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perlfaq5




DESCRIPTION

       This section deals with I/O and the "f" issues: filehan­
       dles, flushing, formats, and footers.

       How do I flush/unbuffer an output filehandle?  Why must I
       do this?

       Perl does not support truly unbuffered output (except
       insofar as you can "syswrite(OUT, $char, 1)"), although it
       does support is "command buffering", in which a physical
       write is performed after every output command.

       The C standard I/O library (stdio) normally buffers char­
       acters sent to devices so that there isn't a system call
       for each byte. In most stdio implementations, the type of
       output buffering and the size of the buffer varies accord­
       ing to the type of device. Perl's print() and write()
       functions normally buffer output, while syswrite()
       bypasses buffering all together.

       If you want your output to be sent immediately when you
       execute print() or write() (for instance, for some network
       protocols), you must set the handle's autoflush flag. This
       flag is the Perl variable $| and when it is set to a true
       value, Perl will flush the handle's buffer after each
       print() or write(). Setting $| affects buffering only for
       the currently selected default file handle. You choose
       this handle with the one argument select() call (see "$|"
       in perlvar and "select" in perlfunc).

       Use select() to choose the desired handle, then set its
       per-filehandle variables.

           $old_fh = select(OUTPUT_HANDLE);
           $| = 1;
           select($old_fh);

       Some idioms can handle this in a single statement:

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

           $| = 1, select $_ for select OUTPUT_HANDLE;

       Some modules offer object-oriented access to handles and
       their variables, although they may be overkill if this is
       the only thing you do with them.  You can use IO::Handle:

           use IO::Handle;
           open(DEV, ">/dev/printer");   # but is this?
           DEV->autoflush(1);


       How do I count the number of lines in a file?

       One fairly efficient way is to count newlines in the file.
       The following program uses a feature of tr///, as docu­
       mented in perlop.  If your text file doesn't end with a
       newline, then it's not really a proper text file, so this
       may report one fewer line than you expect.

           $lines = 0;
           open(FILE, $filename) or die "Can't open `$filename': $!";
           while (sysread FILE, $buffer, 4096) {
               $lines += ($buffer =~ tr/\n//);
           }
           close FILE;

       This assumes no funny games with newline translations.

       How can I use Perl's "-i" option from within a program?

       "-i" sets the value of Perl's $^I variable, which in turn
       affects the behavior of "<>"; see perlrun for more
       details.  By modifying the appropriate variables directly,
       you can get the same behavior within a larger program.
       For example:

            # ...
            {
               local($^I, @ARGV) = ('.orig', glob("*.c"));
               while (<>) {
                  if ($. == 1) {
                      print "This line should appear at the top of each file\n";
                  }
                  s/\b(p)earl\b/${1}erl/i;        # Correct typos, preserving case
                  print;
                  close ARGV if eof;              # Reset $.
               }
            }
            # $^I and @ARGV return to their old values here

       This block modifies all the ".c" files in the current
       directory, leaving a backup of the original data from each
       file in a new ".c.orig" file.

       How do I make a temporary file name?

       Use the File::Temp module, see File::Temp for more infor­
       mation.

         use File::Temp qw/ tempfile tempdir /;

         $dir = tempdir( CLEANUP => 1 );
               or die "Unable to make new temporary file: $!";

       If you're committed to creating a temporary file by hand,
       use the process ID and/or the current time-value.  If you
       need to have many temporary files in one process, use a
       counter:

           BEGIN {
               use Fcntl;
               my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
               my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
               sub temp_file {
                   local *FH;
                   my $count = 0;
                   until (defined(fileno(FH)) || $count++ > 100) {
                       $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
                       sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
                   }
                   if (defined(fileno(FH))
                       return (*FH, $base_name);
                   } else {
                       return ();
                   }
               }
           }

       How can I manipulate fixed-record-length files?

       The most efficient way is using pack() and unpack().  This
       is faster than using substr() when taking many, many
       strings.  It is slower for just a few.

       Here is a sample chunk of code to break up and put back
       together again some fixed-format input lines, in this case
       from the output of a normal, Berkeley-style ps:

           # sample input line:
           #   15158 p5  T      0:00 perl /home/tchrist/scripts/now-what
           $PS_T = 'A6 A4 A7 A5 A*';
           open(PS, "ps|");
           print scalar <PS>;
           while (<PS>) {
               ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
               for $var (qw!pid tt stat time command!) {
                   print "$var: <$$var>\n";
               }
               print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
                       "\n";
           }

       We've used $$var in a way that forbidden by "use strict
       'refs'".  That is, we've promoted a string to a scalar
       variable.  You can then pass these references just like
       any other scalar, and use them in the place of named han­
       dles.

               open my    $fh, $file_name;

               open local $fh, $file_name;

               print $fh "Hello World!\n";

               process_file( $fh );

       Before perl5.6, you had to deal with various typeglob
       idioms which you may see in older code.

               open FILE, "> $filename";
               process_typeglob(   *FILE );
               process_reference( \*FILE );

               sub process_typeglob  { local *FH = shift; print FH  "Typeglob!" }
               sub process_reference { local $fh = shift; print $fh "Reference!" }

       If you want to create many anonymous handles, you should
       check out the Symbol or IO::Handle modules.

       How can I use a filehandle indirectly?

       An indirect filehandle is using something other than a
       symbol in a place that a filehandle is expected.  Here are
       ways to get indirect filehandles:

           $fh =   SOME_FH;       # bareword is strict-subs hostile
           $fh =  "SOME_FH";      # strict-refs hostile; same package only
           $fh =  *SOME_FH;       # typeglob
           $fh = \*SOME_FH;       # ref to typeglob (bless-able)
           $fh =  *SOME_FH{IO};   # blessed IO::Handle from *SOME_FH typeglob

       Or, you can use the "new" method from one of the IO::*
       modules to create an anonymous filehandle, store that in a
       scalar variable, and use it as though it were a normal
       filehandle.

           use IO::Handle;                     # 5.004 or higher
           $fh = IO::Handle->new();

       Then use any of those as you would a normal filehandle.
       Anywhere that Perl is expecting a filehandle, an indirect
       filehandle may be used instead. An indirect filehandle is
       just a scalar variable that contains a filehandle.  Func­
       tions like "print", "open", "seek", or the "<FH>" diamond
       operator will accept either a named filehandle or a scalar
       variable containing one:
       Or it can localize a typeglob and use the filehandle
       directly:

           sub accept_fh {
               local *FH = shift;
               print  FH "Sending to localized filehandle\n";
           }

       Both styles work with either objects or typeglobs of real
       filehandles.  (They might also work with strings under
       some circumstances, but this is risky.)

           accept_fh(*STDOUT);
           accept_fh($handle);

       In the examples above, we assigned the filehandle to a
       scalar variable before using it.  That is because only
       simple scalar variables, not expressions or subscripts of
       hashes or arrays, can be used with built-ins like "print",
       "printf", or the diamond operator.  Using something other
       than a simple scalar variable as a filehandle is illegal
       and won't even compile:

           @fd = (*STDIN, *STDOUT, *STDERR);
           print $fd[1] "Type it: ";                           # WRONG
           $got = <$fd[0]>                                     # WRONG
           print $fd[2] "What was that: $got";                 # WRONG

       With "print" and "printf", you get around this by using a
       block and an expression where you would place the filehan­
       dle:

           print  { $fd[1] } "funny stuff\n";
           printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
           # Pity the poor deadbeef.

       That block is a proper block like any other, so you can
       put more complicated code there.  This sends the message
       out to one of two places:

           $ok = -x "/bin/cat";
           print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
           print { $fd[ 1+ ($ok || 0) ]  } "cat stat $ok\n";

       This approach of treating "print" and "printf" like object
       methods calls doesn't work for the diamond operator.
       That's because it's a real operator, not just a function
       with a comma-less argument.  Assuming you've been storing
       typeglobs in your structure as we did above, you can use
       the built-in function named "readline" to read a record
       just as "<>" does.  Given the initialization shown above
       for @fd, this would work, but only because readline()
       ple of techniques to make it possible for the intrepid
       hacker.

       How can I write() into a string?

       See "Accessing Formatting Internals" in perlform for an
       swrite() function.

       How can I output my numbers with commas added?

       This subroutine will add commas to your number:

               sub commify {
                  local $_  = shift;
                  1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
                  return $_;
                  }

       This regex from Benjamin Goldberg will add commas to num­
       bers:

          s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;

       It is easier to see with comments:

          s/(
              ^[-+]?            # beginning of number.
              \d{1,3}?          # first digits before first comma
              (?=               # followed by, (but not included in the match) :
                 (?>(?:\d{3})+) # some positive multiple of three digits.
                 (?!\d)         # an *exact* multiple, not x * 3 + 1 or whatever.
              )
             |                  # or:
              \G\d{3}           # after the last group, get three digits
              (?=\d)            # but they have to have more digits after them.
          )/$1,/xg;

       How can I translate tildes (~) in a filename?

       Use the <> (glob()) operator, documented in perlfunc.
       Older versions of Perl require that you have a shell
       installed that groks tildes.  Recent perl versions have
       this feature built in. The File::KGlob module (available
       from CPAN) gives more portable glob functionality.

       Within Perl, you may use this directly:

               $filename =~ s{
                 ^ ~             # find a leading tilde
                 (               # save this in $1
                     [^/]        # a non-slash character
                           *     # repeated 0 or more times (0 means me)

       Whoops.  You should instead use this, which will fail if
       the file doesn't exist.

           open(FH, "+< /path/name");          # open for update

       Using ">" always clobbers or creates.  Using "<" never
       does either.  The "+" doesn't change this.

       Here are examples of many kinds of file opens.  Those
       using sysopen() all assume

           use Fcntl;

       To open file for reading:

           open(FH, "< $path")                                 || die $!;
           sysopen(FH, $path, O_RDONLY)                        || die $!;

       To open file for writing, create new file if needed or
       else truncate old file:

           open(FH, "> $path") || die $!;
           sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT)        || die $!;
           sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666)  || die $!;

       To open file for writing, create new file, file must not
       exist:

           sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT)         || die $!;
           sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666)   || die $!;

       To open file for appending, create if necessary:

           open(FH, ">> $path") || die $!;
           sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT)       || die $!;
           sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;

       To open file for appending, file must exist:

           sysopen(FH, $path, O_WRONLY|O_APPEND)               || die $!;

       To open file for update, file must exist:

           open(FH, "+< $path")                                || die $!;
           sysopen(FH, $path, O_RDWR)                          || die $!;

       To open file for update, create file if necessary:

           sysopen(FH, $path, O_RDWR|O_CREAT)                  || die $!;
           sysopen(FH, $path, O_RDWR|O_CREAT, 0666)            || die $!;

       might wish.

       See also the new perlopentut if you have it (new for 5.6).

       Why do I sometimes get an "Argument list too long" when I
       use <*>?

       The "<>" operator performs a globbing operation (see
       above).  In Perl versions earlier than v5.6.0, the inter­
       nal glob() operator forks csh(1) to do the actual glob
       expansion, but csh can't handle more than 127 items and so
       gives the error message "Argument list too long".  People
       who installed tcsh as csh won't have this problem, but
       their users may be surprised by it.

       To get around this, either upgrade to Perl v5.6.0 or
       later, do the glob yourself with readdir() and patterns,
       or use a module like File::KGlob, one that doesn't use the
       shell to do globbing.

       Is there a leak/bug in glob()?

       Due to the current implementation on some operating sys­
       tems, when you use the glob() function or its angle-
       bracket alias in a scalar context, you may cause a memory
       leak and/or unpredictable behavior.  It's best therefore
       to use glob() only in list context.

       How can I open a file with a leading ">" or trailing
       blanks?

       Normally perl ignores trailing blanks in filenames, and
       interprets certain leading characters (or a trailing "|")
       to mean something special.

       The three argument form of open() lets you specify the
       mode separately from the filename.  The open() function
       treats special mode characters and whitespace in the file­
       name as literals

               open FILE, "<", "  file  ";  # filename is "   file   "
               open FILE, ">", ">file";     # filename is ">file"

       It may be a lot clearer to use sysopen(), though:

           use Fcntl;
           $badpath = "<<How can I reliably rename a file?

       How can I lock a file?

       Perl's builtin flock() function (see perlfunc for details)
       will call flock(2) if that exists, fcntl(2) if it doesn't
       (on perl version 5.004 and later), and lockf(3) if neither
       of the two previous system calls exists.  On some systems,
       it may even use a different form of native locking.  Here
       are some gotchas with Perl's flock():

       1   Produces a fatal error if none of the three system
           calls (or their close equivalent) exists.

       2   lockf(3) does not provide shared locking, and requires
           that the filehandle be open for writing (or appending,
           or read/writing).

       3   Some versions of flock() can't lock files over a net­
           work (e.g. on NFS file systems), so you'd need to
           force the use of fcntl(2) when you build Perl.  But
           even this is dubious at best.  See the flock entry of
           perlfunc and the INSTALL file in the source distribu­
           tion for information on building Perl to do this.

           Two potentially non-obvious but traditional flock
           semantics are that it waits indefinitely until the
           lock is granted, and that its locks are merely advi­
           sory.  Such discretionary locks are more flexible, but
           offer fewer guarantees.  This means that files locked
           with flock() may be modified by programs that do not
           also use flock().  Cars that stop for red lights get
           on well with each other, but not with cars that don't
           stop for red lights.  See the perlport manpage, your
           port's specific documentation, or your system-specific
           local manpages for details.  It's best to assume tra­
           ditional behavior if you're writing portable programs.
           (If you're not, you should as always feel perfectly
           free to write for your own system's idiosyncrasies
           (sometimes called "features").  Slavish adherence to
           portability concerns shouldn't get in the way of your
           getting your job done.)

           For more information on file locking, see also "File
           Locking" in perlopentut if you have it (new for 5.6).

       Why can't I just open(FH, ">file.lock")?

       A common bit of code NOT TO USE is this:

           sleep(3) while -e "file.lock";      # PLEASE DO NOT USE
           open(LCK, "> file.lock");           # THIS BROKEN CODE

       This is a classic race condition: you take two steps to do
       number in the file.  How can I do this?

       Didn't anyone ever tell you web-page hit counters were
       useless?  They don't count number of hits, they're a waste
       of time, and they serve only to stroke the writer's van­
       ity.  It's better to pick a random number; they're more
       realistic.

       Anyway, this is what you can do if you can't help your­
       self.

           use Fcntl qw(:DEFAULT :flock);
           sysopen(FH, "numfile", O_RDWR|O_CREAT)       or die "can't open numfile: $!";
           flock(FH, LOCK_EX)                           or die "can't flock numfile: $!";
           $num = <FH> || 0;
           seek(FH, 0, 0)                               or die "can't rewind numfile: $!";
           truncate(FH, 0)                              or die "can't truncate numfile: $!";
           (print FH $num+1, "\n")                      or die "can't write numfile: $!";
           close FH                                     or die "can't close numfile: $!";

       Here's a much better web-page hit counter:

           $hits = int( (time() - 850_000_000) / rand(1_000) );

       If the count doesn't impress your friends, then the code
       might.  :-)

       All I want to do is append a small amount of text to the
       end of a file.  Do I still have to use locking?

       If you are on a system that correctly implements flock()
       and you use the example appending code from "perldoc -f
       flock" everything will be OK even if the OS you are on
       doesn't implement append mode correctly (if such a system
       exists.) So if you are happy to restrict yourself to OSs
       that implement flock() (and that's not really much of a
       restriction) then that is what you should do.

       If you know you are only going to use a system that does
       correctly implement appending (i.e. not Win32) then you
       can omit the seek() from the above code.

       If you know you are only writing code to run on an OS and
       filesystem that does implement append mode correctly (a
       local filesystem on a modern Unix for example), and you
       keep the file in block-buffered mode and you write less
       than one buffer-full of output between each manual flush­
       ing of the buffer then each bufferload is almost guaran­
       teed to be written to the end of the file in one chunk
       without getting intermingled with anyone else's output.
       You can also use the syswrite() function which is simply a
       wrapper around your systems write(2) system call.

       However, if you have fixed sized records, then you might
       do something more like this:

           $RECSIZE = 220; # size of record, in bytes
           $recno   = 37;  # which record to update
           open(FH, "+<somewhere") || die "can't update somewhere: $!";
           seek(FH, $recno * $RECSIZE, 0);
           read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
           # munge the record
           seek(FH, -$RECSIZE, 1);
           print FH $record;
           close FH;

       Locking and error checking are left as an exercise for the
       reader.  Don't forget them or you'll be quite sorry.

       How do I get a file's timestamp in perl?

       If you want to retrieve the time at which the file was
       last read, written, or had its meta-data (owner, etc)
       changed, you use the -M, -A, or -C file test operations as
       documented in perlfunc.  These retrieve the age of the
       file (measured against the start-time of your program) in
       days as a floating point number. Some platforms may not
       have all of these times.  See perlport for details. To
       retrieve the "raw" time in seconds since the epoch, you
       would call the stat function, then use localtime(),
       gmtime(), or POSIX::strftime() to convert this into human-
       readable form.

       Here's an example:

           $write_secs = (stat($file))[9];
           printf "file %s updated at %s\n", $file,
               scalar localtime($write_secs);

       If you prefer something more legible, use the File::stat
       module (part of the standard distribution in version 5.004
       and later):

           # error checking left as an exercise for reader.
           use File::stat;
           use Time::localtime;
           $date_string = ctime(stat($file)->mtime);
           print "file $file updated at $date_string\n";

       The POSIX::strftime() approach has the benefit of being,
       in theory, independent of the current locale.  See perllo­
       cale for details.

       How do I set a file's timestamp in perl?
       Error checking is, as usual, left as an exercise for the
       reader.

       Note that utime() currently doesn't work correctly with
       Win95/NT ports.  A bug has been reported.  Check it care­
       fully before using utime() on those platforms.

       How do I print to more than one file at once?

       To connect one filehandle to several output filehandles,
       you can use the IO::Tee or Tie::FileHandle::Multiplex mod­
       ules.

       If you only have to do this once, you can print individu­
       ally to each filehandle.

           for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }

       How can I read in an entire file all at once?

       You can use the File::Slurp module to do it in one step.

               use File::Slurp;

               $all_of_it = read_file($filename); # entire file in scalar
           @all_lines = read_file($filename); # one line perl element

       The customary Perl approach for processing all the lines
       in a file is to do so one line at a time:

           open (INPUT, $file)         || die "can't open $file: $!";
           while (<INPUT>) {
               chomp;
               # do something with $_
           }
           close(INPUT)                || die "can't close $file: $!";

       This is tremendously more efficient than reading the
       entire file into memory as an array of lines and then pro­
       cessing it one element at a time, which is often--if not
       almost always--the wrong approach.  Whenever you see some­
       one do this:

           @lines = <INPUT>;

       you should think long and hard about why you need every­
       thing loaded at once.  It's just not a scalable solution.
       You might also find it more fun to use the standard
       Tie::File module, or the DB_File module's $DB_RECNO bind­
       ings, which allow you to tie an array to a file so that
       accessing an element the array actually accesses the cor­
       responding line in the file.
       automatically close the file at block exit.  If the file
       is already open, just use this:

           $var = do { local $/; <INPUT> };

       For ordinary files you can also use the read function.

               read( INPUT, $var, -s INPUT );

       The third argument tests the byte size of the data on the
       INPUT filehandle and reads that many bytes into the buffer
       $var.

       How can I read in a file by paragraphs?

       Use the $/ variable (see perlvar for details).  You can
       either set it to "" to eliminate empty paragraphs
       ("abc\n\n\n\ndef", for instance, gets treated as two para­
       graphs and not three), or "\n\n" to accept empty para­
       graphs.

       Note that a blank line must have no blanks in it.  Thus
       "fred\n \nstuff\n\n" is one paragraph, but
       "fred\n\nstuff\n\n" is two.

       How can I read a single character from a file?  From the
       keyboard?

       You can use the builtin "getc()" function for most file­
       handles, but it won't (easily) work on a terminal device.
       For STDIN, either use the Term::ReadKey module from CPAN
       or use the sample code in "getc" in perlfunc.

       If your system supports the portable operating system pro­
       gramming interface (POSIX), you can use the following
       code, which you'll note turns off echo processing as well.

           #!/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);
               }

               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() }

       The Term::ReadKey module from CPAN may be easier to use.
       Recent versions include also support for non-portable sys­
       tems as well.

           use Term::ReadKey;
           open(TTY, "</dev/tty");
           print "Gimme a char: ";
           ReadMode "raw";
           $key = ReadKey 0, *TTY;
           ReadMode "normal";
           printf "\nYou said %s, char number %03d\n",
               $key, ord $key;

       How can I tell whether there's a character waiting on a
       filehandle?

       The very first thing you should do is look into getting
       the Term::ReadKey extension from CPAN.  As we mentioned
       earlier, it now even has limited support for non-portable
       (read: not open systems, closed, proprietary, not POSIX,
       not Unix, etc) systems.

       You should also check out the Frequently Asked Questions
       list in comp.unix.* for things like this: the answer is
       essentially the same.  It's very system dependent.  Here's
       one solution that works on BSD systems:

           sub key_ready {
               my($rin, $nfd);
               vec($rin, fileno(STDIN), 1) = 1;
               return $nfd = select($rin,undef,undef,0);
           }

       grep the include files by hand:

           % grep FIONREAD /usr/include/*/*
           /usr/include/asm/ioctls.h:#define FIONREAD      0x541B

       Or write a small C program using the editor of champions:

           % cat > fionread.c
           #include <sys/ioctl.h>
           main() {
               printf("%#08x\n", FIONREAD);
           }
           ^D
           % cc -o fionread fionread.c
           % ./fionread
           0x4004667f

       And then hard code it, leaving porting as an exercise to
       your successor.

           $FIONREAD = 0x4004667f;         # XXX: opsys dependent

           $size = pack("L", 0);
           ioctl(FH, $FIONREAD, $size)     or die "Couldn't call ioctl: $!\n";
           $size = unpack("L", $size);

       FIONREAD requires a filehandle connected to a stream,
       meaning that sockets, pipes, and tty devices work, but not
       files.

       How do I do a "tail -f" in perl?

       First try

           seek(GWFILE, 0, 1);

       The statement "seek(GWFILE, 0, 1)" doesn't change the cur­
       rent position, but it does clear the end-of-file condition
       on the handle, so that the next <GWFILE> makes Perl try
       again to read something.

       If that doesn't work (it relies on features of your stdio
       implementation), then you need something more like this:

               for (;;) {
                 for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
                   # search for some stuff and put it into files
                 }
                 # sleep for a while
                 seek(GWFILE, $curpos, 0);  # seek to where we had been
               }

           open(LOG, ">>/tmp/logfile");
           open(STDERR, ">&LOG");

       Or even with a literal numeric descriptor:

          $fd = $ENV{MHCONTEXTFD};
          open(MHCONTEXT, "<&=$fd");   # like fdopen(3S)

       Note that "<&STDIN" makes a copy, but "<&=STDIN" make an
       alias.  That means if you close an aliased handle, all
       aliases become inaccessible.  This is not true with a
       copied one.

       Error checking, as always, has been left as an exercise
       for the reader.

       How do I close a file descriptor by number?

       This should rarely be necessary, as the Perl close() func­
       tion is to be used for things that Perl opened itself,
       even if it was a dup of a numeric descriptor as with
       MHCONTEXT above.  But if you really have to, you may be
       able to do this:

           require 'sys/syscall.ph';
           $rc = syscall(&SYS_close, $fd + 0);  # must force numeric
           die "can't sysclose $fd: $!" unless $rc == -1;

       Or, just use the fdopen(3S) feature of open():

           {
               local *F;
               open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
               close F;
           }

       Why can't I use "C:\temp\foo" in DOS paths?  Why doesn't
       `C:\temp\foo.exe` work?

       Whoops!  You just put a tab and a formfeed into that file­
       name!  Remember that within double quoted strings
       ("like\this"), the backslash is an escape character.  The
       full list of these is in "Quote and Quote-like Operators"
       in perlop.  Unsurprisingly, you don't have a file called
       "c:(tab)emp(formfeed)oo" or "c:(tab)emp(formfeed)oo.exe"
       on your legacy DOS filesystem.

       Either single-quote your strings, or (preferably) use for­
       ward slashes.  Since all DOS and Windows versions since
       something like MS-DOS 2.0 or so have treated "/" and "\"
       the same in a path, you might as well use the one that
       doesn't clash with Perl--or the POSIX shell, ANSI C and

       This is elaborately and painstakingly described in the
       file-dir-perms article in the "Far More Than You Ever
       Wanted To Know" collection in
       http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .

       The executive summary: learn how your filesystem works.
       The permissions on a file say what can happen to the data
       in that file.  The permissions on a directory say what can
       happen to the list of files in that directory.  If you
       delete a file, you're removing its name from the directory
       (so the operation depends on the permissions of the direc­
       tory, not of the file).  If you try to write to the file,
       the permissions of the file govern whether you're allowed
       to.

       How do I select a random line from a file?

       Here's an algorithm from the Camel Book:

           srand;
           rand($.) < 1 && ($line = $_) while <>;

       This has a significant advantage in space over reading the
       whole file in.  You can find a proof of this method in The
       Art of Computer Programming, Volume 2, Section 3.4.2, by
       Donald E. Knuth.

       You can use the File::Random module which provides a func­
       tion for that algorithm:

               use File::Random qw/random_line/;
               my $line = random_line($filename);

       Another way is to use the Tie::File module, which treats
       the entire file as an array.  Simply access a random array
       element.

       Why do I get weird spaces when I print an array of lines?

       Saying

           print "@lines\n";

       joins together the elements of @lines with a space between
       them.  If @lines were "("little", "fluffy", "clouds")"
       then the above statement would print

           little fluffy clouds

       but if each element of @lines was a line of text, ending a
       newline character "("little\n", "fluffy\n", "clouds\n")"
       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 here
       are in the public domain.  You are permitted and encour­
       aged to use this code and any derivatives thereof in your
       own programs for fun or for profit as you see fit.  A sim­
       ple comment in the code giving credit to the FAQ would be
       courteous but is not required.

perl v5.8.1                 2003-09-02                PERLFAQ5(1)

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