PERLFAQ5(1)
NNAAMMEE
perlfaq5 - Files and Formats ($Revision: 1.22 $, $Date:
1997/04/24 22:44:02 $)
DDEESSCCRRIIPPTTIIOONN
This section deals with I/O and the "f" issues:
filehandles, flushing, formats, and footers.
HHooww ddoo II fflluusshh//uunnbbuuffffeerr aa ffiilleehhaannddllee?? WWhhyy mmuusstt II ddoo tthhiiss??
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 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 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, as
in the older:
use FileHandle;
open(DEV, "<+/dev/tty"); # ceci n'est pas une pipe
DEV->autoflush(1);
or the newer IO::* modules:
use IO::Handle;
open(DEV, ">/dev/printer"); # but is this?
DEV->autoflush(1);
or even this:
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;
$sock->autoflush();
$sock->print("GET /\015\012");
$document = join('', $sock->getlines());
print "DOC IS: $document\n";
Note the 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.
You can use select() and the $| variable to control
autoflushing (see the section on $| in the perlvar manpage
and the select entry in the perlfunc manpage):
$oldh = select(DEV);
$| = 1;
select($oldh);
You'll also see code that does this without a temporary
variable, as in
select((select(DEV), $| = 1)[0]);
HHooww ddoo II cchhaannggee oonnee lliinnee iinn aa ffiillee//ddeelleettee aa lliinnee iinn aa
ffiillee//iinnsseerrtt aa lliinnee iinn tthhee mmiiddddllee ooff aa ffiillee//aappppeenndd ttoo tthhee
bbeeggiinnnniinngg ooff aa ffiillee??
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. Replacing
a sequence of bytes with another sequence of the same
length is one. Another is using the $DB_RECNO array
bindings as documented in the DB_File manpage. 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.
$old = $file;
$new = "$file.tmp.$$";
$bak = "$file.bak";
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 -i command-line switch or the closely-related $^I
variable (see the perlrun manpage for more details). Note
that -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) = ('.bak', 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.
HHooww ddoo II ccoouunntt tthhee nnuummbbeerr ooff lliinneess iinn aa ffiillee??
One fairly efficient way is to count newlines in the file.
The following program uses a feature of tr///, as
documented in the perlop manpage. 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;
HHooww ddoo II mmaakkee aa tteemmppoorraarryy ffiillee nnaammee??
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 IO::File;
use Fcntl;
my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP} || $ENV{TEMP};
my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
sub temp_file {
my $fh = undef;
my $count = 0;
until (defined($fh) || $count > 100) {
$base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
$fh = IO::File->new($base_name, O_WRONLY|O_EXCL|O_CREAT, 0644)
}
if (defined($fh)) {
return ($fh, $base_name);
} else {
return ();
}
}
}
Or you could simply use IO::Handle::new_tmpfile.
HHooww ccaann II mmaanniippuullaattee ffiixxeedd--rreeccoorrdd--lleennggtthh ffiilleess??
The most efficient way is using pack() and unpack(). This
is faster than using substr(). 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|");
$_ = <PS>; print;
while (<PS>) {
($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";
}
HHooww ccaann II mmaakkee aa ffiilleehhaannddllee llooccaall ttoo aa ssuubbrroouuttiinnee?? HHooww ddoo
II ppaassss ffiilleehhaannddlleess bbeettwweeeenn ssuubbrroouuttiinneess?? HHooww ddoo II mmaakkee aann
aarrrraayy ooff ffiilleehhaannddlleess??
You may have some success with typeglobs, as we always had
to use in days of old:
local(*FH);
But while still supported, that isn't the best to go about
getting local filehandles. Typeglobs have their
drawbacks. You may well want to use the FileHandle
module, which creates new filehandles for you (see the
FileHandle manpage):
use FileHandle;
sub findme {
my $fh = FileHandle->new();
open($fh, "IO::Handle manpage), or one of its more
specific derived classes.
Once you have IO::File or FileHandle objects, you can pass
them between subroutines or store them in hashes as you
would any other scalar values:
use FileHandle;
# Storing filehandles in a hash and array
foreach $filename (@names) {
my $fh = new FileHandle($filename) or die;
$file{$filename} = $fh;
push(@files, $fh);
}
# Using the filehandles in the array
foreach $file (@files) {
print $file "Testing\n";
}
# You have to do the { } ugliness when you're specifying the
# filehandle by anything other than a simple scalar variable.
print { $files[2] } "Testing\n";
# Passing filehandles to subroutines
sub debug {
my $filehandle = shift;
printf $filehandle "DEBUG: ", @_;
}
debug($fh, "Testing\n");
HHooww ccaann II sseett uupp aa ffooootteerr ffoorrmmaatt ttoo bbee uusseedd wwiitthh write()?
There's no builtin way to do this, but the perlform
manpage has a couple of techniques to make it possible for
the intrepid hacker.
HHooww ccaann II write() into a string?
See the perlform manpage for an swrite() function.
HHooww ccaann II oouuttppuutt mmyy nnuummbbeerrss wwiitthh ccoommmmaass aaddddeedd??
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 commifies all numbers in a line
regardless of whether they have decimal portions, are
preceded by + or -, or whatever:
# from Andrew Johnson lt;ajohnson@gpu.srv.ualberta.ca
sub commify {
my $input = shift;
$input = reverse $input;
$input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g;
return reverse $input;
}
HHooww ccaann II ttrraannssllaattee ttiillddeess ((~~)) iinn aa ffiilleennaammee??
Use the <> (glob()) operator, documented in the perlfunc
manpage. 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.
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;
HHooww ccoommee wwhheenn II ooppeenn tthhee ffiillee rreeaadd--wwrriittee iitt wwiippeess iitt oouutt??
Because you're using something like this, which truncates
the file and then gives you read-write access:
open(FH, "+> /path/name"); # WRONG
Whoops. You should instead use this, which will fail if
the file doesn't exist.
open(FH, "+< /path/name"); # open for update
If this is an issue, try:
sysopen(FH, "/path/name", O_RDWR|O_CREAT, 0644);
Error checking is left as an exercise for the reader.
WWhhyy ddoo II ssoommeettiimmeess ggeett aann """"AArrgguummeenntt lliisstt ttoooo lloonngg"""" wwhheenn
II uussee <<<<**>>>>??
The <> operator performs a globbing operation (see above).
By default glob() 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 do the glob yourself with
Dirhandles and patterns, or use a module like Glob::KGlob,
one that doesn't use the shell to do globbing.
IIss tthheerree aa lleeaakk//bbuugg iinn 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 best therefore to use
glob() only in list context.
HHooww ccaann II ooppeenn aa ffiillee wwiitthh aa lleeaaddiinngg """">>>>"""" oorr ttrraaiilliinngg
bbllaannkkss??
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 $_ = shift;
return m#^/#
? "$_\0"
: "./$_\0";
}
$fn = safe_filename("<< $fn") or "couldn't open $fn: $!";
You could also use the sysopen() function (see the sysopen
entry in the perlfunc manpage).
HHooww ccaann II rreelliiaabbllyy rreennaammee aa ffiillee??
Well, usually you just use Perl's rename() function. But
that may not work everywhere, in particular, renaming
files across file systems. If your operating system
supports a mv(1) program or its moral equivalent, this
works:
rename($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 semantics as a real rename(),
though, which preserves metainformation like permissions,
timestamps, inode info, etc.
HHooww ccaann II lloocckk aa ffiillee??
Perl's builtin flock() function (see the perlfunc manpage
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():
1 Produces a fatal error if none of the three system
calls (or their close equivalent) exists.
2 lockf(3) does not provide shared locking, and requires
that the filehandle be open for writing (or appending,
or read/writing).
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. See
the flock entry of the perlfunc manpage, and the
INSTALL file in the source distribution for
information on building Perl to do this.
The CPAN module File::Lock offers similar functionality
and (if you have dynamic loading) won't require you to
rebuild perl if your flock() can't lock network files.
WWhhaatt ccaann''tt II jjuusstt open(FH, "">file.lock")?
A common bit of code NNOOTT TTOO UUSSEE 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, 0644)
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
involving link() have been suggested, but these tend to
involve busy-wait, which is also subdesirable.
II ssttiillll ddoonn''tt ggeett lloocckkiinngg.. II jjuusstt wwaanntt ttoo iinnccrreemmeenntt tthhee
nnuummbbeerr iinn tthhee ffiillee.. HHooww ccaann II ddoo tthhiiss??
Didn't anyone ever tell you web-page hit counters were
useless?
Anyway, this is what to do:
use Fcntl;
sysopen(FH, "numfile", O_RDWR|O_CREAT, 0644) or die "can't open numfile: $!";
flock(FH, 2) 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: $!";
# DO NOT UNLOCK THIS UNTIL YOU CLOSE
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. :-)
HHooww ddoo II rraannddoommllyy uuppddaattee aa bbiinnaarryy ffiillee??
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, "+binmode() under DOS-like platforms
when operating on files that have anything other than
straight text in them. See the docs on open() and on
binmode() for more details.
HHooww ddoo II ggeett aa ffiillee''ss ttiimmeessttaammpp iinn ppeerrll??
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 --MM, --AA, or --CC filetest operations as
documented in the perlfunc manpage. 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];
print "file $file updated at ", scalar(localtime($file)), "\n";
If you prefer something more legible, use the File::stat
module (part of the standard distribution in version 5.004
and later):
use File::stat;
use Time::localtime;
$date_string = ctime(stat($file)->mtime);
print "file $file updated at $date_string\n";
Error checking is left as an exercise for the reader.
HHooww ddoo II sseett aa ffiillee''ss ttiimmeessttaammpp iinn ppeerrll??
You use the utime() function documented in the utime entry
in the perlfunc manpage. 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 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.
HHooww ddoo II pprriinntt ttoo mmoorree tthhaann oonnee ffiillee aatt oonnccee??
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");
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.
In theory a IO::Tee class could be written, but to date we
haven't seen such.
HHooww ccaann II rreeaadd iinn aa ffiillee bbyy ppaarraaggrraapphhss??
Use the $\ variable (see the perlvar manpage 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.
HHooww ccaann II rreeaadd aa ssiinnggllee cchhaarraacctteerr ffrroomm aa ffiillee?? FFrroomm tthhee
kkeeyybbooaarrdd??
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 the getc entry in the
perlfunc manpage.
If your system supports 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() }
The Term::ReadKey module from CPAN may be easier to use:
use Term::ReadKey;
open(TTY, "lt;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 $c. If ord($c) == 0, you have a two byte
code, which means you hit a special key. Read another
byte with sysread(STDIN,$c,1), and that value tells you
what combination it was according to this table:
# PC 2-byte keycodes = ^@ + the following:
# HEX KEYS
# --- ----
# 0F SHF TAB
# 10-19 ALT QWERTYUIOP
# 1E-26 ALT ASDFGHJKL
# 2C-32 ALT ZXCVBNM
# 3B-44 F1-F10
# 47-49 HOME,UP,PgUp
# 4B LEFT
# 4D RIGHT
# 4F-53 END,DOWN,PgDn,Ins,Del
# 54-5D SHF F1-F10
# 5E-67 CTR F1-F10
# 68-71 ALT F1-F10
# 73-77 CTR LEFT,RIGHT,END,PgDn,HOME
# 78-83 ALT 1234567890-=
# 84 CTR PgUp
This is all trial and error I did a long time ago, I hope
I'm reading the file that worked.
HHooww ccaann II tteellll iiff tthheerree''ss aa cchhaarraacctteerr wwaaiittiinngg oonn aa
ffiilleehhaannddllee??
You should check out the Frequently Asked Questions list
in comp.unix.* for things like this: the answer is
essentially the same. It's very system dependent. Here's
one solution that works on BSD systems:
sub key_ready {
my($rin, $nfd);
vec($rin, fileno(STDIN), 1) = 1;
return $nfd = select($rin,undef,undef,0);
}
You should look into getting the Term::ReadKey extension
from CPAN.
HHooww ddoo II ooppeenn aa ffiillee wwiitthhoouutt bblloocckkiinngg??
You need to use the O_NDELAY or O_NONBLOCK flag from the
Fcntl module in conjunction with sysopen():
use Fcntl;
sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
or die "can't open /tmp/somefile: $!":
HHooww ddoo II ccrreeaattee aa ffiillee oonnllyy iiff iitt ddooeessnn''tt eexxiisstt??
You need to use the O_CREAT and O_EXCL flags from the
Fcntl module in conjunction with sysopen():
use Fcntl;
sysopen(FH, "/tmp/somefile", O_WRONLY|O_EXCL|O_CREAT, 0644)
or die "can't open /tmp/somefile: $!":
Be warned that neither creation nor deletion of files is
guaranteed to be an atomic operation over NFS. That is,
two processes might both successful create or unlink the
same file!
HHooww ddoo II ddoo aa ttaaiill --ff in perl?
First try
seek(GWFILE, 0, 1);
The statement seek(GWFILE, 0, 1) doesn't change the
current position, but it does clear the end-of-file
condition on the handle, so that the next <GWFILE> makes
Perl try again to read something.
If that doesn't work (it relies on features of your stdio
implementation), then you need something more like this:
for (;;) {
for ($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
}
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.
HHooww ddoo II dup() a filehandle in Perl?
If you check the open entry in the perlfunc manpage,
you'll see that several of the ways to call open() should
do the trick. For example:
open(LOG, ">>/tmp/logfile");
open(STDERR, ">&LOG");
Or even with a literal numeric descriptor:
$fd = $ENV{MHCONTEXTFD};
open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
Error checking has been left as an exercise for the
reader.
HHooww ddoo II cclloossee aa ffiillee ddeessccrriippttoorr bbyy nnuummbbeerr??
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 $rc == -1;
WWhhyy ccaann''tt II uussee """"CC::\\tteemmpp\\ffoooo"""" iinn DDOOSS ppaatthhss?? WWhhaatt
ddooeessnn''tt ``CC::\\tteemmpp\\ffoooo..eexxee`` wwoorrkk??
Whoops! You just put a tab and a formfeed into that
filename! Remember that within double quoted strings
("like\this"), the backslash is an escape character. The
full list of these is in the section on Quote and Quote-
like Operators in the perlop manpage. Unsurprisingly, you
don't have a file called "c:(tab)emp(formfeed)oo" or
"c:(tab)emp(formfeed)oo.exe" on your DOS filesystem.
Either single-quote your strings, or (preferably) use
forward slashes. Since all DOS and Windows versions since
something like MS-DOS 2.0 or so have treated / and \ the
same in a path, you might as well use the one that doesn't
clash with Perl -- or the POSIX shell, ANSI C and C++,
awk, Tcl, Java, or Python, just to mention a few.
WWhhyy ddooeessnn''tt glob("*.*") get all the files?
Because even on non-Unix ports, Perl's glob function
follows standard Unix globbing semantics. You'll need
glob("*") to get all (non-hidden) files.
WWhhyy ddooeess PPeerrll lleett mmee ddeelleettee rreeaadd--oonnllyy ffiilleess?? WWhhyy ddooeess --ii
clobber protected files? Isn't this a bug in Perl?
This is elaborately and painstakingly described in the
"Far More Than You Every Wanted To Know" in
http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms .
The executive summary: learn how your filesystem works.
The permissions on a file say what can happen to the data
in that file. The permissions on a directory say what can
happen to the list of files in that directory. If you
delete a file, you're removing its name from the directory
(so the operation depends on the permissions of the
directory, not of the file). If you try to write to the
file, the permissions of the file govern whether you're
allowed to.
HHooww ddoo II sseelleecctt aa rraannddoomm lliinnee ffrroomm aa ffiillee??
Here's an algorithm from the Camel Book:
srand;
rand($.) < 1 && ($line = $_) while <>;
This has a significant advantage in space over reading the
whole file in.
AAUUTTHHOORR AANNDD CCOOPPYYRRIIGGHHTT
Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
All rights reserved. See the perlfaq manpage for
distribution information.