=head1 NAME
-perlfaq5 - Files and Formats ($Revision: 6019 $)
+perlfaq5 - Files and Formats
=head1 DESCRIPTION
=head2 How do I flush/unbuffer an output filehandle? Why must I do this?
X<flush> X<buffer> X<unbuffer> X<autoflush>
-Perl does not support truly unbuffered output (except
-insofar as you can C<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
-characters 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
-normally buffer output, while syswrite() bypasses buffering
-all together.
-
-If you want your output to be sent immediately when you
-execute print() or write() (for instance, for some network
-protocols), 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 $| affects buffering only for
-the currently selected default file handle. You choose this
-handle with the one argument select() call (see
-L<perlvar/$E<verbar>> and L<perlfunc/select>).
-
-Use select() to choose the desired handle, then set its
-per-filehandle variables.
-
- $old_fh = select(OUTPUT_HANDLE);
- $| = 1;
- select($old_fh);
+(contributed by brian d foy)
-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:
+You might like to read Mark Jason Dominus's "Suffering From Buffering"
+at http://perl.plover.com/FAQs/Buffering.html .
- use IO::Handle;
- open(DEV, ">/dev/printer"); # but is this?
- DEV->autoflush(1);
+Perl normally buffers output so it doesn't make a system call for every
+bit of output. By saving up output, it makes fewer expensive system calls.
+For instance, in this little bit of code, you want to print a dot to the
+screen for every line you process to watch the progress of your program.
+Instead of seeing a dot for every line, Perl buffers the output and you
+have a long wait before you see a row of 50 dots all at once:
+
+ # long wait, then row of dots all at once
+ while( <> ) {
+ print ".";
+ print "\n" unless ++$count % 50;
+
+ #... expensive line processing operations
+ }
+
+To get around this, you have to unbuffer the output filehandle, in this
+case, C<STDOUT>. You can set the special variable C<$|> to a true value
+(mnemonic: making your filehandles "piping hot"):
+
+ $|++;
+
+ # dot shown immediately
+ while( <> ) {
+ print ".";
+ print "\n" unless ++$count % 50;
+
+ #... expensive line processing operations
+ }
+
+The 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):
+
+ {
+ my $previous_default = select(STDOUT); # save previous default
+ $|++; # autoflush STDOUT
+ select(STDERR);
+ $|++; # autoflush STDERR, to be sure
+ select($previous_default); # restore previous default
+ }
+
+ # now should alternate . and +
+ while( 1 )
+ {
+ sleep 1;
+ print STDOUT ".";
+ print STDERR "+";
+ print STDOUT "\n" unless ++$count % 25;
+ }
+
+Besides the C<$|> special variable, you can use C<binmode> to give
+your filehandle a C<:unix> layer, which is unbuffered:
+
+ binmode( STDOUT, ":unix" );
+
+ while( 1 ) {
+ sleep 1;
+ print ".";
+ print "\n" unless ++$count % 50;
+ }
+
+For more information on output layers, see the entries for C<binmode>
+and C<open> in L<perlfunc>, and the C<PerlIO> module documentation.
-or IO::Socket:
+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:
- use IO::Socket; # this one is kinda a pipe?
- my $sock = IO::Socket::INET->new( 'www.example.com:80' );
+ use IO::Handle;
+ open my( $io_fh ), ">", "output.txt";
+ $io_fh->autoflush(1);
+
+The C<IO::Handle> objects also have a C<flush> method. You can flush
+the buffer any time you want without auto-buffering
- $sock->autoflush();
+ $io_fh->flush;
-=head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
+=head2 How do I change, delete, or insert a line in a file, or append to the beginning of a file?
X<file, editing>
-Use the Tie::File module, which is included in the standard
-distribution since Perl 5.8.0.
+(contributed by brian d foy)
+
+The basic idea of inserting, changing, or deleting a line from a text
+file involves reading and printing the file to the point you want to
+make the change, making the change, then reading and printing the rest
+of the file. Perl doesn't provide random access to lines (especially
+since the record input separator, C<$/>, is mutable), although modules
+such as C<Tie::File> can fake it.
+
+A Perl program to do these tasks takes the basic form of opening a
+file, printing its lines, then closing the file:
+
+ open my $in, '<', $file or die "Can't read old file: $!";
+ open my $out, '>', "$file.new" or die "Can't write new file: $!";
+
+ while( <$in> )
+ {
+ print $out $_;
+ }
+
+ close $out;
+
+Within that basic form, add the parts that you need to insert, change,
+or delete lines.
+
+To prepend lines to the beginning, print those lines before you enter
+the loop that prints the existing lines.
+
+ open my $in, '<', $file or die "Can't read old file: $!";
+ open my $out, '>', "$file.new" or die "Can't write new file: $!";
+
+ print $out "# Add this line to the top\n"; # <--- HERE'S THE MAGIC
+
+ while( <$in> )
+ {
+ print $out $_;
+ }
+
+ close $out;
+
+To change existing lines, insert the code to modify the lines inside
+the 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:
+
+ perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt
+
+To add lines before a certain line, you can add a line (or lines!)
+before Perl prints C<$_>:
+
+ perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt
+
+You can even add a line to the beginning of a file, since the current
+line prints at the end of the loop:
+
+ perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt
+
+To insert a line after one already in the file, use the C<-n> switch.
+It's just like C<-p> except that it doesn't print C<$_> 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>
-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.
+(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:
- $lines = 0;
- open(FILE, $filename) or die "Can't open `$filename': $!";
- while (sysread FILE, $buffer, 4096) {
- $lines += ($buffer =~ tr/\n//);
+ 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;
-This assumes no funny games with newline translations.
+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, $bw->tell );
+
+The C<File::ReadBackwards> module also has the advantage of setting
+the input record separator to a regular expression.
+
+You can also use the C<Tie::File> module which lets you access
+the lines through a tied array. You can use normal array operations
+to modify your file, including setting the last index and using
+C<splice>.
=head2 How can I use Perl's C<-i> option from within a program?
X<-i> X<in-place>
C<-i> sets the value of Perl's C<$^I> variable, which in turn affects
-the behavior of C<< <> >>; see L<perlrun> for more details. By
+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:
+behavior within a larger program. For example:
# ...
{
C<.c.orig> file.
=head2 How can I copy a file?
-X<copy> X<file, copy>
+X<copy> X<file, copy> X<File::Copy>
(contributed by brian d foy)
-Use the File::Copy module. It comes with Perl and can do a
+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.
copy( $original, $new_copy ) or die "Copy failed: $!";
-If you can't use File::Copy, you'll have to do the work yourself:
+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.
+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. The C<open()> function
-creates an anonymous temporary file.
+with C<undef> in place of the file name. In Perl 5.8 or later, the
+C<open()> function creates an anonymous temporary file:
open my $tmp, '+>', undef or die $!;
use File::Temp qw/ tempfile tempdir /;
- $dir = tempdir( CLEANUP => 1 );
+ my $dir = tempdir( CLEANUP => 1 );
($fh, $filename) = tempfile( DIR => $dir );
# or if you don't need to know the filename
- $fh = tempfile( DIR => $dir );
+ my $fh = tempfile( DIR => $dir );
-The File::Temp has been a standard module since Perl 5.6.1. If you
+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:
+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: $!";
+ 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
+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, $$, time());
+ my $base_name = sprintf "%s/%d-%d-0000", $temp_dir, $$, time;
sub temp_file {
- local *FH;
+ 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, $base_name, O_WRONLY|O_EXCL|O_CREAT);
+ until( defined(fileno($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 ();
+ }
}
- if (defined(fileno(FH))
- return (*FH, $base_name);
- }
- else {
- return ();
- }
- }
}
=head2 How can I manipulate fixed-record-length files?
X<fixed-length> X<file, fixed-length records>
The most efficient way is using L<pack()|perlfunc/"pack"> and
-L<unpack()|perlfunc/"unpack">. This is faster than using
-L<substr()|perlfunc/"substr"> when taking many, many strings. It is
+L<unpack()|perlfunc/"unpack">. This is faster than using
+L<substr()|perlfunc/"substr"> when taking many, many strings. It is
slower for just a few.
Here is a sample chunk of code to break up and put back together again
print {$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.
X<filehandle, indirect>
An indirect filehandle is using something other than a symbol
-in a place that a filehandle is expected. Here are ways
+in a place that a filehandle is expected. Here are ways
to get indirect filehandles:
$fh = SOME_FH; # bareword is strict-subs hostile
and use it as though it were a normal filehandle.
use IO::Handle; # 5.004 or higher
- $fh = IO::Handle->new();
+ my $fh = IO::Handle->new();
-Then use any of those as you would a normal filehandle. Anywhere that
+Then use any of those as you would a normal filehandle. Anywhere that
Perl is expecting a filehandle, an indirect filehandle may be used
instead. An indirect filehandle is just a scalar variable that contains
-a filehandle.
Functions like C<print>, C<open>, C<seek>, or
+a filehandle. Functions like C<print>, C<open>, C<seek>, or
the C<< <FH> >> diamond operator will accept either a named filehandle
or a scalar variable containing one:
($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
print $ofh "Type it: ";
- $got = <$ifh>
+ my $got = <$ifh>
print $efh "What was that: $got";
If you're passing a filehandle to a function, you can write
accept_fh($handle);
In the examples above, we assigned the filehandle to a scalar variable
-before using it. That is because only simple scalar variables, not
+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
+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:
- @fd = (*STDIN, *STDOUT, *STDERR);
+ my @fd = (*STDIN, *STDOUT, *STDERR);
print $fd[1] "Type it: "; # WRONG
- $got = <$fd[0]> # 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
# 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:
+complicated code there. This sends the message out to one of two places:
- $ok = -x "/bin/cat";
+ 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
+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
+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
+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()?
=head2 How can I write() into a string?
X<write, into a string>
-See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
+(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 $string;
+
+There are also some tricks that you can play with C<formline> and the
+accumulator variable C<$^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>
=head2 How can I translate tildes (~) in a filename?
X<tilde> X<tilde expansion>
-Use the <> (glob()) operator, documented in L<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 more portable glob
-functionality.
+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:
=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 (almost always)
+ open my $fh, '+>', '/path/name'; # WRONG (almost always)
-Whoops. You should instead use this, which will fail if the file
-doesn't exist.
+Whoops. You should instead use this, which will fail if the file
+doesn't exist:
- open(FH, "+< /path/name"); # open for update
+ open my $fh, '+<', '/path/name'; # open for update
-Using ">" always clobbers or creates. Using "<" never does
-either. The "+" doesn't change this.
+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
+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(FH, "< $path") || die $!;
- sysopen(FH, $path, O_RDONLY) || die $!;
+ 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(FH, "> $path") || die $!;
- sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
- sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
+ open my $fh, '>', $path or die $!;
+ sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT or die $!;
+ sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666 or die $!;
To open file for writing, create new file, file must not exist:
- sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
- sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
+ sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT or die $!;
+ sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT, 0666 or die $!;
To open file for appending, create if necessary:
- open(FH, ">> $path") || die $!;
- sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
- sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
+ 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 $!;
To open file for appending, file must exist:
- sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
+ sysopen my $fh, $path, O_WRONLY|O_APPEND or die $!;
To open file for update, file must exist:
- open(FH, "+< $path") || die $!;
- sysopen(FH, $path, O_RDWR) || die $!;
+ open my $fh, '+<', $path or die $!;
+ sysopen my $fh, $path, O_RDWR or 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 $!;
+ 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(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
- sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
+ 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(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
+ 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
+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 L<perlopentut> if you have it (new for 5.6).
+See also the new 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>
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 upgrade to Perl v5.6.0 or later, do the glob
=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 memory leak and/or unpredictable behavior. It's
-best therefore to use glob() only in list context.
+(contributed by brian d foy)
+
+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.
=head2 How can I open a file with a leading ">" or trailing blanks?
X<filename, special characters>
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.
+characters in the filename as special.
- open FILE, "<", " file "; # filename is " file "
- open FILE, ">", ">file"; # filename is ">file"
+ open my $fh, "<", " file "; # filename is " file "
+ open my $fh, ">", ">file"; # filename is ">file"
=head2 How can I reliably rename a file?
-X<rename> X<mv> X<move> X<file, rename> X<ren>
+X<rename> X<mv> X<move> X<file, rename>
If your operating system supports a proper mv(1) utility or its
functional equivalent, this works:
rename($old, $new) or system("mv", $old, $new);
-It may be more portable to use the File::Copy module instead.
+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 rename(), which preserves meta-information like
+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.
-Newer versions of File::Copy export a move() function.
-
=head2 How can I lock a file?
X<lock> X<file, lock> X<flock>
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>
+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
+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
+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").
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)
+ sysopen my $fh, "file.lock", O_WRONLY|O_EXCL|O_CREAT
or die "can't open file.lock: $!";
except that lamentably, file creation (and deletion) is not atomic
over NFS, so this won't work (at least, not every time) over the net.
Various schemes involving link() have been suggested, but
-these tend to involve busy-wait, which is also subdesirable.
+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?
+=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;
+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: $!";
+ 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. :-)
+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?
+=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 flock() and you use the
-example appending code from "perldoc -f flock" everything will be OK
-even if the OS you are on doesn't implement append mode correctly (if
-such a system exists.) So if you are happy to restrict yourself to OSs
-that implement flock() (and that's not really much of a restriction)
-then that is what you should do.
+If you 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 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 buffer then each bufferload is almost guaranteed to be written to
-the end of the file in one chunk without getting intermingled with
-anyone else's output. You can also use the syswrite() function which is
-simply a wrapper around your systems write(2) system call.
+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 write() operation before completion. There is also a
-possibility that some STDIO implementations may call multiple system
-level write()s even if the buffer was empty to start. There may be some
-systems where this probability is reduced to zero.
+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>
$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: $!";
+ 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;
+ 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.
=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 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 localtime(),
-gmtime(), or POSIX::strftime() to convert this into
-human-readable form.
+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 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];
+ my $write_secs = (stat($file))[9];
printf "file %s updated at %s\n", $file,
scalar localtime($write_secs);
# error checking left as an exercise for reader.
use File::stat;
use Time::localtime;
- $date_string = ctime(stat($file)->mtime);
+ my $date_string = ctime(stat($file)->mtime);
print "file $file updated at $date_string\n";
The POSIX::strftime() approach has the benefit of being,
-in theory, independent of the current locale.
See L<perllocale>
+in theory, independent of the current locale. See L<perllocale>
for details.
=head2 How do I set a file's timestamp in perl?
if (@ARGV < 2) {
die "usage: cptimes timestamp_file other_files ...\n";
}
- $timestamp = shift;
- ($atime, $mtime) = (stat($timestamp))[8,9];
+ 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.
effect as touch(1) on files that I<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
+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
+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?
If you only have to do this once, you can print individually
to each filehandle.
- for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
+ 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>
-You can use the File::Slurp module to do it in one step.
+Are you sure you want to read the entire file and store it in memory?
+If you mmap the file, you can virtually load the entire file into a
+string without actually storing it in memory:
+
+ use File::Map qw(map_file);
+
+ map_file my $string, $filename;
+
+Once mapped, you can treat C<$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.
+
+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:
use File::Slurp;
- $all_of_it = read_file($filename); # entire file in scalar
- @all_lines = read_file($filename); # one line perl element
+ my $all_of_it = read_file($filename); # entire file in scalar
+ my @all_lines = read_file($filename); # one line per element
The customary Perl approach for processing all the lines in a file is to
do so one line at a time:
- open (INPUT, $file) || die "can't open $file: $!";
- while (<INPUT>) {
+ open my $input, '<', $file or die "can't open $file: $!";
+ while (<$input>) {
chomp;
# do something with $_
}
- close(INPUT) || die "can't close $file: $!";
+ close $input or die "can't close $file: $!";
This is tremendously more efficient than reading the entire file into
memory as an array of lines and then processing it one element at a time,
-which is often--if not almost always--the wrong approach. Whenever
+which is often--if not almost always--the wrong approach. Whenever
you see someone do this:
- @lines = <INPUT>;
+ my @lines = <INPUT>;
-you should think long and hard about why you need everything loaded at
-once. It's just not a scalable solution. You might also find it more
-fun to use the standard Tie::File module, or the DB_File module's
-$DB_RECNO bindings, which allow you to tie an array to a file so that
+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 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 the array actually accesses the corresponding
line in the file.
You can read the entire filehandle contents into a scalar.
+ my $var;
{
- local(*INPUT, $/);
- open (INPUT, $file) || die "can't open $file: $!";
- $var = <INPUT>;
+ 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:
+close the file at block exit. If the file is already open, just use this:
+
+ my $var = do { local $/; <$fh> };
+
+You can do that one better by using a localized C<@ARGV> so you can
+eliminate the C<open>:
- $var = do { local $/; <INPUT> };
+ my $var = do { local( @ARGV, $/ ) = $file; <> };
-For ordinary files you can also use the read function.
+For ordinary files you can also use the C<read> function.
- read( INPUT, $var, -s INPUT );
+ read( $fh, $var, -s $fh );
-The third argument tests the byte size of the data on the INPUT filehandle
-and reads that many bytes into the buffer $var.
+That third argument tests the byte size of the data on the C<INPUT> filehandle
+and reads that many bytes into the buffer C<$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
+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
+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>.
use strict;
$| = 1;
for (1..4) {
- my $got;
print "gimme: ";
- $got = getone();
+ my $got = getone();
print "--> $got\n";
}
exit;
my ($term, $oterm, $echo, $noecho, $fd_stdin);
- $fd_stdin = fileno(STDIN);
+ my $fd_stdin = fileno(STDIN);
$term = POSIX::Termios->new();
$term->getattr($fd_stdin);
END { cooked() }
-The Term::ReadKey module from CPAN may be easier to use. Recent versions
+The Term::ReadKey module from CPAN may be easier to use. Recent versions
include also support for non-portable systems as well.
use Term::ReadKey;
- open(TTY, "</dev/tty");
+ open my $tty, '<', '/dev/tty';
print "Gimme a char: ";
ReadMode "raw";
- $key = ReadKey 0, *TTY;
+ my $key = ReadKey 0, $tty;
ReadMode "normal";
printf "\nYou said %s, char number %03d\n",
$key, ord $key;
=head2 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
+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.
+not POSIX, not Unix, etc.) systems.
You should also check out the Frequently Asked Questions list in
comp.unix.* for things like this: the answer is essentially the same.
-It's very system dependent. Here's one solution that works on BSD
+It's very system dependent. Here's one solution that works on BSD
systems:
sub key_ready {
}
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
+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
+can be C<require>d. FIONREAD ends up defined as a function in the
I<sys/ioctl.ph> file:
require 'sys/ioctl.ph';
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, ">>/foo/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" make
-an alias. That means if you close an aliased handle, all
-aliases become inaccessible. This is not true with
+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.
=head2 How do I close a file descriptor by number?
-X<file, closing file descriptors>
+X<file, closing file descriptors> X<POSIX> X<close>
-This should rarely be necessary, as the Perl close() function is to be
+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 ();
+
+ 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
+ my $rc = syscall(&SYS_close, $fd + 0); # must force numeric
die "can't sysclose $fd: $!" unless $rc == -1;
-Or, just use the fdopen(3S) feature of open():
+Or, just use the fdopen(3S) feature of C<open()>:
{
- local *F;
- open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
- close F;
+ open my( $fh ), "<&=$fd" or die "Cannot reopen fd=$fd: $!";
+ close $fh;
}
=head2 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
-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 legacy DOS filesystem.
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. POSIX paths
+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 (non-hidden)
-files. This makes glob() portable even to legacy systems. Your
-port may include proprietary globbing functions as well. Check its
+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?
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>
-Here's an algorithm from the Camel Book:
+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
+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 File::Random module which provides a function
+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 Tie::File module, which treats the entire
-file as an array. Simply access a random array element.
+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?
-Saying
+(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:
- print "@lines\n";
+ my @animals = qw(camel llama alpaca vicuna);
+ print "animals are: @animals\n";
-joins together the elements of C<@lines> with a space between them.
-If C<@lines> were C<("little", "fluffy", "clouds")> then the above
-statement would print
+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):
- little fluffy clouds
+ animals are: camel llama alpaca vicuna
-but if each element of C<@lines> was a line of text, ending a newline
-character C<("little\n", "fluffy\n", "clouds\n")> then it would print:
+This is different than printing the array without the interpolation:
- little
- fluffy
- clouds
+ my @animals = qw(camel llama alpaca vicuna);
+ print "animals are: ", @animals, "\n";
-If your array contains lines, just print them:
+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;
-=head1 REVISION
+=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.
-Revision: $Revision: 6019 $
+=head2 How do I copy an entire directory?
-Date: $Date: 2006-05-04 19:04:31 +0200 (jeu, 04 mai 2006) $
+(contributed by Shlomi Fish)
-See L<perlfaq> for source control details and availability.
+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) 1997-2006 Tom Christiansen, Nathan Torkington, and
+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
+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
+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