[12347] in Perl-Users-Digest
Perl-Users Digest, Issue: 5948 Volume: 8
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Thu Jun 10 12:07:25 1999
Date: Thu, 10 Jun 99 09:02:33 -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 Thu, 10 Jun 1999 Volume: 8 Number: 5948
Today's topics:
Re: Q: How unlock flock of a other process <tchrist@mox.perl.com>
Re: Regexpr for loop to handle e-address list <gellyfish@gellyfish.com>
Re: Substitution with functions (Tad McClellan)
Thanks to everyone who's helped <jhecker@iago.nac.net>
Re: Weird Warning from conditional <aqumsieh@matrox.com>
Special: Digest Administrivia (Last modified: 12 Dec 98 (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: 10 Jun 1999 09:39:04 -0700
From: Tom Christiansen <tchrist@mox.perl.com>
Subject: Re: Q: How unlock flock of a other process
Message-Id: <375fdc18@cs.colorado.edu>
[courtesy cc of this posting mailed to cited author]
In comp.lang.perl.misc,
periat@ens.ascom.ch writes:
:I have a perl script to launch programs, that log used programs in a file.
:The log file is locked with 'flock(FH,2)'. The lock is blocking and if
:it takes longer than 5s to get back from the 'flock()' the script should
:unlock the file and try again.
% man perlfaq8
How do I timeout a slow event?
--tom
NAME
perlfaq8 - System Interaction ($Revision: 1.39 $, $Date:
1999/05/23 18:37:57 $)
DESCRIPTION
This section of the Perl FAQ covers questions involving
operating system interaction. This involves interprocess
communication (IPC), control over the user-interface (keyboard,
screen and pointing devices), and most anything else not related
to data manipulation.
Read the FAQs and documentation specific to the port of perl to
your operating system (eg, the perlvms manpage, the perlplan9
manpage, ...). These should contain more detailed information on
the vagaries of your perl.
How do I find out which operating system I'm running under?
The $^O variable ($OSNAME if you use English) contains an
indication of the name of the operating system (not its release
number) that your perl binary was built for.
How come exec() doesn't return?
Because that's what it does: it replaces your currently running
program with a different one. If you want to keep going (as is
probably the case if you're asking this question) use system()
instead.
How do I do fancy stuff with the keyboard/screen/mouse?
How you access/control keyboards, screens, and pointing devices
("mice") is system-dependent. Try the following modules:
Keyboard
Term::Cap Standard perl distribution
Term::ReadKey CPAN
Term::ReadLine::Gnu CPAN
Term::ReadLine::Perl CPAN
Term::Screen CPAN
Screen
Term::Cap Standard perl distribution
Curses CPAN
Term::ANSIColor CPAN
Mouse
Tk CPAN
Some of these specific cases are shown below.
How do I print something out in color?
In general, you don't, because you don't know whether the
recipient has a color-aware display device. If you know that
they have an ANSI terminal that understands color, you can use
the Term::ANSIColor module from CPAN:
use Term::ANSIColor;
print color("red"), "Stop!\n", color("reset");
print color("green"), "Go!\n", color("reset");
Or like this:
use Term::ANSIColor qw(:constants);
print RED, "Stop!\n", RESET;
print GREEN, "Go!\n", RESET;
How do I read just one key without waiting for a return key?
Controlling input buffering is a remarkably system-dependent
matter. On many systems, you can just use the stty command as
shown in the "getc" entry in the perlfunc manpage, but as you
see, that's already getting you into portability snags.
open(TTY, "+</dev/tty") or die "no tty: $!";
system "stty cbreak </dev/tty >/dev/tty 2>&1";
$key = getc(TTY); # perhaps this works
# OR ELSE
sysread(TTY, $key, 1); # probably this does
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
The Term::ReadKey module from CPAN offers an easy-to-use
interface that should be more efficient than shelling out to
stty for each key. It even includes limited support for Windows.
use Term::ReadKey;
ReadMode('cbreak');
$key = ReadKey(0);
ReadMode('normal');
However, that requires that you have a working C compiler and
can use it to build and install a CPAN module. Here's a solution
using the standard POSIX module, which is already on your
systems (assuming your system supports POSIX).
use HotKey;
$key = readkey();
And here's the HotKey module, which hides the somewhat
mystifying calls to manipulate the POSIX termios structures.
# HotKey.pm
package HotKey;
@ISA = qw(Exporter);
@EXPORT = qw(cbreak cooked readkey);
use strict;
use POSIX qw(:termios_h);
my ($term, $oterm, $echo, $noecho, $fd_stdin);
$fd_stdin = fileno(STDIN);
$term = POSIX::Termios->new();
$term->getattr($fd_stdin);
$oterm = $term->getlflag();
$echo = ECHO | ECHOK | ICANON;
$noecho = $oterm & ~$echo;
sub cbreak {
$term->setlflag($noecho); # ok, so i don't want echo either
$term->setcc(VTIME, 1);
$term->setattr($fd_stdin, TCSANOW);
}
sub cooked {
$term->setlflag($oterm);
$term->setcc(VTIME, 0);
$term->setattr($fd_stdin, TCSANOW);
}
sub readkey {
my $key = '';
cbreak();
sysread(STDIN, $key, 1);
cooked();
return $key;
}
END { cooked() }
1;
How do I check whether input is ready on the keyboard?
The easiest way to do this is to read a key in nonblocking mode
with the Term::ReadKey module from CPAN, passing it an argument
of -1 to indicate not to block:
use Term::ReadKey;
ReadMode('cbreak');
if (defined ($char = ReadKey(-1)) ) {
# input was waiting and it was $char
} else {
# no input was waiting
}
ReadMode('normal'); # restore normal tty settings
How do I clear the screen?
If you only have do so infrequently, use `system':
system("clear");
If you have to do this a lot, save the clear string so you can
print it 100 times without calling a program 100 times:
$clear_string = `clear`;
print $clear_string;
If you're planning on doing other screen manipulations, like
cursor positions, etc, you might wish to use Term::Cap module:
use Term::Cap;
$terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
$clear_string = $terminal->Tputs('cl');
How do I get the screen size?
If you have Term::ReadKey module installed from CPAN, you can
use it to fetch the width and height in characters and in
pixels:
use Term::ReadKey;
($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
This is more portable than the raw `ioctl', but not as
illustrative:
require 'sys/ioctl.ph';
die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
open(TTY, "+</dev/tty") or die "No tty: $!";
unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) {
die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ;
}
($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
print "(row,col) = ($row,$col)";
print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
print "\n";
How do I ask the user for a password?
(This question has nothing to do with the web. See a different
FAQ for that.)
There's an example of this in the "crypt" entry in the perlfunc
manpage). First, you put the terminal into "no echo" mode, then
just read the password normally. You may do this with an old-
style ioctl() function, POSIX terminal control (see the POSIX
manpage, and Chapter 7 of the Camel), or a call to the stty
program, with varying degrees of portability.
You can also do this for most systems using the Term::ReadKey
module from CPAN, which is easier to use and in theory more
portable.
use Term::ReadKey;
ReadMode('noecho');
$password = ReadLine(0);
How do I read and write the serial port?
This depends on which operating system your program is running
on. In the case of Unix, the serial ports will be accessible
through files in /dev; on other systems, the devices names will
doubtless differ. Several problem areas common to all device
interaction are the following
lockfiles
Your system may use lockfiles to control multiple access.
Make sure you follow the correct protocol. Unpredictable
behaviour can result from multiple processes reading from
one device.
open mode
If you expect to use both read and write operations on the
device, you'll have to open it for update (see the section
on "open" in the perlfunc manpage for details). You may wish
to open it without running the risk of blocking by using
sysopen() and `O_RDWR|O_NDELAY|O_NOCTTY' from the Fcntl
module (part of the standard perl distribution). See the
section on "sysopen" in the perlfunc manpage for more on
this approach.
end of line
Some devices will be expecting a "\r" at the end of each
line rather than a "\n". In some ports of perl, "\r" and
"\n" are different from their usual (Unix) ASCII values of
"\012" and "\015". You may have to give the numeric values
you want directly, using octal ("\015"), hex ("0x0D"), or as
a control-character specification ("\cM").
print DEV "atv1\012"; # wrong, for some devices
print DEV "atv1\015"; # right, for some devices
Even though with normal text files, a "\n" will do the
trick, there is still no unified scheme for terminating a
line that is portable between Unix, DOS/Win, and Macintosh,
except to terminate *ALL* line ends with "\015\012", and
strip what you don't need from the output. This applies
especially to socket I/O and autoflushing, discussed next.
flushing output
If you expect characters to get to your device when you
print() them, you'll want to autoflush that filehandle. You
can use select() and the `$|' 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(8) 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.
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?
Because the pipe open takes place in two steps: first Perl calls
fork() to start a new process, then this new process calls
exec() to run the program you really wanted to open. The first
step reports success or failure to your process, so open() can
only tell you whether the fork() succeeded or not.
To find out if the exec() step succeeded, you have to catch
SIGCHLD and wait() to get the exit status. You should also catch
SIGPIPE if you're writing to the child--you may not have found
out the exec() failed by the time you write. This is documented
in the perlipc manpage.
In some cases, even this won't work. If the second argument to a
piped open() contains shell metacharacters, perl fork()s, then
exec()s a shell to decode the metacharacters and eventually run
the desired program. Now when you call wait(), you only learn
whether or not the *shell* could be successfully started. Best
to avoid shell metacharacters.
On systems that follow the spawn() paradigm, open() *might* do
what you expect--unless perl uses a shell to start your command.
In this case the fork()/exec() description still applies.
What's wrong with using backticks in a void context?
Strictly speaking, nothing. Stylistically speaking, it's not a
good way to write maintainable code because backticks have a
(potentially humongous) return value, and you're ignoring it.
It's may also not be very efficient, because you have to read in
all the lines of output, allocate memory for them, and then
throw it away. Too often people are lulled to writing:
`cp file file.bak`;
And now they think "Hey, I'll just always use backticks to run
programs." Bad idea: backticks are for capturing a program's
output; the system() function is for running programs.
Consider this line:
`cat /etc/termcap`;
You haven't assigned the output anywhere, so it just wastes
memory (for a little while). Plus you forgot to check `$?' to
see whether the program even ran correctly. Even if you wrote
print `cat /etc/termcap`;
In most cases, this could and probably should be written as
system("cat /etc/termcap") == 0
or die "cat program failed!";
Which will get the output quickly (as it is generated, instead
of only at the end) and also check the return value.
system() also provides direct control over whether shell
wildcard processing may take place, whereas backticks do not.
How can I call backticks without shell processing?
This is a bit tricky. Instead of writing
@ok = `grep @opts '$search_string' @filenames`;
You have to do this:
my @ok = ();
if (open(GREP, "-|")) {
while (<GREP>) {
chomp;
push(@ok, $_);
}
close GREP;
} else {
exec 'grep', @opts, $search_string, @filenames;
}
Just as with system(), no shell escapes happen when you exec() a
list. Further examples of this can be found in the section on
"Safe Pipe Opens" in the perlipc manpage.
Note that if you're stuck on Microsoft, no solution to this
vexing issue is even possible. Even if Perl were to emulate
fork(), you'd still be hosed, because Microsoft gives no
argc/argv-style API. Their API always reparses from a single
string, which is fundamentally wrong, but you're not likely to
get the Gods of Redmond to acknowledge this and fix it for you.
Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
Because some stdio's set error and eof flags that need clearing.
The POSIX module defines clearerr() that you can use. That is
the technically correct way to do it. Here are some less
reliable workarounds:
1 Try keeping around the seekpointer and go there, like this:
$where = tell(LOG);
seek(LOG, $where, 0);
2 If that doesn't work, try seeking to a different part of the
file and then back.
3 If that doesn't work, try seeking to a different part of the
file, reading something, and then seeking back.
4 If that doesn't work, give up on your stdio package and use
sysread.
How can I convert my shell script to perl?
Learn Perl and rewrite it. Seriously, there's no simple
converter. Things that are awkward to do in the shell are easy
to do in Perl, and this very awkwardness is what would make a
shell->perl converter nigh-on impossible to write. By rewriting
it, you'll think about what you're really trying to do, and
hopefully will escape the shell's pipeline datastream paradigm,
which while convenient for some matters, causes many
inefficiencies.
Can I use perl to run a telnet or ftp session?
Try the Net::FTP, TCP::Client, and Net::Telnet modules
(available from CPAN).
http://www.perl.com/CPAN/scripts/netstuff/telnet.emul.shar will
also help for emulating the telnet protocol, but Net::Telnet is
quite probably easier to use..
If all you want to do is pretend to be telnet but don't need the
initial telnet handshaking, then the standard dual-process
approach will suffice:
use IO::Socket; # new in 5.004
$handle = IO::Socket::INET->new('www.perl.com:80')
|| die "can't connect to port 80 on www.perl.com: $!";
$handle->autoflush(1);
if (fork()) { # XXX: undef means failure
select($handle);
print while <STDIN>; # everything from stdin to socket
} else {
print while <$handle>; # everything from socket to stdout
}
close $handle;
exit;
How can I write expect in Perl?
Once upon a time, there was a library called chat2.pl (part of
the standard perl distribution), which never really got
finished. If you find it somewhere, *don't use it*. These days,
your best bet is to look at the Expect module available from
CPAN, which also requires two other modules from CPAN, IO::Pty
and IO::Stty.
Is there a way to hide perl's command line from programs such as "ps"?
First of all note that if you're doing this for security reasons
(to avoid people seeing passwords, for example) then you should
rewrite your program so that critical information is never given
as an argument. Hiding the arguments won't make your program
completely secure.
To actually alter the visible command line, you can assign to
the variable $0 as documented in the perlvar manpage. This won't
work on all operating systems, though. Daemon programs like
sendmail place their state there, as in:
$0 = "orcus [accepting connections]";
I {changed directory, modified my environment} in a perl script. How come the change disappeared when I exited the script? How do I get my changes to be visible?
Unix
In the strictest sense, it can't be done -- the script
executes as a different process from the shell it was
started from. Changes to a process are not reflected in its
parent, only in its own children created after the change.
There is shell magic that may allow you to fake it by
eval()ing the script's output in your shell; check out the
comp.unix.questions FAQ for details.
How do I close a process's filehandle without waiting for it to complete?
Assuming your system supports such things, just send an
appropriate signal to the process (see the 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 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;
The Proc::Daemon module, available from CPAN, provides a
function to perform these actions for you.
How do I make my program run with sh and csh?
See the eg/nih script (part of the perl source distribution).
How do I find out if I'm running interactively or not?
Good question. Sometimes `-t STDIN' and `-t STDOUT' can give
clues, sometimes not.
if (-t STDIN && -t STDOUT) {
print "Now what? ";
}
On POSIX systems, you can test whether your own process group
matches the current process group of your controlling terminal
as follows:
use POSIX qw/getpgrp tcgetpgrp/;
open(TTY, "/dev/tty") or die $!;
$tpgrp = tcgetpgrp(fileno(*TTY));
$pgrp = getpgrp();
if ($tpgrp == $pgrp) {
print "foreground\n";
} else {
print "background\n";
}
How do I timeout a slow event?
Use the alarm() function, probably in conjunction with a signal
handler, as documented 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 module from CPAN?
The easiest way is to have a module also named CPAN do it for
you. This module comes with perl version 5.004 and later. To
manually install the CPAN module, or any well-behaved CPAN
module for that matter, follow these steps:
1 Unpack the source into a temporary area.
2
perl Makefile.PL
3
make
4
make test
5
make install
If your version of perl is compiled without dynamic loading,
then you just need to replace step 3 (make) with make perl and
you will get a new perl binary with your extension linked in.
See the ExtUtils::MakeMaker manpage for more details on building
extensions. See also the next question.
What's the difference between require and use?
Perl offers several different ways to include code from one file
into another. Here are the deltas between the various inclusion
constructs:
1) do $file is like eval `cat $file`, except the former:
1.1: searches @INC and updates %INC.
1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
2) require $file is like do $file, except the former:
2.1: checks for redundant loading, skipping already loaded files.
2.2: raises an exception on failure to find, compile, or execute $file.
3) require Module is like require "Module.pm", except the former:
3.1: translates each "::" into your system's directory separator.
3.2: primes the parser to disambiguate class Module as an indirect object.
4) use Module is like require Module, except the former:
4.1: loads the module at compile time, not run-time.
4.2: imports symbols and semantics from that package to the current one.
In general, you usually want `use' and a proper Perl module.
How do I keep my own module/library directory?
When you build modules, use the PREFIX option when generating
Makefiles:
perl Makefile.PL PREFIX=/u/mydir/perl
then either set the PERL5LIB environment variable before you run
scripts that use the modules/libraries (see the perlrun manpage)
or say
use lib '/u/mydir/perl';
This is almost the same as:
BEGIN {
unshift(@INC, '/u/mydir/perl');
}
except that the lib module checks for machine-dependent
subdirectories. See Perl's the lib manpage for more information.
How do I add the directory my program lives in to the module/library search path?
use FindBin;
use lib "$FindBin::Bin";
use your_own_modules;
How do I add a directory to my include path at runtime?
Here are the suggested ways of modifying your include path:
the PERLLIB environment variable
the PERL5LIB environment variable
the perl -Idir command line flag
the use lib pragma, as in
use lib "$ENV{HOME}/myown_perllib";
The latter is particularly useful because it knows about machine
dependent architectures. The lib.pm pragmatic module was first
included with the 5.002 release of Perl.
What is socket.ph and where do I get it?
It's a perl4-style file defining values for system networking
constants. Sometimes it is built using h2ph when Perl is
installed, but other times it is not. Modern programs `use
Socket;' instead.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
All rights reserved.
When included as part of the Standard Version of Perl, or as
part of its complete documentation whether printed or otherwise,
this work may be distributed only under the terms of Perl's
Artistic License. Any distribution of this file or derivatives
thereof *outside* of that package require that special
arrangements be made with copyright holder.
Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain. You are permitted and
encouraged to use this code in your own programs for fun or for
profit as you see fit. A simple comment in the code giving
credit would be courteous but is not required.
--
X-Windows: The Cutting Edge of Obsolescence.
--Jamie Zawinski
------------------------------
Date: 10 Jun 1999 16:13:41 +0100
From: Jonathan Stowe <gellyfish@gellyfish.com>
Subject: Re: Regexpr for loop to handle e-address list
Message-Id: <375fd625@newsread3.dircon.co.uk>
dalehend@flash.net wrote:
> On 9 Jun 1999 22:04:36 -0000, Jonathan Stowe <gellyfish@gellyfish.com>
> wrote:
>
>>On Wed, 09 Jun 1999 16:34:35 GMT dalehend@flash.net wrote:
>>> On Wed, 9 Jun 1999 09:07:08 -0700, Tom Phoenix <rootbeer@redcat.com>
>>> wrote:
>>>>On Wed, 9 Jun 1999 dalehend@flash.net wrote:
>>>>
>>>>> Does have an example that I can expand on in order to parse a quoted
>>>>> email list that can be 1 or more lines. In other words -
>>>>> "john\@town.com ben\@town.com younme\@work.com
>>>>> sebastian\@town.org isaac\@town.org"
>>>>
>>>>What do you do with e-mail addresses containing quote marks or spaces?
>>>>Write to <"fred & barney"@redcat.com> if you don't believe that there are
>>>>such addresses. :-)
>>>>
>>>>To do this properly, you'd need to parse the addresses. See whether the
>>>>Mail::Address module from CPAN will help you to do what you need. Good
>>>>luck!
>>>>
>>>
>>> The email list is the value for a $variable, so all I am wanting to do
>>> is parse a quoted list.
>>>
>>
>>Yeah but you still want to use Mail::Address (part of the Mail-tools
>>package from CPAN) :
>>
>>
>
>
>
> No, I did not mean to imply that there were quotes as part of the
> email address. In fact, there are not.
But the whole point is that there *may well be* quotes in an e-mail
address and there are unlikely to be any backslashes in the place
that you describe (I'm not saying there wouldnt because there could.)
> But I am reading a file that has a variable defined using a quoted
> list, like so -
> $emaillist = "younme\@work.com shenhim\@work.com";
>
> I would like to have the regexpr to get what is between the quotes.
>
>
But why dow want to anything with that - the variable $emaillist
now contains :
younme@work.com shenhim@work.com
No slashes no quotes. The point of the example I gave you was to
demonstrate that Mail::Address can handle the full range of mail
addresses - Including those with spaces quotes funny characters and all ...
Infact in your assignment there you could lose the backslashes
altogether if you changed to single quotes.
If you have a file with some Perl source in it why dont you just use
do() on the file to get $emaillist defined and then use Mail::Address to
split the addresses up.
/J\
--
"Over the years I've always had Max Factor in my box" - Tina Earnshaw,
Chief Make-Up Artist, Titanic
------------------------------
Date: Thu, 10 Jun 1999 05:59:43 -0400
From: tadmc@metronet.com (Tad McClellan)
Subject: Re: Substitution with functions
Message-Id: <fa2oj7.b95.ln@magna.metronet.com>
Herve Foucher (Herve.Foucher___NO_SPAM@helio.org) wrote:
: > Firstly, you don't need to escape `#'
: OK. Sorry. There are so many strange caracters in Perl...
: > and you need the /e switch to evaluate the subroutine
: > (and other statements that need to be evaluated).
: >
: > $a_string =~ s/##(\w+)#/&MyPerlFunction($1)/ge;
: It works! But I prefer using $&
: Why do you use $1 ?
Did you even try to answer your question yourself first?
Since this is regex stuff, you search for '$&' in perlre.pod,
and you find:
----------------------
Once perl sees that you need one of C<$&>, C<$`> or C<$'> anywhere in
the program, it has to provide them on each and every pattern match.
This can slow your program down. The same mechanism that handles
these provides for the use of $1, $2, etc., so you pay the same price
for each pattern that contains capturing parentheses. But if you never
use $&, etc., in your script, then patterns I<without> capturing
parentheses won't be penalized. So avoid $&, $', and $` if you can,
but if you can't (and some algorithms really appreciate them), once
you've used them once, use them at will, because you've already paid
the price. As of 5.005, $& is not so costly as the other two.
----------------------
So the answer to your question is: It is faster.
--
Tad McClellan SGML Consulting
tadmc@metronet.com Perl programming
Fort Worth, Texas
------------------------------
Date: Thu, 10 Jun 1999 15:53:01 GMT
From: Jared Hecker <jhecker@iago.nac.net>
Subject: Thanks to everyone who's helped
Message-Id: <xbR73.338$tr.2268@nntp1>
I just wanted to say "Thank you" to all and sundry who have been kind
enough to offer their help and assorted wisdom as I have stumbled up the
learning curve of perl and oraperl these last few weeks. This newsgroup
is a remembrance of what the net (and the profession) used to be. All the
knowledgeable people in this group should give themselves a pat on the
back for bringing along those of us new to the speed and benefits of perl.
Thanks!
Regards,
jh
--
Jared Hecker | HWA Inc. - Oracle architecture and Administration
jared@hwai.com | ** serving NYC and New Jersey **
------------------------------
Date: Thu, 10 Jun 1999 10:21:27 -0400
From: Ala Qumsieh <aqumsieh@matrox.com>
Subject: Re: Weird Warning from conditional
Message-Id: <x3yd7z4qgpk.fsf@tigre.matrox.com>
"Terry Mealy" <spike1965@worldnet.att.net> writes:
> I am writing a rather large script to generate some prices, and I am having
> problems with the following snippet which recurs in a for loop:
> ************************************
>
> if ( $cities[$#flight] ne $cities[$#flight + 1] ) {
> if($con_data{'ANCHOR'.$con_name} eq "Zero"){ #see if contract requires
> anchor
> if($city_data{$contract.$cities[$#flight]} ne "Zero" ||
> $city_data{$contract.$cities[$#flight + 1]} ne "Zero"){ #check to see if
> either city is not an anchor city
> $rej_cons{$valid_cons[$contract]} .= '/"Mixed" contracts not allowed';
> #Add contract to rejected contracts & append reason for rejection
> }
> }
> }
>
> ************************************
> I thought that very few of you would wish to see the entire script.
>
> In any case, the problem occurs when I used the -w switch. The first line
> generates a warning of "Use of uninitialized value at...".
I can't make out what the problem is, but I think it's not where you
think it is. Most probably, the problem is in one of the lines above,
but not in the first one. The obvious (and most tedious, but surest)
thing to do is to add print() statements everywhere to print out each
of your variables, and check their values.
HTH,
Ala
------------------------------
Date: 12 Dec 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 Dec 98)
Message-Id: <null>
Administrivia:
Well, after 6 months, here's the answer to the quiz: what do we do about
comp.lang.perl.moderated. Answer: nothing.
]From: Russ Allbery <rra@stanford.edu>
]Date: 21 Sep 1998 19:53:43 -0700
]Subject: comp.lang.perl.moderated available via e-mail
]
]It is possible to subscribe to comp.lang.perl.moderated as a mailing list.
]To do so, send mail to majordomo@eyrie.org with "subscribe clpm" in the
]body. Majordomo will then send you instructions on how to confirm your
]subscription. This is provided as a general service for those people who
]cannot receive the newsgroup for whatever reason or who just prefer to
]receive messages via e-mail.
The Perl-Users Digest is a retransmission of the USENET newsgroup
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------------------------------
End of Perl-Users Digest V8 Issue 5948
**************************************
