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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 system interac-
tion. Topics include 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 sys-
tem (eg, perlvms, perlplan9, ...). These should contain more detailed information
on the vagaries of your perl.
How do I find out which operating system Iâ€â€™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() doesnâ€â€™t return?
Because that’s what it does: it replaces your currently running program with a dif-
ferent 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 sys-
tem-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 sys-
tems, you can just use the stty command as shown in "getc" in perlfunc, 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, 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;
$key = readkey();
And here’s the HotKey module, which hides the somewhat mystifying calls to manipu-
late the POSIX termios structures.
# HotKey.pm
package HotKey;
@ISA = qw(Exporter);
@EXPORT = qw(cbreak cooked readkey);
use strict;
use POSIX qw(:termios_h);
my ($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 control (see POSIX or its documentation
the Camel Book), or a call to the stty program, with varying degrees of portabil-
ity.
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 pro-
cesses reading from one device.
open mode
If you expect to use both read and write operations on the device, you’ll have
to open it for update (see "open" in perlfunc for details). You may wish to
open it without running the risk of blocking by using sysopen() and
"O_RDWR│O_NDELAY│O_NOCTTY" from the Fcntl module (part of the standard perl
distribution). See "sysopen" in perlfunc for more on this approach.
end of line
Some devices will be expecting a "\r" at the end of each line rather than a
"\n". In some ports of perl, "\r" and "\n" are different from their usual
(Unix) ASCII values of "\012" and "\015". You may have to give the numeric
values you want directly, using octal ("\015"), hex ("0x0D"), or as a control-
character specification ("\cM").
print DEV "atv1\012"; # wrong, for some devices
print DEV "atv1\015"; # right, for some devices
Even though with normal text files a "\n" will do the trick, there is still no
unified scheme for terminating a line that is portable between Unix, DOS/Win,
and Macintosh, except to terminate ALL line ends with "\015\012", and strip
what you don’t need from the output. This applies especially to socket I/O and
autoflushing, discussed next.
flushing output
If you expect characters to get to your device when you print() them, you’ll
want to autoflush that filehandle. You can use select() and the $│ variable to
control autoflushing (see "$│" in perlvar and "select" in perlfunc, or perl-
faq5, ‘‘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 termina-
tors, 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 Zawinski
<jwz AT 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>;
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 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 intelli-
gently) 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?
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" 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 "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 finishes.
$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 foregrounded 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 vio-
lated 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) 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 time zone, 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 "select" in perl-
func. 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 distribution) provides this func-
tionality 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 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 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 doesnâ€â€™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 systemâ€â€™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 argu-
ments to your functions. It doesn’t work perfectly, but it usually gets most of
the job done. Simple files like errno.h, syscall.h, and socket.h were fine, but
the hard ones like ioctl.h nearly always need to hand-edited. Here’s how to
install the *.ph files:
1. become super-user
2. cd /usr/include
3. h2ph *.h */*.h
If your system supports dynamic loading, for reasons of portability and sanity you
probably ought to use h2xs (also part of the standard perl distribution). This
tool converts C header files to Perl extensions. See perlxstut for how to get
started with h2xs.
If your system doesn’t support dynamic loading, you still probably ought to use
h2xs. See perlxstut and ExtUtils::MakeMaker for more information (in brief, just
use make perl instead of a plain make to rebuild perl with a new static extension).
Why do setuid perl scripts complain about kernel problems?
Some operating systems have bugs in the kernel that make setuid scripts inherently
insecure. Perl gives you a number of options (described in perlsec) to work around
such systems.
How can I open a pipe both to and from a command?
The IPC::Open2 module (part of the standard perl distribution) is an easy-to-use
approach that internally uses pipe(), fork(), and exec() to do the job. Make sure
you read the deadlock warnings in its documentation, though (see IPC::Open2). See
"Bidirectional Communication with Another Process" in perlipc and "Bidirectional
Communication with Yourself" in perlipc
You may also use the IPC::Open3 module (part of the standard perl distribution),
but be warned that it has a different order of arguments from IPC::Open2 (see
IPC::Open3).
Why canâ€â€™t I get the output of a command with system()?
You’re confusing the purpose of system() and backticks (‘‘). system() runs a com-
mand 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 Goldberg 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 imme-
diately, 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 backticks, 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 article 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 doesnâ€â€™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 mes-
sage, 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 messages. See "How can I capture STDERR from an external command?"
elsewhere in this document, or use the IPC::Open3 module.
If there are no shell metacharacters in the argument of open(), Perl runs the com-
mand directly, without using the shell, and can correctly report whether the com-
mand started.
Whatâ€â€™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 pro-
gram. 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 $? to see whether the program even ran correctly. Even if you
wrote
print ‘cat /etc/termcap‘;
this code could and probably should be written as
system("cat /etc/termcap") == 0
or die "cat program failed!";
which will get the output quickly (as it is generated, instead of only at the end)
and also check the return value.
system() also provides direct control over whether shell wildcard processing may
take place, whereas backticks do not.
How can I call backticks without shell processing?
This is a bit tricky. You can’t simply write the command like this:
@ok = ‘grep @opts ’$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 possi-
ble. Even if Perl were to emulate fork(), you’d still be stuck, because Microsoft
does not have a argc/argv-style API.
Why canâ€â€™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 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.cpan.org/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 perlâ€â€™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 criti-
cal information is never given as an argument. Hiding the arguments won’t make
your program completely secure.
To actually alter the visible command line, you can assign to the variable $0 as
documented in perlvar. This won’t work on all operating systems, though. Daemon
programs like sendmail place their state there, as in:
$0 = "orcus [accepting connections]";
I {changed directory, modified my environment} in a perl script. How come the
change disappeared when I exited the script? How do I get my changes to be visi-
ble?
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 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 processâ€â€™s filehandle without waiting for it to complete?
Assuming your system supports such things, just send an appropriate signal to the
process (see "kill" in perlfunc). It’s common to first send a TERM signal, wait a
little bit, and then send a KILL signal to finish it off.
How do I fork a daemon process?
If by daemon process you mean one that’s detached (disassociated from its tty),
then the following process is reported to work on most Unixish systems. Non-Unix
users should check their Your_OS::Process module for other solutions.
· Open /dev/tty and use the TIOCNOTTY ioctl on it. See tty for details. Or bet-
ter 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 find out if Iâ€â€™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 docu-
mented in "Signals" in perlipc and the section on ‘‘Signals’’ in the Camel. You
may instead use the more flexible Sys::AlarmCall module available from CPAN.
The alarm() function is not implemented on all versions of Windows. Check the doc-
umentation 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 pro-
cess 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-mod-
ule/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 sample code) and
then have a signal handler for the INT signal that passes the signal on to the sub-
process. 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 AT 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 out-
puts its warnings by defining a custom warning and die functions.
Consider this script, which has an error you may not notice immediately.
#!/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 mat-
ter, 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?’’.
Whatâ€â€™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 dependent archi-
tectures. 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)
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