=head1 NAME
-perlfaq5 - Files and Formats ($Revision: 1.18 $, $Date: 2002/05/30 07:04:25 $)
+perlfaq5 - Files and Formats ($Revision: 1.28 $, $Date: 2003/01/26 17:45:46 $)
=head1 DESCRIPTION
print() or write(). Setting $| affects buffering only for
the currently selected default file handle. You choose this
handle with the one argument select() call (see
-L<perlvar/$|> and L<perlfunc/select>).
+L<perlvar/$E<verbar>> and L<perlfunc/select>).
Use select() to choose the desired handle, then set its
per-filehandle variables.
Use the File::Temp module, see L<File::Temp> for more information.
- use File::Temp qw/ tempfile tempdir /;
+ use File::Temp qw/ tempfile tempdir /;
$dir = tempdir( CLEANUP => 1 );
($fh, $filename) = tempfile( DIR => $dir );
# 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
$PS_T = 'A6 A4 A7 A5 A*';
open(PS, "ps|");
- print scalar <PS>;
+ print scalar <PS>;
while (<PS>) {
($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
for $var (qw!pid tt stat time command!) {
That block is a proper block like any other, so you can put more
complicated code there. This sends the message out to one of two places:
- $ok = -x "/bin/cat";
+ $ok = -x "/bin/cat";
print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
- print { $fd[ 1+ ($ok || 0) ] } "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
=head2 How can I output my numbers with commas added?
-This one from Benjamin Goldberg will do it for you:
+This subroutine will add commas to your number:
+
+ sub commify {
+ local $_ = shift;
+ 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
+ return $_;
+ }
+
+This regex from Benjamin Goldberg will add commas to numbers:
s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
-or written verbosely:
+It is easier to see with comments:
s/(
^[-+]? # beginning of number.
open(FH, "+> /path/name"); # WRONG (almost always)
Whoops. You should instead use this, which will fail if the file
-doesn't exist.
+doesn't exist.
open(FH, "+< /path/name"); # open for update
See also the new L<perlopentut> if you have it (new for 5.6).
-=head2 Why do I sometimes get an "Argument list too long" when I use <*>?
+=head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
The C<< <> >> operator performs a globbing operation (see above).
In Perl versions earlier than v5.6.0, the internal glob() operator forks
Normally perl ignores trailing blanks in filenames, and interprets
certain leading characters (or a trailing "|") to mean something
-special.
+special.
The three argument form of open() lets you specify the mode
separately from the filename. The open() function treats
-special mode characters and whitespace in the filename as
+special mode characters and whitespace in the filename as
literals
open FILE, "<", " file "; # filename is " file "
=head2 How can I reliably rename a file?
-If your operating system supports a proper mv(1) utility or its functional
-equivalent, this works:
+If your operating system supports a proper mv(1) utility or its
+functional equivalent, this works:
rename($old, $new) or system("mv", $old, $new);
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
+For more information on file locking, see also
L<perlopentut/"File Locking"> if you have it (new for 5.6).
=back
-=head2 Why can't I just open(FH, ">file.lock")?
+=head2 Why can't I just open(FH, "E<gt>file.lock")?
A common bit of code B<NOT TO USE> is this:
=head2 How do I print to more than one file at once?
-If you only have to do this once, you can do this:
-
- for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
+To connect one filehandle to several output filehandles,
+you can use the IO::Tee or Tie::FileHandle::Multiplex modules.
-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:
+If you only have to do this once, you can print individually
+to each filehandle.
- open (FH, "| tee file1 file2 file3");
+ for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
-Or even:
+=head2 How can I read in an entire file all at once?
- # make STDOUT go to three files, plus original STDOUT
- open (STDOUT, "| tee file1 file2 file3") or die "Teeing off: $!\n";
- print "whatever\n" or die "Writing: $!\n";
- close(STDOUT) or die "Closing: $!\n";
+You can use the File::Slurp module to do it in one step.
-Otherwise you'll have to write your own multiplexing print
-function--or your own tee program--or use Tom Christiansen's,
-at http://www.cpan.org/authors/id/TOMC/scripts/tct.gz , which is
-written in Perl and offers much greater functionality
-than the stock version.
+ use File::Slurp;
-=head2 How can I read in an entire file all at once?
+ $all_of_it = read_file($filename); # entire file in scalar
+ @all_lines = read_file($filename); # one line perl element
The customary Perl approach for processing all the lines in a file is to
do so one line at a time:
while (<INPUT>) {
chomp;
# do something with $_
- }
+ }
close(INPUT) || die "can't close $file: $!";
This is tremendously more efficient than reading the entire file into
$var = <INPUT>;
}
-That temporarily undefs your record separator, and will automatically
+That temporarily undefs your record separator, and will automatically
close the file at block exit. If the file is already open, just use this:
$var = do { local $/; <INPUT> };
for instance, gets treated as two paragraphs and not three), or
C<"\n\n"> to accept empty paragraphs.
-Note that a blank line must have no blanks in it. Thus
+Note that a blank line must have no blanks in it. Thus
S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
=head2 How can I read a single character from a file? From the keyboard?
Note that "<&STDIN" makes a copy, but "<&=STDIN" make
an alias. That means if you close an aliased handle, all
-aliases become inaccessible. This is not true with
+aliases become inaccessible. This is not true with
a copied one.
Error checking, as always, has been left as an exercise for the reader.
Or, just use the fdopen(3S) feature of open():
- {
- local *F;
+ {
+ local *F;
open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
close F;
}
This is elaborately and painstakingly described in the
F<file-dir-perms> article in the "Far More Than You Ever Wanted To
-Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz .
+Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
The executive summary: learn how your filesystem works. The
permissions on a file say what can happen to the data in that file.
srand;
rand($.) < 1 && ($line = $_) while <>;
-This has a significant advantage in space over reading the whole
-file in. A simple proof by induction is available upon
-request if you doubt the algorithm's correctness.
+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 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.
=head2 Why do I get weird spaces when I print an array of lines?
projects / p5sagit/p5-mst-13.2.git / blobdiff