IRIX Base Documentation 2001 May/SGI IRIX Base Documentation 2001 May.iso/usr/share/catman/u_man/cat1/perlfaq8.z/perlfaq8

Text File | 1998-10-30 | 49.2 KB | 1,189 lines

PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



NNNNAAAAMMMMEEEE
     perlfaq8 - System Interaction ($Revision: 1.21 $, $Date: 1997/04/24
     22:44:19 $)

DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
     This section of the Perl FAQ covers questions involving operating system
     interaction.  This involves interprocess communication (IPC), control
     over the user-interface (keyboard, screen and pointing devices), and most
     anything else not related to data manipulation.

     Read the FAQs and documentation specific to the port of perl to your
     operating system (eg, the _p_e_r_l_v_m_s manpage, the _p_e_r_l_p_l_a_n_9 manpage, ...).
     These should contain more detailed information on the vagaries of your
     perl.

     HHHHoooowwww ddddoooo IIII ffffiiiinnnndddd oooouuuutttt wwwwhhhhiiiicccchhhh ooooppppeeeerrrraaaattttiiiinnnngggg ssssyyyysssstttteeeemmmm IIII''''mmmm rrrruuuunnnnnnnniiiinnnngggg uuuunnnnddddeeeerrrr????

     The $^O variable ($OSTYPE if you use English) contains the operating
     system that your perl binary was built for.

     HHHHoooowwww ccccoooommmmeeee _e_x_e_c() 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 _s_y_s_t_e_m() instead.

     HHHHoooowwww ddddoooo IIII ddddoooo ffffaaaannnnccccyyyy ssssttttuuuuffffffff wwwwiiiitttthhhh tttthhhheeee kkkkeeeeyyyybbbbooooaaaarrrrdddd////ssssccccrrrreeeeeeeennnn////mmmmoooouuuusssseeee????

     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





                                                                        PPPPaaaaggggeeee 1111






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



     HHHHoooowwww ddddoooo IIII aaaasssskkkk tttthhhheeee uuuusssseeeerrrr ffffoooorrrr aaaa ppppaaaasssssssswwwwoooorrrrdddd????

     (This question has nothing to do with the web.  See a different FAQ for
     that.)

     There's an example of this in the crypt entry in the _p_e_r_l_f_u_n_c manpage).
     First, you put the terminal into "no echo" mode, then just read the
     password normally.  You may do this with an old-style _i_o_c_t_l() function,
     POSIX terminal control (see the _P_O_S_I_X manpage, and Chapter 7 of the
     Camel), or a call to the ssssttttttttyyyy 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.

     HHHHoooowwww ddddoooo IIII rrrreeeeaaaadddd aaaannnndddd wwwwrrrriiiitttteeee tttthhhheeee sssseeeerrrriiiiaaaallll ppppoooorrrrtttt????

     This depends on which operating system your program is running on.  In
     the case of Unix, the serial ports will be accessible through files in
     /dev; on other systems, the devices names will doubtless differ.  Several
     problem areas common to all device interaction are the following

     lockfiles
         Your system may use lockfiles to control multiple access.  Make sure
         you follow the correct protocol.  Unpredictable behaviour can result
         from multiple processes reading from one device.

     open mode
         If you expect to use both read and write operations on the device,
         you'll have to open it for update (see the section on _o_p_e_n in the
         _p_e_r_l_f_u_n_c manpage for details).  You may wish to open it without
         running the risk of blocking by using _s_y_s_o_p_e_n() and
         O_RDWR|O_NDELAY|O_NOCTTY from the Fcntl module (part of the standard
         perl distribution).  See the section on _s_y_s_o_p_e_n in the _p_e_r_l_f_u_n_c
         manpage for more on this approach.

     end of line
         Some devices will be expecting a "\r" at the end of each line rather
         than a "\n".  In some ports of perl, "\r" and "\n" are different from
         their usual (Unix) ASCII values of "\012" and "\015".  You may have
         to give the numeric values you want directly, using octal ("\015"),
         hex ("0x0D"), or as a control-character specification ("\cM").

             print DEV "atv1\012";       # wrong, for some devices
             print DEV "atv1\015";       # right, for some devices

         Even though with normal text files, a "\n" will do the trick, there
         is still no unified scheme for terminating a line that is portable
         between Unix, DOS/Win, and Macintosh, except to terminate _A_L_L 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.



                                                                        PPPPaaaaggggeeee 2222






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



     flushing output
         If you expect characters to get to your device when you _p_r_i_n_t() them,
         you'll want to autoflush that filehandle, as in the older

             use FileHandle;
             DEV->autoflush(1);

         and the newer

             use IO::Handle;
             DEV->autoflush(1);

         You can use _s_e_l_e_c_t() and the $| variable to control autoflushing (see
         the section on $| in the _p_e_r_l_v_a_r manpage and the select entry in the
         _p_e_r_l_f_u_n_c manpage):

             $oldh = select(DEV);
             $| = 1;
             select($oldh);

         You'll also see code that does this without a temporary variable, as
         in

             select((select(DEV), $| = 1)[0]);

         As mentioned in the previous item, this still doesn't work when using
         socket I/O between Unix and Macintosh.  You'll need to hardcode your
         line terminators, in that case.

     non-blocking input
         If you are doing a blocking _r_e_a_d() or _s_y_s_r_e_a_d(), you'll have to
         arrange for an alarm handler to provide a timeout (see the alarm
         entry in the _p_e_r_l_f_u_n_c manpage).  If you have a non-blocking open,
         you'll likely have a non-blocking read, which means you may have to
         use a 4-arg _s_e_l_e_c_t() to determine whether I/O is ready on that device
         (see the section on _s_e_l_e_c_t in the _p_e_r_l_f_u_n_c manpage.

     HHHHoooowwww ddddoooo IIII ddddeeeeccccooooddddeeee eeeennnnccccrrrryyyypppptttteeeedddd ppppaaaasssssssswwwwoooorrrrdddd ffffiiiilllleeeessss????

     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.  Programs like Crack can forcibly (and
     intelligently) try to guess passwords, but don't (can't) guarantee quick
     success.

     If you're worried about users selecting bad passwords, you should
     proactively check when they try to change their password (by modifying
     _p_a_s_s_w_d(1), for example).





                                                                        PPPPaaaaggggeeee 3333






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



     HHHHoooowwww ddddoooo IIII ssssttttaaaarrrrtttt aaaa pppprrrroooocccceeeessssssss iiiinnnn tttthhhheeee bbbbaaaacccckkkkggggrrrroooouuuunnnndddd????

     You could use

         system("cmd &")

     or you could use fork as documented in the section on _f_o_r_k in the
     _p_e_r_l_f_u_n_c manpage, with further examples in the _p_e_r_l_i_p_c manpage.  Some
     things to be aware of, if you're on a Unix-like system:

     STDIN, STDOUT and STDERR are shared
         Both the main process and the backgrounded one (the "child" process)
         share the same STDIN, STDOUT and STDERR filehandles.  If both try to
         access them at once, strange things can happen.  You may want to
         close or reopen these for the child.  You can get around this with
         opening a pipe (see the section on _o_p_e_n in the _p_e_r_l_f_u_n_c manpage) but
         on some systems this means that the child process cannot outlive the
         parent.

     Signals
         You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
         SIGCHLD is sent when the backgrounded process finishes.  SIGPIPE is
         sent when you write to a filehandle whose child process has closed
         (an untrapped SIGPIPE can cause your program to silently die).  This
         is not an issue with system("cmd&").

     Zombies
         You have to be prepared to "reap" the child process when it finishes

             $SIG{CHLD} = sub { wait };

         See the section on _S_i_g_n_a_l_s in the _p_e_r_l_i_p_c manpage for other examples
         of code to do this.  Zombies are not an issue with system("prog &").

     HHHHoooowwww ddddoooo IIII ttttrrrraaaapppp ccccoooonnnnttttrrrroooollll cccchhhhaaaarrrraaaacccctttteeeerrrrssss////ssssiiiiggggnnnnaaaallllssss????

     You don't actually "trap" a control character.  Instead, that character
     generates a signal, which you then trap.  Signals are documented in the
     section on _S_i_g_n_a_l_s in the _p_e_r_l_i_p_c manpage and chapter 6 of the Camel.

     Be warned that very few C libraries are re-entrant.  Therefore, if you
     attempt to _p_r_i_n_t() 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 _s_y_s_w_r_i_t_e() instead of _p_r_i_n_t().

     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 _m_a_l_l_o_c() is not called (eg,
     by setting a variable that already has a value).





                                                                        PPPPaaaaggggeeee 4444






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



     For example:

         $Interrupted = 0;   # to ensure it has a value
         $SIG{INT} = sub {
             $Interrupted++;
             syswrite(STDERR, "ouch\n", 5);
         }

     However, because syscalls restart by default, you'll find that if you're
     in a "slow" call, such as <FH>, _r_e_a_d(), _c_o_n_n_e_c_t(), or _w_a_i_t(), 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 _f_l_o_c_k() in the
     section on _S_i_g_n_a_l_s in the _p_e_r_l_i_p_c manpage or chapter 6 of the Camel.

     HHHHoooowwww ddddoooo IIII mmmmooooddddiiiiffffyyyy tttthhhheeee sssshhhhaaaaddddoooowwww ppppaaaasssssssswwwwoooorrrrdddd ffffiiiilllleeee oooonnnn aaaa UUUUnnnniiiixxxx ssssyyyysssstttteeeemmmm????

     If perl was installed correctly, the getpw*() functions described in the
     _p_e_r_l_f_u_n_c manpage provide (read-only) access to the shadow password file.
     To change the file, make a new shadow password file (the format varies
     from system to system - see the _p_a_s_s_w_d(_5) manpage for specifics) and use
     _p_w_d__m_k_d_b(8) to install it (see the _p_w_d__m_k_d_b(_5) manpage for more details).

     HHHHoooowwww ddddoooo IIII sssseeeetttt tttthhhheeee ttttiiiimmmmeeee aaaannnndddd ddddaaaatttteeee????

     Assuming you're running under sufficient permissions, you should be able
     to set the system-wide date and time by running the _d_a_t_e(1) program.
     (There is no way to set the time and date on a per-process basis.)  This
     mechanism will work for Unix, MS-DOS, Windows, and NT; the VMS equivalent
     is set time.

     However, if all you want to do is change your timezone, you can probably
     get away with setting an environment variable:

         $ENV{TZ} = "MST7MDT";                  # unixish
         $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
         system "trn comp.lang.perl";


     HHHHoooowwww ccccaaaannnn IIII _s_l_e_e_p() or _a_l_a_r_m() for under a second?

     If you want finer granularity than the 1 second that the _s_l_e_e_p() function
     provides, the easiest way is to use the _s_e_l_e_c_t() function as documented
     in the section on _s_e_l_e_c_t in the _p_e_r_l_f_u_n_c manpage.  If your system has
     itimers and _s_y_s_c_a_l_l() support, you can check out the old example in
     http://www.perl.com/CPAN/doc/misc/ancient/tutorial/eg/itimers.pl .

     HHHHoooowwww ccccaaaannnn IIII mmmmeeeeaaaassssuuuurrrreeee ttttiiiimmmmeeee uuuunnnnddddeeeerrrr aaaa sssseeeeccccoooonnnndddd????

     In general, you may not be able to.  The Time::HiRes module (available
     from CPAN) provides this functionality for some systems.





                                                                        PPPPaaaaggggeeee 5555






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



     In general, you may not be able to.  But if you system supports both the
     _s_y_s_c_a_l_l() function in Perl as well as a system call like _g_e_t_t_i_m_e_o_f_d_a_y(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] );


     HHHHoooowwww ccccaaaannnn IIII ddddoooo aaaannnn _a_t_e_x_i_t() or _s_e_t_j_m_p()/_l_o_n_g_j_m_p()? (Exception handling)

     Release 5 of Perl added the END block, which can be used to simulate
     _a_t_e_x_i_t().  Each package's END block is called when the program or thread
     ends (see the _p_e_r_l_m_o_d manpage manpage for more details).  It 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 _e_v_a_l() operator.  You can use
     _e_v_a_l() as setjmp and _d_i_e() as longjmp.  For details of this, see the
     section on signals, especially the time-out handler for a blocking
     _f_l_o_c_k() in the section on _S_i_g_n_a_l_s in the _p_e_r_l_i_p_c manpage and chapter 6 of
     the Camel.

     If exception handling is all you're interested in, try the exceptions.pl
     library (part of the standard perl distribution).






                                                                        PPPPaaaaggggeeee 6666






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



     If you want the _a_t_e_x_i_t() syntax (and an _r_m_e_x_i_t() as well), try the AtExit
     module available from CPAN.

     WWWWhhhhyyyy ddddooooeeeessssnnnn''''tttt mmmmyyyy ssssoooocccckkkkeeeettttssss pppprrrrooooggggrrrraaaammmm wwwwoooorrrrkkkk uuuunnnnddddeeeerrrr SSSSyyyysssstttteeeemmmm VVVV ((((SSSSoooollllaaaarrrriiiissss))))???? WWWWhhhhaaaatttt ddddooooeeeessss
     tttthhhheeee eeeerrrrrrrroooorrrr mmmmeeeessssssssaaaaggggeeee """"PPPPrrrroooottttooooccccoooollll nnnnooootttt ssssuuuuppppppppoooorrrrtttteeeedddd"""" mmmmeeeeaaaannnn????

     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.

     HHHHoooowwww ccccaaaannnn IIII ccccaaaallllllll mmmmyyyy ssssyyyysssstttteeeemmmm''''ssss uuuunnnniiiiqqqquuuueeee CCCC ffffuuuunnnnccccttttiiiioooonnnnssss ffffrrrroooommmm PPPPeeeerrrrllll????

     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 _s_y_s_c_a_l_l(), you
     can use the syscall function (documented in the _p_e_r_l_f_u_n_c manpage).

     Remember to check the modules that came with your distribution, and CPAN
     as well - someone may already have written a module to do it.

     WWWWhhhheeeerrrreeee ddddoooo IIII ggggeeeetttt tttthhhheeee iiiinnnncccclllluuuuddddeeee ffffiiiilllleeeessss ttttoooo ddddoooo _i_o_c_t_l() or _s_y_s_c_a_l_l()?

     Historically, these would be generated by the h2ph tool, part of the
     standard perl distribution.  This program converts _c_p_p(1) directives in C
     header files to files containing subroutine definitions, like
     &SYS_getitimer, which you can use as arguments to your functions.  It
     doesn't work perfectly, but it usually gets most of the job done.  Simple
     files like _e_r_r_n_o._h, _s_y_s_c_a_l_l._h, and _s_o_c_k_e_t._h were fine, but the hard ones
     like _i_o_c_t_l._h nearly always need to hand-edited.  Here's how to install
     the *.ph files:

         1.  become super-user
         2.  cd /usr/include
         3.  h2ph *.h */*.h

     If your system supports dynamic loading, for reasons of portability and
     sanity you probably ought to use h2xs (also part of the standard perl
     distribution).  This tool converts C header files to Perl extensions.
     See the _p_e_r_l_x_s_t_u_t manpage for how to get started with h2xs.

     If your system doesn't support dynamic loading, you still probably ought
     to use h2xs.  See the _p_e_r_l_x_s_t_u_t manpage and the _E_x_t_U_t_i_l_s::_M_a_k_e_M_a_k_e_r
     manpage for more information (in brief, just use mmmmaaaakkkkeeee ppppeeeerrrrllll instead of a
     plain mmmmaaaakkkkeeee to rebuild perl with a new static extension).







                                                                        PPPPaaaaggggeeee 7777






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



     WWWWhhhhyyyy ddddoooo sssseeeettttuuuuiiiidddd ppppeeeerrrrllll ssssccccrrrriiiippppttttssss ccccoooommmmppppllllaaaaiiiinnnn aaaabbbboooouuuutttt kkkkeeeerrrrnnnneeeellll pppprrrroooobbbblllleeeemmmmssss????

     Some operating systems have bugs in the kernel that make setuid scripts
     inherently insecure.  Perl gives you a number of options (described in
     the _p_e_r_l_s_e_c manpage) to work around such systems.

     HHHHoooowwww ccccaaaannnn IIII ooooppppeeeennnn aaaa ppppiiiippppeeee bbbbooootttthhhh ttttoooo aaaannnndddd ffffrrrroooommmm aaaa ccccoooommmmmmmmaaaannnndddd????

     The IPC::Open2 module (part of the standard perl distribution) is an
     easy-to-use approach that internally uses _p_i_p_e(), _f_o_r_k(), and _e_x_e_c() to
     do the job.  Make sure you read the deadlock warnings in its
     documentation, though (see the _I_P_C::_O_p_e_n_2 manpage).

     WWWWhhhhyyyy ccccaaaannnn''''tttt IIII ggggeeeetttt tttthhhheeee oooouuuuttttppppuuuutttt ooooffff aaaa ccccoooommmmmmmmaaaannnndddd wwwwiiiitttthhhh _s_y_s_t_e_m()?

     You're confusing the purpose of _s_y_s_t_e_m() and backticks (``).  _s_y_s_t_e_m()
     runs a command and returns exit status information (as a 16 bit value:
     the low 8 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`;


     HHHHoooowwww ccccaaaannnn IIII ccccaaaappppttttuuuurrrreeee SSSSTTTTDDDDEEEERRRRRRRR ffffrrrroooommmm aaaannnn eeeexxxxtttteeeerrrrnnnnaaaallll ccccoooommmmmmmmaaaannnndddd????

     There are three basic ways of running external commands:

         system $cmd;                # using system()
         $output = `$cmd`;           # using backticks (``)
         open (PIPE, "cmd |");       # using open()

     With _s_y_s_t_e_m(), both STDOUT and STDERR will go the same place as the
     script's versions of these, unless the command redirects them.  Backticks
     and _o_p_e_n() read oooonnnnllllyyyy the STDOUT of your command.

     With any of these, you can change file descriptors before the call:

         open(STDOUT, ">logfile");
         system("ls");

     or you can use Bourne shell file-descriptor redirection:

         $output = `$cmd 2>some_file`;
         open (PIPE, "cmd 2>some_file |");

     You can also use file-descriptor redirection to make STDERR a duplicate
     of STDOUT:






                                                                        PPPPaaaaggggeeee 8888






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



         $output = `$cmd 2>&1`;
         open (PIPE, "cmd 2>&1 |");

     Note that you _c_a_n_n_o_t 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 _o_p_e_n() makes STDERR go to where STDOUT was going
     at the time of the _o_p_e_n().  The backticks then make STDOUT go to a
     string, but don't change STDERR (which still goes to the old STDOUT).

     Note that you _m_u_s_t use Bourne shell (_s_h(1)) redirection syntax in
     backticks, not _c_s_h(1)!  Details on why Perl's _s_y_s_t_e_m() and backtick and
     pipe opens all use the Bourne shell are in
     http://www.perl.com/CPAN/doc/FMTEYEWTK/versus/csh.whynot .

     You may also use the IPC::Open3 module (part of the standard perl
     distribution), but be warned that it has a different order of arguments
     from IPC::Open2 (see the _I_P_C::_O_p_e_n_3 manpage).

     WWWWhhhhyyyy ddddooooeeeessssnnnn''''tttt _o_p_e_n() return an error when a pipe open fails?

     It does, but probably not how you expect it to.  On systems that follow
     the standard _f_o_r_k()/_e_x_e_c() paradigm (eg, Unix), it works like this:
     _o_p_e_n() causes a _f_o_r_k().  In the parent, _o_p_e_n() returns with the process
     ID of the child.  The child _e_x_e_c()s the command to be piped to/from.  The
     parent can't know whether the _e_x_e_c() was successful or not - all it can
     return is whether the _f_o_r_k() succeeded or not.  To find out if the
     command succeeded, you have to catch SIGCHLD and _w_a_i_t() to get the exit
     status.  You should also catch SIGPIPE if you're writing to the child --
     you may not have found out the _e_x_e_c() failed by the time you write.  This
     is documented in the _p_e_r_l_i_p_c manpage.

     On systems that follow the _s_p_a_w_n() paradigm, _o_p_e_n() _m_i_g_h_t do what you
     expect - unless perl uses a shell to start your command. In this case the
     _f_o_r_k()/_e_x_e_c() description still applies.

     WWWWhhhhaaaatttt''''ssss wwwwrrrroooonnnngggg wwwwiiiitttthhhh uuuussssiiiinnnngggg bbbbaaaacccckkkkttttiiiicccckkkkssss iiiinnnn aaaa vvvvooooiiiidddd ccccoooonnnntttteeeexxxxtttt????

     Strictly speaking, nothing.  Stylistically speaking, it's not a good way
     to write maintainable code because backticks have a (potentially
     humungous) return value, and you're ignoring it.  It's may also not be
     very efficient, because you have to read in all the lines of output,
     allocate memory for them, and then throw it away.  Too often people are
     lulled to writing:

         `cp file file.bak`;

     And now they think "Hey, I'll just always use backticks to run programs."



                                                                        PPPPaaaaggggeeee 9999






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



     Bad idea: backticks are for capturing a program's output; the _s_y_s_t_e_m()
     function is for running programs.

     Consider this line:

         `cat /etc/termcap`;

     You haven't assigned the output anywhere, so it just wastes memory (for a
     little while).  Plus you forgot to check $? to see whether the program
     even ran correctly.  Even if you wrote

         print `cat /etc/termcap`;

     In most cases, this could and probably should be written as

         system("cat /etc/termcap") == 0
             or die "cat program failed!";

     Which will get the output quickly (as its generated, instead of only at
     the end ) and also check the return value.

     _s_y_s_t_e_m() also provides direct control over whether shell wildcard
     processing may take place, whereas backticks do not.

     HHHHoooowwww ccccaaaannnn IIII ccccaaaallllllll bbbbaaaacccckkkkttttiiiicccckkkkssss wwwwiiiitttthhhhoooouuuutttt sssshhhheeeellllllll pppprrrroooocccceeeessssssssiiiinnnngggg????

     This is a bit tricky.  Instead of writing

         @ok = `grep @opts '$search_string' @filenames`;

     You have to do this:

         my @ok = ();
         if (open(GREP, "-|")) {
             while (<GREP>) {
                 chomp;
                 push(@ok, $_);
             }
             close GREP;
         } else {
             exec 'grep', @opts, $search_string, @filenames;
         }

     Just as with _s_y_s_t_e_m(), no shell escapes happen when you _e_x_e_c() a list.

     WWWWhhhhyyyy ccccaaaannnn''''tttt mmmmyyyy ssssccccrrrriiiipppptttt rrrreeeeaaaadddd ffffrrrroooommmm SSSSTTTTDDDDIIIINNNN aaaafffftttteeeerrrr IIII ggggaaaavvvveeee iiiitttt EEEEOOOOFFFF ((((^^^^DDDD oooonnnn UUUUnnnniiiixxxx,,,, ^^^^ZZZZ
     oooonnnn MMMMSSSS----DDDDOOOOSSSS))))????

     Because some stdio's set error and eof flags that need clearing.  The
     POSIX module defines _c_l_e_a_r_e_r_r() that you can use.  That is the
     technically correct way to do it.  Here are some less reliable
     workarounds:



                                                                       PPPPaaaaggggeeee 11110000






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



     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.

     HHHHoooowwww ccccaaaannnn IIII ccccoooonnnnvvvveeeerrrrtttt mmmmyyyy sssshhhheeeellllllll ssssccccrrrriiiipppptttt ttttoooo ppppeeeerrrrllll????

     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.

     CCCCaaaannnn IIII uuuusssseeee ppppeeeerrrrllll ttttoooo rrrruuuunnnn aaaa tttteeeellllnnnneeeetttt oooorrrr ffffttttpppp sssseeeessssssssiiiioooonnnn????

     Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
     CPAN).  http://www.perl.com/CPAN/scripts/netstuff/telnet.emul.shar will
     also help for emulating the telnet protocol, but Net::Telnet is quite
     probably easier to use..

     If all you want to do is pretend to be telnet but don't need the initial
     telnet handshaking, then the standard dual-process approach will suffice:

         use IO::Socket;             # new in 5.004
         $handle = IO::Socket::INET->new('www.perl.com:80')
                 || die "can't connect to port 80 on www.perl.com: $!";
         $handle->autoflush(1);
         if (fork()) {               # XXX: undef means failure
             select($handle);
             print while <STDIN>;    # everything from stdin to socket
         } else {
             print while <$handle>;  # everything from socket to stdout
         }
         close $handle;
         exit;


     HHHHoooowwww ccccaaaannnn IIII wwwwrrrriiiitttteeee eeeexxxxppppeeeecccctttt iiiinnnn PPPPeeeerrrrllll????






                                                                       PPPPaaaaggggeeee 11111111






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



     Once upon a time, there was a library called chat2.pl (part of the
     standard perl distribution), which never really got finished.  These
     days, your best bet is to look at the Comm.pl library available from
     CPAN.

     IIIIssss tttthhhheeeerrrreeee aaaa wwwwaaaayyyy ttttoooo hhhhiiiiddddeeee ppppeeeerrrrllll''''ssss ccccoooommmmmmmmaaaannnndddd lllliiiinnnneeee ffffrrrroooommmm pppprrrrooooggggrrrraaaammmmssss ssssuuuucccchhhh aaaassss """"ppppssss""""????

     First of all note that if you're doing this for security reasons (to
     avoid people seeing passwords, for example) then you should rewrite your
     program so that critical information is never given as an argument.
     Hiding the arguments won't make your program completely secure.

     To actually alter the visible command line, you can assign to the
     variable $0 as documented in the _p_e_r_l_v_a_r manpage.  This won't work on all
     operating systems, though.  Daemon programs like sendmail place their
     state there, as in:

         $0 = "orcus [accepting connections]";


     IIII {{{{cccchhhhaaaannnnggggeeeedddd ddddiiiirrrreeeeccccttttoooorrrryyyy,,,, mmmmooooddddiiiiffffiiiieeeedddd mmmmyyyy eeeennnnvvvviiiirrrroooonnnnmmmmeeeennnntttt}}}} iiiinnnn aaaa ppppeeeerrrrllll ssssccccrrrriiiipppptttt....  HHHHoooowwww
     ccccoooommmmeeee tttthhhheeee cccchhhhaaaannnnggggeeee ddddiiiissssaaaappppppppeeeeaaaarrrreeeedddd wwwwhhhheeeennnn IIII eeeexxxxiiiitttteeeedddd tttthhhheeee ssssccccrrrriiiipppptttt????  HHHHoooowwww ddddoooo IIII ggggeeeetttt mmmmyyyy
     cccchhhhaaaannnnggggeeeessss ttttoooo bbbbeeee vvvviiiissssiiiibbbblllleeee????

     Unix
         In the strictest sense, it can't be done -- the script executes as a
         different process from the shell it was started from.  Changes to a
         process are not reflected in its parent, only in its own children
         created after the change.  There is shell magic that may allow you to
         fake it by _e_v_a_l()ing the script's output in your shell; check out the
         comp.unix.questions FAQ for details.

     VMS Change to %ENV persist after Perl exits, but directory changes do
         not.

     HHHHoooowwww ddddoooo IIII cccclllloooosssseeee aaaa pppprrrroooocccceeeessssssss''''ssss ffffiiiilllleeeehhhhaaaannnnddddlllleeee wwwwiiiitttthhhhoooouuuutttt wwwwaaaaiiiittttiiiinnnngggg ffffoooorrrr iiiitttt ttttoooo ccccoooommmmpppplllleeeetttteeee????

     Assuming your system supports such things, just send an appropriate
     signal to the process (see the section on _k_i_l_l in the _p_e_r_l_f_u_n_c manpage.
     It's common to first send a TERM signal, wait a little bit, and then send
     a KILL signal to finish it off.

     HHHHoooowwww ddddoooo IIII ffffoooorrrrkkkk aaaa ddddaaaaeeeemmmmoooonnnn pppprrrroooocccceeeessssssss????

     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.

     +o   Open /dev/tty and use the the TIOCNOTTY ioctl on it.  See the _t_t_y(_4)
         manpage for details.




                                                                       PPPPaaaaggggeeee 11112222






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



     +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;


     HHHHoooowwww ddddoooo IIII mmmmaaaakkkkeeee mmmmyyyy pppprrrrooooggggrrrraaaammmm rrrruuuunnnn wwwwiiiitttthhhh sssshhhh aaaannnndddd ccccsssshhhh????

     See the _e_g/_n_i_h script (part of the perl source distribution).

     HHHHoooowwww ddddoooo IIII ffffiiiinnnndddd oooouuuutttt iiiiffff IIII''''mmmm rrrruuuunnnnnnnniiiinnnngggg iiiinnnntttteeeerrrraaaaccccttttiiiivvvveeeellllyyyy oooorrrr nnnnooootttt????

     Good question.  Sometimes -t STDIN and -t STDOUT can give clues,
     sometimes not.

         if (-t STDIN && -t STDOUT) {
             print "Now what? ";
         }

     On POSIX systems, you can test whether your own process group matches the
     current process group of your controlling terminal as follows:

         use POSIX qw/getpgrp tcgetpgrp/;
         open(TTY, "/dev/tty") or die $!;
         $tpgrp = tcgetpgrp(TTY);
         $pgrp = getpgrp();
         if ($tpgrp == $pgrp) {
             print "foreground\n";
         } else {
             print "background\n";
         }


     HHHHoooowwww ddddoooo IIII ttttiiiimmmmeeeeoooouuuutttt aaaa sssslllloooowwww eeeevvvveeeennnntttt????

     Use the _a_l_a_r_m() function, probably in conjunction with a signal handler,
     as documented the section on _S_i_g_n_a_l_s in the _p_e_r_l_i_p_c manpage and chapter 6
     of the Camel.  You may instead use the more flexible Sys::AlarmCall
     module available from CPAN.

     HHHHoooowwww ddddoooo IIII sssseeeetttt CCCCPPPPUUUU lllliiiimmmmiiiittttssss????

     Use the BSD::Resource module from CPAN.

     HHHHoooowwww ddddoooo IIII aaaavvvvooooiiiidddd zzzzoooommmmbbbbiiiieeeessss oooonnnn aaaa UUUUnnnniiiixxxx ssssyyyysssstttteeeemmmm????






                                                                       PPPPaaaaggggeeee 11113333






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



     Use the reaper code from the section on _S_i_g_n_a_l_s in the _p_e_r_l_i_p_c manpage to
     call _w_a_i_t() when a SIGCHLD is received, or else use the double-fork
     technique described in the fork entry in the _p_e_r_l_f_u_n_c manpage.

     HHHHoooowwww ddddoooo IIII uuuusssseeee aaaannnn SSSSQQQQLLLL ddddaaaattttaaaabbbbaaaasssseeee????

     There are a number of excellent interfaces to SQL databases.  See the
     DBD::* modules available from http://www.perl.com/CPAN/modules/dbperl/DBD
     .

     HHHHoooowwww ddddoooo IIII mmmmaaaakkkkeeee aaaa _s_y_s_t_e_m() exit on control-C?

     You can't.  You need to imitate the _s_y_s_t_e_m() call (see the _p_e_r_l_i_p_c
     manpage for sample code) and then have a signal handler for the INT
     signal that passes the signal on to the subprocess.

     HHHHoooowwww ddddoooo IIII ooooppppeeeennnn aaaa ffffiiiilllleeee wwwwiiiitttthhhhoooouuuutttt bbbblllloooocccckkkkiiiinnnngggg????

     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 _s_y_s_o_p_e_n():

         use Fcntl;
         sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
             or die "can't open /tmp/somefile: $!":


     HHHHoooowwww ddddoooo IIII iiiinnnnssssttttaaaallllllll aaaa CCCCPPPPAAAANNNN mmmmoooodddduuuulllleeee????

     The easiest way is to have the CPAN module do it for you.  This module
     comes with perl version 5.004 and later.  To manually install the CPAN
     module, or any well-behaved CPAN module for that matter, follow these
     steps:

     1   Unpack the source into a temporary area.

     2

             perl Makefile.PL


     3

             make


     4

             make test






                                                                       PPPPaaaaggggeeee 11114444






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



     5

             make install


     If your version of perl is compiled without dynamic loading, then you
     just need to replace step 3 (mmmmaaaakkkkeeee) with mmmmaaaakkkkeeee ppppeeeerrrrllll and you will get a new
     _p_e_r_l binary with your extension linked in.

     See the _E_x_t_U_t_i_l_s::_M_a_k_e_M_a_k_e_r manpage for more details on building
     extensions, the question "How do I keep my own module/library directory?"

     HHHHoooowwww ddddoooo IIII kkkkeeeeeeeepppp mmmmyyyy oooowwwwnnnn mmmmoooodddduuuulllleeee////lllliiiibbbbrrrraaaarrrryyyy ddddiiiirrrreeeeccccttttoooorrrryyyy????

     When you build modules, use the PREFIX option when generating Makefiles:

         perl Makefile.PL PREFIX=/u/mydir/perl

     then either set the PERL5LIB environment variable before you run scripts
     that use the modules/libraries (see the _p_e_r_l_r_u_n manpage) or say

         use lib '/u/mydir/perl';

     See Perl's the _l_i_b manpage for more information.

     HHHHoooowwww ddddoooo IIII aaaadddddddd tttthhhheeee ddddiiiirrrreeeeccccttttoooorrrryyyy mmmmyyyy pppprrrrooooggggrrrraaaammmm lllliiiivvvveeeessss iiiinnnn ttttoooo tttthhhheeee mmmmoooodddduuuulllleeee////lllliiiibbbbrrrraaaarrrryyyy
     sssseeeeaaaarrrrcccchhhh ppppaaaatttthhhh????

         use FindBin;
         use lib "$FindBin:Bin";
         use your_own_modules;


     HHHHoooowwww ddddoooo IIII aaaadddddddd aaaa ddddiiiirrrreeeeccccttttoooorrrryyyy ttttoooo mmmmyyyy iiiinnnncccclllluuuuddddeeee ppppaaaatttthhhh aaaatttt rrrruuuunnnnttttiiiimmmmeeee????

     Here are the suggested ways of modifying your include path:

         the PERLLIB environment variable
         the PERL5LIB environment variable
         the perl -Idir commpand line flag
         the use lib pragma, as in
             use lib "$ENV{HOME}/myown_perllib";

     The latter is particularly useful because it knows about machine
     dependent architectures.  The lib.pm pragmatic module was first included
     with the 5.002 release of Perl.

HHHHoooowwww ddddoooo IIII ggggeeeetttt oooonnnneeee kkkkeeeeyyyy ffffrrrroooommmm tttthhhheeee tttteeeerrrrmmmmiiiinnnnaaaallll aaaatttt aaaa ttttiiiimmmmeeee,,,, uuuunnnnddddeeeerrrr PPPPOOOOSSSSIIIIXXXX????







                                                                       PPPPaaaaggggeeee 11115555






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



         #!/usr/bin/perl -w
         use strict;
         $| = 1;
         for (1..4) {
             my $got;
             print "gimme: ";
             $got = getone();
             print "--> $got\n";
         }
         exit;

         BEGIN {
             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);
                 $term->setcc(VTIME, 1);
                 $term->setattr($fd_stdin, TCSANOW);
             }

             sub cooked {
                 $term->setlflag($oterm);
                 $term->setcc(VTIME, 0);
                 $term->setattr($fd_stdin, TCSANOW);
             }

             sub getone {
                 my $key = '';
                 cbreak();
                 sysread(STDIN, $key, 1);
                 cooked();
                 return $key;
             }

         }
         END { cooked() }








                                                                       PPPPaaaaggggeeee 11116666






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))



AUTHOR AND COPYRIGHT
     Copyright (c) 1997 Tom Christiansen and Nathan Torkington.  All rights
     reserved.  See the _p_e_r_l_f_a_q manpage for distribution information.




















































                                                                       PPPPaaaaggggeeee 11117777






PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))                                                        PPPPEEEERRRRLLLLFFFFAAAAQQQQ8888((((1111))))























































                                                                       PPPPaaaaggggeeee 11118888