perlfaq5.pod

MSYS在windows下模拟了一个类unix的终端

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
as C<< <> >> does.  Given the initialization shown above for @fd, this
would work, but only because readline() require a typeglob.  It doesn't
work with objects or strings, which might be a bug we haven't fixed yet.

    $got = readline($fd[0]);

Let it be noted that the flakiness of indirect filehandles is not
related to whether they're strings, typeglobs, objects, or anything else.
It's the syntax of the fundamental operators.  Playing the object
game doesn't help you at all here.

=head2 How can I set up a footer format to be used with write()?

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?

See L<perlform/"Accessing Formatting Internals"> for an swrite() function.

=head2 How can I output my numbers with commas added?

This one will do it for you:

    sub commify {
	local $_  = shift;
	1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
	return $_;
    }

    $n = 23659019423.2331;
    print "GOT: ", commify($n), "\n";

    GOT: 23,659,019,423.2331

You can't just:

    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 <[e-mail]>
    sub commify {
       my $input = shift;
        $input = reverse $input;
        $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g;
        return scalar reverse $input;
    }

=head2 How can I translate tildes (~) in a filename?

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
Glob::KGlob module (available from CPAN) gives more portable glob
functionality.

Within Perl, you may use this directly:

	$filename =~ s{
	  ^ ~             # find a leading tilde
	  (               # save this in $1
	      [^/]        # a non-slash character
	            *     # repeated 0 or more times (0 means me)
	  )
	}{
	  $1
	      ? (getpwnam($1))[7]
	      : ( $ENV{HOME} || $ENV{LOGDIR} )
	}ex;

=head2 How come when I open a file read-write it wipes it out?

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.  

    open(FH, "+< /path/name");  	# open for update

Using ">" always clobbers or creates.  Using "<" never does
either.  The "+" doesn't change this.

Here are examples of many kinds of file opens.  Those using sysopen()
all assume

    use Fcntl;

To open file for reading:

    open(FH, "< $path")                                 || die $!;
    sysopen(FH, $path, O_RDONLY)                        || 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 $!;

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 $!;

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 $!;

To open file for appending, file must exist:

    sysopen(FH, $path, O_WRONLY|O_APPEND)               || die $!;

To open file for update, file must exist:

    open(FH, "+< $path")                                || die $!;
    sysopen(FH, $path, O_RDWR)                          || 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 $!;

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 $!;

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: $!":

Be warned that neither creation nor deletion of files is guaranteed to
be an atomic operation over NFS.  That is, two processes might both
successfully create or unlink the same file!  Therefore O_EXCL
isn't as exclusive as you might wish.

See also 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 <*>?

The C<< <> >> operator performs a globbing operation (see above).
In Perl versions earlier than v5.6.0, the internal glob() operator forks
csh(1) to do the actual glob expansion, but
csh can't handle more than 127 items and so gives the error message
C<Argument list too long>.  People who installed tcsh as csh won't
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,
one that doesn't use the shell to do globbing.

=head2 Is there a leak/bug in 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.

=head2 How can I open a file with a leading ">" or trailing blanks?

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 $_;
    }

    $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:

    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
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
permissions, timestamps, inode info, etc.

Newer versions of File::Copy exports a move() function.

=head2 How can I lock a file?

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
later), and lockf(3) if neither of the two previous system calls exists.
On some systems, it may even use a different form of native locking.
Here are some gotchas with Perl's flock():

=over 4

=item 1

Produces a fatal error if none of the three system calls (or their
close equivalent) exists.

=item 2

lockf(3) does not provide shared locking, and requires that the
filehandle be open for writing (or appending, or read/writing).

=item 3

Some versions of flock() can't lock files over a network (e.g. on NFS file
systems), so you'd need to force the use of fcntl(2) when you build Perl.
But even this is dubious at best.  See the flock entry of L<perlfunc>
and the F<INSTALL> file in the source distribution for information on
building Perl to do this.

Two potentially non-obvious but traditional flock semantics are that
it waits indefinitely until the lock is granted, and that its locks are
I<merely advisory>.  Such discretionary locks are more flexible, but
offer fewer guarantees.  This means that files locked with flock() may
be modified by programs that do not also use flock().  Cars that stop
for red lights get on well with each other, but not with cars that don't
stop for red lights.  See the perlport manpage, your port's specific
documentation, or your system-specific local manpages for details.  It's
best to assume traditional behavior if you're writing portable programs.
(If you're not, you should as always feel perfectly free to write
for your own system's idiosyncrasies (sometimes called "features").
Slavish adherence to portability concerns shouldn't get in the way of
your getting your job done.)

For more information on file locking, see also 
L<perlopentut/"File Locking"> if you have it (new for 5.6).

=back

=head2 Why can't I just open(FH, ">file.lock")?

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

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:

    sysopen(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.

=head2 I still don't get locking.  I just want to increment the number in the file.  How can I do this?

Didn't anyone ever tell you web-page hit counters were useless?
They don't count number of hits, they're a waste of time, and they serve
only to stroke the writer's vanity.  It's better to pick a random number;
they're more realistic.

Anyway, this is what you can do if you can't help yourself.

    use Fcntl qw(:DEFAULT :flock);
    sysopen(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: $!";

Here's a much better web-page hit counter:

    $hits = int( (time() - 850_000_000) / rand(1_000) );

If the count doesn't impress your friends, then the code might.  :-)

=head2 How do I randomly update a binary file?

If you're just trying to patch a binary, in many cases something as
simple as this works:

    perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs

However, if you have fixed sized records, then you might do something more
like this:

    $RECSIZE = 220; # size of record, in bytes
    $recno   = 37;  # which record to update
    open(FH, "+<somewhere") || die "can't update somewhere: $!";
    seek(FH, $recno * $RECSIZE, 0);
    read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
    # munge the record
    seek(FH, -$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?

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.

Here's an example:

    $write_secs = (stat($file))[9];
    printf "file %s updated at %s\n", $file,
	scalar localtime($write_secs);

If you prefer something more legible, use the File::stat module
(part of the standard distribution in version 5.004 and later):

    # error checking left as an exercise for reader.
    use File::stat;
    use Time::localtime;
    $date_string = ctime(stat($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>
for details.

=head2 How do I set a file's timestamp in perl?

You use the utime() function documented in L<perlfunc/utime>.
By way of example, here's a little program that copies the
read and write times from its first argument to all the rest
of them.

    if (@ARGV < 2) {
	die "usage: cptimes timestamp_file other_files ...\n";
    }
    $timestamp = shift;
    ($atime, $mtime) = (stat($timestamp))[8,9];
    utime $atime, $mtime, @ARGV;

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
utime() on those platforms.

=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 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:

    open (FH, "| tee file1 file2 file3");

Or even: