=head1 NAME
-perlfaq5 - Files and Formats ($Revision: 1.3 $, $Date: 2001/10/16 13:27:22 $)
+perlfaq5 - Files and Formats ($Revision: 1.15 $, $Date: 2002/04/12 02:02:05 $)
=head1 DESCRIPTION
=head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
-Those are operations of a text editor. Perl is not a text editor.
-Perl is a programming language. You have to decompose the problem into
-low-level calls to read, write, open, close, and seek.
-
-Although humans have an easy time thinking of a text file as being a
-sequence of lines that operates much like a stack of playing cards--or
-punch cards--computers usually see the text file as a sequence of bytes.
-In general, there's no direct way for Perl to seek to a particular line
-of a file, insert text into a file, or remove text from a file.
-
-(There are exceptions in special circumstances. You can add or remove
-data at the very end of the file. A sequence of bytes can be replaced
-with another sequence of the same length. The C<$DB_RECNO> array
-bindings as documented in L<DB_File> also provide a direct way of
-modifying a file. Files where all lines are the same length are also
-easy to alter.)
-
-The general solution is to create a temporary copy of the text file with
-the changes you want, then copy that over the original. This assumes
-no locking.
-
- $old = $file;
- $new = "$file.tmp.$$";
- $bak = "$file.orig";
-
- open(OLD, "< $old") or die "can't open $old: $!";
- open(NEW, "> $new") or die "can't open $new: $!";
-
- # Correct typos, preserving case
- while (<OLD>) {
- s/\b(p)earl\b/${1}erl/i;
- (print NEW $_) or die "can't write to $new: $!";
- }
-
- close(OLD) or die "can't close $old: $!";
- close(NEW) or die "can't close $new: $!";
-
- rename($old, $bak) or die "can't rename $old to $bak: $!";
- rename($new, $old) or die "can't rename $new to $old: $!";
-
-Perl can do this sort of thing for you automatically with the C<-i>
-command-line switch or the closely-related C<$^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
-
- # form a script
- 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 $.
- }
-
-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).
-
-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:
-
- open (FH, "+< $file");
- while ( <FH> ) { $addr = tell(FH) unless eof(FH) }
- truncate(FH, $addr);
-
-Error checking is left as an exercise for the reader.
+Use the Tie::File module, which is included in the standard
+distribution since Perl 5.8.0.
=head2 How do I count the number of lines in a file?
=head2 How can I output my numbers with commas added?
-This one will do it for you:
-
- sub commify {
- my $number = shift;
- 1 while ($number =~ s/^([-+]?\d+)(\d{3})/$1,$2/);
- return $number;
- }
-
- $n = 23659019423.2331;
- print "GOT: ", commify($n), "\n";
+This one from Benjamin Goldberg will do it for you:
- GOT: 23,659,019,423.2331
+ s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
-You can't just:
+or written verbosely:
- s/^([-+]?\d+)(\d{3})/$1,$2/g;
-
-because you have to put the comma in and then recalculate your
-position.
-
-Alternatively, this code commifies all numbers in a line regardless of
-whether they have decimal portions, are preceded by + or -, or
-whatever:
-
- # from Andrew Johnson <ajohnson@gpu.srv.ualberta.ca>
- sub commify {
- my $input = shift;
- $input = reverse $input;
- $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g;
- return scalar reverse $input;
- }
+ s/(
+ ^[-+]? # beginning of number.
+ \d{1,3}? # 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?
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 the one below.
-It turns incomplete pathnames into explicit relative ones, and tacks a
-trailing null byte on the name to make perl leave it alone:
-
- sub safe_filename {
- local $_ = shift;
- s#^([^./])#./$1#;
- $_ .= "\0";
- return $_;
- }
+special.
- $badpath = "<<<something really wicked ";
- $fn = safe_filename($badpath");
- open(FH, "> $fn") or "couldn't open $badpath: $!";
+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
+literals
-This assumes that you are using POSIX (portable operating systems
-interface) paths. If you are on a closed, non-portable, proprietary
-system, you may have to adjust the C<"./"> above.
+ open FILE, "<", " file "; # filename is " file "
+ open FILE, ">", ">file"; # filename is ">file"
-It would be a lot clearer to use sysopen(), though:
+It may be a lot clearer to use sysopen(), though:
use Fcntl;
$badpath = "<<<something really wicked ";
sysopen (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
or die "can't open $badpath: $!";
-For more information, see also the new L<perlopentut> if you have it
-(new for 5.6).
-
=head2 How can I reliably rename a file?
-Well, usually you just use Perl's rename() function. That may not
-work everywhere, though, particularly when renaming files across file systems.
-Some sub-Unix systems have broken ports that corrupt the semantics of
-rename()--for example, WinNT does this right, but Win95 and Win98
-are broken. (The last two parts are not surprising, but the first is. :-)
-
-If your operating system supports a proper mv(1) program or its moral
+If your operating system supports a proper mv(1) utility or its functional
equivalent, this works:
rename($old, $new) or system("mv", $old, $new);
-It may be more 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 semantically as a
-real rename(), though, which preserves metainformation like
+It may be more portable to use the File::Copy module instead.
+You just copy to the new file to the new name (checking return
+values), then delete the old one. This isn't really the same
+semantically as a rename(), which preserves meta-information like
permissions, timestamps, inode info, etc.
-Newer versions of File::Copy exports a move() function.
+Newer versions of File::Copy export a move() function.
=head2 How can I lock a file?
=head2 How do I get a file's timestamp in perl?
-If you want to retrieve the time at which the file was last read,
-written, or had its meta-data (owner, etc) changed, you use the B<-M>,
-B<-A>, 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. 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<-M>, B<-A>, 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.
Here's an example:
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
+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.
@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 DB_File module's $DB_RECNO bindings,
-which allow you to tie an array to a file so that accessing an element
-the array actually accesses the corresponding line in the file.
+you should think long and hard about why you need everything loaded at
+once. It's just not a scalable solution. You might also find it more
+fun to use the standard Tie::File module, or the DB_File module's
+$DB_RECNO bindings, which allow you to tie an array to a file so that
+accessing an element the array actually accesses the corresponding
+line in the file.
-On very rare occasion, you may have an algorithm that demands that
-the entire file be in memory at once as one scalar. The simplest solution
-to that is
-
- $var = `cat $file`;
-
-Being in scalar context, you get the whole thing. In list context,
-you'd get a list of all the lines:
-
- @lines = `cat $file`;
-
-This tiny but expedient solution is neat, clean, and portable to
-all systems on which decent tools have been installed. For those
-who prefer not to use the toolbox, you can of course read the file
-manually, although this makes for more complicated code.
+You can read the entire filehandle contents into a scalar.
{
local(*INPUT, $/);
$var = do { local $/; <INPUT> };
+For ordinary files you can also use the read function.
+
+ read( INPUT, $var, -s INPUT );
+
+The third argument tests the byte size of the data on the INPUT filehandle
+and reads that many bytes into the buffer $var.
+
=head2 How can I read in a file by paragraphs?
Use the C<$/> variable (see L<perlvar> for details). You can either
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 C<"fred\n
-\nstuff\n\n"> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
+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?
close F;
}
-=head2 Why can't I use "C:\temp\foo" in 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?
Whoops! You just put a tab and a formfeed into that filename!
Remember that within double quoted strings ("like\this"), the
=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 Ever 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/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.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
+Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
All rights reserved.
This documentation is free; you can redistribute it and/or modify it
projects / p5sagit/p5-mst-13.2.git / blobdiff