MirOS Manual: perlfaq5(1)


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?

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

     The C standard I/O library (stdio) normally buffers charac-
     ters sent to devices so that there isn't a system call for
     each byte. In most stdio implementations, the type of output
     buffering and the size of the buffer varies according to the
     type of device. Perl's print() and write() functions nor-
     mally buffer output, while syswrite() bypasses buffering all
     together.

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

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

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

     Some idioms can handle this in a single statement:

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

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

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

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

     or IO::Socket:

         use IO::Socket;               # this one is kinda a pipe?
             my $sock = IO::Socket::INET->new( 'www.example.com:80' );

         $sock->autoflush();

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

     Use the Tie::File module, which is included in the standard
     distribution since Perl 5.8.0.

     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:

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          # ...
          {
             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 direc-
     tory, 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: $!";

     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.

     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.  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

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       $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);
                 }
                 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:

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         # 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 han-
     dles 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 );

     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.

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     How can I use a filehandle indirectly?

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

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

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

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

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

         ($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.)

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         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 sim-
     ple scalar variables, not expressions or subscripts of
     hashes or arrays, can be used with built-ins like "print",
     "printf", or the diamond operator.  Using something other
     than a simple scalar variable as a filehandle is illegal and
     won't even compile:

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

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

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

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

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

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

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     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 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:
            \G\d{3}           # after the last group, get three digits
            (?=\d)            # but they have to have more digits after them.
        )/$1,/xg;

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

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

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     more portable glob functionality.

     Within Perl, you may use this directly:

             $filename =~ s{
               ^ ~             # find a leading tilde
               (               # save this in $1
                   [^/]        # a non-slash character
                         *     # repeated 0 or more times (0 means me)
               )
             }{
               $1
                   ? (getpwnam($1))[7]
                   : ( $ENV{HOME} || $ENV{LOGDIR} )
             }ex;

     How come when I open 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:

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

     To open file for appending, create if necessary:

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

     To open file for appending, file must exist:

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

     To open file for update, file must exist:

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

     To open file for update, create file if necessary:

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

     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.

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     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 open() which
     does not treat any charcters 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

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     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 informa-
         tion on building Perl to do this.

         Two potentially non-obvious but traditional flock seman-
         tics 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(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: $!";

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     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 subdesirable.

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

     Didn't anyone ever tell you web-page hit counters were use-
     less? 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) );

     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 imple-
     ment flock() (and that's not really much of a restriction)
     then that is what you should do.

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

     If you know you are only writing code to run on an OS and
     filesystem that does implement append mode correctly (a
     local filesystem on a modern Unix for example), and you keep
     the file in block-buffered mode and you write less than one
     buffer-full of output between each manual flushing of the

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     buffer then each bufferload is almost guaranteed to be writ-
     ten to the end of the file in one chunk without getting
     intermingled with anyone else's output. You can also use the
     syswrite() function which is simply a wrapper around your
     systems write(2) system call.

     There is still a small theoretical chance that a signal will
     interrupt the system level write() operation before comple-
     tion.  There is also a possibility that some STDIO implemen-
     tations 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.

     How do I randomly update a binary file?

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

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

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

         $RECSIZE = 220; # size of record, in bytes
         $recno   = 37;  # which record to update
         open(FH, "+<somewhere") || die "can't update somewhere: $!";
         seek(FH, $recno * $RECSIZE, 0);
         read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
         # munge the record
         seek(FH, -$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 float-
     ing 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 func-
     tion, then use localtime(), gmtime(), or POSIX::strftime()
     to convert this into human-readable form.

     Here's an example:

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         $write_secs = (stat($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 perl-
     func. By way of example, here's a little program that copies
     the read and write times from its first argument to all the
     rest of them.

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

     Error checking is, 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.

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

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

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

             use File::Slurp;

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

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

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

     This is tremendously more efficient than reading the entire
     file into memory as an array of lines and then 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>;

     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> };

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     For ordinary files you can also use the read function.

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

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

     How can I read in a file by paragraphs?

     Use the $/ variable (see perlvar for details).  You can
     either set it to "" to eliminate empty paragraphs
     ("abc\n\n\n\ndef", for instance, gets treated as two para-
     graphs and not three), or "\n\n" to accept empty 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 filehan-
     dles, but it won't (easily) work on a terminal device.  For
     STDIN, either use the Term::ReadKey module from CPAN or use
     the sample code in "getc" in perlfunc.

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

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

         BEGIN {
             use POSIX qw(:termios_h);

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

             $fd_stdin = fileno(STDIN);

             $term     = POSIX::Termios->new();
             $term->getattr($fd_stdin);
             $oterm     = $term->getlflag();

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             $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 sys-
     tems as well.

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

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

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

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

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     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);
         }
         ^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, mean-
     ing that sockets, pipes, and tty devices work, but not
     files.

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     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 condi-
     tion on the handle, so that the next <GWFILE> makes Perl try
     again to read something.

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

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

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

     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(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.

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     How do I close a file descriptor by number?

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

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

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

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

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

     Whoops!  You just put a tab and a formfeed into that
     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 for-
     ward slashes. Since all DOS and Windows versions since some-
     thing 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?

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     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 hap-
     pen 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 permis-
     sions of the file govern whether you're allowed to.

     How do I select a random line from a file?

     Here's an algorithm from the Camel Book:

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

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

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

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

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

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

     Saying

         print "@lines\n";

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

         little fluffy clouds

     but if each element of @lines was a line of text, ending a
     newline character "("little\n", "fluffy\n", "clouds\n")"
     then it would print:

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         little
          fluffy
          clouds

     If your array contains lines, just print them:

         print @lines;

AUTHOR AND COPYRIGHT

     Copyright (c) 1997-2006 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 pro-
     grams for fun or for profit as you see fit.  A simple com-
     ment in the code giving credit to the FAQ would be courteous
     but is not required.

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