tail -f in perl?
-i clobber protected files? Isn't this a bug in Perl?
perlfaq5 - Files and Formats ($Revision: 1.24 $, $Date: 1998/07/05 15:07:20 $)
This section deals with I/O and the ``f'' issues: filehandles, flushing, formats, and footers.
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 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
$| variable to control autoflushing (see perlvar/$ and select):
$old_fh = select(OUTPUT_HANDLE);
$| = 1;
select($old_fh);
Or using the traditional idiom:
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 $| variable:
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();
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 the way things work in network programming: you really should specify the exact bit pattern on the network line terminator. In practice,
"\n\n" often works, but this is not portable.
See the perlfaq9 manpage for other examples of fetching URLs over the web.
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 $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. This assumes no locking.
$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.
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;
This assumes no funny games with newline translations.
Use the 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.
use IO::File;
$fh = IO::File->new_tmpfile()
or die "Unable to make new temporary file: $!";
Or you can use the 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.
use Fcntl;
use POSIX qw(tmpnam);
# 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);
# 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 : $!" }
# now go on to use the 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:
BEGIN {
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 {
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 ();
}
}
}
The most efficient way is using
pack() and
unpack(). This is faster than using
substr() when take 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: <$$var>\n";
}
print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
"\n";
}
We've used $$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 ok in small programs, but doesn't scale well.
It also only works on global variables, not lexicals.
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 ];
}
# 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";
}
For passing filehandles to functions, the easiest way is to prefer them with a star, as in
func(*STDIN). See
Passing Filehandles for details.
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:
foreach $filename (@names) {
use Symbol;
my $fh = gensym();
open($fh, "/etc/$filename") || die "open /etc/$filename: $!";
$file{$filename} = [ $i++, $fh ];
}
Or here using the semi-object-oriented FileHandle, which certainly isn't light-weight:
use FileHandle;
foreach $filename (@names) {
my $fh = FileHandle->new("/etc/$filename") or die "$filename: $!";
$file{$filename} = [ $i++, $fh ];
}
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.
An indirect filehandle is using something other than a symbol in a place that a filehandle is expected. Here are ways to get those:
$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
Or to use the new method from the FileHandle or
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
$fh = IO::Handle->new();
Then use any of those as you would a normal filehandle. Anywhere that Perl
is expecting a filehandle, an indirect filehandle may be used instead. An
indirect filehandle is just a scalar variable that contains a filehandle.
Functions like print, open, seek, or the functions or the <FH> diamond operator will accept either a read filehandle or a scalar variable
containing one:
($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
print $ofh "Type it: ";
$got = <$ifh>
print $efh "What was that: $got";
Of you're passing a filehandle to a function, you can write the function in two ways:
sub accept_fh {
my $fh = shift;
print $fh "Sending to indirect filehandle\n";
}
Or it can localize a typeglob and use the filehandle directly:
sub accept_fh {
local *FH = shift;
print FH "Sending to localized filehandle\n";
}
Both styles work with either objects or typeglobs of real filehandles. (They might also work with strings under some circumstances, but this is risky.)
accept_fh(*STDOUT);
accept_fh($handle);
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 print, printf, or the diamond operator. These are 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
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.
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";
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 reads a record just as <> 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.
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.
See the perlform manpage for an
swrite() function.
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 <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;
}
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;
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)
Whoops. You should instead use this, which will fail if the file doesn't exist. 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 successful create or unlink the same file! Therefore O_EXCL isn't so exclusive as you might wish.
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.
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.
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("<<<something really wicked ");
open(FH, "> $fn") or "couldn't open $fn: $!";
You could also use the
sysopen() function (see sysopen).
Well, usually you just use Perl's
rename() function. But that may not work everywhere, in particular, renaming files across file systems. If your operating system supports a
mv(1) program or its moral equivalent, this works:
rename($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.
The newer version of File::Copy export a
move() function.
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():
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).
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.
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
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 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. Better to pick a random number. It's more realistic.
Anyway, this is what you can do if you can't help yourself.
use Fcntl;
sysopen(FH, "numfile", O_RDWR|O_CREAT) 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. :-)
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, $recno * $RECSIZE, 0);
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.
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 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];
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):
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.
You use the
utime() function documented in 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 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.
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:
# 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";
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.
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.
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 getc.
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, "</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;
For 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 $c. If ord($c) == 0, you have a two byte code, which means you hit a special key. Read another
byte with sysread(STDIN,$c,1), and that value tells you what combination it was according to this table:
# PC 2-byte keycodes = ^@ + the following:
# HEX KEYS
# --- ----
# 0F SHF TAB
# 10-19 ALT QWERTYUIOP
# 1E-26 ALT ASDFGHJKL
# 2C-32 ALT ZXCVBNM
# 3B-44 F1-F10
# 47-49 HOME,UP,PgUp
# 4B LEFT
# 4D RIGHT
# 4F-53 END,DOWN,PgDn,Ins,Del
# 54-5D SHF F1-F10
# 5E-67 CTR F1-F10
# 68-71 ALT F1-F10
# 73-77 CTR LEFT,RIGHT,END,PgDn,HOME
# 78-83 ALT 1234567890-=
# 84 CTR PgUp
This is all trial and error I did a long time ago, I hope I'm reading the file that worked.
The very first thing you should do is look into getting the Term::ReadKey extension from CPAN. It now even has limited support for closed, proprietary (read: not open systems, not POSIX, not Unix, etc) systems.
You should also check out the Frequently Asked Questions list in comp.unix.* for things like this: the answer is essentially the same. It's very system dependent. Here's one solution that works on BSD systems:
sub key_ready {
my($rin, $nfd);
vec($rin, fileno(STDIN), 1) = 1;
return $nfd = select($rin,undef,undef,0);
}
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);
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
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
% ./fionread
0x4004667f
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", $size);
FIONREAD requires a filehandle connected to a stream, meaning sockets, pipes, and tty devices work, but not files.
tail -f in perl?First try
seek(GWFILE, 0, 1);
The statement seek(GWFILE, 0, 1) doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next <GWFILE> makes Perl try again to read something.
If that doesn't work (it relies on features of your stdio implementation), then you need something more like this:
for (;;) {
for ($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.
If you check 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");
Or even with a literal numeric descriptor:
$fd = $ENV{MHCONTEXTFD};
open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
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 $rc == -1;
Whoops! You just put a tab and a formfeed into that filename! Remember that within double quoted strings (``like\this''), the backslash is an escape character. The full list of these is in Quote and Quote-like Operators. Unsurprisingly, you don't have a file called ``c:(tab)emp(formfeed)oo'' or ``c:(tab)emp(formfeed)oo.exe'' on your DOS filesystem.
Either single-quote your strings, or (preferably) use forward slashes. Since all
DOS and Windows versions since something like
MS-DOS 2.0 or so have treated
/ and \ the same in a path, you might as well use the one that doesn't clash with Perl -- or the
POSIX shell,
ANSI
C and
C++, awk, Tcl, Java, or Python, just to mention a few.
Because even on non-Unix ports, Perl's glob function follows standard Unix
globbing semantics. You'll need glob("*") to get all (non-hidden) files. This makes
glob() portable.
-i clobber protected files? Isn't this a bug in Perl?This is elaborately and painstakingly described in the ``Far More Than You Ever Wanted To Know'' in http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms .
The executive summary: learn how your filesystem works. The permissions on a file say what can happen to the data in that file. The permissions on a directory say what can happen to the list of files in that directory. If you delete a file, you're removing its name from the directory (so the operation depends on the permissions of the directory, not of the file). If you try to write to the file, the permissions of the file govern whether you're allowed to.
Here's an algorithm from the Camel Book:
srand;
rand($.) < 1 && ($line = $_) while <>;
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.
Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington. 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 Licence. For separate distributions of all or part of this FAQ outside of that, see the perlfaq manpage.
Irrespective of its distribution, all code examples here are 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.
If rather than formatting bugs, you encounter substantive content errors in these documents, such as mistakes in the explanations or code, please use the perlbug utility included with the Perl distribution.