[9765] in Perl-Users-Digest
Perl-Users Digest, Issue: 3358 Volume: 8
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Wed Aug 5 09:07:11 1998
Date: Wed, 5 Aug 98 06:05:32 -0700
From: Perl-Users Digest <Perl-Users-Request@ruby.OCE.ORST.EDU>
To: Perl-Users@ruby.OCE.ORST.EDU (Perl-Users Digest)
Perl-Users Digest Wed, 5 Aug 1998 Volume: 8 Number: 3358
Today's topics:
perlfaq5 - Files and Formats (part 5 of 9) <perlfaq-suggestions@mox.perl.com>
Special: Digest Administrivia (Last modified: 12 Mar 98 (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: 5 Aug 1998 12:29:23 GMT
From: Tom Christiansen <perlfaq-suggestions@mox.perl.com>
Subject: perlfaq5 - Files and Formats (part 5 of 9)
Message-Id: <6q9j73$26r$1@csnews.cs.colorado.edu>
+-----------------------------------------------------------------------+
| The following document was generated from its original pod using |
| the pod2text program included with the standard perl release, plus |
| small cosmetic mark-ups. The FAQ is also distributed with all Perl |
| releases as standard manpages; their latest versions can be retrieved |
| from http://language.perl.com/misc/faqs.tar.gz if you'd like. |
+-----------------------------------------------------------------------+
NAME
perlfaq5 - Files and Formats ($Revision: 1.24 $, $Date: 1998/07/05
15:07:20 $)
DESCRIPTION
This section deals with I/O and the "f" issues: filehandles,
flushing, formats, and footers.
How do I flush/unbuffer an output filehandle? Why must I do this?
The C standard I/O library (stdio) normally buffers characters sent
to devices. This is done for efficiency reasons, so that there
isn't a system call for each byte. Any time you use print() or
write() in Perl, you go though this buffering. syswrite()
circumvents stdio and buffering.
In most stdio implementations, the type of output buffering and the
size of the buffer varies according to the type of device. Disk
files are block buffered, often with a buffer size of more than 2k.
Pipes and sockets are often buffered with a buffer size between 1/2
and 2k. Serial devices (e.g. modems, terminals) are normally line-
buffered, and stdio sends the entire line when it gets the newline.
Perl does not support truly unbuffered output (except insofar as
you can `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 the section on "$|" in
the perlvar manpage and the "select" entry in the perlfunc
manpage):
$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: $document\n";
Note the bizarrely hardcoded carriage return and newline in their
octal equivalents. This is the ONLY way (currently) to assure a
proper flush on all platforms, including Macintosh. That the way
things work in network programming: you really should specify the
exact bit pattern on the network line terminator. In practice,
`"\n\n"' often works, but this is not portable.
See the perlfaq9 manpage for other examples of fetching URLs over
the web.
How do I change one line in a file/delete a line in a file/insert a
line in the middle of a file/append to the beginning of a file?
Although humans have an easy time thinking of a text file as being
a sequence of lines that operates much like a stack of playing
cards -- or punch cards -- computers usually see the text file as a
sequence of bytes. In general, there's no direct way for Perl to
seek to a particular line of a file, insert text into a file, or
remove text from a file.
(There are exceptions in special circumstances. You can add or
remove at the very end of the file. Another is replacing a sequence
of bytes with another sequence of the same length. Another is using
the `$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.
How do I count the number of lines in a file?
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.
How do I make a temporary file name?
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 ();
}
}
}
How can I manipulate fixed-record-length files?
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.
How can I make a filehandle local to a subroutine? How do I pass
filehandles between subroutines? How do I make an array of
filehandles?
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 the section on "Passing
Filehandles" in the perlfaq7 manpage 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.
How can I use a filehandle indirectly?
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.
How can I set up a footer format to be used with 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.
How can I write() into a string?
See the perlform manpage for an swrite() function.
How can I output my numbers with commas added?
This one will do it for you:
sub commify {
local $_ = shift;
1 while s/^(-?\d+)(\d{3})/$1,$2/;
return $_;
}
$n = 23659019423.2331;
print "GOT: ", commify($n), "\n";
GOT: 23,659,019,423.2331
You can't just:
s/^(-?\d+)(\d{3})/$1,$2/g;
because you have to put the comma in and then recalculate your
position.
Alternatively, this 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;
}
How can I translate tildes (~) in a filename?
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;
How come when I open a file read-write it wipes it out?
Because you're using something like this, which truncates the file
and *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.
Why do I sometimes get an "Argument list too long" when I use <*>?
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 `Dirhandle's
and patterns, or use a module like Glob::KGlob, one that doesn't
use the shell to do globbing.
Is there a leak/bug in glob()?
Due to the current implementation on some operating systems, when
you use the glob() function or its angle-bracket alias in a scalar
context, you may cause a leak and/or unpredictable behavior. It's
best therefore to use glob() only in list context.
How can I open a file with a leading ">" or trailing blanks?
Normally perl ignores trailing blanks in filenames, and interprets
certain leading characters (or a trailing "|") to mean something
special. To avoid this, you might want to use a routine like 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: $!";
You could also use the sysopen() function (see the "sysopen" entry
in the perlfunc manpage).
How can I reliably rename a file?
Well, usually you just use Perl's rename() function. But that may
not work everywhere, in particular, renaming files across file
systems. If your operating system supports a mv(1) program or its
moral equivalent, this works:
rename($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.
How can I lock a file?
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.
What can't I just open(FH, ">file.lock")?
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.
I still don't get locking. I just want to increment the number in
the file. How can I do this?
Didn't anyone ever tell you web-page hit counters were useless?
They don't count number of hits, they're a waste of time, and they
serve only to stroke the writer's vanity. 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. :-)
How do I randomly update a binary file?
If you're just trying to patch a binary, in many cases something as
simple as this works:
perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
However, if you have fixed sized records, then you might do
something more like this:
$RECSIZE = 220; # size of record, in bytes
$recno = 37; # which record to update
open(FH, "+<somewhere") || die "can't update somewhere: $!";
seek(FH, $recno * $RECSIZE, 0);
read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
# munge the record
seek(FH, $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.
How do I get a file's timestamp in perl?
If you want to retrieve the time at which the file was last read,
written, or had its meta-data (owner, etc) changed, you use the -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.
How do I set a file's timestamp in perl?
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.
How do I print to more than one file at once?
If you only have to do this once, you can do this:
for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
To connect up to one filehandle to several output filehandles, it's
easiest to use the tee(1) program if you have it, and let it take
care of the multiplexing:
open (FH, "| tee file1 file2 file3");
Or even:
# 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: $!\n";
Otherwise you'll have to write your own multiplexing print function
-- or your own tee program -- or use Tom Christiansen's, at
http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz, which is
written in Perl and offers much greater functionality than the
stock version.
How can I read in a file by paragraphs?
Use the `$\' 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.
How can I read a single character from a file? From the keyboard?
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, "</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,$c,1)', and that value tells you what combination it
was according to this table:
# PC 2-byte keycodes = ^@ + the following:
# HEX KEYS
# --- ----
# 0F SHF TAB
# 10-19 ALT QWERTYUIOP
# 1E-26 ALT ASDFGHJKL
# 2C-32 ALT ZXCVBNM
# 3B-44 F1-F10
# 47-49 HOME,UP,PgUp
# 4B LEFT
# 4D RIGHT
# 4F-53 END,DOWN,PgDn,Ins,Del
# 54-5D SHF F1-F10
# 5E-67 CTR F1-F10
# 68-71 ALT F1-F10
# 73-77 CTR LEFT,RIGHT,END,PgDn,HOME
# 78-83 ALT 1234567890-=
# 84 CTR PgUp
This is all trial and error I did a long time ago, I hope I'm
reading the file that worked.
How can I tell if there's a character waiting on a filehandle?
The very first thing you should do is look into getting the
Term::ReadKey extension from CPAN. It now even has limited support
for closed, proprietary (read: not open systems, not POSIX, not
Unix, etc) systems.
You should also check out the Frequently Asked Questions list in
comp.unix.* for things like this: the answer is essentially the
same. It's very system dependent. Here's one solution that works on
BSD systems:
sub key_ready {
my($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 `require'd. 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", $size);
FIONREAD requires a filehandle connected to a stream, meaning
sockets, pipes, and tty devices work, but *not* files.
How do I do a `tail -f' in perl?
First try
seek(GWFILE, 0, 1);
The statement `seek(GWFILE, 0, 1)' doesn't change the current
position, but it does clear the end-of-file condition on the
handle, so that the next <GWFILE> makes Perl try again to read
something.
If that doesn't work (it relies on features of your stdio
implementation), then you need something more like this:
for (;;) {
for ($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.
How do I 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)
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.
How do I close a file descriptor by number?
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 $rc == -1;
Why can't I use "C:\temp\foo" in DOS paths? What doesn't
`C:\temp\foo.exe` work?
Whoops! You just put a tab and a formfeed into that filename!
Remember that within double quoted strings ("like\this"), the
backslash is an escape character. The full list of these is in the
section on "Quote and Quote-like Operators" in the perlop manpage.
Unsurprisingly, you don't have a file called
"c:(tab)emp(formfeed)oo" or "c:(tab)emp(formfeed)oo.exe" on your
DOS filesystem.
Either single-quote your strings, or (preferably) use forward
slashes. Since all DOS and Windows versions since something like
MS-DOS 2.0 or so have treated `/' and `\' the same in a path, you
might as well use the one that doesn't clash with Perl -- or the
POSIX shell, ANSI C and C++, awk, Tcl, Java, or Python, just to
mention a few.
Why doesn't glob("*.*") get all the files?
Because even on non-Unix ports, Perl's glob function follows
standard Unix globbing semantics. You'll need `glob("*")' to get
all (non-hidden) files. This makes glob() portable.
Why does Perl let me delete read-only files? Why does `-i' clobber
protected files? Isn't this a bug in Perl?
This is elaborately and painstakingly described in the "Far More
Than You Ever Wanted To Know" in
http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms .
The executive summary: learn how your filesystem works. The
permissions on a file say what can happen to the data in that file.
The permissions on a directory say what can happen to the list of
files in that directory. If you delete a file, you're removing its
name from the directory (so the operation depends on the
permissions of the directory, not of the file). If you try to write
to the file, the permissions of the file govern whether you're
allowed to.
How do I select a random line from a file?
Here's an algorithm from the Camel Book:
srand;
rand($.) < 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.
AUTHOR AND COPYRIGHT
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.
------------------------------
Date: 12 Jul 98 21:33:47 GMT (Last modified)
From: Perl-Request@ruby.oce.orst.edu (Perl-Users-Digest Admin)
Subject: Special: Digest Administrivia (Last modified: 12 Mar 98)
Message-Id: <null>
Administrivia:
Special notice: in a few days, the new group comp.lang.perl.moderated
should be formed. I would rather not support two different groups, and I
know of no other plans to create a digested moderated group. This leaves
me with two options: 1) keep on with this group 2) change to the
moderated one.
If you have opinions on this, send them to
perl-users-request@ruby.oce.orst.edu.
The Perl-Users Digest is a retransmission of the USENET newsgroup
comp.lang.perl.misc. For subscription or unsubscription requests, send
the single line:
subscribe perl-users
or:
unsubscribe perl-users
to almanac@ruby.oce.orst.edu.
To submit articles to comp.lang.perl.misc (and this Digest), send your
article to perl-users@ruby.oce.orst.edu.
To submit articles to comp.lang.perl.announce, send your article to
clpa@perl.com.
To request back copies (available for a week or so), send your request
to almanac@ruby.oce.orst.edu with the command "send perl-users x.y",
where x is the volume number and y is the issue number.
The Meta-FAQ, an article containing information about the FAQ, is
available by requesting "send perl-users meta-faq". The real FAQ, as it
appeared last in the newsgroup, can be retrieved with the request "send
perl-users FAQ". Due to their sizes, neither the Meta-FAQ nor the FAQ
are included in the digest.
The "mini-FAQ", which is an updated version of the Meta-FAQ, is
available by requesting "send perl-users mini-faq". It appears twice
weekly in the group, but is not distributed in the digest.
For other requests pertaining to the digest, send mail to
perl-users-request@ruby.oce.orst.edu. Do not waste your time or mine
sending perl questions to the -request address, I don't have time to
answer them even if I did know the answer.
------------------------------
End of Perl-Users Digest V8 Issue 3358
**************************************
