perlfaq5


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

   How do I flush/unbuffer an output filehandle?  Why must I do this?
       (contributed by brian d foy)

       You might like to read Mark Jason Dominus's "Suffering From Buffering"
       at http://perl.plover.com/FAQs/Buffering.html .

       Perl normally buffers output so it doesn't make a system call for every
       bit of output. By saving up output, it makes fewer expensive system
       calls.  For instance, in this little bit of code, you want to print a
       dot to the screen for every line you process to watch the progress of
       your program.  Instead of seeing a dot for every line, Perl buffers the
       output and you have a long wait before you see a row of 50 dots all at
       once:

               # long wait, then row of dots all at once
               while( <> ) {
                       print ".";
                       print "\n" unless ++$count % 50;

                       #... expensive line processing operations
                       }

       To get around this, you have to unbuffer the output filehandle, in this
       case, "STDOUT". You can set the special variable $| to a true value
       (mnemonic: making your filehandles "piping hot"):

               $|++;

               # dot shown immediately
               while( <> ) {
                       print ".";
                       print "\n" unless ++$count % 50;

                       #... expensive line processing operations
                       }

       The $| is one of the per-filehandle special variables, so each
       filehandle has its own copy of its value. If you want to merge standard
       output and standard error for instance, you have to unbuffer each
       (although STDERR might be unbuffered by default):

               {
               my $previous_default = select(STDOUT);  # save previous default
               $|++;                                   # autoflush STDOUT
               select(STDERR);
               $|++;                                   # autoflush STDERR, to be sure
               select($previous_default);              # restore previous default
               }

               binmode( STDOUT, ":unix" );

               while( 1 ) {
                       sleep 1;
                       print ".";
                       print "\n" unless ++$count % 50;
                       }

       For more information on output layers, see the entries for "binmode"
       and "open" in perlfunc, and the "PerlIO" module documentation.

       If you are using "IO::Handle" or one of its subclasses, you can call
       the "autoflush" method to change the settings of the filehandle:

               use IO::Handle;
               open my( $io_fh ), ">", "output.txt";
               $io_fh->autoflush(1);

       The "IO::Handle" objects also have a "flush" method. You can flush the
       buffer any time you want without auto-buffering

               $io_fh->flush;

   How do I change, delete, or insert a line in a file, or append to the
       beginning of a file?
       (contributed by brian d foy)

       The basic idea of inserting, changing, or deleting a line from a text
       file involves reading and printing the file to the point you want to
       make the change, making the change, then reading and printing the rest
       of the file. Perl doesn't provide random access to lines (especially
       since the record input separator, $/, is mutable), although modules
       such as "Tie::File" can fake it.

       A Perl program to do these tasks takes the basic form of opening a
       file, printing its lines, then closing the file:

               open my $in,  '<',  $file      or die "Can't read old file: $!";
               open my $out, '>', "$file.new" or die "Can't write new file: $!";

               while( <$in> )
                       {
                       print $out $_;
                       }

          close $out;

       Within that basic form, add the parts that you need to insert, change,
       or delete lines.

       To prepend lines to the beginning, print those lines before you enter
       the loop that prints the existing lines.

               open my $in,  '<',  $file      or die "Can't read old file: $!";

       the "while" loop. In this case, the code finds all lowercased versions
       of "perl" and uppercases them. The happens for every line, so be sure
       that you're supposed to do that on every line!

               open my $in,  '<',  $file      or die "Can't read old file: $!";
               open my $out, '>', "$file.new" or die "Can't write new file: $!";

               print $out "# Add this line to the top\n";

               while( <$in> )
                       {
                       s/\b(perl)\b/Perl/g;
                       print $out $_;
                       }

          close $out;

       To change only a particular line, the input line number, $., is useful.
       First read and print the lines up to the one you  want to change. Next,
       read the single line you want to change, change it, and print it. After
       that, read the rest of the lines and print those:

               while( <$in> )   # print the lines before the change
                       {
                       print $out $_;
                       last if $. == 4; # line number before change
                       }

               my $line = <$in>;
               $line =~ s/\b(perl)\b/Perl/g;
               print $out $line;

               while( <$in> )   # print the rest of the lines
                       {
                       print $out $_;
                       }

       To skip lines, use the looping controls. The "next" in this example
       skips comment lines, and the "last" stops all processing once it
       encounters either "__END__" or "__DATA__".

               while( <$in> )
                       {
                       next if /^\s+#/;             # skip comment lines
                       last if /^__(END|DATA)__$/;  # stop at end of code marker
                       print $out $_;
                       }

       Do the same sort of thing to delete a particular line by using "next"
       to skip the lines you don't want to show up in the output. This example
       skips every fifth line:

               while( <$in> )
                       {

                       # do your magic here

               print $out @lines;

       Modules such as "File::Slurp" and "Tie::File" can help with that too.
       If you can, however, avoid reading the entire file at once. Perl won't
       give that memory back to the operating system until the process
       finishes.

       You can also use Perl one-liners to modify a file in-place. The
       following changes all 'Fred' to 'Barney' in inFile.txt, overwriting the
       file with the new contents. With the "-p" switch, Perl wraps a "while"
       loop around the code you specify with "-e", and "-i" turns on in-place
       editing. The current line is in $_. With "-p", Perl automatically
       prints the value of $_ at the end of the loop. See perlrun for more
       details.

               perl -pi -e 's/Fred/Barney/' inFile.txt

       To make a backup of "inFile.txt", give "-i" a file extension to add:

               perl -pi.bak -e 's/Fred/Barney/' inFile.txt

       To change only the fifth line, you can add a test checking $., the
       input line number, then only perform the operation when the test
       passes:

               perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt

       To add lines before a certain line, you can add a line (or lines!)
       before Perl prints $_:

               perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt

       You can even add a line to the beginning of a file, since the current
       line prints at the end of the loop:

               perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt

       To insert a line after one already in the file, use the "-n" switch.
       It's just like "-p" except that it doesn't print $_ at the end of the
       loop, so you have to do that yourself. In this case, print $_ first,
       then print the line that you want to add.

               perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt

       To delete lines, only print the ones that you want.

               perl -ni -e 'print unless /d/' inFile.txt

                       ... or ...

               perl -pi -e 'next unless /d/' inFile.txt

       number from the last filehandle read:

               1 while( <$fh> );
               my $count = $.;

       If you want to use $., you can reduce it to a simple one-liner, like
       one of these:

               % perl -lne '} print $.; {'    file

               % perl -lne 'END { print $. }' file

       Those can be rather inefficient though. If they aren't fast enough for
       you, you might just read chunks of data and count the number of
       newlines:

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

       However, that doesn't work if the line ending isn't a newline. You
       might change that "tr///" to a "s///" so you can count the number of
       times the input record separator, $/, shows up:

               my $lines = 0;
               open my($fh), '<:raw', $filename or die "Can't open $filename: $!";
               while( sysread $fh, $buffer, 4096 ) {
                       $lines += ( $buffer =~ s|$/||g; );
                       }
               close FILE;

       If you don't mind shelling out, the "wc" command is usually the
       fastest, even with the extra interprocess overhead. Ensure that you
       have an untainted filename though:

               #!perl -T

               $ENV{PATH} = undef;

               my $lines;
               if( $filename =~ /^([0-9a-z_.]+)\z/ ) {
                       $lines = `/usr/bin/wc -l $1`
                       chomp $lines;
                       }

   How do I delete the last N lines from a file?
       (contributed by brian d foy)

       The easiest conceptual solution is to count the lines in the file then
       start at the beginning and print the number of lines (minus the last N)
       to a new file.

               use File::ReadBackwards;

               my $filename = 'test.txt';
               my $Lines_to_truncate = 2;

               my $bw = File::ReadBackwards->new( $filename )
                       or die "Could not read backwards in [$filename]: $!";

               my $lines_from_end = 0;
               until( $bw->eof or $lines_from_end == $Lines_to_truncate )
                       {
                       print "Got: ", $bw->readline;
                       $lines_from_end++;
                       }

               truncate( $filename, $bw->tell );

       The "File::ReadBackwards" module also has the advantage of setting the
       input record separator to a regular expression.

       You can also use the "Tie::File" module which lets you access the lines
       through a tied array. You can use normal array operations to modify
       your file, including setting the last index and using "splice".

   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 can I copy a file?
       (contributed by brian d foy)

       Use the "File::Copy" module. It comes with Perl and can do a true copy
       across file systems, and it does its magic in a portable fashion.
       with "undef" in place of the file name. In Perl 5.8 or later, the
       "open()" function creates an anonymous temporary file:

               open my $tmp, '+>', undef or die $!;

       Otherwise, you can use the File::Temp module.

               use File::Temp qw/ tempfile tempdir /;

               my $dir = tempdir( CLEANUP => 1 );
               ($fh, $filename) = tempfile( DIR => $dir );

               # or if you don't need to know the filename

               my $fh = tempfile( DIR => $dir );

       The File::Temp has been a standard module since Perl 5.6.1. If you
       don't have a modern enough Perl installed, use the "new_tmpfile" class
       method from the IO::File module to get a filehandle opened for reading
       and writing. Use it if you don't need to know the file's name:

               use IO::File;
               my $fh = IO::File->new_tmpfile()
                       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 {
                       my $fh;
                       my $count = 0;
                       until( defined(fileno($fh)) || $count++ > 100 ) {
                               $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
                               # O_EXCL is required for security reasons.
                               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?
               print scalar <$ps>;
               my @fields = qw( pid tt stat time command );
               while (<$ps>) {
                       my %process;
                       @process{@fields} = unpack($PS_T, $_);
               for my $field ( @fields ) {
                       print "$field: <$process{$field}>\n";
               }
               print 'line=', pack($PS_T, @process{@fields} ), "\n";
               }

       We've used a hash slice in order to easily handle the fields of each
       row.  Storing the keys in an array makes it easy to operate on them as
       a group or loop over them with "for". It also avoids polluting the
       program with global variables and using symbolic references.

   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?
       As of perl5.6, open() autovivifies file and directory handles as
       references if you pass it an uninitialized scalar variable.  You can
       then pass these references just like any other scalar, and use them in
       the place of named handles.

               open my    $fh, $file_name;

               open local $fh, $file_name;

               print $fh "Hello World!\n";

               process_file( $fh );

       If you like, you can store these filehandles in an array or a hash.  If
       you access them directly, they aren't simple scalars and you need to
       give "print" a little help by placing the filehandle reference in
       braces. Perl can only figure it out on its own when the filehandle
       reference is a simple scalar.

               my @fhs = ( $fh1, $fh2, $fh3 );

               for( $i = 0; $i <= $#fhs; $i++ ) {
                       print {$fhs[$i]} "just another Perl answer, \n";
                       }

       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!" }

               $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 and store that in a scalar variable.

               use IO::Handle;                     # 5.004 or higher
               my $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. Functions like "print", "open", "seek", or the "<FH>"
       diamond operator will accept either a named filehandle or a scalar
       variable containing one:

               ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
               print $ofh "Type it: ";
               my $got = <$ifh>
               print $efh "What was that: $got";

       If you're passing a filehandle to a function, you can write the
       function in two ways:

               sub accept_fh {
                       my $fh = shift;
                       print $fh "Sending to indirect filehandle\n";
               }

       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:

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

               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() requires a typeglob. It doesn't work
       with objects or strings, which might be a bug we haven't fixed yet.

               $got = readline($fd[0]);

       Let it be noted that the flakiness of indirect filehandles is not
       related to whether they're strings, typeglobs, objects, or anything
       else.  It's the syntax of the fundamental operators. Playing the object
       game doesn't help you at all here.

   How can I set up a footer format to be used with write()?
       There's no builtin way to do this, but perlform has a couple of
       techniques to make it possible for the intrepid hacker.

   How can I write() into a string?
       (contributed by brian d foy)

       If you want to "write" into a string, you just have to <open> a
       filehandle to a string, which Perl has been able to do since Perl 5.6:

               open FH, '>', \my $string;
               write( FH );

       Since you want to be a good programmer, you probably want to use a
       lexical filehandle, even though formats are designed to work with
       bareword filehandles since the default format names take the filehandle
       name. However, you can control this with some Perl special per-
       filehandle variables: $^, which names the top-of-page format, and $~
       which shows the line format. You have to change the default filehandle
       to set these variables:

               open my($fh), '>', \my $string;

               { # set per-filehandle variables
               my $old_fh = select( $fh );
               $~ = 'ANIMAL';
               $^ = 'ANIMAL_TOP';
               select( $old_fh );
               }

               format ANIMAL_TOP =
                ID  Type    Name
               .

               format ANIMAL =

               print $string;

       There are also some tricks that you can play with "formline" and the
       accumulator variable $^A, but you lose a lot of the value of formats
       since "formline" won't handle paging and so on. You end up
       reimplementing formats when you use them.

   How can I open a filehandle to a string?
       (contributed by Peter J. Holzer, hjp-usenet2@hjp.at)

       Since Perl 5.8.0 a file handle referring to a string can be created by
       calling open with a reference to that string instead of the filename.
       This file handle can then be used to read from or write to the string:

               open(my $fh, '>', \$string) or die "Could not open string for writing";
               print $fh "foo\n";
               print $fh "bar\n";      # $string now contains "foo\nbar\n"

               open(my $fh, '<', \$string) or die "Could not open string for reading";
               my $x = <$fh>;  # $x now contains "foo\n"

       With older versions of Perl, the "IO::String" module provides similar
       functionality.

   How can I output my numbers with commas added?
       (contributed by brian d foy and Benjamin Goldberg)

       You can use Number::Format to separate places in a number.  It handles
       locale information for those of you who want to insert full stops
       instead (or anything else that they want to use, really).

       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 numbers:

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

       It is easier to see with comments:

               s/(
                       ^[-+]?             # beginning of number.
                       \d+?               # 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:

               $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 a file read-write it wipes it out?
       Because you're using something like this, which truncates the file then
       gives you read-write access:

               open my $fh, '+>', '/path/name'; # WRONG (almost always)

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

               open my $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 that you've pulled in the constants from "Fcntl":

               use Fcntl;

       To open file for reading:

               open my $fh, '<', $path                               or die $!;
               sysopen my $fh, $path, O_RDONLY                       or die $!;

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

               open my $fh, '>', $path                               or die $!;
               sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT       or die $!;
               sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666 or die $!;

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

               sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT        or die $!;
               sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT, 0666  or die $!;

       To open file for appending, create if necessary:

               open my $fh, '>>' $path                               or die $!;
               sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT      or die $!;
               sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT, 0666 or die $!;
               sysopen my $fh, $path, O_RDWR|O_CREAT                 or die $!;
               sysopen my $fh, $path, O_RDWR|O_CREAT, 0666           or die $!;

       To open file for update, file must not exist:

               sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT          or die $!;
               sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT, 0666    or die $!;

       To open a file without blocking, creating if necessary:

               sysopen my $fh, '/foo/somefile', O_WRONLY|O_NDELAY|O_CREAT
                   or die "can't open /foo/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
       successfully create or unlink the same file! Therefore O_EXCL isn't as
       exclusive as you might wish.

       See also perlopentut.

   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 internal 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()?
       (contributed by brian d foy)

       Starting with Perl 5.6.0, "glob" is implemented internally rather than
       relying on an external resource. As such, memory issues with "glob"
       aren't a problem in modern perls.

   How can I open a file with a leading ">" or trailing blanks?
       (contributed by Brian McCauley)

       The special two-argument form of Perl's open() function ignores
       trailing blanks in filenames and infers the mode from certain leading
       characters (or a trailing "|"). In older versions of Perl this was the
       only version of open() and so it is prevalent in old code and books.

       Unless you have a particular reason to use the two-argument form you
       should use the three-argument form of open() which does not treat any
       characters in the filename as special.

               open my $fh, "<", "  file  ";  # filename is "   file   "
               open my $fh, ">", ">file";     # filename is ">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 network (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 distribution
           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 advisory. 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
           traditional 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 my $lock, '>', '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 my $fh, "file.lock", O_WRONLY|O_EXCL|O_CREAT
                       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 link() have been suggested, but these tend to
       involve busy-wait, which is also less than desirable.
               flock $fh, LOCK_EX                        or die "can't flock numfile: $!";
               my $num = <$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 code in the previous answer.

       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
       flushing of the buffer then each bufferload is almost guaranteed 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 system's write(2) system
       call.

       There is still a small theoretical chance that a signal will interrupt
       the system-level "write()" operation before completion. There is also a
       possibility that some STDIO implementations may call multiple system
       level "write()"s even if the buffer was empty to start. There may be
       some systems where this probability is reduced to zero, and this is not
       a concern when using ":perlio" instead of your system's STDIO.

   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

       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 -A, -M,
       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:

               my $write_secs = (stat($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;
               my $date_string = ctime(stat($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 perllocale for details.

   How do I set a file's timestamp in perl?
       You use the utime() function documented in "utime" in perlfunc.  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";
                       }
               my $timestamp = shift;
               my($atime, $mtime) = (stat($timestamp))[8,9];
               utime $atime, $mtime, @ARGV;

       Error checking is, as usual, left as an exercise for the reader.

       The perldoc for utime also has an example that has the same effect as
       touch(1) on files that already exist.

       Certain file systems have a limited ability to store the times on a
       file at the expected level of precision. For example, the FAT and HPFS
       filesystem are unable to create dates on files with a finer granularity
       than two seconds. This is a limitation of the filesystems, not of
       utime().

   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 modules.
                       chomp;
                       # do something with $_
                       }
               close $input or die "can't close $file: $!";

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

               my @lines = <INPUT>;

       You should think long and hard about why you need everything loaded at
       once. It's just not a scalable solution.

       If you "mmap" the file with the File::Map module from CPAN, you can
       virtually load the entire file into a string without actually storing
       it in memory:

               use File::Map qw(map_file);

               map_file my $string, $filename;

       Once mapped, you can treat $string as you would any other string.
       Since you don't necessarily have to load the data, mmap-ing can be very
       fast and may not increase your memory footprint.

       You might also find it more fun to use the standard "Tie::File" module,
       or the "DB_File" module's $DB_RECNO bindings, which allow you to tie an
       array to a file so that accessing an element of the array actually
       accesses the corresponding line in the file.

       If you want to load the entire file, you can use the "File::Slurp"
       module to do it in one one simple and efficient step:

               use File::Slurp;

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

       Or you can read the entire file contents into a scalar like this:

               my $var;
               {
               local $/;
               open my $fh, '<', $file or die "can't open $file: $!";
               $var = <$fh>;
               }

       That temporarily undefs your record separator, and will automatically
       close the file at block exit. If the file is already open, just use
       this:

               my $var = do { local $/; <$fh> };

   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 paragraphs and not three), or "\n\n" to accept empty
       paragraphs.

       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 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 "getc" in
       perlfunc.

       If your system supports the portable operating system programming
       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) {
                       print "gimme: ";
                       my $got = getone();
                       print "--> $got\n";
                       }
           exit;

               BEGIN {
               use POSIX qw(:termios_h);

               my ($term, $oterm, $echo, $noecho, $fd_stdin);

               my $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);
                       }

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

               use Term::ReadKey;
               open my $tty, '<', '/dev/tty';
               print "Gimme a char: ";
               ReadMode "raw";
               my $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);
                       }

       If you want to find out how many characters are waiting, there's also
       the FIONREAD ioctl call to be looked at. The h2ph tool that comes with
       Perl tries to convert C include files to Perl code, which can be
       "require"d. FIONREAD ends up defined as a function in the sys/ioctl.ph
       file:

               require 'sys/ioctl.ph';

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

       If h2ph wasn't installed or doesn't work for you, you can 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);

               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 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 "clearerr" method from
       "IO::Handle", which resets the error and end-of-file states on the
       handle.

       There's also a "File::Tail" module from CPAN.

   How do I dup() a filehandle in Perl?
       If you check "open" in perlfunc, you'll see that several of the ways to
       call open() should do the trick. For example:

               open my $log, '>>', '/foo/logfile';
               open STDERR, '>&LOG';

       Or even with a literal numeric descriptor:

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

       Note that "<&STDIN" makes a copy, but "<&=STDIN" makes 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?
       If, for some reason, you have a file descriptor instead of a filehandle
       (perhaps you used "POSIX::open"), you can use the "close()" function
       from the "POSIX" module:
               die "can't sysclose $fd: $!" unless $rc == -1;

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

               {
               open my( $fh ), "<&=$fd" or die "Cannot reopen fd=$fd: $!";
               close $fh;
               }

   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 filename!
       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 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. POSIX paths are
       more portable, too.

   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. This makes glob() portable even to legacy systems. Your
       port may include proprietary globbing functions as well. Check its
       documentation for details.

   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 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
       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?
       Short of loading the file into a database or pre-indexing the lines in
       the file, there are a couple of things that you can do.

       Here's a reservoir-sampling algorithm from the Camel Book:

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

       file as an array. Simply access a random array element.

   Why do I get weird spaces when I print an array of lines?
       (contributed by brian d foy)

       If you are seeing spaces between the elements of your array when you
       print the array, you are probably interpolating the array in double
       quotes:

               my @animals = qw(camel llama alpaca vicuna);
               print "animals are: @animals\n";

       It's the double quotes, not the "print", doing this. Whenever you
       interpolate an array in a double quote context, Perl joins the elements
       with spaces (or whatever is in $", which is a space by default):

               animals are: camel llama alpaca vicuna

       This is different than printing the array without the interpolation:

               my @animals = qw(camel llama alpaca vicuna);
               print "animals are: ", @animals, "\n";

       Now the output doesn't have the spaces between the elements because the
       elements of @animals simply become part of the list to "print":

               animals are: camelllamaalpacavicuna

       You might notice this when each of the elements of @array end with a
       newline. You expect to print one element per line, but notice that
       every line after the first is indented:

               this is a line
                this is another line
                this is the third line

       That extra space comes from the interpolation of the array. If you
       don't want to put anything between your array elements, don't use the
       array in double quotes. You can send it to print without them:

               print @lines;

   How do I traverse a directory tree?
       (contributed by brian d foy)

       The "File::Find" module, which comes with Perl, does all of the hard
       work to traverse a directory structure. It comes with Perl. You simply
       call the "find" subroutine with a callback subroutine and the
       directories you want to traverse:

               use File::Find;

               find( \&wanted, @directories );


               use File::Find;
               use File::Finder;

               my $deep_dirs = File::Finder->depth->type('d')->ls->exec('rmdir','{}');

               find( $deep_dirs->as_options, @places );

       The "File::Find::Rule" module, which you can download from CPAN, has a
       similar interface, but does the traversal for you too:

               use File::Find::Rule;

               my @files = File::Find::Rule->file()
                                                                ->name( '*.pm' )
                                                                ->in( @INC );

   How do I delete a directory tree?
       (contributed by brian d foy)

       If you have an empty directory, you can use Perl's built-in "rmdir".
       If the directory is not empty (so, no files or subdirectories), you
       either have to empty it yourself (a lot of work) or use a module to
       help you.

       The "File::Path" module, which comes with Perl, has a "remove_tree"
       which can take care of all of the hard work for you:

               use File::Path qw(remove_tree);

               remove_tree( @directories );

       The "File::Path" module also has a legacy interface to the older
       "rmtree" subroutine.

   How do I copy an entire directory?
       (contributed by Shlomi Fish)

       To do the equivalent of "cp -R" (i.e. copy an entire directory tree
       recursively) in portable Perl, you'll either need to write something
       yourself or find a good CPAN module such as  File::Copy::Recursive.

AUTHOR AND COPYRIGHT
       Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and other
       authors as noted. All rights reserved.

       This documentation is free; you can redistribute it and/or modify it
       under the same terms as Perl itself.

       Irrespective of its distribution, all code examples here are in the
       public domain. You are permitted and encouraged to use this code and
       any derivatives thereof in your own programs for fun or for profit as
       you see fit. A simple comment in the code giving credit to the FAQ
       would be courteous but is not required.
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