safe Pipe Opens" in
the perlipc manpage.
Why can't my script read from STDIN after I gave it EOF (^D on Unix,
^Z on MS-DOS)?
Because some stdio's set error and eof flags that need clearing.
The POSIX module defines clearerr() that you can use. That is the
technically correct way to do it. Here are some less reliable
workarounds:
1 Try keeping around the seekpointer and go there, like this:
$where = tell(LOG);
seek(LOG, $where, 0);
2 If that doesn't work, try seeking to a different part of the file
and then back.
3 If that doesn't work, try seeking to a different part of the file,
reading something, and then seeking back.
4 If that doesn't work, give up on your stdio package and use
sysread.
How can I convert my shell script to perl?
Learn Perl and rewrite it. Seriously, there's no simple converter.
Things that are awkward to do in the shell are easy to do in Perl,
and this very awkwardness is what would make a shell->perl
converter nigh-on impossible to write. By rewriting it, you'll
think about what you're really trying to do, and hopefully will
escape the shell's pipeline datastream paradigm, which while
convenient for some matters, causes many inefficiencies.
Can I use perl to run a telnet or ftp session?
Try the Net::FTP, TCP::Client, and Net::Telnet modules (available
from CPAN).
http://www.perl.com/CPAN/scripts/netstuff/telnet.emul.shar will
also help for emulating the telnet protocol, but Net::Telnet is
quite probably easier to use..
If all you want to do is pretend to be telnet but don't need the
initial telnet handshaking, then the standard dual-process approach
will suffice:
use IO::Socket; # new in 5.004
$handle = IO::Socket::INET->new('www.perl.com:80')
|| die "can't connect to port 80 on www.perl.com: $!";
$handle->autoflush(1);
if (fork()) { # XXX: undef means failure
select($handle);
print while <STDIN>; # everything from stdin to socket
} else {
print while <$handle>; # everything from socket to stdout
}
close $handle;
exit;
How can I write expect in Perl?
Once upon a time, there was a library called chat2.pl (part of the
standard perl distribution), which never really got finished. If
you find it somewhere, *don't use it*. These days, your best bet is
to look at the Expect module available from CPAN, which also
requires two other modules from CPAN, IO::Pty and IO::Stty.
Is there a way to hide perl's command line from programs such as "ps"?
First of all note that if you're doing this for security reasons
(to avoid people seeing passwords, for example) then you should
rewrite your program so that critical information is never given as
an argument. Hiding the arguments won't make your program
completely secure.
To actually alter the visible command line, you can assign to the
variable $0 as documented in the perlvar manpage. This won't work
on all operating systems, though. Daemon programs like sendmail
place their state there, as in:
$0 = "orcus [accepting connections]";
I {changed directory, modified my environment} in a perl script.
How come the change disappeared when I exited the script? How do
I get my changes to be visible?
Unix
In the strictest sense, it can't be done -- the script executes
as a different process from the shell it was started from.
Changes to a process are not reflected in its parent, only in
its own children created after the change. There is shell magic
that may allow you to fake it by eval()ing the script's output
in your shell; check out the comp.unix.questions FAQ for
details.
How do I close a process's filehandle without waiting for it to complete?
Assuming your system supports such things, just send an appropriate
signal to the process (see the 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. Or better yet, you can just use the
POSIX::setsid() function, so you don't have to worry about
process groups.
* 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?
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 . A lot of information
on this can be found at
http://www.hermetica.com/technologia/perl/DBI/index.html .
How do I make a system() exit on control-C?
You can't. You need to imitate the system() call (see the perlipc
manpage for sample code) and then have a signal handler for the INT
signal that passes the signal on to the subprocess. Or you can
check for it:
$rc = system($cmd);
if ($rc & 127) { die "signal death" }
How do I open a file without blocking?
If you're lucky enough to be using a system that supports non-
blocking reads (most Unixish systems do), you need only to use the
O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction
with sysopen():
use Fcntl;
sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
or die "can't open /tmp/somefile: $!":
How do I install a 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 the ExtUtils::MakeMaker manpage for more details on building
extensions. See also the next question.
What's the difference between require and use?
Perl offers several different ways to include code from one file
into another. Here are the deltas between the various inclusion
constructs:
1) do $file is like eval `cat $file`, except the former:
1.1: searches @INC and updates %INC.
1.2: bequeaths an *unrelated* lexical scope on the eval'ed
code.
2) require $file is like do $file, except the former:
2.1: checks for redundant loading, skipping already loaded
files.
2.2: raises an exception on failure to find, compile, or
execute $file.
3) require Module is like require "Module.pm", except the former:
3.1: translates each "::" into your system's directory
separator.
3.2: primes the parser to disambiguate class Module as an
indirect object.
4) use Module is like require Module, except the former:
4.1: loads the module at compile time, not run-time.
4.2: imports symbols and semantics from that package to the
current one.
In general, you usually want `use' and a proper Perl module.
How do I keep my own module/library directory?
When you build modules, use the PREFIX option when generating
Makefiles:
perl Makefile.PL PREFIX=/u/mydir/perl
then either set the PERL5LIB environment variable before you run
scripts that use the modules/libraries (see the perlrun manpage) or
say
use lib '/u/mydir/perl';
See Perl's the lib manpage for more information.
How do I add the directory my program lives in to the module/library
search path?
use FindBin;
use lib "$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.
AUTHOR AND COPYRIGHT
Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.
All rights reserved.
When included as part of the Standard Version of Perl, or as part
of its complete documentation whether printed or otherwise, this
work may be distributed only under the terms of Perl's Artistic
License. Any distribution of this file or derivatives thereof
*outside* of that package require that special arrangements be made
with copyright holder.
Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain. You are permitted and
encouraged to use this code in your own programs for fun or for
profit as you see fit. A simple comment in the code giving credit
would be courteous but is not required.
------------------------------
Date: 12 Jul 98 21:33:47 GMT (Last modified)
From: Perl-Request@ruby.oce.orst.edu (Perl-Users-Digest Admin)
Subject: Special: Digest Administrivia (Last modified: 12 Mar 98)
Message-Id: <null>
Administrivia:
Special notice: in a few days, the new group comp.lang.perl.moderated
should be formed. I would rather not support two different groups, and I
know of no other plans to create a digested moderated group. This leaves
me with two options: 1) keep on with this group 2) change to the
moderated one.
If you have opinions on this, send them to
perl-users-request@ruby.oce.orst.edu.
The Perl-Users Digest is a retransmission of the USENET newsgroup
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the single line:
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To submit articles to comp.lang.perl.misc (and this Digest), send your
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To request back copies (available for a week or so), send your request
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The Meta-FAQ, an article containing information about the FAQ, is
available by requesting "send perl-users meta-faq". The real FAQ, as it
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perl-users FAQ". Due to their sizes, neither the Meta-FAQ nor the FAQ
are included in the digest.
The "mini-FAQ", which is an updated version of the Meta-FAQ, is
available by requesting "send perl-users mini-faq". It appears twice
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For other requests pertaining to the digest, send mail to
perl-users-request@ruby.oce.orst.edu. Do not waste your time or mine
sending perl questions to the -request address, I don't have time to
answer them even if I did know the answer.
------------------------------
End of Perl-Users Digest V8 Issue 3361
**************************************
