=head1 NAME
-perlfaq5 - Files and Formats ($Revision: 1.38 $, $Date: 1999/05/23 16:08:30 $)
+perlfaq5 - Files and Formats ($Revision: 1.38 $, $Date: 2005/10/13 19:49:13 $)
=head1 DESCRIPTION
formats, and footers.
=head2 How do I flush/unbuffer an output filehandle? Why must I do this?
-
-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.
-
-In most stdio implementations, the type of output 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.
-
-Perl does not support truly unbuffered output (except insofar as you can
-C<syswrite(OUT, $char, 1)>). What it does instead support is "command
-buffering", in which a physical write is performed after every output
-command. This isn't as hard on your system as unbuffering, but does
-get the output where you want it when you want it.
-
-If you expect characters to get to your device when you print them there,
-you'll want to autoflush its handle.
-Use select() and the C<$|> variable to control autoflushing
-(see L<perlvar/$|> and L<perlfunc/select>):
+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);
-Or using the traditional idiom:
+Some idioms can handle this in a single statement:
select((select(OUTPUT_HANDLE), $| = 1)[0]);
-Or if don't mind slowly loading several thousand lines of module code
-just because you're afraid of the C<$|> variable:
-
- use FileHandle;
- open(DEV, "+</dev/tty"); # ceci n'est pas une pipe
- DEV->autoflush(1);
+ $| = 1, select $_ for select OUTPUT_HANDLE;
-or the newer IO::* modules:
+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:
use IO::Handle;
open(DEV, ">/dev/printer"); # but is this?
DEV->autoflush(1);
-or even this:
+or IO::Socket:
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;
+ my $sock = IO::Socket::INET->new( 'www.example.com:80' ) ;
$sock->autoflush();
- print $sock "GET / HTTP/1.0" . "\015\012" x 2;
- $document = join('', <$sock>);
- print "DOC IS: $document\n";
-
-Note the bizarrely hardcoded carriage return and newline in their octal
-equivalents. This is the ONLY way (currently) to assure a proper flush
-on all platforms, including Macintosh. That's the way things work in
-network programming: you really should specify the exact bit pattern
-on the network line terminator. In practice, C<"\n\n"> often works,
-but this is not portable.
-
-See L<perlfaq9> for other examples of fetching URLs over the web.
=head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
+X<file, editing>
-Those are operations of a text editor. Perl is not a text editor.
-Perl is a programming language. You have to decompose the problem into
-low-level calls to read, write, open, close, and seek.
-
-Although humans have an easy time thinking of a text file as being a
-sequence of lines that operates much like a stack of playing cards -- or
-punch cards -- computers usually see the text file as a sequence of bytes.
-In general, there's no direct way for Perl to seek to a particular line
-of a file, insert text into a file, or remove text from a file.
-
-(There are exceptions in special circumstances. You can add or remove at
-the very end of the file. Another is replacing a sequence of bytes with
-another sequence of the same length. Another is using the C<$DB_RECNO>
-array bindings as documented in L<DB_File>. Yet another is manipulating
-files with all lines the same length.)
-
-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?
+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>.
This assumes no funny games with newline translations.
+=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>
+
+(contributed by brian d foy)
+
+Use the File::Copy module. It comes with Perl and can do a
+true copy across file systems, and it does its magic in
+a portable fashion.
+
+ use File::Copy;
+
+ copy( $original, $new_copy ) or die "Copy failed: $!";
+
+If you can't use File::Copy, you'll have to do the work yourself:
+open the original file, open the destination file, then print
+to the destination file as you read the original.
+
=head2 How do I make a temporary file name?
+X<file, temporary>
-Use the C<new_tmpfile> class method from the IO::File module to get a
-filehandle opened for reading and writing. Use this if you don't
-need to know the file's name.
+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.
- use IO::File;
- $fh = IO::File->new_tmpfile()
- or die "Unable to make new temporary file: $!";
+ open my $tmp, '+>', undef or die $!;
-Or you can use the C<tmpnam> function from the POSIX module to get a
-filename that you then open yourself. Use this if you do need to know
-the file's name.
+Otherwise, you can use the File::Temp module.
- use Fcntl;
- use POSIX qw(tmpnam);
+ use File::Temp qw/ tempfile tempdir /;
+
+ $dir = tempdir( CLEANUP => 1 );
+ ($fh, $filename) = tempfile( DIR => $dir );
+
+ # or if you don't need to know the filename
- # try new temporary filenames until we get one that didn't already
- # exist; the check should be unnecessary, but you can't be too careful
- do { $name = tmpnam() }
- until sysopen(FH, $name, O_RDWR|O_CREAT|O_EXCL);
+ $fh = tempfile( DIR => $dir );
- # install atexit-style handler so that when we exit or die,
- # we automatically delete this temporary file
- END { unlink($name) or die "Couldn't unlink $name : $!" }
+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:
- # now go on to use the file ...
+ use IO::File;
+ $fh = IO::File->new_tmpfile()
+ or die "Unable to make new temporary file: $!";
-If you're committed to doing this 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:
+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 $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
sub temp_file {
local *FH;
my $count = 0;
until (defined(fileno(FH)) || $count++ > 100) {
$base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
+ # O_EXCL is required for security reasons.
sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
}
if (defined(fileno(FH))
}
=head2 How can I manipulate fixed-record-length files?
+X<fixed-length> X<file, fixed-length records>
-The most efficient way is using pack() and unpack(). This is faster than
-using substr() when taking many, many strings. It is slower for just a few.
+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,
# sample input line:
# 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
- $PS_T = 'A6 A4 A7 A5 A*';
- open(PS, "ps|");
- print scalar <PS>;
- while (<PS>) {
- ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
- for $var (qw!pid tt stat time command!) {
- print "$var: <$$var>\n";
+ my $PS_T = 'A6 A4 A7 A5 A*';
+ open my $ps, '-|', 'ps';
+ print scalar <$ps>;
+ my @fields = qw( pid tt stat time command );
+ while (<$ps>) {
+ my %process;
+ @process{@fields} = unpack($PS_T, $_);
+ for my $field ( @fields ) {
+ print "$field: <$process{$field}>\n";
}
- print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
- "\n";
+ print 'line=', pack($PS_T, @process{@fields} ), "\n";
}
-We've used C<$$var> in a way that forbidden by C<use strict 'refs'>.
-That is, we've promoted a string to a scalar variable reference using
-symbolic references. This is ok in small programs, but doesn't scale
-well. It also only works on global variables, not lexicals.
+We've used a hash slice in order to easily handle the fields of each row.
+Storing the keys in an array means it's easy to operate on them as a
+group or loop over them with for. It also avoids polluting the program
+with global variables and using symbolic references.
=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>
-The fastest, simplest, and most direct way is to localize the typeglob
-of the filehandle in question:
-
- local *TmpHandle;
-
-Typeglobs are fast (especially compared with the alternatives) and
-reasonably easy to use, but they also have one subtle drawback. If you
-had, for example, a function named TmpHandle(), or a variable named
-%TmpHandle, you just hid it from yourself.
-
- sub findme {
- local *HostFile;
- open(HostFile, "</etc/hosts") or die "no /etc/hosts: $!";
- local $_; # <- VERY IMPORTANT
- while (<HostFile>) {
- print if /\b127\.(0\.0\.)?1\b/;
- }
- # *HostFile automatically closes/disappears here
- }
-
-Here's how to use this in a loop to open and store a bunch of
-filehandles. We'll use as values of the hash an ordered
-pair to make it easy to sort the hash in insertion order.
-
- @names = qw(motd termcap passwd hosts);
- my $i = 0;
- foreach $filename (@names) {
- local *FH;
- open(FH, "/etc/$filename") || die "$filename: $!";
- $file{$filename} = [ $i++, *FH ];
- }
+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.
- # Using the filehandles in the array
- foreach $name (sort { $file{$a}[0] <=> $file{$b}[0] } keys %file) {
- my $fh = $file{$name}[1];
- my $line = <$fh>;
- print "$name $. $line";
- }
+ open my $fh, $file_name;
-For passing filehandles to functions, the easiest way is to
-preface them with a star, as in func(*STDIN). See L<perlfaq7/"Passing
-Filehandles"> for details.
+ open local $fh, $file_name;
-If you want to create many anonymous handles, you should check out the
-Symbol, FileHandle, or IO::Handle (etc.) modules. Here's the equivalent
-code with Symbol::gensym, which is reasonably light-weight:
+ print $fh "Hello World!\n";
- foreach $filename (@names) {
- use Symbol;
- my $fh = gensym();
- open($fh, "/etc/$filename") || die "open /etc/$filename: $!";
- $file{$filename} = [ $i++, $fh ];
- }
+ process_file( $fh );
-Or here using the semi-object-oriented FileHandle module, which certainly
-isn't light-weight:
+Before perl5.6, you had to deal with various typeglob idioms
+which you may see in older code.
- use FileHandle;
+ open FILE, "> $filename";
+ process_typeglob( *FILE );
+ process_reference( \*FILE );
- foreach $filename (@names) {
- my $fh = FileHandle->new("/etc/$filename") or die "$filename: $!";
- $file{$filename} = [ $i++, $fh ];
- }
+ sub process_typeglob { local *FH = shift; print FH "Typeglob!" }
+ sub process_reference { local $fh = shift; print $fh "Reference!" }
-Please understand that whether the filehandle happens to be a (probably
-localized) typeglob or an anonymous handle from one of the modules,
-in no way affects the bizarre rules for managing indirect handles.
-See the next question.
+If you want to create many anonymous handles, you should
+check out the Symbol or IO::Handle modules.
=head2 How can I use a filehandle indirectly?
+X<filehandle, indirect>
An indirect filehandle is using something other than a symbol
in a place that a filehandle is expected. Here are ways
-to get those:
+to get indirect filehandles:
$fh = SOME_FH; # bareword is strict-subs hostile
$fh = "SOME_FH"; # strict-refs hostile; same package only
$fh = \*SOME_FH; # ref to typeglob (bless-able)
$fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
-Or to use the C<new> method from the FileHandle or IO modules to
+Or, you can use the C<new> method from one of the IO::* modules to
create an anonymous filehandle, store that in a scalar variable,
and use it as though it were a normal filehandle.
- use FileHandle;
- $fh = FileHandle->new();
-
use IO::Handle; # 5.004 or higher
$fh = IO::Handle->new();
Perl is expecting a filehandle, an indirect filehandle may be used
instead. An indirect filehandle is just a scalar variable that contains
a filehandle. Functions like C<print>, C<open>, C<seek>, or
-the C<< <FH> >> diamond operator will accept either a read filehandle
+the C<< <FH> >> diamond operator will accept either a named filehandle
or a scalar variable containing one:
($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
accept_fh($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 into hashes or arrays, can be used with
-built-ins like C<print>, C<printf>, or the diamond operator. These are
+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:
@fd = (*STDIN, *STDOUT, *STDERR);
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
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 reads a record just
+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() require a typeglob. It doesn't
+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]);
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>
See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
=head2 How can I output my numbers with commas added?
+X<number, commify>
-This one will do it for you:
+(contributed by brian d foy and Benjamin Goldberg)
- sub commify {
- local $_ = shift;
- 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
- return $_;
- }
+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).
- $n = 23659019423.2331;
- print "GOT: ", commify($n), "\n";
+This subroutine will add commas to your number:
- GOT: 23,659,019,423.2331
+ sub commify {
+ local $_ = shift;
+ 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
+ return $_;
+ }
-You can't just:
+This regex from Benjamin Goldberg will add commas to numbers:
- s/^([-+]?\d+)(\d{3})/$1,$2/g;
+ s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
-because you have to put the comma in and then recalculate your
-position.
+It is easier to see with comments:
-Alternatively, this 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+? # 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 <> (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
-portable glob functionality.
+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.
Within Perl, you may use this directly:
}ex;
=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
I<then> gives you read-write access:
open(FH, "+> /path/name"); # WRONG (almost always)
Whoops. You should instead use this, which will fail if the file
-doesn't exist.
+doesn't exist.
open(FH, "+< /path/name"); # open for update
To open a file without blocking, creating if necessary:
- sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT)
- or die "can't open /tmp/somefile: $!":
+ sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
+ or die "can't open /foo/somefile: $!":
Be warned that neither creation nor deletion of files is guaranteed to
be an atomic operation over NFS. That is, two processes might both
-successful create or unlink the same file! Therefore O_EXCL
-isn't so exclusive as you might wish.
+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).
-=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>?
+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
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
-yourself with readdir() and patterns, or use a module like Glob::KGlob,
+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
+context, you may cause a memory leak and/or unpredictable behavior. It's
best therefore to use glob() only in list context.
=head2 How can I open a file with a leading ">" or trailing blanks?
+X<filename, special characters>
-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:
-
- sub safe_filename {
- local $_ = shift;
- s#^([^./])#./$1#;
- $_ .= "\0";
- return $_;
- }
-
- $badpath = "<<<something really wicked ";
- $fn = safe_filename($badpath");
- open(FH, "> $fn") or "couldn't open $badpath: $!";
-
-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.
-
-It would be a lot clearer to use sysopen(), though:
+(contributed by Brian McCauley)
- use Fcntl;
- $badpath = "<<<something really wicked ";
- open (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
- or die "can't open $badpath: $!";
+The special two argument form of Perl's open() function ignores
+trailing blanks in filenames and infers the mode from certain leading
+characters (or a trailing "|"). In older versions of Perl this was the
+only version of open() and so it is prevalent in old code and books.
-For more information, see also the new L<perlopentut> if you have it
-(new for 5.6).
+Unless you have a particular reason to use the two argument form you
+should use the three argument form of open() which does not treat any
+charcters in the filename as special.
+
+ open FILE, "<", " file "; # filename is " file "
+ open FILE, ">", ">file"; # filename is ">file"
=head2 How can I reliably rename a file?
+X<rename> X<mv> X<move> X<file, rename> X<ren>
-Well, usually you just use Perl's rename() function. But that may not
-work everywhere, in particular, renaming files across file systems.
-Some sub-Unix systems have broken ports that corrupt the semantics of
-rename() -- for example, WinNT does this right, but Win95 and Win98
-are broken. (The last two parts are not surprising, but the first is. :-)
-
-If your operating system supports a proper mv(1) program or its moral
-equivalent, this works:
+If your operating system supports a proper mv(1) utility or its
+functional equivalent, this works:
rename($old, $new) or system("mv", $old, $new);
-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 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.
-The newer version of File::Copy exports a move() function.
+Newer versions of File::Copy export a move() function.
=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
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
+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
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.
-(But if you're not, you should as always feel perfectly free to write
+(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).
+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")?
+X<lock, lockfile race condition>
A common bit of code B<NOT TO USE> is this:
atomic test-and-set instruction. In theory, this "ought" to work:
sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
- or die "can't open file.lock: $!":
+ 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.
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?
+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. Better to pick a random number.
-It's more realistic.
+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 ':flock';
+ 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: $!";
- # Perl as of 5.004 automatically flushes before unlocking
- flock(FH, LOCK_UN) or die "can't flock numfile: $!";
close FH or die "can't close numfile: $!";
Here's a much better web-page hit counter:
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 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 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.
+
+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.
+
=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:
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 them or you'll be quite sorry.
=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> 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.
+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<-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:
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
Error checking is, as usual, left as an exercise for the reader.
-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.
+The perldoc for utime also has an example that has the same
+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
+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>
-If you only have to do this once, you can do this:
+To connect one filehandle to several output filehandles,
+you can use the IO::Tee or Tie::FileHandle::Multiplex modules.
- for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
+If you only have to do this once, you can print individually
+to each filehandle.
-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:
+ for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
- open (FH, "| tee file1 file2 file3");
+=head2 How can I read in an entire file all at once?
+X<slurp> X<file, slurping>
-Or even:
+You can use the File::Slurp module to do it in one step.
- # 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";
+ use File::Slurp;
-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 and offers much greater functionality
-than the stock version.
-
-=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
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>;
-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 = <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 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 $var.
+
=head2 How can I read in a file by paragraphs?
+X<file, reading by paragraphs>
Use the C<$/> 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 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?
+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
+the Term::ReadKey module from CPAN or use the sample code in
L<perlfunc/getc>.
If your system supports the portable operating system programming
END { cooked() }
-The Term::ReadKey module from CPAN may be easier to use. Recent version
+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;
printf "\nYou said %s, char number %03d\n",
$key, ord $key;
-For legacy 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
-
-Then to read a single character:
-
- sysread(STDIN,$c,1); # Read a single character
-
-And to put the PC back to "cooked" mode:
-
- ioctl(STDIN,1,$old_ioctl); # Sets it back to cooked mode.
-
-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:
-
- # PC 2-byte keycodes = ^@ + the following:
-
- # 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
-
-This is all trial and error I did a long time ago, I hope I'm reading the
-file that worked.
-
=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
% ./fionread
0x4004667f
-And then hard-code it, leaving porting as an exercise to your successor.
+And then hard code it, leaving porting as an exercise to your successor.
$FIONREAD = 0x4004667f; # XXX: opsys dependent
ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
$size = unpack("L", $size);
-FIONREAD requires a filehandle connected to a stream, meaning sockets,
+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>
First try
There's also a 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:
- open(LOG, ">>/tmp/logfile");
+ open(LOG, ">>/foo/logfile");
open(STDERR, ">&LOG");
Or even with a literal numeric descriptor:
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.
=head2 How do I close a file descriptor by number?
+X<file, closing file descriptors>
This should rarely be necessary, as the Perl 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 MHCONTEXT above. But if you really have
to, you may be able to do this:
require 'sys/syscall.ph';
$rc = syscall(&SYS_close, $fd + 0); # must force numeric
die "can't sysclose $fd: $!" unless $rc == -1;
-Or just use the fdopen(3S) feature of open():
+Or, just use the fdopen(3S) feature of open():
- {
- local *F;
+ {
+ local *F;
open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
close F;
}
-=head2 Why can't I use "C:\temp\foo" in DOS paths? What doesn't `C:\temp\foo.exe` work?
+=head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
+X<filename, DOS issues>
Whoops! You just put a tab and a formfeed into that filename!
Remember that within double quoted strings ("like\this"), the
Either single-quote your strings, or (preferably) use forward slashes.
Since all DOS and Windows versions since something like MS-DOS 2.0 or so
have treated C</> and C<\> the same in a path, you might as well use the
-one that doesn't clash with Perl -- or the POSIX shell, ANSI C and C++,
+one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++,
awk, Tcl, Java, or Python, just to mention a few. POSIX paths
are more portable, too.
=head2 Why doesn't glob("*.*") get all the files?
+X<glob>
Because even on non-Unix ports, Perl's glob function follows standard
Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
=head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
-This is elaborately and painstakingly described in the "Far More Than
-You Ever Wanted To Know" in
-http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms .
+This is elaborately and painstakingly described in the
+F<file-dir-perms> article in the "Far More Than You Ever Wanted To
+Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
The executive summary: learn how your filesystem works. The
permissions on a file say what can happen to the data in that file.
the permissions of the file govern whether you're allowed to.
=head2 How do I select a random line from a file?
+X<file, selecting a random line>
Here's an algorithm from the Camel Book:
srand;
rand($.) < 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 its 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?
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:
+statement would print
little fluffy clouds
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
-All rights reserved.
+Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
+other authors as noted. All rights reserved.
-When included as an integrated part of the Standard Distribution
-of Perl or of its documentation (printed or otherwise), this works is
-covered under Perl's Artistic License. For separate distributions of
-all or part of this FAQ outside of that, see L<perlfaq>.
+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
projects / p5sagit/p5-mst-13.2.git / blobdiff