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perlfaq5 - Files and Formats
This section deals with I/O and the "f" issues: filehandles, flushing, formats, and
footers.
Perl does not support truly unbuffered output (except insofar as you can 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 $| affects buffering only for the currently selected
default file handle. You choose this handle with the one argument select() call (see perlvar/$| and perlfunc/select).
Use select() to choose the desired handle, then set its per-filehandle variables.
$old_fh = select(OUTPUT_HANDLE);
$| = 1;
select($old_fh);
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Some idioms can handle this in a single statement:
select((select(OUTPUT_HANDLE), $| = 1)[0]);
$| = 1, select $_ for select OUTPUT_HANDLE;
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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);
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or IO::Socket:
use IO::Socket; # this one is kinda a pipe?
my $sock = IO::Socket::INET->new( 'www.example.com:80' ) ;
$sock->autoflush();
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Use the Tie::File module, which is included in the standard distribution since Perl 5.8.0.
One fairly efficient way is to count newlines in the file. The following program uses a
feature of tr///, as documented in 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.
$lines = 0;
open(FILE, $filename) or die "Can't open `$filename': $!";
while (sysread FILE, $buffer, 4096) {
$lines += ($buffer =~ tr/\n//);
}
close FILE;
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This assumes no funny games with newline translations.
Use the File::Temp module, see File::Temp
for more information.
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
$fh = tempfile( DIR => $dir );
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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 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:
use IO::File;
$fh = IO::File->new_tmpfile()
or die "Unable to make new temporary file: $!";
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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{TMPDIR} || $ENV{TEMP};
my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
sub temp_file {
local *FH;
my $count = 0;
until (defined(fileno(FH)) || $count++ > 100) {
$base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
}
if (defined(fileno(FH))
return (*FH, $base_name);
} else {
return ();
}
}
}
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The most efficient way is using pack() and unpack(). This is faster than using substr() when
taking many, many strings. It is slower for just a few.
Here is a sample chunk of code to break up and put back together again some fixed-format
input lines, in this case from the output of a normal, Berkeley-style ps:
# sample input line:
# 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
$PS_T = 'A6 A4 A7 A5 A*';
open(PS, "ps|");
print scalar <PS>;
while (<PS>) {
($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
for $var (qw!pid tt stat time command!) {
print "$var: <$$var>\n";
}
print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
"\n";
}
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We've used $$var in a way that forbidden by use strict 'refs'. That
is, we've promoted a string to a scalar variable reference using symbolic references. This is
okay in small programs, but doesn't scale well. It also only works on global variables, not
lexicals.
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.
open my $fh, $file_name;
open local $fh, $file_name;
print $fh "Hello World!\n";
process_file( $fh );
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Before perl5.6, you had to deal with various typeglob idioms which you may see in older code.
open FILE, "> $filename";
process_typeglob( *FILE );
process_reference( \*FILE );
sub process_typeglob { local *FH = shift; print FH "Typeglob!" }
sub process_reference { local $fh = shift; print $fh "Reference!" }
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If you want to create many anonymous handles, you should check out the Symbol or IO::Handle
modules.
An indirect filehandle is using something other than a symbol in a place that a filehandle is
expected. Here are ways to get indirect filehandles:
$fh = SOME_FH; # bareword is strict-subs hostile
$fh = "SOME_FH"; # strict-refs hostile; same package only
$fh = *SOME_FH; # typeglob
$fh = \*SOME_FH; # ref to typeglob (bless-able)
$fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
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Or, you can use the 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 IO::Handle; # 5.004 or higher
$fh = IO::Handle->new();
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Then use any of those as you would a normal filehandle. Anywhere that Perl is expecting a
filehandle, an indirect filehandle may be used instead. An indirect filehandle is just a scalar
variable that contains a filehandle. Functions like print, open, seek,
or the <FH> diamond operator will accept either a named filehandle or a
scalar variable containing one:
($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
print $ofh "Type it: ";
$got = <$ifh>
print $efh "What was that: $got";
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If you're passing a filehandle to a function, you can write the function in two ways:
sub accept_fh {
my $fh = shift;
print $fh "Sending to indirect filehandle\n";
}
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Or it can localize a typeglob and use the filehandle directly:
sub accept_fh {
local *FH = shift;
print FH "Sending to localized filehandle\n";
}
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Both styles work with either objects or typeglobs of real filehandles. (They might also work
with strings under some circumstances, but this is risky.)
accept_fh(*STDOUT);
accept_fh($handle);
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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 of hashes or arrays, can
be used with built-ins like print, 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);
print $fd[1] "Type it: "; # WRONG
$got = <$fd[0]> # WRONG
print $fd[2] "What was that: $got"; # WRONG
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With print and printf, you get around this by using a block and an
expression where you would place the filehandle:
print { $fd[1] } "funny stuff\n";
printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
# Pity the poor deadbeef.
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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";
print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
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This approach of treating print and 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 readline to read a record
just as <> 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.
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.
There's no builtin way to do this, but perlform has a couple of
techniques to make it possible for the intrepid hacker.
See perlform/"Accessing
Formatting Internals" for an swrite() function.
This one from Benjamin Goldberg will do it for you:
s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
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or written verbosely:
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;
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Use the <> (glob()) operator, documented in 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:
$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;
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Because you're using something like this, which truncates the file and then gives you
read-write access:
open(FH, "+> /path/name"); # WRONG (almost always)
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Whoops. You should instead use this, which will fail if the file doesn't exist.
open(FH, "+< /path/name"); # open for update
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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
To open file for reading:
open(FH, "< $path") || die $!;
sysopen(FH, $path, O_RDONLY) || die $!;
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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 $!;
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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 $!;
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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 $!;
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To open file for appending, file must exist:
sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
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To open file for update, file must exist:
open(FH, "+< $path") || die $!;
sysopen(FH, $path, O_RDWR) || die $!;
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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 $!;
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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 $!;
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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: $!":
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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 perlopentut
if you have it (new for 5.6).
The <> 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 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 File::KGlob, one that doesn't use the shell to do globbing.
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.
Normally perl ignores trailing blanks in filenames, and interprets certain leading characters
(or a trailing "|") to mean something 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 literals
open FILE, "<", " file "; # filename is " file "
open FILE, ">", ">file"; # filename is ">file"
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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: $!";
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If your operating system supports a proper mv(1) utility or its functional equivalent, this
works:
rename($old, $new) or system("mv", $old, $new);
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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 export a move() function.
Perl's builtin flock() function (see 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():
- Produces a fatal error if none of the three system calls (or their close equivalent)
exists.
- lockf(3) does not provide shared locking, and requires that the filehandle be open for
writing (or appending, or read/writing).
-
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 perlfunc
and the 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 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 perlopentut/"File
Locking" if you have it (new for 5.6).
A common bit of code NOT TO USE is this:
sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
open(LCK, "> file.lock"); # THIS BROKEN CODE
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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: $!":
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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.
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: $!";
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Here's a much better web-page hit counter:
$hits = int( (time() - 850_000_000) / rand(1_000) );
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If the count doesn't impress your friends, then the code might. :-)
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.
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
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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;
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Locking and error checking are left as an exercise for the reader. Don't forget them or
you'll be quite sorry.
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 -M, -A, or -C file test
operations as documented in 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 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:
$write_secs = (stat($file))[9];
printf "file %s updated at %s\n", $file,
scalar localtime($write_secs);
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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";
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The POSIX::strftime() approach has the benefit of being, in theory, independent of the
current locale. See perllocale
for details.
You use the utime() function documented in 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;
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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.
If you only have to do this once, you can do this:
for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
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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");
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Or even:
# 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";
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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.
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>) {
chomp;
# do something with $_
}
close(INPUT) || die "can't close $file: $!";
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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 you see someone do this:
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.
You can read the entire filehandle contents into a scalar.
{
local(*INPUT, $/);
open (INPUT, $file) || die "can't open $file: $!";
$var = <INPUT>;
}
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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> };
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For ordinary files you can also use the read function.
read( INPUT, $var, -s INPUT );
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The third argument tests the byte size of the data on the INPUT filehandle and reads that
many bytes into the buffer $var.
Use the $/ variable (see perlvar for details). You can
either set it to "" to eliminate empty paragraphs ("abc\n\n\n\ndef",
for instance, gets treated as two paragraphs and not three), or "\n\n" to
accept empty paragraphs.
Note that a blank line must have no blanks in it. Thus "fred\n \nstuff\n\n"
is one paragraph, but "fred\n\nstuff\n\n" is two.
You can use the builtin 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 perlfunc/getc.
If your system supports the portable operating system programming interface (POSIX), you can
use the following code, which you'll note turns off echo processing as well.
#!/usr/bin/perl -w
use strict;
$| = 1;
for (1..4) {
my $got;
print "gimme: ";
$got = getone();
print "--> $got\n";
}
exit;
BEGIN {
use POSIX qw(:termios_h);
my ($term, $oterm, $echo, $noecho, $fd_stdin);
$fd_stdin = fileno(STDIN);
$term = POSIX::Termios->new();
$term->getattr($fd_stdin);
$oterm = $term->getlflag();
$echo = ECHO | ECHOK | ICANON;
$noecho = $oterm & ~$echo;
sub cbreak {
$term->setlflag($noecho);
$term->setcc(VTIME, 1);
$term->setattr($fd_stdin, TCSANOW);
}
sub cooked {
$term->setlflag($oterm);
$term->setcc(VTIME, 0);
$term->setattr($fd_stdin, TCSANOW);
}
sub getone {
my $key = '';
cbreak();
sysread(STDIN, $key, 1);
cooked();
return $key;
}
}
END { cooked() }
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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");
print "Gimme a char: ";
ReadMode "raw";
$key = ReadKey 0, *TTY;
ReadMode "normal";
printf "\nYou said %s, char number %03d\n",
$key, ord $key;
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The very first thing you should do is look into getting the Term::ReadKey extension from CPAN.
As we mentioned earlier, it now even has limited support for non-portable (read: not open
systems, closed, proprietary, not POSIX, not Unix, etc) systems.
You should also check out the Frequently Asked Questions list in comp.unix.* for things like
this: the answer is essentially the same. It's very system dependent. Here's one solution that
works on BSD systems:
sub key_ready {
my($rin, $nfd);
vec($rin, fileno(STDIN), 1) = 1;
return $nfd = select($rin,undef,undef,0);
}
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If you want to find out how many characters are waiting, there's also the FIONREAD ioctl call
to be looked at. The h2ph tool that comes with Perl tries to convert C include files to
Perl code, which can be required. FIONREAD ends up defined as a function in the sys/ioctl.ph
file:
require 'sys/ioctl.ph';
$size = pack("L", 0);
ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
$size = unpack("L", $size);
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If h2ph wasn't installed or doesn't work for you, you can grep the include
files by hand:
% grep FIONREAD /usr/include/*/*
/usr/include/asm/ioctls.h:#define FIONREAD 0x541B
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Or write a small C program using the editor of champions:
% cat > fionread.c
#include <sys/ioctl.h>
main() {
printf("%#08x\n", FIONREAD);
}
^D
% cc -o fionread fionread.c
% ./fionread
0x4004667f
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And then hard code it, leaving porting as an exercise to your successor.
$FIONREAD = 0x4004667f; # XXX: opsys dependent
$size = pack("L", 0);
ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
$size = unpack("L", $size);
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FIONREAD requires a filehandle connected to a stream, meaning that sockets, pipes, and tty
devices work, but not files.
First try
The statement seek(GWFILE, 0, 1) doesn't change the current position, but it
does clear the end-of-file condition on the handle, so that the next <GWFILE> makes Perl
try again to read something.
If that doesn't work (it relies on features of your stdio implementation), then you need
something more like this:
for (;;) {
for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
# search for some stuff and put it into files
}
# sleep for a while
seek(GWFILE, $curpos, 0); # seek to where we had been
}
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If this still doesn't work, look into the POSIX module. POSIX defines the clearerr() method,
which can remove the end of file condition on a filehandle. The method: read until end of file,
clearerr(), read some more. Lather, rinse, repeat.
There's also a File::Tail module from CPAN.
If you check perlfunc/open,
you'll see that several of the ways to call open() should do the trick. For example:
open(LOG, ">>/tmp/logfile");
open(STDERR, ">&LOG");
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Or even with a literal numeric descriptor:
$fd = $ENV{MHCONTEXTFD};
open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
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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 a copied one.
Error checking, as always, has been left as an exercise for the reader.
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 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 $rc == -1;
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Or, just use the fdopen(3S) feature of open():
{
local *F;
open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
close F;
}
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Whoops! You just put a tab and a formfeed into that filename! Remember that within double
quoted strings ("like\this"), the backslash is an escape character. The full list of
these is in perlop/Quote
and Quote-like Operators. Unsurprisingly, you don't have a file called "c:(tab)emp(formfeed)oo"
or "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
Either single-quote your strings, or (preferably) use forward slashes. Since all DOS and
Windows versions since something like MS-DOS 2.0 or so have treated / and \
the same in a path, you might as well use the one that doesn't clash with Perl--or the POSIX
shell, ANSI C and C++, awk, Tcl, Java, or Python, just to mention a few. POSIX paths are more
portable, too.
Because even on non-Unix ports, Perl's glob function follows standard Unix globbing
semantics. You'll need glob("*") to get all (non-hidden) files. This
makes glob() portable even to legacy systems. Your port may include proprietary globbing
functions as well. Check its documentation for details.
This is elaborately and painstakingly described in the file-dir-perms article in the
"Far More Than You Ever Wanted To Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz
.
The executive summary: learn how your filesystem works. The permissions on a file say what
can happen to the data in that file. The permissions on a directory say what can happen to the
list of files in that directory. If you delete a file, you're removing its name from the
directory (so the operation depends on the permissions of the directory, not of the file). If
you try to write to the file, the permissions of the file govern whether you're allowed to.
Here's an algorithm from the Camel Book:
srand;
rand($.) < 1 && ($line = $_) while <>;
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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.
Saying
joins together the elements of @lines with a space between them. If @lines
were ("little", "fluffy", "clouds") then the above
statement would print
but if each element of @lines was a line of text, ending a newline character ("little\n",
"fluffy\n", "clouds\n") then it would print:
If your array contains lines, just print them:
Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington. 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 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 be
courteous but is not required.
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