=head1 NAME
-perlfaq5 - Files and Formats ($Revision: 1.20 $, $Date: 1997/03/19 17:24:51 $)
+perlfaq5 - Files and Formats
=head1 DESCRIPTION
This section deals with I/O and the "f" issues: filehandles, flushing,
formats, and footers.
-=head2 How do I flush/unbuffer a filehandle? Why must I do this?
+=head2 How do I flush/unbuffer an output filehandle? Why must I do this?
+X<flush> X<buffer> X<unbuffer> X<autoflush>
-The C standard I/O library (stdio) normally buffers characters sent to
-devices. This is done for efficiency reasons, so that there isn't a
-system call for each byte. Any time you use print() or write() in
-Perl, you go though this buffering. syswrite() circumvents stdio and
-buffering.
+(contributed by brian d foy)
-In most stdio implementations, the type of buffering and the size of
-the buffer varies according to the type of device. Disk files are block
-buffered, often with a buffer size of more than 2k. Pipes and sockets
-are often buffered with a buffer size between 1/2 and 2k. Serial devices
-(e.g. modems, terminals) are normally line-buffered, and stdio sends
-the entire line when it gets the newline.
+You might like to read Mark Jason Dominus's "Suffering From Buffering"
+at http://perl.plover.com/FAQs/Buffering.html .
-Perl does not support truly unbuffered output (except insofar as you can
-C<syswrite(OUT, $char, 1)>). What it does instead support is "command
-buffering", in which a physical write is performed after every output
-command. This isn't as hard on your system as unbuffering, but does
-get the output where you want it when you want it.
+Perl normally buffers output so it doesn't make a system call for every
+bit of output. By saving up output, it makes fewer expensive system calls.
+For instance, in this little bit of code, you want to print a dot to the
+screen for every line you process to watch the progress of your program.
+Instead of seeing a dot for every line, Perl buffers the output and you
+have a long wait before you see a row of 50 dots all at once:
-If you expect characters to get to your device when you print them there,
-you'll want to autoflush its handle, as in the older:
+ # long wait, then row of dots all at once
+ while( <> ) {
+ print ".";
+ print "\n" unless ++$count % 50;
- use FileHandle;
- open(DEV, "<+/dev/tty"); # ceci n'est pas une pipe
- DEV->autoflush(1);
+ #... expensive line processing operations
+ }
-or the newer IO::* modules:
+To get around this, you have to unbuffer the output filehandle, in this
+case, C<STDOUT>. You can set the special variable C<$|> to a true value
+(mnemonic: making your filehandles "piping hot"):
- use IO::Handle;
- open(DEV, ">/dev/printer"); # but is this?
- DEV->autoflush(1);
+ $|++;
-or even this:
+ # dot shown immediately
+ while( <> ) {
+ print ".";
+ print "\n" unless ++$count % 50;
- use IO::Socket; # this one is kinda a pipe?
- $sock = IO::Socket::INET->new(PeerAddr => 'www.perl.com',
- PeerPort => 'http(80)',
- Proto => 'tcp');
- die "$!" unless $sock;
+ #... expensive line processing operations
+ }
- $sock->autoflush();
- $sock->print("GET /\015\012");
- $document = join('', $sock->getlines());
- print "DOC IS: $document\n";
+The C<$|> 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):
-Note the hardcoded carriage return and newline in their octal
-equivalents. This is the ONLY way (currently) to assure a proper
-flush on all platforms, including Macintosh.
+ {
+ my $previous_default = select(STDOUT); # save previous default
+ $|++; # autoflush STDOUT
+ select(STDERR);
+ $|++; # autoflush STDERR, to be sure
+ select($previous_default); # restore previous default
+ }
-You can use select() and the C<$|> variable to control autoflushing
-(see L<perlvar/$|> and L<perlfunc/select>):
+ # now should alternate . and +
+ while( 1 )
+ {
+ sleep 1;
+ print STDOUT ".";
+ print STDERR "+";
+ print STDOUT "\n" unless ++$count % 25;
+ }
- $oldh = select(DEV);
- $| = 1;
- select($oldh);
+Besides the C<$|> special variable, you can use C<binmode> to give
+your filehandle a C<:unix> layer, which is unbuffered:
-You'll also see code that does this without a temporary variable, as in
+ binmode( STDOUT, ":unix" );
- select((select(DEV), $| = 1)[0]);
+ while( 1 ) {
+ sleep 1;
+ print ".";
+ print "\n" unless ++$count % 50;
+ }
-=head2 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?
+For more information on output layers, see the entries for C<binmode>
+and C<open> in L<perlfunc>, and the C<PerlIO> module documentation.
-Although humans have an easy time thinking of a text file as being a
-sequence of lines that operates much like a stack of playing cards --
-or punch cards -- computers usually see the text file as a sequence of
-bytes. In general, there's no direct way for Perl to seek to a
-particular line of a file, insert text into a file, or remove text
-from a file.
+If you are using C<IO::Handle> or one of its subclasses, you can
+call the C<autoflush> method to change the settings of the
+filehandle:
-(There are exceptions in special circumstances. Replacing a sequence
-of bytes with another sequence of the same length is one. Another is
-using the C<$DB_RECNO> array bindings as documented in L<DB_File>.
-Yet another is manipulating files with all lines the same length.)
+ use IO::Handle;
+ open my( $io_fh ), ">", "output.txt";
+ $io_fh->autoflush(1);
-The general solution is to create a temporary copy of the text file with
-the changes you want, then copy that over the original.
+The C<IO::Handle> objects also have a C<flush> method. You can flush
+the buffer any time you want without auto-buffering
- $old = $file;
- $new = "$file.tmp.$$";
- $bak = "$file.bak";
+ $io_fh->flush;
- open(OLD, "< $old") or die "can't open $old: $!";
- open(NEW, "> $new") or die "can't open $new: $!";
+=head2 How do I change, delete, or insert a line in a file, or append to the beginning of a file?
+X<file, editing>
- # Correct typos, preserving case
- while (<OLD>) {
- s/\b(p)earl\b/${1}erl/i;
- (print NEW $_) or die "can't write to $new: $!";
- }
+(contributed by brian d foy)
- close(OLD) or die "can't close $old: $!";
- close(NEW) or die "can't close $new: $!";
+The basic idea of inserting, changing, or deleting a line from a text
+file involves reading and printing the file to the point you want to
+make the change, making the change, then reading and printing the rest
+of the file. Perl doesn't provide random access to lines (especially
+since the record input separator, C<$/>, is mutable), although modules
+such as C<Tie::File> can fake it.
- rename($old, $bak) or die "can't rename $old to $bak: $!";
- rename($new, $old) or die "can't rename $new to $old: $!";
+A Perl program to do these tasks takes the basic form of opening a
+file, printing its lines, then closing the file:
-Perl can do this sort of thing for you automatically with the C<-i>
-command line switch or the closely-related C<$^I> variable (see
-L<perlrun> for more details). Note that
-C<-i> may require a suffix on some non-Unix systems; see the
-platform-specific documentation that came with your port.
-
- # Renumber a series of tests from the command line
- perl -pi -e 's/(^\s+test\s+)\d+/ $1 . ++$count /e' t/op/taint.t
+ open my $in, '<', $file or die "Can't read old file: $!";
+ open my $out, '>', "$file.new" or die "Can't write new file: $!";
- # form a script
- local($^I, @ARGV) = ('.bak', glob("*.c"));
- while (<>) {
- if ($. == 1) {
- print "This line should appear at the top of each file\n";
- }
- s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
- print;
- close ARGV if eof; # Reset $.
- }
+ 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 C<while> loop. In this case, the code finds all lowercased
+versions of "perl" and uppercases them. The happens for every line, so
+be sure that you're supposed to do that on every line!
+
+ open my $in, '<', $file or die "Can't read old file: $!";
+ open my $out, '>', "$file.new" or die "Can't write new file: $!";
+
+ print $out "# Add this line to the top\n";
+
+ while( <$in> )
+ {
+ s/\b(perl)\b/Perl/g;
+ print $out $_;
+ }
+
+ close $out;
+
+To change only a particular line, the input line number, C<$.>, 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 C<next> in this example
+skips comment lines, and the C<last> stops all processing once it
+encounters either C<__END__> or C<__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 C<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
+
+ print $out @lines;
+
+Modules such as C<File::Slurp> and C<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 F<inFile.txt>, overwriting
+the file with the new contents. With the C<-p> switch, Perl wraps a
+C<while> loop around the code you specify with C<-e>, and C<-i> turns
+on in-place editing. The current line is in C<$_>. With C<-p>, Perl
+automatically prints the value of C<$_> at the end of the loop. See
+L<perlrun> for more details.
+
+ perl -pi -e 's/Fred/Barney/' inFile.txt
+
+To make a backup of C<inFile.txt>, give C<-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 C<$.>, the
+input line number, then only perform the operation when the test
+passes:
-If you need to seek to an arbitrary line of a file that changes
-infrequently, you could build up an index of byte positions of where
-the line ends are in the file. If the file is large, an index of
-every tenth or hundredth line end would allow you to seek and read
-fairly efficiently. If the file is sorted, try the look.pl library
-(part of the standard perl distribution).
+ perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt
-In the unique case of deleting lines at the end of a file, you
-can use tell() and truncate(). The following code snippet deletes
-the last line of a file without making a copy or reading the
-whole file into memory:
+To add lines before a certain line, you can add a line (or lines!)
+before Perl prints C<$_>:
- open (FH, "+< $file");
- while ( <FH> ) { $addr = tell(FH) unless eof(FH) }
- truncate(FH, $addr);
+ perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt
-Error checking is left as an exercise for the reader.
+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 C<-n> switch.
+It's just like C<-p> except that it doesn't print C<$_> at the end of
+the loop, so you have to do that yourself. In this case, print C<$_>
+first, then print the line that you want to add.
+
+ perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt
+
+To delete lines, only print the ones that you want.
+
+ perl -ni -e 'print unless /d/' inFile.txt
+
+ ... or ...
+
+ perl -pi -e 'next unless /d/' inFile.txt
=head2 How do I count the number of lines in a file?
+X<file, counting lines> X<lines> X<line>
+
+(contributed by brian d foy)
+
+Conceptually, the easiest way to count the lines in a file is to
+simply read them and count them:
+
+ my $count = 0;
+ while( <$fh> ) { $count++; }
+
+You don't really have to count them yourself, though, since Perl
+already does that with the C<$.> variable, which is the current line
+number from the last filehandle read:
+
+ 1 while( <$fh> );
+ my $count = $.;
+
+If you want to use C<$.>, you can reduce it to a simple one-liner,
+like one of these:
+
+ % perl -lne '} print $.; {' file
+
+ % perl -lne 'END { print $. }' file
+
+Those can be rather inefficient though. If they aren't fast enough for
+you, you might just read chunks of data and count the number of
+newlines:
+
+ my $lines = 0;
+ open my($fh), '<:raw', $filename or die "Can't open $filename: $!";
+ while( sysread $fh, $buffer, 4096 ) {
+ $lines += ( $buffer =~ tr/\n// );
+ }
+ close FILE;
+
+However, that doesn't work if the line ending isn't a newline. You
+might change that C<tr///> to a C<s///> so you can count the number of
+times the input record separator, C<$/>, shows up:
+
+ my $lines = 0;
+ open my($fh), '<:raw', $filename or die "Can't open $filename: $!";
+ while( sysread $fh, $buffer, 4096 ) {
+ $lines += ( $buffer =~ s|$/||g; );
+ }
+ close FILE;
+
+If you don't mind shelling out, the C<wc> command is usually the
+fastest, even with the extra interprocess overhead. Ensure that you
+have an untainted filename though:
+
+ #!perl -T
+
+ $ENV{PATH} = undef;
+
+ my $lines;
+ if( $filename =~ /^([0-9a-z_.]+)\z/ ) {
+ $lines = `/usr/bin/wc -l $1`
+ chomp $lines;
+ }
+
+=head2 How do I delete the last N lines from a file?
+X<lines> X<file>
+
+(contributed by brian d foy)
+
+The easiest conceptual solution is to count the lines in the
+file then start at the beginning and print the number of lines
+(minus the last N) to a new file.
+
+Most often, the real question is how you can delete the last N lines
+without making more than one pass over the file, or how to do it
+without a lot of copying. The easy concept is the hard reality when
+you might have millions of lines in your file.
+
+One trick is to use C<File::ReadBackwards>, which starts at the end of
+the file. That module provides an object that wraps the real filehandle
+to make it easy for you to move around the file. Once you get to the
+spot you need, you can get the actual filehandle and work with it as
+normal. In this case, you get the file position at the end of the last
+line you want to keep and truncate the file to that point:
+
+ use File::ReadBackwards;
+
+ my $filename = 'test.txt';
+ my $Lines_to_truncate = 2;
+
+ my $bw = File::ReadBackwards->new( $filename )
+ or die "Could not read backwards in [$filename]: $!";
+
+ my $lines_from_end = 0;
+ until( $bw->eof or $lines_from_end == $Lines_to_truncate )
+ {
+ print "Got: ", $bw->readline;
+ $lines_from_end++;
+ }
+
+ truncate( $filename, $bw->tell );
-One fairly efficient way is to count newlines in the file. The
-following program uses a feature of tr///, as documented in L<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.
+The C<File::ReadBackwards> module also has the advantage of setting
+the input record separator to a regular expression.
- $lines = 0;
- open(FILE, $filename) or die "Can't open `$filename': $!";
- while (sysread FILE, $buffer, 4096) {
- $lines += ($buffer =~ tr/\n//);
- }
- close FILE;
+You can also use the C<Tie::File> module which lets you access
+the lines through a tied array. You can use normal array operations
+to modify your file, including setting the last index and using
+C<splice>.
+
+=head2 How can I use Perl's C<-i> option from within a program?
+X<-i> X<in-place>
+
+C<-i> sets the value of Perl's C<$^I> variable, which in turn affects
+the behavior of C<< <> >>; see L<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<.c> files in the current directory,
+leaving a backup of the original data from each file in a new
+C<.c.orig> file.
+
+=head2 How can I copy a file?
+X<copy> X<file, copy> X<File::Copy>
+
+(contributed by brian d foy)
+
+Use the C<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 C<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.
=head2 How do I make a temporary file name?
+X<file, temporary>
+
+If you don't need to know the name of the file, you can use C<open()>
+with C<undef> in place of the file name. In Perl 5.8 or later, the
+C<open()> function creates an anonymous temporary file:
-Use the process ID and/or the current time-value. If you need to have
-many temporary files in one process, use a counter:
+ open my $tmp, '+>', undef or die $!;
+
+Otherwise, you can use the File::Temp module.
+
+ use File::Temp qw/ tempfile tempdir /;
+
+ my $dir = tempdir( CLEANUP => 1 );
+ ($fh, $filename) = tempfile( DIR => $dir );
+
+ # or if you don't need to know the filename
+
+ my $fh = tempfile( DIR => $dir );
+
+The File::Temp has been a standard module since Perl 5.6.1. If you
+don't have a modern enough Perl installed, use the C<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:
- BEGIN {
use IO::File;
+ my $fh = IO::File->new_tmpfile()
+ or die "Unable to make new temporary file: $!";
+
+If you're committed to creating a temporary file by hand, use the
+process ID and/or the current time-value. If you need to have many
+temporary files in one process, use a counter:
+
+ BEGIN {
use Fcntl;
- my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP} || $ENV{TEMP};
- my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
+ my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
+ my $base_name = sprintf "%s/%d-%d-0000", $temp_dir, $$, time;
+
sub temp_file {
- my $fh = undef;
- my $count = 0;
- until (defined($fh) || $count > 100) {
- $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
- $fh = IO::File->new($base_name, O_WRONLY|O_EXCL|O_CREAT, 0644)
- }
- if (defined($fh)) {
- return ($fh, $base_name);
- } else {
- return ();
- }
- }
- }
+ my $fh;
+ my $count = 0;
+ until( defined(fileno($fh)) || $count++ > 100 ) {
+ $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
+ # O_EXCL is required for security reasons.
+ sysopen $fh, $base_name, O_WRONLY|O_EXCL|O_CREAT;
+ }
+
+ if( defined fileno($fh) ) {
+ return ($fh, $base_name);
+ }
+ else {
+ return ();
+ }
+ }
-Or you could simply use IO::Handle::new_tmpfile.
+ }
=head2 How can I manipulate fixed-record-length files?
-
-The most efficient way is using pack() and unpack(). This is faster
-than using substr(). Here is a sample chunk of code to break up and
-put back together again some fixed-format input lines, in this case
-from the output of a normal, Berkeley-style ps:
-
- # sample input line:
- # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
- $PS_T = 'A6 A4 A7 A5 A*';
- open(PS, "ps|");
- $_ = <PS>; print;
- while (<PS>) {
- ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
- for $var (qw!pid tt stat time command!) {
- print "$var: <$$var>\n";
+X<fixed-length> X<file, fixed-length records>
+
+The most efficient way is using L<pack()|perlfunc/"pack"> and
+L<unpack()|perlfunc/"unpack">. This is faster than using
+L<substr()|perlfunc/"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, $pid, $tt, $stat, $time, $command),
- "\n";
- }
+ print 'line=', pack($PS_T, @process{@fields} ), "\n";
+ }
+
+We've used a hash slice in order to easily handle the fields of each row.
+Storing the keys in an array makes it easy to operate on them as a
+group or loop over them with C<for>. It also avoids polluting the program
+with global variables and using symbolic references.
=head2 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?
+X<filehandle, local> X<filehandle, passing> X<filehandle, reference>
+
+As of perl5.6, open() autovivifies file and directory handles
+as references if you pass it an uninitialized scalar variable.
+You can then pass these references just like any other scalar,
+and use them in the place of named handles.
+
+ open my $fh, $file_name;
+
+ open local $fh, $file_name;
+
+ print $fh "Hello World!\n";
+
+ process_file( $fh );
+
+If you like, you can store these filehandles in an array or a hash.
+If you access them directly, they aren't simple scalars and you
+need to give C<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 );
-You may have some success with typeglobs, as we always had to use
-in days of old:
+ sub process_typeglob { local *FH = shift; print FH "Typeglob!" }
+ sub process_reference { local $fh = shift; print $fh "Reference!" }
- local(*FH);
+If you want to create many anonymous handles, you should
+check out the Symbol or IO::Handle modules.
-But while still supported, that isn't the best to go about getting
-local filehandles. Typeglobs have their drawbacks. You may well want
-to use the C<FileHandle> module, which creates new filehandles for you
-(see L<FileHandle>):
+=head2 How can I use a filehandle indirectly?
+X<filehandle, indirect>
- use FileHandle;
- sub findme {
- my $fh = FileHandle->new();
- open($fh, "</etc/hosts") or die "no /etc/hosts: $!";
- while (<$fh>) {
- print if /\b127\.(0\.0\.)?1\b/;
+An indirect filehandle is the use of 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 C<new> method from one of the IO::* modules to
+create an anonymous filehandle and store that in a scalar variable.
+
+ use IO::Handle; # 5.004 or higher
+ my $fh = IO::Handle->new();
+
+Then use any of those as you would a normal filehandle. Anywhere that
+Perl is expecting a filehandle, an indirect filehandle may be used
+instead. An indirect filehandle is just a scalar variable that contains
+a filehandle. Functions like C<print>, C<open>, C<seek>, or
+the C<< <FH> >> diamond operator will accept either a named filehandle
+or a scalar variable containing one:
+
+ ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
+ print $ofh "Type it: ";
+ my $got = <$ifh>
+ print $efh "What was that: $got";
+
+If you're passing a filehandle to a function, you can write
+the function in two ways:
+
+ sub accept_fh {
+ my $fh = shift;
+ print $fh "Sending to indirect filehandle\n";
}
- # $fh automatically closes/disappears here
- }
-Internally, Perl believes filehandles to be of class IO::Handle. You
-may use that module directly if you'd like (see L<IO::Handle>), or
-one of its more specific derived classes.
+Or it can localize a typeglob and use the filehandle directly:
+
+ sub accept_fh {
+ local *FH = shift;
+ print FH "Sending to localized filehandle\n";
+ }
+
+Both styles work with either objects or typeglobs of real filehandles.
+(They might also work with strings under some circumstances, but this
+is risky.)
+
+ accept_fh(*STDOUT);
+ accept_fh($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 C<print>, C<printf>, or the diamond operator. Using
+something other than a simple scalar variable as a filehandle is
+illegal and won't even compile:
+
+ my @fd = (*STDIN, *STDOUT, *STDERR);
+ print $fd[1] "Type it: "; # WRONG
+ my $got = <$fd[0]> # WRONG
+ print $fd[2] "What was that: $got"; # WRONG
+
+With C<print> and C<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:
+
+ my $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 C<print> and C<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 C<readline> to read a record just
+as C<< <> >> 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.
=head2 How can I set up a footer format to be used with write()?
+X<footer>
There's no builtin way to do this, but L<perlform> has a couple of
techniques to make it possible for the intrepid hacker.
=head2 How can I write() into a string?
+X<write, into a string>
+
+(contributed by brian d foy)
+
+If you want to C<write> into a string, you just have to <open> a
+filehandle to a string, which Perl has been able to do since Perl 5.6:
+
+ open FH, '>', \my $string;
+ write( FH );
+
+Since you want to be a good programmer, you probably want to use a lexical
+filehandle, even though formats are designed to work with bareword filehandles
+since the default format names take the filehandle name. However, you can
+control this with some Perl special per-filehandle variables: C<$^>, which
+names the top-of-page format, and C<$~> which shows the line format. You have
+to change the default filehandle to set these variables:
+
+ open my($fh), '>', \my $string;
+
+ { # set per-filehandle variables
+ my $old_fh = select( $fh );
+ $~ = 'ANIMAL';
+ $^ = 'ANIMAL_TOP';
+ select( $old_fh );
+ }
+
+ format ANIMAL_TOP =
+ ID Type Name
+ .
+
+ format ANIMAL =
+ @## @<<< @<<<<<<<<<<<<<<
+ $id, $type, $name
+ .
+
+Although write can work with lexical or package variables, whatever variables
+you use have to scope in the format. That most likely means you'll want to
+localize some package variables:
+
+ {
+ local( $id, $type, $name ) = qw( 12 cat Buster );
+ write( $fh );
+ }
+
+ print $string;
-See L<perlform> for an swrite() function.
+There are also some tricks that you can play with C<formline> and the
+accumulator variable C<$^A>, but you lose a lot of the value of formats
+since C<formline> won't handle paging and so on. You end up reimplementing
+formats when you use them.
+
+=head2 How can I open a filehandle to a string?
+X<string> X<open> X<IO::String> X<filehandle>
+
+(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 C<IO::String> module provides similar
+functionality.
=head2 How can I output my numbers with commas added?
+X<number, commify>
+
+(contributed by brian d foy and Benjamin Goldberg)
-This one will do it for you:
+You can use L<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).
- sub commify {
- local $_ = shift;
- 1 while s/^(-?\d+)(\d{3})/$1,$2/;
- return $_;
- }
+This subroutine will add commas to your number:
- $n = 23659019423.2331;
- print "GOT: ", commify($n), "\n";
+ sub commify {
+ local $_ = shift;
+ 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
+ return $_;
+ }
- GOT: 23,659,019,423.2331
+This regex from Benjamin Goldberg will add commas to numbers:
-You can't just:
+ s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
- s/^(-?\d+)(\d{3})/$1,$2/g;
+It is easier to see with comments:
-because you have to put the comma in and then recalculate your
-position.
+ 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;
=head2 How can I translate tildes (~) in a filename?
+X<tilde> X<tilde expansion>
-Use the E<lt>E<gt> (glob()) operator, documented in L<perlfunc>. This
-requires that you have a shell installed that groks tildes, meaning
-csh or tcsh or (some versions of) ksh, and thus may have portability
-problems. The Glob::KGlob module (available from CPAN) gives more
+Use the E<lt>E<gt> (C<glob()>) operator, documented in L<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 C<File::KGlob> module (available from CPAN) gives more
portable glob functionality.
Within Perl, you may use this directly:
: ( $ENV{HOME} || $ENV{LOGDIR} )
}ex;
-=head2 How come when I open the file read-write it wipes it out?
+=head2 How come when I open a file read-write it wipes it out?
+X<clobber> X<read-write> X<clobbering> X<truncate> X<truncating>
-Because you're using something like this, which truncates the file and
+Because you're using something like this, which truncates the file
I<then> gives you read-write access:
- open(FH, "+> /path/name"); # WRONG
+ open my $fh, '+>', '/path/name'; # WRONG (almost always)
+
+Whoops. You should instead use this, which will fail if the file
+doesn't exist:
+
+ open my $fh, '+<', '/path/name'; # open for update
+
+Using ">" always clobbers or creates. Using "<" never does
+either. The "+" doesn't change this.
+
+Here are examples of many kinds of file opens. Those using C<sysopen>
+all assume that you've pulled in the constants from C<Fcntl>:
+
+ use Fcntl;
+
+To open file for reading:
+
+ open my $fh, '<', $path or die $!;
+ sysopen my $fh, $path, O_RDONLY or die $!;
+
+To open file for writing, create new file if needed or else truncate old file:
+
+ open my $fh, '>', $path or die $!;
+ sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT or die $!;
+ sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666 or die $!;
-Whoops. You should instead use this, which will fail if the file
-doesn't exist.
+To open file for writing, create new file, file must not exist:
- open(FH, "+< /path/name"); # open for update
+ sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT or die $!;
+ sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT, 0666 or die $!;
-If this is an issue, try:
+To open file for appending, create if necessary:
- sysopen(FH, "/path/name", O_RDWR|O_CREAT, 0644);
+ open my $fh, '>>' $path or die $!;
+ sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT or die $!;
+ sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT, 0666 or die $!;
-Error checking is left as an exercise for the reader.
+To open file for appending, file must exist:
-=head2 Why do I sometimes get an "Argument list too long" when I use <*>?
+ sysopen my $fh, $path, O_WRONLY|O_APPEND or die $!;
-The C<E<lt>E<gt>> operator performs a globbing operation (see above).
-By default glob() forks csh(1) to do the actual glob expansion, but
+To open file for update, file must exist:
+
+ open my $fh, '+<', $path or die $!;
+ sysopen my $fh, $path, O_RDWR or die $!;
+
+To open file for update, create file if necessary:
+
+ sysopen my $fh, $path, O_RDWR|O_CREAT or die $!;
+ sysopen my $fh, $path, O_RDWR|O_CREAT, 0666 or die $!;
+
+To open file for update, file must not exist:
+
+ sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT or die $!;
+ sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT, 0666 or die $!;
+
+To open a file without blocking, creating if necessary:
+
+ sysopen my $fh, '/foo/somefile', O_WRONLY|O_NDELAY|O_CREAT
+ or die "can't open /foo/somefile: $!":
+
+Be warned that neither creation nor deletion of files is guaranteed to
+be an atomic operation over NFS. That is, two processes might both
+successfully create or unlink the same file! Therefore O_EXCL
+isn't as exclusive as you might wish.
+
+See also L<perlopentut>.
+
+=head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
+X<argument list too long>
+
+The C<< <> >> 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
-C<Argument list too long>. People who installed tcsh as csh won't
+C<Argument list too long>. People who installed tcsh as csh won't
have this problem, but their users may be surprised by it.
-To get around this, either do the glob yourself with C<Dirhandle>s and
-patterns, or use a module like Glob::KGlob, one that doesn't use the
-shell to do globbing.
+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.
=head2 Is there a leak/bug in glob()?
+X<glob>
-Due to the current implementation on some operating systems, when you
-use the glob() function or its angle-bracket alias in a scalar
-context, you may cause a leak and/or unpredictable behavior. It's
-best therefore to use glob() only in list context.
+(contributed by brian d foy)
-=head2 How can I open a file with a leading "E<gt>" or trailing blanks?
+Starting with Perl 5.6.0, C<glob> is implemented internally rather
+than relying on an external resource. As such, memory issues with
+C<glob> aren't a problem in modern perls.
-Normally perl ignores trailing blanks in filenames, and interprets
-certain leading characters (or a trailing "|") to mean something
-special. To avoid this, you might want to use a routine like this.
-It makes incomplete pathnames into explicit relative ones, and tacks a
-trailing null byte on the name to make perl leave it alone:
+=head2 How can I open a file with a leading ">" or trailing blanks?
+X<filename, special characters>
- sub safe_filename {
- local $_ = shift;
- return m#^/#
- ? "$_\0"
- : "./$_\0";
- }
+(contributed by Brian McCauley)
- $fn = safe_filename("<<<something really wicked ");
- open(FH, "> $fn") or "couldn't open $fn: $!";
+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.
-You could also use the sysopen() function (see L<perlfunc/sysopen>).
+Unless you have a particular reason to use the two-argument form you
+should use the three-argument form of open() which does not treat any
+characters in the filename as special.
+
+ open my $fh, "<", " file "; # filename is " file "
+ open my $fh, ">", ">file"; # filename is ">file"
=head2 How can I reliably rename a file?
+X<rename> X<mv> X<move> X<file, rename>
-Well, usually you just use Perl's rename() function. But that may
-not work everywhere, in particular, renaming files across file systems.
-If your operating system supports a mv(1) program or its moral equivalent,
-this works:
+If your operating system supports a proper mv(1) utility or its
+functional equivalent, this works:
- rename($old, $new) or system("mv", $old, $new);
+ rename($old, $new) or system("mv", $old, $new);
-It may be more compelling to use the File::Copy module instead. You
-just copy to the new file to the new name (checking return values),
-then delete the old one. This isn't really the same semantics as a
-real rename(), though, which preserves metainformation like
+It may be more portable to use the C<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 C<rename()>, which preserves meta-information like
permissions, timestamps, inode info, etc.
=head2 How can I lock a file?
+X<lock> X<file, lock> X<flock>
Perl's builtin flock() function (see L<perlfunc> for details) will call
flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
=item 3
-Some versions of flock() can't lock files over a network (e.g. on NFS
-file systems), so you'd need to force the use of fcntl(2) when you
-build Perl. See the flock entry of L<perlfunc>, and the F<INSTALL>
-file in the source distribution for information on building Perl to do
-this.
+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 L<perlfunc>
+and the F<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
+I<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
+L<perlopentut/"File Locking"> if you have it (new for 5.6).
=back
-The CPAN module File::Lock offers similar functionality and (if you
-have dynamic loading) won't require you to rebuild perl if your
-flock() can't lock network files.
-
-=head2 What can't I just open(FH, ">file.lock")?
+=head2 Why can't I just open(FH, "E<gt>file.lock")?
+X<lock, lockfile race condition>
A common bit of code B<NOT TO USE> is this:
- sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
- open(LCK, "> file.lock"); # THIS BROKEN CODE
+ sleep(3) while -e 'file.lock'; # PLEASE DO NOT USE
+ open my $lock, '>', 'file.lock'; # THIS BROKEN CODE
This is a classic race condition: you take two steps to do something
-which must be done in one. That's why computer hardware provides an
-atomic test-and-set instruction. In theory, this "ought" to work:
+which must be done in one. That's why computer hardware provides an
+atomic test-and-set instruction. In theory, this "ought" to work:
- sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT, 0644)
- or die "can't open file.lock: $!":
+ sysopen my $fh, "file.lock", O_WRONLY|O_EXCL|O_CREAT
+ or die "can't open file.lock: $!";
except that lamentably, file creation (and deletion) is not atomic
over NFS, so this won't work (at least, not every time) over the net.
-Various schemes involving involving link() have been suggested, but
-these tend to involve busy-wait, which is also subdesirable.
-
-=head2 I still don't get locking. I just want to increment the number
-in the file. How can I do this?
-
-Didn't anyone ever tell you web page hit counters were useless?
-
-Anyway, this is what to do:
-
- use Fcntl;
- sysopen(FH, "numfile", O_RDWR|O_CREAT, 0644) or die "can't open numfile: $!";
- flock(FH, 2) or die "can't flock numfile: $!";
- $num = <FH> || 0;
- seek(FH, 0, 0) or die "can't rewind numfile: $!";
- truncate(FH, 0) or die "can't truncate numfile: $!";
- (print FH $num+1, "\n") or die "can't write numfile: $!";
- # DO NOT UNLOCK THIS UNTIL YOU CLOSE
- close FH or die "can't close numfile: $!";
-
-Here's a much better web page hit counter:
-
- $hits = int( (time() - 850_000_000) / rand(1_000) );
-
-If the count doesn't impress your friends, then the code might. :-)
+Various schemes involving link() have been suggested, but
+these tend to involve busy-wait, which is also less than desirable.
+
+=head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
+X<counter> X<file, counter>
+
+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 my $fh, "numfile", O_RDWR|O_CREAT or die "can't open numfile: $!";
+ flock $fh, LOCK_EX or die "can't flock numfile: $!";
+ my $num = <$fh> || 0;
+ seek $fh, 0, 0 or die "can't rewind numfile: $!";
+ truncate $fh, 0 or die "can't truncate numfile: $!";
+ (print $fh $num+1, "\n") or die "can't write numfile: $!";
+ close $fh or die "can't close numfile: $!";
+
+Here's a much better web-page hit counter:
+
+ $hits = int( (time() - 850_000_000) / rand(1_000) );
+
+If the count doesn't impress your friends, then the code might. :-)
+
+=head2 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?
+X<append> X<file, append>
+
+If you are on a system that correctly implements C<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 C<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 C<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
+C<syswrite> function which is simply a wrapper around your system's
+C<write(2)> system call.
+
+There is still a small theoretical chance that a signal will interrupt
+the system-level C<write()> operation before completion. There is also
+a possibility that some STDIO implementations may call multiple system
+level C<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 C<:perlio> instead of your system's STDIO.
=head2 How do I randomly update a binary file?
+X<file, binary patch>
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
+ perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
However, if you have fixed sized records, then you might do something more
like this:
- $RECSIZE = 220; # size of record, in bytes
- $recno = 37; # which record to update
- open(FH, "+<somewhere") || die "can't update somewhere: $!";
- seek(FH, $recno * $RECSIZE, 0);
- read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
- # munge the record
- seek(FH, $recno * $RECSIZE, 0);
- print FH $record;
- close FH;
+ $RECSIZE = 220; # size of record, in bytes
+ $recno = 37; # which record to update
+ open my $fh, '+<', 'somewhere' or die "can't update somewhere: $!";
+ seek $fh, $recno * $RECSIZE, 0;
+ read $fh, $record, $RECSIZE == $RECSIZE or 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.
-
-Don't forget to set binmode() under MS-DOS-like platforms when operating
-on files that have anything other than straight text in them. See the
-docs on open() and on binmode() for more details.
+Don't forget them or you'll be quite sorry.
=head2 How do I get a file's timestamp in perl?
+X<timestamp> X<file, timestamp>
If you want to retrieve the time at which the file was last read,
-written, or had its metadata (owner, etc) changed, you use the B<-M>,
-B<-A>, or B<-C> filetest operations as documented in L<perlfunc>. These
-retrieve the age of the file (measured against the start-time of your
-program) in days as a floating point number. To retrieve the "raw"
-time in seconds since the epoch, you would call the stat function,
-then use localtime(), gmtime(), or POSIX::strftime() to convert this
-into human-readable form.
+written, or had its meta-data (owner, etc) changed, you use the B<-A>,
+B<-M>, or B<-C> file test operations as documented in L<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 L<perlport> for details. To retrieve
+the "raw" time in seconds since the epoch, you would call the stat
+function, then use C<localtime()>, C<gmtime()>, or
+C<POSIX::strftime()> to convert this into human-readable form.
Here's an example:
- $write_secs = (stat($file))[9];
- print "file $file updated at ", scalar(localtime($file)), "\n";
+ my $write_secs = (stat($file))[9];
+ printf "file %s updated at %s\n", $file,
+ scalar localtime($write_secs);
If you prefer something more legible, use the File::stat module
(part of the standard distribution in version 5.004 and later):
- use File::stat;
- use Time::localtime;
- $date_string = ctime(stat($file)->mtime);
- print "file $file updated at $date_string\n";
+ # error checking left as an exercise for reader.
+ use File::stat;
+ use Time::localtime;
+ my $date_string = ctime(stat($file)->mtime);
+ print "file $file updated at $date_string\n";
-Error checking is left as an exercise for the reader.
+The POSIX::strftime() approach has the benefit of being,
+in theory, independent of the current locale. See L<perllocale>
+for details.
=head2 How do I set a file's timestamp in perl?
+X<timestamp> X<file, timestamp>
You use the utime() function documented in L<perlfunc/utime>.
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;
+ if (@ARGV < 2) {
+ die "usage: cptimes timestamp_file other_files ...\n";
+ }
+ my $timestamp = shift;
+ my($atime, $mtime) = (stat($timestamp))[8,9];
+ utime $atime, $mtime, @ARGV;
+
+Error checking is, as usual, left as an exercise for the reader.
-Error checking is 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 I<already exist>.
-Note that utime() currently doesn't work correctly with Win95/NT
-ports. A bug has been reported. Check it carefully before using
-it on those platforms.
+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().
=head2 How do I print to more than one file at once?
+X<print, to multiple files>
+
+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 my $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
+
+=head2 How can I read in an entire file all at once?
+X<slurp> X<file, slurping>
+
+The customary Perl approach for processing all the lines in a file is to
+do so one line at a time:
+
+ open my $input, '<', $file or die "can't open $file: $!";
+ while (<$input>) {
+ chomp;
+ # do something with $_
+ }
+ close $input or die "can't close $file: $!";
+
+This is tremendously more efficient than reading the entire file into
+memory as an array of lines and then processing it one element at a time,
+which is often--if not almost always--the wrong approach. Whenever
+you see someone do this:
+
+ my @lines = <INPUT>;
+
+You should think long and hard about why you need everything loaded at
+once. It's just not a scalable solution.
+
+If you "mmap" the file with the File::Map module from
+CPAN, you can virtually load the entire file into a
+string without actually storing it in memory:
-If you only have to do this once, you can do this:
+ use File::Map qw(map_file);
- for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
+ map_file my $string, $filename;
-To connect up to one filehandle to several output filehandles, it's
-easiest to use the tee(1) program if you have it, and let it take care
-of the multiplexing:
+Once mapped, you can treat C<$string> as you would any other string.
+Since you don't necessarily have to load the data, mmap-ing can be
+very fast and may not increase your memory footprint.
- open (FH, "| tee file1 file2 file3");
+You might also find it more
+fun to use the standard C<Tie::File> module, or the C<DB_File> module's
+C<$DB_RECNO> bindings, which allow you to tie an array to a file so that
+accessing an element of the array actually accesses the corresponding
+line in the file.
-Otherwise you'll have to write your own multiplexing print function --
-or your own tee program -- or use Tom Christiansen's, at
-http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz, which is
-written in Perl.
+If you want to load the entire file, you can use the C<File::Slurp>
+module to do it in one one simple and efficient step:
-In theory a IO::Tee class could be written, but to date we haven't
-seen such.
+ use File::Slurp;
+
+ my $all_of_it = read_file($filename); # entire file in scalar
+ my @all_lines = read_file($filename); # one line per element
+
+Or you can read the entire file contents into a scalar like this:
+
+ my $var;
+ {
+ local $/;
+ open my $fh, '<', $file or die "can't open $file: $!";
+ $var = <$fh>;
+ }
+
+That temporarily undefs your record separator, and will automatically
+close the file at block exit. If the file is already open, just use this:
+
+ my $var = do { local $/; <$fh> };
+
+You can also use a localized C<@ARGV> to eliminate the C<open>:
+
+ my $var = do { local( @ARGV, $/ ) = $file; <> };
+
+For ordinary files you can also use the C<read> function.
+
+ read( $fh, $var, -s $fh );
+
+That third argument tests the byte size of the data on the C<$fh> filehandle
+and reads that many bytes into the buffer C<$var>.
=head2 How can I read in a file by paragraphs?
+X<file, reading by paragraphs>
-Use the C<$\> variable (see L<perlvar> for details). You can either
+Use the C<$/> variable (see L<perlvar> for details). You can either
set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
for instance, gets treated as two paragraphs and not three), or
C<"\n\n"> to accept empty paragraphs.
+Note that a blank line must have no blanks in it. Thus
+S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
+
=head2 How can I read a single character from a file? From the keyboard?
+X<getc> X<file, reading one character at a time>
You can use the builtin C<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
+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
L<perlfunc/getc>.
-If your system supports POSIX, you can use the following code, which
-you'll note turns off echo processing as well.
-
- #!/usr/bin/perl -w
- use strict;
- $| = 1;
- for (1..4) {
- my $got;
- print "gimme: ";
- $got = getone();
- print "--> $got\n";
- }
+If your system supports the portable operating system programming
+interface (POSIX), you can use the following code, which you'll note
+turns off echo processing as well.
+
+ #!/usr/bin/perl -w
+ use strict;
+ $| = 1;
+ for (1..4) {
+ print "gimme: ";
+ my $got = getone();
+ print "--> $got\n";
+ }
exit;
- BEGIN {
+ BEGIN {
use POSIX qw(:termios_h);
my ($term, $oterm, $echo, $noecho, $fd_stdin);
- $fd_stdin = fileno(STDIN);
+ my $fd_stdin = fileno(STDIN);
$term = POSIX::Termios->new();
$term->getattr($fd_stdin);
$noecho = $oterm & ~$echo;
sub cbreak {
- $term->setlflag($noecho);
- $term->setcc(VTIME, 1);
- $term->setattr($fd_stdin, TCSANOW);
- }
+ $term->setlflag($noecho);
+ $term->setcc(VTIME, 1);
+ $term->setattr($fd_stdin, TCSANOW);
+ }
sub cooked {
- $term->setlflag($oterm);
- $term->setcc(VTIME, 0);
- $term->setattr($fd_stdin, TCSANOW);
- }
+ $term->setlflag($oterm);
+ $term->setcc(VTIME, 0);
+ $term->setattr($fd_stdin, TCSANOW);
+ }
sub getone {
- my $key = '';
- cbreak();
- sysread(STDIN, $key, 1);
- cooked();
- return $key;
- }
-
- }
+ my $key = '';
+ cbreak();
+ sysread(STDIN, $key, 1);
+ cooked();
+ return $key;
+ }
- END { cooked() }
-
-The Term::ReadKey module from CPAN may be easier to use:
-
- use Term::ReadKey;
- open(TTY, "</dev/tty");
- print "Gimme a char: ";
- ReadMode "raw";
- $key = ReadKey 0, *TTY;
- ReadMode "normal";
- printf "\nYou said %s, char number %03d\n",
- $key, ord $key;
-
-For MS-DOS systems, Dan Carson <dbc@tc.fluke.COM> reports the following:
-
-To put the PC in "raw" mode, use ioctl with some magic numbers gleaned
-from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes
-across the net every so often):
+ }
- $old_ioctl = ioctl(STDIN,0,0); # Gets device info
- $old_ioctl &= 0xff;
- ioctl(STDIN,1,$old_ioctl | 32); # Writes it back, setting bit 5
+ END { cooked() }
-Then to read a single character:
+The Term::ReadKey module from CPAN may be easier to use. Recent versions
+include also support for non-portable systems as well.
- sysread(STDIN,$c,1); # Read a single character
+ use Term::ReadKey;
+ open my $tty, '<', '/dev/tty';
+ print "Gimme a char: ";
+ ReadMode "raw";
+ my $key = ReadKey 0, $tty;
+ ReadMode "normal";
+ printf "\nYou said %s, char number %03d\n",
+ $key, ord $key;
-And to put the PC back to "cooked" mode:
+=head2 How can I tell whether there's a character waiting on a filehandle?
- ioctl(STDIN,1,$old_ioctl); # Sets it back to cooked mode.
+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.
-So now you have $c. If C<ord($c) == 0>, you have a two byte code, which
-means you hit a special key. Read another byte with C<sysread(STDIN,$c,1)>,
-and that value tells you what combination it was according to this
-table:
+You should also check out the Frequently Asked Questions list in
+comp.unix.* for things like this: the answer is essentially the same.
+It's very system-dependent. Here's one solution that works on BSD
+systems:
- # PC 2-byte keycodes = ^@ + the following:
+ sub key_ready {
+ my($rin, $nfd);
+ vec($rin, fileno(STDIN), 1) = 1;
+ return $nfd = select($rin,undef,undef,0);
+ }
- # HEX KEYS
- # --- ----
- # 0F SHF TAB
- # 10-19 ALT QWERTYUIOP
- # 1E-26 ALT ASDFGHJKL
- # 2C-32 ALT ZXCVBNM
- # 3B-44 F1-F10
- # 47-49 HOME,UP,PgUp
- # 4B LEFT
- # 4D RIGHT
- # 4F-53 END,DOWN,PgDn,Ins,Del
- # 54-5D SHF F1-F10
- # 5E-67 CTR F1-F10
- # 68-71 ALT F1-F10
- # 73-77 CTR LEFT,RIGHT,END,PgDn,HOME
- # 78-83 ALT 1234567890-=
- # 84 CTR PgUp
+If you want to find out how many characters are waiting, there's
+also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
+comes with Perl tries to convert C include files to Perl code, which
+can be C<require>d. FIONREAD ends up defined as a function in the
+I<sys/ioctl.ph> file:
-This is all trial and error I did a long time ago, I hope I'm reading the
-file that worked.
+ require 'sys/ioctl.ph';
-=head2 How can I tell if there's a character waiting on a filehandle?
+ $size = pack("L", 0);
+ ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
+ $size = unpack("L", $size);
-You should check out the Frequently Asked Questions list in
-comp.unix.* for things like this: the answer is essentially the same.
-It's very system dependent. Here's one solution that works on BSD
-systems:
+If I<h2ph> wasn't installed or doesn't work for you, you can
+I<grep> the include files by hand:
- sub key_ready {
- my($rin, $nfd);
- vec($rin, fileno(STDIN), 1) = 1;
- return $nfd = select($rin,undef,undef,0);
- }
+ % grep FIONREAD /usr/include/*/*
+ /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
-You should look into getting the Term::ReadKey extension from CPAN.
+Or write a small C program using the editor of champions:
-=head2 How do I open a file without blocking?
-
-You need to use the O_NDELAY or O_NONBLOCK flag from the Fcntl module
-in conjunction with sysopen():
-
- use Fcntl;
- sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
- or die "can't open /tmp/somefile: $!":
+ % cat > fionread.c
+ #include <sys/ioctl.h>
+ main() {
+ printf("%#08x\n", FIONREAD);
+ }
+ ^D
+ % cc -o fionread fionread.c
+ % ./fionread
+ 0x4004667f
-=head2 How do I create a file only if it doesn't exist?
+And then hard-code it, leaving porting as an exercise to your successor.
-You need to use the O_CREAT and O_EXCL flags from the Fcntl module in
-conjunction with sysopen():
+ $FIONREAD = 0x4004667f; # XXX: opsys dependent
- use Fcntl;
- sysopen(FH, "/tmp/somefile", O_WRONLY|O_EXCL|O_CREAT, 0644)
- or die "can't open /tmp/somefile: $!":
+ $size = pack("L", 0);
+ ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
+ $size = unpack("L", $size);
-Be warned that neither creation nor deletion of files is guaranteed to
-be an atomic operation over NFS. That is, two processes might both
-successful create or unlink the same file!
+FIONREAD requires a filehandle connected to a stream, meaning that sockets,
+pipes, and tty devices work, but I<not> files.
=head2 How do I do a C<tail -f> in perl?
+X<tail> X<IO::Handle> X<File::Tail> X<clearerr>
First try
- seek(GWFILE, 0, 1);
+ seek(GWFILE, 0, 1);
The statement C<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.
+next C<< <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:
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.
+If this still doesn't work, look into the C<clearerr> method
+from C<IO::Handle>, which resets the error and end-of-file states
+on the handle.
+
+There's also a C<File::Tail> module from CPAN.
=head2 How do I dup() a filehandle in Perl?
+X<dup>
If you check L<perlfunc/open>, you'll see that several of the ways
-to call open() should do the trick. For example:
+to call open() should do the trick. For example:
- open(LOG, ">>/tmp/logfile");
- open(STDERR, ">&LOG");
+ open my $log, '>>', '/foo/logfile';
+ open STDERR, '>&LOG';
Or even with a literal numeric descriptor:
- $fd = $ENV{MHCONTEXTFD};
- open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
+ my $fd = $ENV{MHCONTEXTFD};
+ open $mhcontext, "<&=$fd"; # like fdopen(3S)
+
+Note that "<&STDIN" makes a copy, but "<&=STDIN" makes
+an alias. That means if you close an aliased handle, all
+aliases become inaccessible. This is not true with
+a copied one.
-Error checking has been left as an exercise for the reader.
+Error checking, as always, has been left as an exercise for the reader.
=head2 How do I close a file descriptor by number?
+X<file, closing file descriptors> X<POSIX> X<close>
+
+If, for some reason, you have a file descriptor instead of a
+filehandle (perhaps you used C<POSIX::open>), you can use the
+C<close()> function from the C<POSIX> module:
+
+ use POSIX ();
-This should rarely be necessary, as the Perl close() function is to be
+ POSIX::close( $fd );
+
+This should rarely be necessary, as the Perl C<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
+numeric descriptor as with C<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;
+ require 'sys/syscall.ph';
+ my $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 C<open()>:
+
+ {
+ open my( $fh ), "<&=$fd" or die "Cannot reopen fd=$fd: $!";
+ close $fh;
+ }
-=head2 Why can't I use "C:\temp\foo" in MS-DOS paths? What doesn't `C:\temp\foo.exe` work?
+=head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
+X<filename, DOS issues>
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
-L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
+backslash is an escape character. The full list of these is in
+L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
have a file called "c:(tab)emp(formfeed)oo" or
-"c:(tab)emp(formfeed)oo.exe" on your MS-DOS filesystem.
+"c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
Either single-quote your strings, or (preferably) use forward slashes.
-Since all MS-DOS and Windows versions since something like MS-DOS 2.0 or so
+Since all DOS and Windows versions since something like MS-DOS 2.0 or so
have treated C</> and C<\> 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.
+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.
=head2 Why doesn't glob("*.*") get all the files?
+X<glob>
Because even on non-Unix ports, Perl's glob function follows standard
-Unix globbing semantics. You'll need C<glob("*")> to get all (nonhidden)
-files.
+Unix globbing semantics. You'll need C<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.
=head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
-This is elaborately and painstakingly described in the "Far More Than
-You Every Wanted To Know" in
-http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms .
+This is elaborately and painstakingly described in the
+F<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
+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
+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,
+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.
=head2 How do I select a random line from a file?
+X<file, selecting a random line>
+
+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 I<The Art of Computer
+Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth.
+
+You can use the C<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 C<Tie::File> module, which treats the entire
+file as an array. Simply access a random array element.
+
+=head2 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 C<print>, doing this. Whenever you
+interpolate an array in a double quote context, Perl joins the
+elements with spaces (or whatever is in C<$">, which is a space by
+default):
-Here's an algorithm from the Camel Book:
+ animals are: camel llama alpaca vicuna
- srand;
- rand($.) < 1 && ($line = $_) while <>;
+This is different than printing the array without the interpolation:
-This has a significant advantage in space over reading the whole
-file in.
+ 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 C<@animals> simply become part of the list to
+C<print>:
+
+ animals are: camelllamaalpacavicuna
+
+You might notice this when each of the elements of C<@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;
+
+=head2 How do I traverse a directory tree?
+
+(contributed by brian d foy)
+
+The C<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 C<find> subroutine with a callback subroutine and the
+directories you want to 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 C<File::Find::Closures>, which you can download from CPAN, provides
+many ready-to-use subroutines that you can use with C<File::Find>.
+
+The C<File::Finder>, which you can download from CPAN, can help you
+create the callback subroutine using something closer to the syntax of
+the C<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 C<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 );
+
+=head2 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 C<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 C<File::Path> module, which comes with Perl, has a C<remove_tree>
+which can take care of all of the hard work for you:
+
+ use File::Path qw(remove_tree);
+
+ remove_tree( @directories );
+
+The C<File::Path> module also has a legacy interface to the older
+C<rmtree> subroutine.
+
+=head2 How do I copy an entire directory?
+
+(contributed by Shlomi Fish)
+
+To do the equivalent of C<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 L<File::Copy::Recursive>.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
-All rights reserved. See L<perlfaq> for distribution information.
+Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and
+other authors as noted. All rights reserved.
+
+This documentation is free; you can redistribute it and/or modify it
+under the same terms as Perl itself.
+
+Irrespective of its distribution, all code examples here are in the public
+domain. You are permitted and encouraged to use this code and any
+derivatives thereof in your own programs for fun or for profit as you
+see fit. A simple comment in the code giving credit to the FAQ would
+be courteous but is not required.
https://perl5.git.perl.org / perl5.git / blobdiff