perlfaq5

Hurricane Electric Internet Services: Accounts starting at $9.95/month
Hurricane Electric Internet Services

NAME

       perlfaq5 - Files and Formats ($Revision: 1.22 $, $Date:
       1997/04/24 22:44:02 $)


DESCRIPTION

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

       How do I flush/unbuffer a filehandle?  Why must I do this?

       The C standard I/O library (stdio) normally buffers
       characters sent to devices.  This is done for efficiency
       reasons, so that there isn't a system call for each byte.
       Any time you use print() or write() in Perl, you go though
       this buffering.  syswrite() circumvents stdio and
       buffering.

       In most stdio implementations, the type of buffering and
       the size of the buffer varies according to the type of
       device.  Disk files are block buffered, often with a
       buffer size of more than 2k.  Pipes and sockets are often
       buffered with a buffer size between 1/2 and 2k.  Serial
       devices (e.g. modems, terminals) are normally line-
       buffered, and stdio sends the entire line when it gets the
       newline.

       Perl does not support truly unbuffered output (except
       insofar as you can syswrite(OUT, $char, 1)).  What it does
       instead support is "command buffering", in which a
       physical write is performed after every output command.
       This isn't as hard on your system as unbuffering, but does
       get the output where you want it when you want it.

       If you expect characters to get to your device when you
       print them there, you'll want to autoflush its handle, as
       in the older:

           use FileHandle;
           open(DEV, "<+/dev/tty");      # ceci n'est pas une pipe
           DEV->autoflush(1);

       or the newer IO::* modules:

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

       or even this:

           use IO::Socket;               # this one is kinda a pipe?
           $sock = IO::Socket::INET->new(PeerAddr => 'www.perl.com',
                                         PeerPort => 'http(80)',
                                         Proto    => 'tcp');
           die "$!" unless $sock;
           $sock->autoflush();
           $sock->print("GET /\015\012");
           $document = join('', $sock->getlines());
           print "DOC IS: $document\n";

       Note the hardcoded carriage return and newline in their
       octal equivalents.  This is the ONLY way (currently) to
       assure a proper flush on all platforms, including
       Macintosh.

       You can use select() and the $| variable to control
       autoflushing (see the section on $| in the perlvar manpage
       and the select entry in the perlfunc 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]);


       How do I change one line in a file/delete a line in a
       file/insert a line in the middle of a file/append to the
       beginning of a file?

       Although humans have an easy time thinking of a text file
       as being a sequence of lines that operates much like a
       stack of playing cards -- or punch cards -- computers
       usually see the text file as a sequence of bytes.  In
       general, there's no direct way for Perl to seek to a
       particular line of a file, insert text into a file, or
       remove text from a file.

       (There are exceptions in special circumstances.  Replacing
       a sequence of bytes with another sequence of the same
       length is one.  Another is using the $DB_RECNO array
       bindings as documented in the DB_File manpage.  Yet
       another is manipulating files with all lines the same
       length.)

       The general solution is to create a temporary copy of the
       text file with the changes you want, then copy that over
       the original.

           $old = $file;
           $new = "$file.tmp.$$";
           $bak = "$file.bak";

           open(OLD, "< $old")         or die "can't open $old: $!";
           open(NEW, "> $new")         or die "can't open $new: $!";

           # Correct typos, preserving case
           while (<OLD>) {
               s/\b(p)earl\b/${1}erl/i;
               (print NEW $_)          or die "can't write to $new: $!";
           }

           close(OLD)                  or die "can't close $old: $!";
           close(NEW)                  or die "can't close $new: $!";

           rename($old, $bak)          or die "can't rename $old to $bak: $!";
           rename($new, $old)          or die "can't rename $new to $old: $!";

       Perl can do this sort of thing for you automatically with
       the -i command-line switch or the closely-related $^I
       variable (see the perlrun manpage for more details).  Note
       that -i may require a suffix on some non-Unix systems; see
       the platform-specific documentation that came with your
       port.

           # Renumber a series of tests from the command line
           perl -pi -e 's/(^\s+test\s+)\d+/ $1 . ++$count /e' t/op/taint.t

           # form a script
           local($^I, @ARGV) = ('.bak', glob("*.c"));
           while (<>) {
               if ($. == 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 $.
           }

       If you need to seek to an arbitrary line of a file that
       changes infrequently, you could build up an index of byte
       positions of where the line ends are in the file.  If the
       file is large, an index of every tenth or hundredth line
       end would allow you to seek and read fairly efficiently.
       If the file is sorted, try the look.pl library (part of
       the standard perl distribution).

       In the unique case of deleting lines at the end of a file,
       you can use tell() and truncate().  The following code
       snippet deletes the last line of a file without making a
       copy or reading the whole file into memory:

               open (FH, "+< $file");
               while ( <FH> ) { $addr = tell(FH) unless eof(FH) }
               truncate(FH, $addr);

       Error checking is left as an exercise for the reader.



       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
       documented in the perlop manpage.  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;


       How do I make a temporary file name?

       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 IO::File;
               use Fcntl;
               my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP} || $ENV{TEMP};
               my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
               sub temp_file {
                   my $fh = undef;
                   my $count = 0;
                   until (defined($fh) || $count > 100) {
                       $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
                       $fh = IO::File->new($base_name, O_WRONLY|O_EXCL|O_CREAT, 0644)
                   }
                   if (defined($fh)) {
                       return ($fh, $base_name);
                   } else {
                       return ();
                   }
               }
           }

       Or you could simply use IO::Handle::new_tmpfile.

       How can I manipulate fixed-record-length files?

       The most efficient way is using pack() and unpack().  This
       is faster than using substr().  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|");
           $_ = <PS>; print;
           while (<PS>) {
               ($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";
           }


       How can I make a filehandle local to a subroutine?  How do
       I pass filehandles between subroutines?  How do I make an
       array of filehandles?

       You may have some success with typeglobs, as we always had
       to use in days of old:

           local(*FH);

       But while still supported, that isn't the best to go about
       getting local filehandles.  Typeglobs have their
       drawbacks.  You may well want to use the FileHandle
       module, which creates new filehandles for you (see the
       FileHandle manpage):

           use FileHandle;
           sub findme {
               my $fh = FileHandle->new();
               open($fh, "</etc/hosts") or die "no /etc/hosts: $!";
               while (<$fh>) {
                   print if /\b127\.(0\.0\.)?1\b/;
               }
               # $fh automatically closes/disappears here
           }

       Internally, Perl believes filehandles to be of class
       IO::Handle.  You may use that module directly if you'd
       like (see the IO::Handle manpage), or one of its more
       specific derived classes.

       Once you have IO::File or FileHandle objects, you can pass
       them between subroutines or store them in hashes as you
       would any other scalar values:

           use FileHandle;




           # Storing filehandles in a hash and array
           foreach $filename (@names) {
               my $fh = new FileHandle($filename)              or die;
               $file{$filename} = $fh;
               push(@files, $fh);
           }

           # Using the filehandles in the array
           foreach $file (@files) {
               print $file "Testing\n";
           }

           # You have to do the { } ugliness when you're specifying the
           # filehandle by anything other than a simple scalar variable.
           print { $files[2] } "Testing\n";

           # Passing filehandles to subroutines
           sub debug {
               my $filehandle = shift;
               printf $filehandle "DEBUG: ", @_;
           }

           debug($fh, "Testing\n");


       How can I set up a footer format to be used with write()?

       There's no builtin way to do this, but the perlform
       manpage has a couple of techniques to make it possible for
       the intrepid hacker.

       How can I write() into a string?

       See the perlform manpage for an swrite() function.

       How can I output my numbers with commas added?

       This one will do it for you:

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

           $n = 23659019423.2331;
           print "GOT: ", commify($n), "\n";

           GOT: 23,659,019,423.2331

       You can't just:

           s/^(-?\d+)(\d{3})/$1,$2/g;

       because you have to put the comma in and then recalculate
       your position.

       Alternatively, this commifies all numbers in a line
       regardless of whether they have decimal portions, are
       preceded by + or -, or whatever:

           # from Andrew Johnson <ajohnson@gpu.srv.ualberta.ca>
           sub commify {
              my $input = shift;
               $input = reverse $input;
               $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g;
               return reverse $input;
           }


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

       Use the <> (glob()) operator, documented in the perlfunc
       manpage.  This requires that you have a shell installed
       that groks tildes, meaning csh or tcsh or (some versions
       of) ksh, and thus may have portability problems.  The
       Glob::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)
                 )
               }{
                 $1
                     ? (getpwnam($1))[7]
                     : ( $ENV{HOME} || $ENV{LOGDIR} )
               }ex;


       How come when I open the file read-write it wipes it out?

       Because you're using something like this, which truncates
       the file and then gives you read-write access:

           open(FH, "+> /path/name");  # WRONG

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

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

       If this is an issue, try:

           sysopen(FH, "/path/name", O_RDWR|O_CREAT, 0644);

       Error checking is left as an exercise for the reader.

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

       The <> operator performs a globbing operation (see above).
       By default glob() 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 do the glob yourself with
       Dirhandles and patterns, or use a module like Glob::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
       systems, when you use the glob() function or its angle-
       bracket alias in a scalar context, you may cause a 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.  To avoid this, you might want
       to use a routine like this.  It makes incomplete pathnames
       into explicit relative ones, and tacks a trailing null
       byte on the name to make perl leave it alone:

           sub safe_filename {
               local $_  = shift;
               return m#^/#
                       ? "$_\0"
                       : "./$_\0";
           }

           $fn = safe_filename("<<sysopen() function (see the sysopen
       entry in the perlfunc manpage).

       How can I reliably rename a file?

       Well, usually you just use Perl's rename() function.  But
       that may not work everywhere, in particular, renaming
       files across file systems.  If your operating system
       supports a mv(1) program or its moral equivalent, this
       works:

           rename($old, $new) or system("mv", $old, $new);

       It may be more compelling to use the File::Copy module
       instead.  You just copy to the new file to the new name
       (checking return values), then delete the old one.  This
       isn't really the same semantics as a real rename(),
       though, which preserves metainformation like permissions,
       timestamps, inode info, etc.

       How can I lock a file?

       Perl's builtin flock() function (see the perlfunc manpage
       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
           network (e.g. on NFS file systems), so you'd need to
           force the use of fcntl(2) when you build Perl.  See
           the flock entry of the perlfunc manpage, and the
           INSTALL file in the source distribution for
           information on building Perl to do this.

       The CPAN module File::Lock offers similar functionality
       and (if you have dynamic loading) won't require you to
       rebuild perl if your flock() can't lock network files.

       What 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
       something which must be done in one.  That's why computer
       hardware provides an atomic test-and-set instruction.   In
       theory, this "ought" to work:

           sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT, 0644)
                       or die "can't open  file.lock: $!":

       except that lamentably, file creation (and deletion) is
       not atomic over NFS, so this won't work (at least, not
       every time) over the net.  Various schemes involving
       involving link() have been suggested, but these tend to
       involve busy-wait, which is also subdesirable.

       I still don't get locking.  I just want to increment the
       number in the file.  How can I do this?

       Didn't anyone ever tell you web-page hit counters were
       useless?

       Anyway, this is what to do:

           use Fcntl;
           sysopen(FH, "numfile", O_RDWR|O_CREAT, 0644) or die "can't open numfile: $!";
           flock(FH, 2)                                 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: $!";
           # DO NOT UNLOCK THIS UNTIL YOU CLOSE
           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.  :-)

       How do I randomly update a binary file?

       If you're just trying to patch a binary, in many cases
       something as simple as this works:

           perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs

       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, $recno * $RECSIZE, 0);
           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.

       Don't forget to set binmode() under DOS-like platforms
       when operating on files that have anything other than
       straight text in them.  See the docs on open() and on
       binmode() for more details.

       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 filetest operations as
       documented in the perlfunc manpage.  These retrieve the
       age of the file (measured against the start-time of your
       program) in days as a floating point number.  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];
           print "file $file updated at ", scalar(localtime($file)), "\n";

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

           use File::stat;
           use Time::localtime;
           $date_string = ctime(stat($file)->mtime);
           print "file $file updated at $date_string\n";

       Error checking is left as an exercise for the reader.

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

       You use the utime() function documented in the utime entry
       in the perlfunc manpage.  By way of example, here's a
       little program that copies the read and write times from
       its first argument to all the rest of them.

           if (@ARGV < 2) {
               die "usage: cptimes timestamp_file other_files ...\n";
           }
           $timestamp = shift;
           ($atime, $mtime) = (stat($timestamp))[8,9];
           utime $atime, $mtime, @ARGV;

       Error checking is 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
       carefully before using it on those platforms.

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

       If you only have to do this once, you can do this:

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

       To connect up to one filehandle to several output
       filehandles, it's easiest to use the tee(1) program if you
       have it, and let it take care of the multiplexing:

           open (FH, "| tee file1 file2 file3");

       Otherwise you'll have to write your own multiplexing print
       function -- or your own tee program -- or use Tom
       Christiansen's, at
       http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz,
       which is written in Perl.

       In theory a IO::Tee class could be written, but to date we
       haven't seen such.

       How can I read in a file by paragraphs?

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

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

       You can use the builtin getc() function for most
       filehandles, 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 the getc entry in the
       perlfunc manpage.

       If your system supports 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);

               $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() }

       The Term::ReadKey module from CPAN may be easier to use:

           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;

       For DOS systems, Dan Carson <dbc@tc.fluke.COM> reports the
       following:

       To put the PC in "raw" mode, use ioctl with some magic
       numbers gleaned from msdos.c (Perl source file) and Ralf
       Brown's interrupt list (comes across the net every so
       often):

           $old_ioctl = ioctl(STDIN,0,0);     # Gets device info
           $old_ioctl &= 0xff;
           ioctl(STDIN,1,$old_ioctl | 32);    # Writes it back, setting bit 5

       Then to read a single character:

           sysread(STDIN,$c,1);               # Read a single character

       And to put the PC back to "cooked" mode:

           ioctl(STDIN,1,$old_ioctl);         # Sets it back to cooked mode.

       So now you have $c.  If ord($c) == 0, you have a two byte
       code, which means you hit a special key.  Read another
       byte with sysread(STDIN,$c,1), and that value tells you
       what combination it was according to this table:

           # PC 2-byte keycodes = ^@ + the following:

           # HEX     KEYS
           # ---     ----
           # 0F      SHF TAB
           # 10-19   ALT QWERTYUIOP
           # 1E-26   ALT ASDFGHJKL
           # 2C-32   ALT ZXCVBNM
           # 3B-44   F1-F10
           # 47-49   HOME,UP,PgUp
           # 4B      LEFT
           # 4D      RIGHT
           # 4F-53   END,DOWN,PgDn,Ins,Del
           # 54-5D   SHF F1-F10
           # 5E-67   CTR F1-F10
           # 68-71   ALT F1-F10
           # 73-77   CTR LEFT,RIGHT,END,PgDn,HOME
           # 78-83   ALT 1234567890-=
           # 84      CTR PgUp

       This is all trial and error I did a long time ago, I hope
       I'm reading the file that worked.

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

       You should 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);
           }

       You should look into getting the Term::ReadKey extension
       from CPAN.

       How do I open a file without blocking?

       You need to use the O_NDELAY or O_NONBLOCK flag from the
       Fcntl module in conjunction with sysopen():

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


       How do I create a file only if it doesn't exist?

       You need to use the O_CREAT and O_EXCL flags from the
       Fcntl module in conjunction with sysopen():

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

       Be warned that neither creation nor deletion of files is
       guaranteed to be an atomic operation over NFS.  That is,
       two processes might both successful create or unlink the
       same file!

       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
       current 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
               }

       If this still doesn't work, look into the POSIX module.
       POSIX defines the clearerr() method, which can remove the
       end of file condition on a filehandle.  The method: read
       until end of file, clearerr(), read some more.  Lather,
       rinse, repeat.
       How do I dup() a filehandle in Perl?

       If you check the open entry in the perlfunc manpage,
       you'll see that several of the ways to call open() should
       do the trick.  For example:

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

       Or even with a literal numeric descriptor:

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

       Error checking 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()
       function 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;


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

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

       Either single-quote your strings, or (preferably) use
       forward 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 C++,
       awk, Tcl, Java, or Python, just to mention a few.

       Why doesn't glob("*.*") get all the files?

       Because even on non-Unix ports, Perl's glob function
       follows standard Unix globbing semantics.  You'll need
       glob("*") to get all (non-hidden) files.

       Why does Perl let me delete read-only files?  Why does -i
       clobber protected files?  Isn't this a bug in Perl?

       This is elaborately and painstakingly described in the
       "Far More Than You Every Wanted To Know" in
       http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms .

       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
       directory, 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.


AUTHOR AND COPYRIGHT

       Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
       All rights reserved.  See the perlfaq manpage for
       distribution information.
Hurricane Electric Internet Services: Accounts starting at $9.95/month
Hurricane Electric Internet Services
Copyright (C) 1998 Hurricane Electric. All Rights Reserved.