<STRONG><P CLASS=block> perlfaq8 - System Interaction</P></STRONG>
</TD></TR>
</TABLE>
<A NAME="__index__"></A>
<!-- INDEX BEGIN -->
<UL>
<LI><A HREF="#name">NAME</A></LI>
<LI><A HREF="#description">DESCRIPTION</A></LI>
<UL>
<LI><A HREF="#how do i find out which operating system i'm running under">How do I find out which operating system I'm running under?</A></LI>
<LI><A HREF="#how come exec() doesn't return">How come <A HREF="../../lib/Pod/perlfunc.html#item_exec"><CODE>exec()</CODE></A> doesn't return?</A></LI>
<LI><A HREF="#how do i do fancy stuff with the keyboard/screen/mouse">How do I do fancy stuff with the keyboard/screen/mouse?</A></LI>
<LI><A HREF="#how do i print something out in color">How do I print something out in color?</A></LI>
<LI><A HREF="#how do i read just one key without waiting for a return key">How do I read just one key without waiting for a return key?</A></LI>
<LI><A HREF="#how do i check whether input is ready on the keyboard">How do I check whether input is ready on the keyboard?</A></LI>
<LI><A HREF="#how do i clear the screen">How do I clear the screen?</A></LI>
<LI><A HREF="#how do i get the screen size">How do I get the screen size?</A></LI>
<LI><A HREF="#how do i ask the user for a password">How do I ask the user for a password?</A></LI>
<LI><A HREF="#how do i read and write the serial port">How do I read and write the serial port?</A></LI>
<LI><A HREF="#how do i decode encrypted password files">How do I decode encrypted password files?</A></LI>
<LI><A HREF="#how do i start a process in the background">How do I start a process in the background?</A></LI>
<LI><A HREF="#how do i trap control characters/signals">How do I trap control characters/signals?</A></LI>
<LI><A HREF="#how do i modify the shadow password file on a unix system">How do I modify the shadow password file on a Unix system?</A></LI>
<LI><A HREF="#how do i set the time and date">How do I set the time and date?</A></LI>
<LI><A HREF="#how can i sleep() or alarm() for under a second">How can I <A HREF="../../lib/Pod/perlfunc.html#item_sleep"><CODE>sleep()</CODE></A> or <A HREF="../../lib/Pod/perlfunc.html#item_alarm"><CODE>alarm()</CODE></A> for under a second?</A></LI>
<LI><A HREF="#how can i measure time under a second">How can I measure time under a second?</A></LI>
<LI><A HREF="#how can i do an atexit() or setjmp()/longjmp() (exception handling)">How can I do an <CODE>atexit()</CODE> or setjmp()/longjmp()? (Exception handling)</A></LI>
<LI><A HREF="#why doesn't my sockets program work under system v (solaris) what does the error message protocol not supported mean">Why doesn't my sockets program work under System V (Solaris)? What does the error message ``Protocol not supported'' mean?</A></LI>
<LI><A HREF="#how can i call my system's unique c functions from perl">How can I call my system's unique C functions from Perl?</A></LI>
<LI><A HREF="#where do i get the include files to do ioctl() or syscall()">Where do I get the include files to do <A HREF="../../lib/Pod/perlfunc.html#item_ioctl"><CODE>ioctl()</CODE></A> or syscall()?</A></LI>
<LI><A HREF="#why do setuid perl scripts complain about kernel problems">Why do setuid perl scripts complain about kernel problems?</A></LI>
<LI><A HREF="#how can i open a pipe both to and from a command">How can I open a pipe both to and from a command?</A></LI>
<LI><A HREF="#why can't i get the output of a command with system()">Why can't I get the output of a command with system()?</A></LI>
<LI><A HREF="#how can i capture stderr from an external command">How can I capture STDERR from an external command?</A></LI>
<LI><A HREF="#why doesn't open() return an error when a pipe open fails">Why doesn't <A HREF="../../lib/Pod/perlfunc.html#item_open"><CODE>open()</CODE></A> return an error when a pipe open fails?</A></LI>
<LI><A HREF="#what's wrong with using backticks in a void context">What's wrong with using backticks in a void context?</A></LI>
<LI><A HREF="#how can i call backticks without shell processing">How can I call backticks without shell processing?</A></LI>
<LI><A HREF="#why can't my script read from stdin after i gave it eof (^d on unix, ^z on msdos)">Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?</A></LI>
<LI><A HREF="#how can i convert my shell script to perl">How can I convert my shell script to perl?</A></LI>
<LI><A HREF="#can i use perl to run a telnet or ftp session">Can I use perl to run a telnet or ftp session?</A></LI>
<LI><A HREF="#how can i write expect in perl">How can I write expect in Perl?</A></LI>
<LI><A HREF="#is there a way to hide perl's command line from programs such as ps">Is there a way to hide perl's command line from programs such as ``ps''?</A></LI>
<LI><A HREF="#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">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?</A></LI>
<LI><A HREF="#how do i close a process's filehandle without waiting for it to complete">How do I close a process's filehandle without waiting for it to complete?</A></LI>
<LI><A HREF="#how do i fork a daemon process">How do I fork a daemon process?</A></LI>
<LI><A HREF="#how do i make my program run with sh and csh">How do I make my program run with sh and csh?</A></LI>
<LI><A HREF="#how do i find out if i'm running interactively or not">How do I find out if I'm running interactively or not?</A></LI>
<LI><A HREF="#how do i timeout a slow event">How do I timeout a slow event?</A></LI>
<LI><A HREF="#how do i set cpu limits">How do I set CPU limits?</A></LI>
<LI><A HREF="#how do i avoid zombies on a unix system">How do I avoid zombies on a Unix system?</A></LI>
<LI><A HREF="#how do i use an sql database">How do I use an SQL database?</A></LI>
<LI><A HREF="#how do i make a system() exit on controlc">How do I make a <A HREF="../../lib/Pod/perlfunc.html#item_system"><CODE>system()</CODE></A> exit on control-C?</A></LI>
<LI><A HREF="#how do i open a file without blocking">How do I open a file without blocking?</A></LI>
<LI><A HREF="#how do i install a module from cpan">How do I install a module from CPAN?</A></LI>
<LI><A HREF="#what's the difference between require and use">What's the difference between require and use?</A></LI>
<LI><A HREF="#how do i keep my own module/library directory">How do I keep my own module/library directory?</A></LI>
<LI><A HREF="#how do i add the directory my program lives in to the module/library search path">How do I add the directory my program lives in to the module/library search path?</A></LI>
<LI><A HREF="#how do i add a directory to my include path at runtime">How do I add a directory to my include path at runtime?</A></LI>
<LI><A HREF="#what is socket.ph and where do i get it">What is socket.ph and where do I get it?</A></LI>
</UL>
<LI><A HREF="#author and copyright">AUTHOR AND COPYRIGHT</A></LI>
# 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";
}
}</PRE>
<P>
<H2><A NAME="how do i decode encrypted password files">How do I decode encrypted password files?</A></H2>
<P>You spend lots and lots of money on dedicated hardware, but this is
bound to get you talked about.</P>
<P>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.</P>
<P>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).</P>
<P>
<H2><A NAME="how do i start a process in the background">How do I start a process in the background?</A></H2>
<P>You could use</P>
<PRE>
system("cmd &")</PRE>
<P>or you could use fork as documented in <A HREF="../../lib/Pod/perlfunc.html#fork">fork in the perlfunc manpage</A>, with
further examples in <A HREF="../../lib/Pod/perlipc.html">the perlipc manpage</A>. Some things to be aware of, if you're
on a Unix-like system:</P>
<DL>
<DT><STRONG><A NAME="item_STDIN%2C_STDOUT%2C_and_STDERR_are_shared">STDIN, STDOUT, and STDERR are shared</A></STRONG><BR>
<DD>
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
<A HREF="../../lib/Pod/perlfunc.html#item_open"><CODE>open</CODE></A>ing a pipe (see <A HREF="../../lib/Pod/perlfunc.html#open">open in the perlfunc manpage</A>) but on some systems this
means that the child process cannot outlive the parent.
You have to be prepared to ``reap'' the child process when it finishes
<PRE>
$SIG{CHLD} = sub { wait };</PRE>
<P>See <A HREF="../../lib/Pod/perlipc.html#signals">Signals in the perlipc manpage</A> for other examples of code to do this.
Zombies are not an issue with <A HREF="../../lib/Pod/perlfunc.html#item_system"><CODE>system("prog &")</CODE></A>.</P>
<P></P></DL>
<P>
<H2><A NAME="how do i trap control characters/signals">How do I trap control characters/signals?</A></H2>
<P>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 <A HREF="../../lib/Pod/perlipc.html#signals">Signals in the perlipc manpage</A> and chapter 6 of the Camel.</P>
<P>Be warned that very few C libraries are re-entrant. Therefore, if you
attempt to <A HREF="../../lib/Pod/perlfunc.html#item_print"><CODE>print()</CODE></A> 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 <A HREF="../../lib/Pod/perlfunc.html#item_syswrite"><CODE>syswrite()</CODE></A> instead of print().</P>
<P>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 <CODE>malloc()</CODE> is not
called (eg, by setting a variable that already has a value).</P>
<P>For example:</P>
<PRE>
$Interrupted = 0; # to ensure it has a value
$SIG{INT} = sub {
$Interrupted++;
syswrite(STDERR, "ouch\n", 5);
}</PRE>
<P>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 <A HREF="../../lib/Pod/perlfunc.html#item_flock"><CODE>flock()</CODE></A> in <A HREF="../../lib/Pod/perlipc.html#signals">Signals in the perlipc manpage</A> or chapter 6 of the Camel.</P>
<P>
<H2><A NAME="how do i modify the shadow password file on a unix system">How do I modify the shadow password file on a Unix system?</A></H2>
<P>If perl was installed correctly, and your shadow library was written
properly, the getpw*() functions described in <A HREF="../../lib/Pod/perlfunc.html">the perlfunc manpage</A> 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 <EM>passwd(5)</EM> for specifics) and use
<CODE>pwd_mkdb(8)</CODE> to install it (see <EM>pwd_mkdb(8)</EM> for more details).</P>
<P>
<H2><A NAME="how do i set the time and date">How do I set the time and date?</A></H2>
<P>Assuming you're running under sufficient permissions, you should be
able to set the system-wide date and time by running the <CODE>date(1)</CODE>
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 <CODE>set time</CODE>.</P>
<P>However, if all you want to do is change your timezone, you can
probably get away with setting an environment variable:</P>
<PRE>
$ENV{TZ} = "MST7MDT"; # unixish
$ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
system "trn comp.lang.perl.misc";</PRE>
<P>
<H2><A NAME="how can i sleep() or alarm() for under a second">How can I <A HREF="../../lib/Pod/perlfunc.html#item_sleep"><CODE>sleep()</CODE></A> or <A HREF="../../lib/Pod/perlfunc.html#item_alarm"><CODE>alarm()</CODE></A> for under a second?</A></H2>
<P>If you want finer granularity than the 1 second that the <A HREF="../../lib/Pod/perlfunc.html#item_sleep"><CODE>sleep()</CODE></A>
function provides, the easiest way is to use the <A HREF="../../lib/Pod/perlfunc.html#item_select"><CODE>select()</CODE></A> function as
documented in <A HREF="../../lib/Pod/perlfunc.html#select">select in the perlfunc manpage</A>. If your system has itimers and
<A HREF="../../lib/Pod/perlfunc.html#item_syscall"><CODE>syscall()</CODE></A> support, you can check out the old example in
<H2><A NAME="how can i do an atexit() or setjmp()/longjmp() (exception handling)">How can I do an <CODE>atexit()</CODE> or setjmp()/longjmp()? (Exception handling)</A></H2>
<P>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 <A HREF="../../lib/Pod/perlmod.html">the perlmod manpage</A> manpage for more details).</P>
<P>For example, you can use this to make sure your filter program
managed to finish its output without filling up the disk:</P>
<PRE>
END {
close(STDOUT) || die "stdout close failed: $!";
}</PRE>
<P>The END block isn't called when untrapped signals kill the program, though, so if
you use END blocks you should also use</P>
<PRE>
use sigtrap qw(die normal-signals);</PRE>
<P>Perl's exception-handling mechanism is its <A HREF="../../lib/Pod/perlfunc.html#item_eval"><CODE>eval()</CODE></A> operator. You can
use <A HREF="../../lib/Pod/perlfunc.html#item_eval"><CODE>eval()</CODE></A> as setjmp and <A HREF="../../lib/Pod/perlfunc.html#item_die"><CODE>die()</CODE></A> as longjmp. For details of this, see
the section on signals, especially the time-out handler for a blocking
<A HREF="../../lib/Pod/perlfunc.html#item_flock"><CODE>flock()</CODE></A> in <A HREF="../../lib/Pod/perlipc.html#signals">Signals in the perlipc manpage</A> and chapter 6 of the Camel.</P>
<P>If exception handling is all you're interested in, try the
exceptions.pl library (part of the standard perl distribution).</P>
<P>If you want the <CODE>atexit()</CODE> syntax (and an <CODE>rmexit()</CODE> as well), try the
AtExit module available from CPAN.</P>
<P>
<H2><A NAME="why doesn't my sockets program work under system v (solaris) what does the error message protocol not supported mean">Why doesn't my sockets program work under System V (Solaris)? What does the error message ``Protocol not supported'' mean?</A></H2>
<P>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.</P>
<P>Note that even though SunOS and Solaris are binary compatible, these
values are different. Go figure.</P>
<P>
<H2><A NAME="how can i call my system's unique c functions from perl">How can I call my system's unique C functions from Perl?</A></H2>
<P>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
<P>Remember to check the modules that came with your distribution, and
CPAN as well - someone may already have written a module to do it.</P>
<P>
<H2><A NAME="where do i get the include files to do ioctl() or syscall()">Where do I get the include files to do <A HREF="../../lib/Pod/perlfunc.html#item_ioctl"><CODE>ioctl()</CODE></A> or syscall()?</A></H2>
<P>Historically, these would be generated by the h2ph tool, part of the
standard perl distribution. This program converts <CODE>cpp(1)</CODE> 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 <EM>errno.h</EM>, <EM>syscall.h</EM>, and <EM>socket.h</EM> were fine,
but the hard ones like <EM>ioctl.h</EM> nearly always need to hand-edited.
Here's how to install the *.ph files:</P>
<PRE>
1. become super-user
2. cd /usr/include
3. h2ph *.h */*.h</PRE>
<P>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 <A HREF="../../lib/Pod/perlxstut.html">the perlxstut manpage</A> for how to get started with h2xs.</P>
<P>If your system doesn't support dynamic loading, you still probably
ought to use h2xs. See <A HREF="../../lib/Pod/perlxstut.html">the perlxstut manpage</A> and <A HREF="../../lib/ExtUtils/MakeMaker.html">the ExtUtils::MakeMaker manpage</A> for
more information (in brief, just use <STRONG>make perl</STRONG> instead of a plain
<STRONG>make</STRONG> to rebuild perl with a new static extension).</P>
<P>
<H2><A NAME="why do setuid perl scripts complain about kernel problems">Why do setuid perl scripts complain about kernel problems?</A></H2>
<P>Some operating systems have bugs in the kernel that make setuid
scripts inherently insecure. Perl gives you a number of options
(described in <A HREF="../../lib/Pod/perlsec.html">the perlsec manpage</A>) to work around such systems.</P>
<P>
<H2><A NAME="how can i open a pipe both to and from a command">How can I open a pipe both to and from a command?</A></H2>
<P>The IPC::Open2 module (part of the standard perl distribution) is an
easy-to-use approach that internally uses pipe(), fork(), and <A HREF="../../lib/Pod/perlfunc.html#item_exec"><CODE>exec()</CODE></A> to do
the job. Make sure you read the deadlock warnings in its documentation,
though (see <A HREF="../../lib/IPC/Open2.html">the IPC::Open2 manpage</A>). See <A HREF="../../lib/Pod/perlipc.html#bidirectional communication with another process">Bidirectional Communication with Another Process in the perlipc manpage</A> and <A HREF="../../lib/Pod/perlipc.html#bidirectional communication with yourself">Bidirectional Communication with Yourself in the perlipc manpage</A></P>
<P>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 <A HREF="../../lib/IPC/Open3.html">the IPC::Open3 manpage</A>).</P>
<P>
<H2><A NAME="why can't i get the output of a command with system()">Why can't I get the output of a command with system()?</A></H2>
<P>You're confusing the purpose of <A HREF="../../lib/Pod/perlfunc.html#item_system"><CODE>system()</CODE></A> and backticks (``). <A HREF="../../lib/Pod/perlfunc.html#item_system"><CODE>system()</CODE></A>
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.</P>
<PRE>
$exit_status = system("mail-users");
$output_string = `ls`;</PRE>
<P>
<H2><A NAME="how can i capture stderr from an external command">How can I capture STDERR from an external command?</A></H2>
<P>There are three basic ways of running external commands:</P>
<PRE>
system $cmd; # using system()
$output = `$cmd`; # using backticks (``)
open (PIPE, "cmd |"); # using open()</PRE>
<P>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 <A HREF="../../lib/Pod/perlfunc.html#item_open"><CODE>open()</CODE></A> read <STRONG>only</STRONG> the STDOUT of your command.</P>
<P>With any of these, you can change file descriptors before the call:</P>
<PRE>
open(STDOUT, ">logfile");
system("ls");</PRE>
<P>or you can use Bourne shell file-descriptor redirection:</P>
<PRE>
$output = `$cmd 2>some_file`;
open (PIPE, "cmd 2>some_file |");</PRE>
<P>You can also use file-descriptor redirection to make STDERR a
duplicate of STDOUT:</P>
<PRE>
$output = `$cmd 2>&1`;
open (PIPE, "cmd 2>&1 |");</PRE>
<P>Note that you <EM>cannot</EM> 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:</P>
<PRE>
open(STDERR, ">&STDOUT");
$alloutput = `cmd args`; # stderr still escapes</PRE>
<P>This fails because the <A HREF="../../lib/Pod/perlfunc.html#item_open"><CODE>open()</CODE></A> 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).</P>
<P>Note that you <EM>must</EM> use Bourne shell (sh(1)) redirection syntax in
backticks, not csh(1)! Details on why Perl's <A HREF="../../lib/Pod/perlfunc.html#item_system"><CODE>system()</CODE></A> and backtick
<P>Ordering is important in all these examples. That's because the shell
processes file descriptor redirections in strictly left to right order.</P>
<PRE>
system("prog args 1>tmpfile 2>&1");
system("prog args 2>&1 1>tmpfile");</PRE>
<P>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.</P>
<P>
<H2><A NAME="why doesn't open() return an error when a pipe open fails">Why doesn't <A HREF="../../lib/Pod/perlfunc.html#item_open"><CODE>open()</CODE></A> return an error when a pipe open fails?</A></H2>
<P>Because the pipe open takes place in two steps: first Perl calls
<A HREF="../../lib/Pod/perlfunc.html#item_fork"><CODE>fork()</CODE></A> to start a new process, then this new process calls <A HREF="../../lib/Pod/perlfunc.html#item_exec"><CODE>exec()</CODE></A> to
run the program you really wanted to open. The first step reports
success or failure to your process, so <A HREF="../../lib/Pod/perlfunc.html#item_open"><CODE>open()</CODE></A> can only tell you
whether the <A HREF="../../lib/Pod/perlfunc.html#item_fork"><CODE>fork()</CODE></A> succeeded or not.</P>
<P>To find out if the <A HREF="../../lib/Pod/perlfunc.html#item_exec"><CODE>exec()</CODE></A> step succeeded, you have to catch SIGCHLD
and <A HREF="../../lib/Pod/perlfunc.html#item_wait"><CODE>wait()</CODE></A> to get the exit status. You should also catch SIGPIPE if
you're writing to the child--you may not have found out the <A HREF="../../lib/Pod/perlfunc.html#item_exec"><CODE>exec()</CODE></A>
failed by the time you write. This is documented in <A HREF="../../lib/Pod/perlipc.html">the perlipc manpage</A>.</P>
<P>In some cases, even this won't work. If the second argument to a
piped <A HREF="../../lib/Pod/perlfunc.html#item_open"><CODE>open()</CODE></A> 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
<EM>shell</EM> could be successfully started. Best to avoid shell
metacharacters.</P>
<P>On systems that follow the <CODE>spawn()</CODE> paradigm, <A HREF="../../lib/Pod/perlfunc.html#item_open"><CODE>open()</CODE></A> <EM>might</EM> do what
you expect--unless perl uses a shell to start your command. In this
case the <A HREF="../../lib/Pod/perlfunc.html#item_fork"><CODE>fork()/exec()</CODE></A> description still applies.</P>
<P>
<H2><A NAME="what's wrong with using backticks in a void context">What's wrong with using backticks in a void context?</A></H2>
<P>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:</P>
<PRE>
`cp file file.bak`;</PRE>
<P>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 <A HREF="../../lib/Pod/perlfunc.html#item_system"><CODE>system()</CODE></A>
function is for running programs.</P>
<P>Consider this line:</P>
<PRE>
`cat /etc/termcap`;</PRE>
<P>You haven't assigned the output anywhere, so it just wastes memory
(for a little while). Plus you forgot to check <CODE>$?</CODE> to see whether
the program even ran correctly. Even if you wrote</P>
<PRE>
print `cat /etc/termcap`;</PRE>
<P>In most cases, this could and probably should be written as</P>
<PRE>
system("cat /etc/termcap") == 0
or die "cat program failed!";</PRE>
<P>Which will get the output quickly (as it is generated, instead of only
at the end) and also check the return value.</P>
<P><A HREF="../../lib/Pod/perlfunc.html#item_system"><CODE>system()</CODE></A> also provides direct control over whether shell wildcard
processing may take place, whereas backticks do not.</P>
<P>
<H2><A NAME="how can i call backticks without shell processing">How can I call backticks without shell processing?</A></H2>
<P>Just as with system(), no shell escapes happen when you <A HREF="../../lib/Pod/perlfunc.html#item_exec"><CODE>exec()</CODE></A> a list.
Further examples of this can be found in <A HREF="../../lib/Pod/perlipc.html#safe pipe opens">Safe Pipe Opens in the perlipc manpage</A>.</P>
<P>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.</P>
<P>
<H2><A NAME="why can't my script read from stdin after i gave it eof (^d on unix, ^z on msdos)">Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?</A></H2>
<P>Because some stdio's set error and eof flags that need clearing. The
POSIX module defines <CODE>clearerr()</CODE> that you can use. That is the
technically correct way to do it. Here are some less reliable
workarounds:</P>
<OL>
<LI>
Try keeping around the seekpointer and go there, like this:
<PRE>
$where = tell(LOG);
seek(LOG, $where, 0);</PRE>
<P></P>
<LI>
If that doesn't work, try seeking to a different part of the file and
then back.
<P></P>
<LI>
If that doesn't work, try seeking to a different part of
the file, reading something, and then seeking back.
<P></P>
<LI>
If that doesn't work, give up on your stdio package and use sysread.
<P></P></OL>
<P>
<H2><A NAME="how can i convert my shell script to perl">How can I convert my shell script to perl?</A></H2>
<P>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.</P>
<P>
<H2><A NAME="can i use perl to run a telnet or ftp session">Can I use perl to run a telnet or ftp session?</A></H2>
<P>Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
|| 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;</PRE>
<P>
<H2><A NAME="how can i write expect in perl">How can I write expect in Perl?</A></H2>
<P>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, <EM>don't use it</EM>. 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.</P>
<P>
<H2><A NAME="is there a way to hide perl's command line from programs such as ps">Is there a way to hide perl's command line from programs such as ``ps''?</A></H2>
<P>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.</P>
<P>To actually alter the visible command line, you can assign to the
variable $0 as documented in <A HREF="../../lib/Pod/perlvar.html">the perlvar manpage</A>. This won't work on all
operating systems, though. Daemon programs like sendmail place their
state there, as in:</P>
<PRE>
$0 = "orcus [accepting connections]";</PRE>
<P>
<H2><A NAME="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">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?</A></H2>
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.
<P></P></DL>
<P>
<H2><A NAME="how do i close a process's filehandle without waiting for it to complete">How do I close a process's filehandle without waiting for it to complete?</A></H2>
<P>Assuming your system supports such things, just send an appropriate signal
to the process (see <A HREF="../../lib/Pod/perlfunc.html#kill">kill in the perlfunc manpage</A>. It's common to first send a TERM
signal, wait a little bit, and then send a KILL signal to finish it off.</P>
<P>
<H2><A NAME="how do i fork a daemon process">How do I fork a daemon process?</A></H2>
<P>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.</P>
<UL>
<LI>
Open /dev/tty and use the TIOCNOTTY ioctl on it. See <EM>tty(4)</EM>
for details. Or better yet, you can just use the POSIX::setsid()
function, so you don't have to worry about process groups.
<P></P>
<LI>
Change directory to /
<P></P>
<LI>
Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
tty.
<P></P>
<LI>
Background yourself like this:
<PRE>
fork && exit;</PRE>
<P></P></UL>
<P>The Proc::Daemon module, available from CPAN, provides a function to
perform these actions for you.</P>
<P>
<H2><A NAME="how do i make my program run with sh and csh">How do I make my program run with sh and csh?</A></H2>
<P>See the <EM>eg/nih</EM> script (part of the perl source distribution).</P>
<P>
<H2><A NAME="how do i find out if i'm running interactively or not">How do I find out if I'm running interactively or not?</A></H2>
<P>Good question. Sometimes <CODE>-t STDIN</CODE> and <CODE>-t STDOUT</CODE> can give clues,
sometimes not.</P>
<PRE>
if (-t STDIN && -t STDOUT) {
print "Now what? ";
}</PRE>
<P>On POSIX systems, you can test whether your own process group matches
the current process group of your controlling terminal as follows:</P>
<PRE>
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";
}</PRE>
<P>
<H2><A NAME="how do i timeout a slow event">How do I timeout a slow event?</A></H2>
<P>Use the <A HREF="../../lib/Pod/perlfunc.html#item_alarm"><CODE>alarm()</CODE></A> function, probably in conjunction with a signal
handler, as documented in <A HREF="../../lib/Pod/perlipc.html#signals">Signals in the perlipc manpage</A> and chapter 6 of the
Camel. You may instead use the more flexible Sys::AlarmCall module
available from CPAN.</P>
<P>
<H2><A NAME="how do i set cpu limits">How do I set CPU limits?</A></H2>
<P>Use the BSD::Resource module from CPAN.</P>
<P>
<H2><A NAME="how do i avoid zombies on a unix system">How do I avoid zombies on a Unix system?</A></H2>
<P>Use the reaper code from <A HREF="../../lib/Pod/perlipc.html#signals">Signals in the perlipc manpage</A> to call <A HREF="../../lib/Pod/perlfunc.html#item_wait"><CODE>wait()</CODE></A> when a
SIGCHLD is received, or else use the double-fork technique described
in <A HREF="../../lib/Pod/perlfunc.html#fork">fork in the perlfunc manpage</A>.</P>
<P>
<H2><A NAME="how do i use an sql database">How do I use an SQL database?</A></H2>
<P>There are a number of excellent interfaces to SQL databases. See the
DBD::* modules available from <A HREF="http://www.perl.com/CPAN/modules/DBD">http://www.perl.com/CPAN/modules/DBD</A> .
<H2><A NAME="how do i make a system() exit on controlc">How do I make a <A HREF="../../lib/Pod/perlfunc.html#item_system"><CODE>system()</CODE></A> exit on control-C?</A></H2>
<P>You can't. You need to imitate the <A HREF="../../lib/Pod/perlfunc.html#item_system"><CODE>system()</CODE></A> call (see <A HREF="../../lib/Pod/perlipc.html">the perlipc manpage</A> 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:</P>
<PRE>
$rc = system($cmd);
if ($rc & 127) { die "signal death" }</PRE>
<P>
<H2><A NAME="how do i open a file without blocking">How do I open a file without blocking?</A></H2>
<P>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
<H2><A NAME="how do i install a module from cpan">How do I install a module from CPAN?</A></H2>
<P>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:</P>
<OL>
<LI>
Unpack the source into a temporary area.
<P></P>
<LI>
<PRE>
perl Makefile.PL</PRE>
<LI>
<PRE>
make</PRE>
<LI>
<PRE>
make test</PRE>
<LI>
<PRE>
make install</PRE>
</OL>
<P>If your version of perl is compiled without dynamic loading, then you
just need to replace step 3 (<STRONG>make</STRONG>) with <STRONG>make perl</STRONG> and you will
get a new <EM>perl</EM> binary with your extension linked in.</P>
<P>See <A HREF="../../lib/ExtUtils/MakeMaker.html">the ExtUtils::MakeMaker manpage</A> for more details on building extensions.
See also the next question.</P>
<P>
<H2><A NAME="what's the difference between require and use">What's the difference between require and use?</A></H2>
<P>Perl offers several different ways to include code from one file into
another. Here are the deltas between the various inclusion constructs:</P>
<PRE>
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.</PRE>
<PRE>
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 $file.</PRE>
<PRE>
3) require Module is like require "Module.pm", except the former:
3.1: translates each "::" into your system's directory separator.
3.2: primes the parser to disambiguate class Module as an indirect object.</PRE>
<PRE>
4) use Module is like require Module, except the former:
4.1: loads the module at compile time, not run-time.
4.2: imports symbols and semantics from that package to the current one.</PRE>
<P>In general, you usually want <A HREF="../../lib/Pod/perlfunc.html#item_use"><CODE>use</CODE></A> and a proper Perl module.</P>
<P>
<H2><A NAME="how do i keep my own module/library directory">How do I keep my own module/library directory?</A></H2>
<P>When you build modules, use the PREFIX option when generating
Makefiles:</P>
<PRE>
perl Makefile.PL PREFIX=/u/mydir/perl</PRE>
<P>then either set the PERL5LIB environment variable before you run
scripts that use the modules/libraries (see <A HREF="../../lib/Pod/perlrun.html">the perlrun manpage</A>) or say</P>
<PRE>
use lib '/u/mydir/perl';</PRE>
<P>This is almost the same as:</P>
<PRE>
BEGIN {
unshift(@INC, '/u/mydir/perl');
}</PRE>
<P>except that the lib module checks for machine-dependent subdirectories.
See Perl's <A HREF="../../lib/lib.html">the lib manpage</A> for more information.</P>
<P>
<H2><A NAME="how do i add the directory my program lives in to the module/library search path">How do I add the directory my program lives in to the module/library search path?</A></H2>
<PRE>
use FindBin;
use lib "$FindBin::Bin";
use your_own_modules;</PRE>
<P>
<H2><A NAME="how do i add a directory to my include path at runtime">How do I add a directory to my include path at runtime?</A></H2>
<P>Here are the suggested ways of modifying your include path:</P>
<PRE>
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";</PRE>
<P>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.</P>
<P>
<H2><A NAME="what is socket.ph and where do i get it">What is socket.ph and where do I get it?</A></H2>
<P>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 <CODE>use Socket;</CODE> instead.</P>
<P>
<HR>
<H1><A NAME="author and copyright">AUTHOR AND COPYRIGHT</A></H1>
<P>Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
All rights reserved.</P>
<P>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 <EM>outside</EM>
of that package require that special arrangements be made with
copyright holder.</P>
<P>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.</P>
