perlfaq8
PERLFAQ8(1) Perl Programmers Reference Guide PERLFAQ8(1)
NAME
perlfaq8 - System Interaction ($Revision: 1.22 $, $Date: 2004/10/05
22:13:49 $)
DESCRIPTION
This section of the Perl FAQ covers questions involving operating sys-
tem interaction. Topics include interprocess communication (IPC), con-
trol over the user-interface (keyboard, screen and pointing devices),
and most anything else not related to data manipulation.
Read the FAQs and documentation specific to the port of perl to your
operating system (eg, perlvms, perlplan9, ...). These should contain
more detailed information on the vagaries of your perl.
How do I find out which operating system I'm running under?
The $^O variable ($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 ter-
minal that understands color, you can use the Term::ANSIColor module
from CPAN:
use Term::ANSIColor;
print color("red"), "Stop!\n", color("reset");
print color("green"), "Go!\n", color("reset");
Or like this:
use Term::ANSIColor qw(:constants);
print RED, "Stop!\n", RESET;
print GREEN, "Go!\n", RESET;
How do I read just one key without waiting for a return key?
Controlling input buffering is a remarkably system-dependent matter.
On many systems, you can just use the stty command as shown in "getc"
in perlfunc, but as you see, that's already getting you into portabil-
ity snags.
open(TTY, "+</dev/tty") or die "no tty: $!";
system "stty cbreak </dev/tty >/dev/tty 2>&1";
$key = getc(TTY); # perhaps this works
# OR ELSE
sysread(TTY, $key, 1); # probably this does
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
The Term::ReadKey module from CPAN offers an easy-to-use interface that
should be more efficient than shelling out to stty for each key. It
even includes limited support for Windows.
use Term::ReadKey;
ReadMode('cbreak');
$key = ReadKey(0);
ReadMode('normal');
However, using the code requires that you have a working C compiler and
can use it to build and install a CPAN module. Here's a solution using
the standard POSIX module, which is already on your systems (assuming
your system supports POSIX).
use HotKey;
$key = readkey();
And here's the HotKey module, which hides the somewhat mystifying calls
to manipulate the POSIX termios structures.
# HotKey.pm
package HotKey;
@ISA = qw(Exporter);
@EXPORT = qw(cbreak cooked readkey);
use strict;
use POSIX qw(:termios_h);
my ($term, $oterm, $echo, $noecho, $fd_stdin);
$fd_stdin = fileno(STDIN);
$term = POSIX::Termios->new();
$term->getattr($fd_stdin);
$oterm = $term->getlflag();
$echo = ECHO | ECHOK | ICANON;
$noecho = $oterm & ~$echo;
sub cbreak {
$term->setlflag($noecho); # ok, so i don't want echo either
$term->setcc(VTIME, 1);
$term->setattr($fd_stdin, TCSANOW);
}
sub cooked {
$term->setlflag($oterm);
$term->setcc(VTIME, 0);
$term->setattr($fd_stdin, TCSANOW);
}
sub readkey {
my $key = '';
cbreak();
sysread(STDIN, $key, 1);
cooked();
return $key;
}
END { cooked() }
1;
How do I check whether input is ready on the keyboard?
The easiest way to do this is to read a key in nonblocking mode with
the Term::ReadKey module from CPAN, passing it an argument of -1 to
indicate not to block:
use Term::ReadKey;
ReadMode('cbreak');
if (defined ($char = ReadKey(-1)) ) {
# input was waiting and it was $char
} else {
# no input was waiting
}
ReadMode('normal'); # restore normal tty settings
How do I clear the screen?
If you only have do so infrequently, use "system":
system("clear");
If you have to do this a lot, save the clear string so you can print it
100 times without calling a program 100 times:
$clear_string = `clear`;
print $clear_string;
If you're planning on doing other screen manipulations, like cursor
positions, etc, you might wish to use Term::Cap module:
use Term::Cap;
$terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
$clear_string = $terminal->Tputs('cl');
How do I get the screen size?
If you have Term::ReadKey module installed from CPAN, you can use it to
fetch the width and height in characters and in pixels:
use Term::ReadKey;
($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
This is more portable than the raw "ioctl", but not as illustrative:
require 'sys/ioctl.ph';
die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
open(TTY, "+</dev/tty") or die "No tty: $!";
unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) {
die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ;
}
($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
print "(row,col) = ($row,$col)";
print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
print "\n";
How do I ask the user for a password?
(This question has nothing to do with the web. See a different FAQ for
that.)
There's an example of this in "crypt" in perlfunc). First, you put the
terminal into "no echo" mode, then just read the password normally.
You may do this with an old-style ioctl() function, POSIX terminal con-
trol (see POSIX or its documentation the Camel Book), or a call to the
stty program, with varying degrees of portability.
You can also do this for most systems using the Term::ReadKey module
from CPAN, which is easier to use and in theory more portable.
use Term::ReadKey;
ReadMode('noecho');
$password = ReadLine(0);
How do I read and write the serial port?
This depends on which operating system your program is running on. In
the case of Unix, the serial ports will be accessible through files in
/dev; on other systems, device names will doubtless differ. Several
problem areas common to all device interaction are the following:
lockfiles
Your system may use lockfiles to control multiple access. Make
sure you follow the correct protocol. Unpredictable behavior can
result from multiple processes reading from one device.
open mode
If you expect to use both read and write operations on the device,
you'll have to open it for update (see "open" in perlfunc for
details). You may wish to open it without running the risk of
blocking by using sysopen() and "O_RDWR|O_NDELAY|O_NOCTTY" from the
Fcntl module (part of the standard perl distribution). See
"sysopen" in perlfunc for more on this approach.
end of line
Some devices will be expecting a "\r" at the end of each line
rather than a "\n". In some ports of perl, "\r" and "\n" are dif-
ferent 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 specifica-
tion ("\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 out-
put. This applies especially to socket I/O and autoflushing, dis-
cussed 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 $| variable to control autoflushing (see "$|" in
perlvar and "select" in perlfunc, or perlfaq5, ``How do I
flush/unbuffer an output filehandle? Why must I do this?''):
$oldh = select(DEV);
$| = 1;
select($oldh);
You'll also see code that does this without a temporary variable,
as in
select((select(DEV), $| = 1)[0]);
Or if you don't mind pulling in a few thousand lines of code just
because you're afraid of a little $| variable:
use IO::Handle;
DEV->autoflush(1);
As mentioned in the previous item, this still doesn't work when
using socket I/O between Unix and Macintosh. You'll need to hard
code your line terminators, in that case.
non-blocking input
If you are doing a blocking read() or sysread(), you'll have to
arrange for an alarm handler to provide a timeout (see "alarm" in
perlfunc). If you have a non-blocking open, you'll likely have a
non-blocking read, which means you may have to use a 4-arg select()
to determine whether I/O is ready on that device (see "select" in
perlfunc.
While trying to read from his caller-id box, the notorious Jamie Zawin-
ski <jwz@netscape.com>, after much gnashing of teeth and fighting with
sysread, sysopen, POSIX's tcgetattr business, and various other func-
tions that go bump in the night, finally came up with this:
sub open_modem {
use IPC::Open2;
my $stty = `/bin/stty -g`;
open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
# starting cu hoses /dev/tty's stty settings, even when it has
# been opened on a pipe...
system("/bin/stty $stty");
$_ = <MODEM_IN>;
chomp;
if ( !m/^Connected/ ) {
print STDERR "$0: cu printed `$_' instead of `Connected'\n";
}
}
How do I decode encrypted password files?
You spend lots and lots of money on dedicated hardware, but this is
bound to get you talked about.
Seriously, you can't if they are Unix password files--the Unix password
system employs one-way encryption. It's more like hashing than encryp-
tion. The best you can check is whether something else hashes to the
same string. You can't turn a hash back into the original string.
Programs like Crack can forcibly (and intelligently) try to guess pass-
words, but don't (can't) guarantee quick success.
If you're worried about users selecting bad passwords, you should
proactively check when they try to change their password (by modifying
passwd(1), for example).
How do I start a process in the background?
Several modules can start other processes that do not block your Perl
program. You can use IPC::Open3, Parallel::Jobs, IPC::Run, and some of
the POE modules. See CPAN for more details.
You could also use
system("cmd &")
or you could use fork as documented in "fork" in perlfunc, with further
examples in perlipc. Some things to be aware of, if you're on a Unix-
like system:
STDIN, STDOUT, and STDERR are shared
Both the main process and the backgrounded one (the "child" pro-
cess) share the same STDIN, STDOUT and STDERR filehandles. If both
try to access them at once, strange things can happen. You may
want to close or reopen these for the child. You can get around
this with "open"ing a pipe (see "open" in perlfunc) but on some
systems this means that the child process cannot outlive the par-
ent.
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 fin-
ishes.
$SIG{CHLD} = sub { wait };
$SIG{CHLD} = 'IGNORE';
You can also use a double fork. You immediately wait() for your
first child, and the init daemon will wait() for your grandchild
once it exits.
unless ($pid = fork) {
unless (fork) {
exec "what you really wanna do";
die "exec failed!";
}
exit 0;
}
waitpid($pid,0);
See "Signals" in perlipc for other examples of code to do this.
Zombies are not an issue with "system("prog &")".
How do I trap control characters/signals?
You don't actually "trap" a control character. Instead, that character
generates a signal which is sent to your terminal's currently fore-
grounded process group, which you then trap in your process. Signals
are documented in "Signals" in perlipc and the section on ``Signals''
in the Camel.
You can set the values of the %SIG hash to be the functions you want to
handle the signal. After perl catches the signal, it looks in %SIG for
a key with the same name as the signal, then calls the subroutine value
for that key.
# as an anonymous subroutine
$SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) };
# or a reference to a function
$SIG{INT} = \&ouch;
# or the name of the function as a string
$SIG{INT} = "ouch";
Perl versions before 5.8 had in its C source code signal handlers which
would catch the signal and possibly run a Perl function that you had
set in %SIG. This violated the rules of signal handling at that level
causing perl to dump core. Since version 5.8.0, perl looks at %SIG
*after* the signal has been caught, rather than while it is being
caught. Previous versions of this answer were incorrect.
How do I modify the shadow password file on a Unix system?
If perl was installed correctly and your shadow library was written
properly, the getpw*() functions described in perlfunc should in theory
provide (read-only) access to entries in the shadow password file. To
change the file, make a new shadow password file (the format varies
from system to system--see passwd for specifics) and use pwd_mkdb(8) to
install it (see pwd_mkdb 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) pro-
gram. (There is no way to set the time and date on a per-process
basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT;
the VMS equivalent is "set time".
However, if all you want to do is change your time zone, you can proba-
bly 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() func-
tion provides, the easiest way is to use the select() function as docu-
mented in "select" in perlfunc. Try the Time::HiRes and the
BSD::Itimer modules (available from CPAN, and starting from Perl 5.8
Time::HiRes is part of the standard distribution).
How can I measure time under a second?
In general, you may not be able to. The Time::HiRes module (available
from CPAN, and starting from Perl 5.8 part of the standard distribu-
tion) 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 some-
thing like this:
require 'sys/syscall.ph';
$TIMEVAL_T = "LL";
$done = $start = pack($TIMEVAL_T, ());
syscall(&SYS_gettimeofday, $start, 0) != -1
or die "gettimeofday: $!";
##########################
# DO YOUR OPERATION HERE #
##########################
syscall( &SYS_gettimeofday, $done, 0) != -1
or die "gettimeofday: $!";
@start = unpack($TIMEVAL_T, $start);
@done = unpack($TIMEVAL_T, $done);
# fix microseconds
for ($done[1], $start[1]) { $_ /= 1_000_000 }
$delta_time = sprintf "%.4f", ($done[0] + $done[1] )
-
($start[0] + $start[1] );
How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
Release 5 of Perl added the END block, which can be used to simulate
atexit(). Each package's END block is called when the program or
thread ends (see perlmod manpage for more details).
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 "Signals" in perlipc or the section on ``Signals'' in the
Camel Book.
If exception handling is all you're interested in, try the excep-
tions.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 archi-
tectures, they were often hardwired into perl code. The proper way to
deal with this is to "use Socket" to get the correct values.
Note that even though SunOS and Solaris are binary compatible, these
values are different. Go figure.
How can I call my system's unique C functions from Perl?
In most cases, you write an external module to do it--see the answer to
"Where can I learn about linking C with Perl? [h2xs, xsubpp]".
However, if the function is a system call, and your system supports
syscall(), you can use the syscall function (documented in perlfunc).
Remember to check the modules that came with your distribution, and
CPAN as well---someone may already have written a module to do it. On
Windows, try Win32::API. On Macs, try Mac::Carbon. If no module has
an interface to the C function, you can inline a bit of C in your Perl
source with Inline::C.
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. Sim-
ple files like errno.h, syscall.h, and socket.h were fine, but the hard
ones like ioctl.h nearly always need to hand-edited. Here's how to
install the *.ph files:
1. become super-user
2. cd /usr/include
3. h2ph *.h */*.h
If your system supports dynamic loading, for reasons of portability and
sanity you probably ought to use h2xs (also part of the standard perl
distribution). This tool converts C header files to Perl extensions.
See perlxstut for how to get started with h2xs.
If your system doesn't support dynamic loading, you still probably
ought to use h2xs. See perlxstut and ExtUtils::MakeMaker for more
information (in brief, just use make perl instead of a plain make to
rebuild perl with a new static extension).
Why do setuid perl scripts complain about kernel problems?
Some operating systems have bugs in the kernel that make setuid scripts
inherently insecure. Perl gives you a number of options (described in
perlsec) to work around such systems.
How can I open a pipe both to and from a command?
The IPC::Open2 module (part of the standard perl distribution) is an
easy-to-use approach that internally uses pipe(), fork(), and exec() to
do the job. Make sure you read the deadlock warnings in its documenta-
tion, though (see IPC::Open2). See "Bidirectional Communication with
Another Process" in perlipc and "Bidirectional Communication with Your-
self" in perlipc
You may also use the IPC::Open3 module (part of the standard perl dis-
tribution), but be warned that it has a different order of arguments
from IPC::Open2 (see IPC::Open3).
Why can't I get the output of a command with system()?
You're confusing the purpose of system() and backticks (``). system()
runs a command and returns exit status information (as a 16 bit value:
the low 7 bits are the signal the process died from, if any, and the
high 8 bits are the actual exit value). Backticks (``) run a command
and return what it sent to STDOUT.
$exit_status = system("mail-users");
$output_string = `ls`;
How can I capture STDERR from an external command?
There are three basic ways of running external commands:
system $cmd; # using system()
$output = `$cmd`; # using backticks (``)
open (PIPE, "cmd |"); # using open()
With system(), both STDOUT and STDERR will go the same place as the
script's STDOUT and STDERR, unless the system() command redirects them.
Backticks and open() read only the STDOUT of your command.
You can also use the open3() function from IPC::Open3. Benjamin Gold-
berg provides some sample code:
To capture a program's STDOUT, but discard its STDERR:
use IPC::Open3;
use File::Spec;
use Symbol qw(gensym);
open(NULL, ">", File::Spec->devnull);
my $pid = open3(gensym, \*PH, ">&NULL", "cmd");
while( <PH> ) { }
waitpid($pid, 0);
To capture a program's STDERR, but discard its STDOUT:
use IPC::Open3;
use File::Spec;
use Symbol qw(gensym);
open(NULL, ">", File::Spec->devnull);
my $pid = open3(gensym, ">&NULL", \*PH, "cmd");
while( <PH> ) { }
waitpid($pid, 0);
To capture a program's STDERR, and let its STDOUT go to our own STDERR:
use IPC::Open3;
use Symbol qw(gensym);
my $pid = open3(gensym, ">&STDERR", \*PH, "cmd");
while( <PH> ) { }
waitpid($pid, 0);
To read both a command's STDOUT and its STDERR separately, you can
redirect them to temp files, let the command run, then read the temp
files:
use IPC::Open3;
use Symbol qw(gensym);
use IO::File;
local *CATCHOUT = IO::File->new_tempfile;
local *CATCHERR = IO::File->new_tempfile;
my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd");
waitpid($pid, 0);
seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
while( <CATCHOUT> ) {}
while( <CATCHERR> ) {}
But there's no real need for *both* to be tempfiles... the following
should work just as well, without deadlocking:
use IPC::Open3;
use Symbol qw(gensym);
use IO::File;
local *CATCHERR = IO::File->new_tempfile;
my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd");
while( <CATCHOUT> ) {}
waitpid($pid, 0);
seek CATCHERR, 0, 0;
while( <CATCHERR> ) {}
And it'll be faster, too, since we can begin processing the program's
stdout immediately, rather than waiting for the program to finish.
With any of these, you can change file descriptors before the call:
open(STDOUT, ">logfile");
system("ls");
or you can use Bourne shell file-descriptor redirection:
$output = `$cmd 2>some_file`;
open (PIPE, "cmd 2>some_file |");
You can also use file-descriptor redirection to make STDERR a duplicate
of STDOUT:
$output = `$cmd 2>&1`;
open (PIPE, "cmd 2>&1 |");
Note that you cannot simply open STDERR to be a dup of STDOUT in your
Perl program and avoid calling the shell to do the redirection. This
doesn't work:
open(STDERR, ">&STDOUT");
$alloutput = `cmd args`; # stderr still escapes
This fails because the open() makes STDERR go to where STDOUT was going
at the time of the open(). The backticks then make STDOUT go to a
string, but don't change STDERR (which still goes to the old STDOUT).
Note that you must use Bourne shell (sh(1)) redirection syntax in back-
ticks, not csh(1)! Details on why Perl's system() and backtick and
pipe opens all use the Bourne shell are in the versus/csh.whynot arti-
cle in the "Far More Than You Ever Wanted To Know" collection in
http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . To capture a command's
STDERR and STDOUT together:
$output = `cmd 2>&1`; # either with backticks
$pid = open(PH, "cmd 2>&1 |"); # or with an open pipe
while (<PH>) { } # plus a read
To capture a command's STDOUT but discard its STDERR:
$output = `cmd 2>/dev/null`; # either with backticks
$pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe
while (<PH>) { } # plus a read
To capture a command's STDERR but discard its STDOUT:
$output = `cmd 2>&1 1>/dev/null`; # either with backticks
$pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe
while (<PH>) { } # plus a read
To exchange a command's STDOUT and STDERR in order to capture the
STDERR but leave its STDOUT to come out our old STDERR:
$output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks
$pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
while (<PH>) { } # plus a read
To read both a command's STDOUT and its STDERR separately, it's easiest
to redirect them separately to files, and then read from those files
when the program is done:
system("program args 1>program.stdout 2>program.stderr");
Ordering is important in all these examples. That's because the shell
processes file descriptor redirections in strictly left to right order.
system("prog args 1>tmpfile 2>&1");
system("prog args 2>&1 1>tmpfile");
The first command sends both standard out and standard error to the
temporary file. The second command sends only the old standard output
there, and the old standard error shows up on the old standard out.
Why doesn't open() return an error when a pipe open fails?
If the second argument to a piped open() contains shell metacharacters,
perl fork()s, then exec()s a shell to decode the metacharacters and
eventually run the desired program. If the program couldn't be run,
it's the shell that gets the message, not Perl. All your Perl program
can find out is whether the shell itself could be successfully started.
You can still capture the shell's STDERR and check it for error mes-
sages. See "How can I capture STDERR from an external command?" else-
where in this document, or use the IPC::Open3 module.
If there are no shell metacharacters in the argument of open(), Perl
runs the command directly, without using the shell, and can correctly
report whether the command started.
What's wrong with using backticks in a void context?
Strictly speaking, nothing. Stylistically speaking, it's not a good
way to write maintainable code. Perl has several operators for running
external commands. Backticks are one; they collect the output from the
command for use in your program. The "system" function is another; it
doesn't do this.
Writing backticks in your program sends a clear message to the readers
of your code that you wanted to collect the output of the command. Why
send a clear message that isn't true?
Consider this line:
`cat /etc/termcap`;
You forgot to check $? to see whether the program even ran correctly.
Even if you wrote
print `cat /etc/termcap`;
this code could and probably should be written as
system("cat /etc/termcap") == 0
or die "cat program failed!";
which will get the output quickly (as it is generated, instead of only
at the end) and also check the return value.
system() also provides direct control over whether shell wildcard pro-
cessing may take place, whereas backticks do not.
How can I call backticks without shell processing?
This is a bit tricky. You can't simply write the command like this:
@ok = `grep @opts '$search_string' @filenames`;
As of Perl 5.8.0, you can use open() with multiple arguments. Just
like the list forms of system() and exec(), no shell escapes happen.
open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
chomp(@ok = <GREP>);
close GREP;
You can also:
my @ok = ();
if (open(GREP, "-|")) {
while (<GREP>) {
chomp;
push(@ok, $_);
}
close GREP;
} else {
exec 'grep', @opts, $search_string, @filenames;
}
Just as with system(), no shell escapes happen when you exec() a list.
Further examples of this can be found in "Safe Pipe Opens" in perlipc.
Note that if you're use Microsoft, no solution to this vexing issue is
even possible. Even if Perl were to emulate fork(), you'd still be
stuck, because Microsoft does not have a argc/argv-style API.
Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z
on MS-DOS)?
Some stdio's set error and eof flags that need clearing. The POSIX
module defines 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 sys-
read.
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.cpan.org/scripts/netstuff/telnet.emul.shar will also
help for emulating the telnet protocol, but Net::Telnet is quite proba-
bly 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 argu-
ment. Hiding the arguments won't make your program completely secure.
To actually alter the visible command line, you can assign to the vari-
able $0 as documented in perlvar. This won't work on all operating
systems, though. Daemon programs like sendmail place their state
there, as in:
$0 = "orcus [accepting connections]";
I {changed directory, modified my environment} in a perl script. How
come the change disappeared when I exited the script? How do I get my
changes to be visible?
Unix
In the strictest sense, it can't be done--the script executes as a
different process from the shell it was started from. Changes to a
process are not reflected in its parent--only in any children cre-
ated 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 com-
plete?
Assuming your system supports such things, just send an appropriate
signal to the process (see "kill" in perlfunc). It's common to first
send a TERM signal, wait a little bit, and then send a KILL signal to
finish it off.
How do I fork a daemon process?
If by daemon process you mean one that's detached (disassociated from
its tty), then the following process is reported to work on most Unix-
ish systems. Non-Unix users should check their Your_OS::Process module
for other solutions.
o Open /dev/tty and use the TIOCNOTTY ioctl on it. See tty for
details. Or better yet, you can just use the POSIX::setsid() func-
tion, so you don't have to worry about process groups.
o Change directory to /
o Reopen STDIN, STDOUT, and STDERR so they're not connected to the
old tty.
o Background yourself like this:
fork && exit;
The Proc::Daemon module, available from CPAN, provides a function to
perform these actions for you.
How do I find out if I'm running interactively or not?
Good question. Sometimes "-t STDIN" and "-t STDOUT" can give clues,
sometimes not.
if (-t STDIN && -t STDOUT) {
print "Now what? ";
}
On POSIX systems, you can test whether your own process group matches
the current process group of your controlling terminal as follows:
use POSIX qw/getpgrp tcgetpgrp/;
open(TTY, "/dev/tty") or die $!;
$tpgrp = tcgetpgrp(fileno(*TTY));
$pgrp = getpgrp();
if ($tpgrp == $pgrp) {
print "foreground\n";
} else {
print "background\n";
}
How do I timeout a slow event?
Use the alarm() function, probably in conjunction with a signal han-
dler, as documented in "Signals" in perlipc and the section on ``Sig-
nals'' in the Camel. You may instead use the more flexible Sys::Alarm-
Call module available from CPAN.
The alarm() function is not implemented on all versions of Windows.
Check the documentation for your specific version of Perl.
How do I set CPU limits?
Use the BSD::Resource module from CPAN.
How do I avoid zombies on a Unix system?
Use the reaper code from "Signals" in perlipc to call wait() when a
SIGCHLD is received, or else use the double-fork technique described in
"How do I start a process in the background?" in perlfaq8.
How do I use an SQL database?
The DBI module provides an abstract interface to most database servers
and types, including Oracle, DB2, Sybase, mysql, Postgresql, ODBC, and
flat files. The DBI module accesses each database type through a
database driver, or DBD. You can see a complete list of available
drivers on CPAN: http://www.cpan.org/modules/by-module/DBD/ . You can
read more about DBI on http://dbi.perl.org .
Other modules provide more specific access: Win32::ODBC, Alzabo, iodbc,
and others found on CPAN Search: http://search.cpan.org .
How do I make a system() exit on control-C?
You can't. You need to imitate the system() call (see perlipc for sam-
ple 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, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
or die "can't open /foo/somefile: $!":
How do I tell the difference between errors from the shell and perl?
(answer contributed by brian d foy, "<bdfoy@cpan.org>"
When you run a Perl script, something else is running the script for
you, and that something else may output error messages. The script
might emit its own warnings and error messages. Most of the time you
cannot tell who said what.
You probably cannot fix the thing that runs perl, but you can change
how perl outputs its warnings by defining a custom warning and die
functions.
Consider this script, which has an error you may not notice immedi-
ately.
#!/usr/locl/bin/perl
print "Hello World\n";
I get an error when I run this from my shell (which happens to be
bash). That may look like perl forgot it has a print() function, but
my shebang line is not the path to perl, so the shell runs the script,
and I get the error.
$ ./test
./test: line 3: print: command not found
A quick and dirty fix involves a little bit of code, but this may be
all you need to figure out the problem.
#!/usr/bin/perl -w
BEGIN {
$SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; };
$SIG{__DIE__} = sub{ print STDERR "Perl: ", @_; exit 1};
}
$a = 1 + undef;
$x / 0;
__END__
The perl message comes out with "Perl" in front. The BEGIN block works
at compile time so all of the compilation errors and warnings get the
"Perl:" prefix too.
Perl: Useless use of division (/) in void context at ./test line 9.
Perl: Name "main::a" used only once: possible typo at ./test line 8.
Perl: Name "main::x" used only once: possible typo at ./test line 9.
Perl: Use of uninitialized value in addition (+) at ./test line 8.
Perl: Use of uninitialized value in division (/) at ./test line 9.
Perl: Illegal division by zero at ./test line 9.
Perl: Illegal division by zero at -e line 3.
If I don't see that "Perl:", it's not from perl.
You could also just know all the perl errors, and although there are
some people who may know all of them, you probably don't. However,
they all should be in the perldiag manpage. If you don't find the error
in there, it probably isn't a perl error.
Looking up every message is not the easiest way, so let perl to do it
for you. Use the diagnostics pragma with turns perl's normal messages
into longer discussions on the topic.
use diagnostics;
If you don't get a paragraph or two of expanded discussion, it might
not be perl's message.
How do I install a module from CPAN?
The easiest way is to have a module also named CPAN do it for you.
This module comes with perl version 5.004 and later.
$ perl -MCPAN -e shell
cpan shell -- CPAN exploration and modules installation (v1.59_54)
ReadLine support enabled
cpan> install Some::Module
To manually install the CPAN module, or any well-behaved CPAN module
for that matter, follow these steps:
1 Unpack the source into a temporary area.
2
perl Makefile.PL
3
make
4
make test
5
make install
If your version of perl is compiled without dynamic loading, then you
just need to replace step 3 (make) with make perl and you will get a
new perl binary with your extension linked in.
See ExtUtils::MakeMaker for more details on building extensions. See
also the next question, ``What's the difference between require and
use?''.
What's the difference between require and use?
Perl offers several different ways to include code from one file into
another. Here are the deltas between the various inclusion constructs:
1) do $file is like eval `cat $file`, except the former
1.1: searches @INC and updates %INC.
1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
2) require $file is like do $file, except the former
2.1: checks for redundant loading, skipping already loaded files.
2.2: raises an exception on failure to find, compile, or execute $file.
3) require Module is like require "Module.pm", except the former
3.1: translates each "::" into your system's directory separator.
3.2: primes the parser to disambiguate class Module as an indirect object.
4) use Module is like require Module, except the former
4.1: loads the module at compile time, not run-time.
4.2: imports symbols and semantics from that package to the current one.
In general, you usually want "use" and a proper Perl module.
How do I keep my own module/library directory?
When you build modules, use the PREFIX and LIB options when generating
Makefiles:
perl Makefile.PL PREFIX=/mydir/perl LIB=/mydir/perl/lib
then either set the PERL5LIB environment variable before you run
scripts that use the modules/libraries (see perlrun) or say
use lib '/mydir/perl/lib';
This is almost the same as
BEGIN {
unshift(@INC, '/mydir/perl/lib');
}
except that the lib module checks for machine-dependent subdirectories.
See Perl's lib for more information.
How do I add the directory my program lives in to the module/library
search path?
use FindBin;
use lib "$FindBin::Bin";
use your_own_modules;
How do I add a directory to my include path (@INC) 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 depen-
dent 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) 1997-2003 Tom Christiansen and Nathan Torkington. All
rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file are
hereby placed into the public domain. You are permitted and encouraged
to use this code in your own programs for fun or for profit as you see
fit. A simple comment in the code giving credit would be courteous but
is not required.
perl v5.8.6 2004-11-05 PERLFAQ8(1)
