


PERLFAQ5(1)      Perl Programmers Reference Guide     PERLFAQ5(1)


NAME
       perlfaq5 - Files and Formats

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):





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

               # now should alternate . and +
               while( 1 )
                       {
                       sleep 1;
                       print STDOUT ".";
                       print STDERR "+";
                       print STDOUT "\n" unless ++$count % 25;
                       }

       Besides the $| special variable, you can use "binmode" to
       give your filehandle a ":unix" layer, which is unbuffered:

               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



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

               print $out "# Add this line to the top\n"; # <--- HERE'S THE MAGIC

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

          close $out;

       To change existing lines, insert the code to modify the
       lines inside 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;




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       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> )
                       {
                       next unless $. % 5;
                       print $out $_;
                       }

       If, for some odd reason, you really want to see the whole
       file at once rather than processing line by line, you can
       slurp it in (as long as you can fit the whole thing in
       memory!):

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

               my @lines = do { local $/; <$in> }; # slurp!

                       # do your magic here



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               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 ...




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

   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 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 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.

               use File::Copy;

               copy( $original, $new_copy ) or die "Copy failed: $!";




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       If you can't use "File::Copy", you'll have to do the work
       yourself: open the original file, open the destination
       file, then print to the destination file as you read the
       original. You also have to remember to copy the
       permissions, owner, and group to the new file.

   How do I make a temporary file name?
       If you don't need to know the name of the file, you can
       use "open()" 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 /;

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

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

               $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;
               $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 {
                       local *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;
                               }




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                       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
               my $PS_T = 'A6 A4 A7 A5 A*';
               open my $ps, '-|', 'ps';
               print scalar <$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 means
       it's 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";



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               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!" }

       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.
       Functions like "print", "open", "seek", or the "<FH>"
       diamond operator will accept either a named filehandle or



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       a scalar variable containing one:

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

               @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
       filehandle:

               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:



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

   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.



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       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:
                       \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.
       Versions of Perl older than 5.6 require that you have a
       shell installed that groks tildes.  Later versions of Perl
       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)
                 )
               }{
                 $1
                     ? (getpwnam($1))[7]
                     : ( $ENV{HOME} || $ENV{LOGDIR} )
               }ex;





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   How come when I open a 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 (almost always)

       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:




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

       To open file for update, file must not exist:

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

       To open a file without blocking, creating if necessary:

               sysopen(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 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
       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()?
       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 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?
       (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



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       open() which does not treat any characters in the filename
       as special.

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

   How can I reliably rename a file?
       If your operating system supports a proper mv(1) utility
       or its functional equivalent, this works:

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

       It may be more portable 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 semantically as a rename(), which
       preserves meta-information like permissions, timestamps,
       inode info, etc.

       Newer versions of File::Copy export a move() function.

   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



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           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(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)
                       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.

   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?  They don't count number of hits, they're a waste
       of time, and they serve only to stroke the writer's
       vanity.  It's better to pick a random number; they're more
       realistic.

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

               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) );



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       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:











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               $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 -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:

               $write_secs = (stat($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
       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.






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               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, 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.

       If you only have to do this once, you can print
       individually 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 per 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
       processing it one element at a time, which is often--if
       not almost always--the wrong approach.  Whenever you see
       someone do this:

               @lines = <INPUT>;



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       you should think long and hard about why you need
       everything 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 bindings, which allow you to tie an array to a
       file so that accessing an element the array actually
       accesses the corresponding line in the file.

       You can read the entire filehandle contents into a scalar.

               {
               local(*INPUT, $/);
               open (INPUT, $file)     || die "can't open $file: $!";
               $var = <INPUT>;
               }

       That temporarily undefs your record separator, and will
       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
       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.






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               #!/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.
       Recent versions include also support for non-portable
       systems as well.





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               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);
                       }

       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:










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               % 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
       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:



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               open(LOG, ">>/foo/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?
       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:

               use POSIX ();

               POSIX::close( $fd );

       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;

       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



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       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 <>;

       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
       function 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



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       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



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       traverse:

               use File::Find;

               find( \&wanted, @directories );

               sub wanted {
                       # full path in $File::Find::name
                       # just filename in $_
                       ... do whatever you want to do ...
                       }

       The "File::Find::Closures", which you can download from
       CPAN, provides many ready-to-use subroutines that you can
       use with "File::Find".

       The "File::Finder", which you can download from CPAN, can
       help you create the callback subroutine using something
       closer to the syntax of the "find" command-line utility:

               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
       "rmtree" which can take care of all of the hard work for
       you:

               use File::Path qw(rmtree);

               rmtree( \@directories, 0, 0 );

       The first argument to "rmtree" is either a string



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       representing a directory path or an array reference. The
       second argument controls progress messages, and the third
       argument controls the handling of files you don't have
       permissions to delete. See the "File::Path" module for the
       details.

   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.  =head1
       REVISION

       Revision: $Revision: 1.11 $

       Date: $Date: 2009/10/12 18:24:47 $

       See perlfaq for source control details and availability.

AUTHOR AND COPYRIGHT
       Copyright (c) 1997-2009 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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