PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)NAMEperlfaq5 - Files and Formats ($Revision: 1.38 $, $Date:
1999/05/23 16:08:30 $)
DESCRIPTION
This section deals with I/O and the "f" issues: filehan
dles, 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 char
acters sent to devices. This is done for efficiency rea
sons 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 buffer
ing.
In most stdio implementations, the type of output buffer
ing 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 the select entry in the perlfunc man
page):
$old_fh = select(OUTPUT_HANDLE);
$| = 1;
select($old_fh);
Or using the traditional idiom:
select((select(OUTPUT_HANDLE), $| = 1)[0]);
Or if don't mind slowly loading several thousand lines of
module code just because you're afraid of the "$|" vari
able:
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: $document\n";
Note the bizarrely hardcoded carriage return and newline
in their octal equivalents. This is the ONLY way (cur
rently) to assure a proper flush on all platforms, includ
ing Macintosh. That's the way things work in network pro
gramming: you really should specify the exact bit pattern
on the network line terminator. In practice, ""\n\n""
often works, but this is not portable.
See the perlfaq9 manpage for other examples of fetching
URLs over the web.
How do I change one line in a file/delete a line in a
file/insert a line in the middle of a file/append to the
beginning of a file?
Those are operations of a text editor. Perl is not a text
editor. Perl is a programming language. You have to
decompose the problem into low-level calls to read, write,
open, close, and seek.
Although humans have an easy time thinking of a text file
as being a sequence of lines that operates much like a
stack of playing cards--or punch cards--computers usually
see the text file as a sequence of bytes. In general,
there's no direct way for Perl to seek to a particular
line of a file, insert text into a file, or remove text
from a file.
(There are exceptions in special circumstances. You can
add or remove data at the very end of the file. A
sequence of bytes can be replaced with another sequence of
the same length. The "$DB_RECNO" array bindings as docu
mented in the DB_File manpage also provide a direct way of
modifying a file. Files where all lines are the same
length are also easy to alter.)
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.orig";
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) = ('.orig', 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 docu
mented 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 mod
ule to get a filehandle opened for reading and writing.
Use it if you don't need to know the file's name:
use IO::File;
$fh = IO::File->new_tmpfile()
or die "Unable to make new temporary file: $!";
If you do need to know the file's name, you can use the
"tmpnam" function from the POSIX module to get a filename
that you then open yourself:
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 creating a temporary file by hand,
use the process ID and/or the current time-value. If you
need to have many temporary files in one process, use a
counter:
BEGIN {
use Fcntl;
my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{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 taking many, many
strings. It is slower for just a few.
Here is a sample chunk of code to break up and put back
together again some fixed-format input lines, in this case
from the output of a normal, Berkeley-style ps:
# sample input line:
# 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
$PS_T = 'A6 A4 A7 A5 A*';
open(PS, "ps|");
print scalar <PS>;
while (<PS>) {
($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
for $var (qw!pid tt stat time command!) {
print "$var: <$$var>\n";
}
print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
"\n";
}
We've used "$$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 alterna
tives) 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 typeglobs 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 preface them with a star, as in func(*STDIN). See the
Passing Filehandles entry 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 ];
}
Here's using the semi-object-oriented FileHandle module,
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 ques
tion.
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 indirect filehandles:
$fh = SOME_FH; # bareword is strict-subs hostile
$fh = "SOME_FH"; # strict-refs hostile; same package only
$fh = *SOME_FH; # typeglob
$fh = \*SOME_FH; # ref to typeglob (bless-able)
$fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
Or, you can 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
$fh = IO::Handle->new();
Then use any of those as you would a normal filehandle.
Anywhere that Perl is expecting a filehandle, an indirect
filehandle may be used instead. An indirect filehandle is
just a scalar variable that contains a filehandle. Func
tions like "print", "open", "seek", or the "<FH>" diamond
operator will accept either a read filehandle or a scalar
variable containing one:
($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
print $ofh "Type it: ";
$got = <$ifh>
print $efh "What was that: $got";
If you're passing a filehandle to a function, you can
write the function in two ways:
sub accept_fh {
my $fh = shift;
print $fh "Sending to indirect filehandle\n";
}
Or it can localize a typeglob and use the filehandle
directly:
sub accept_fh {
local *FH = shift;
print FH "Sending to localized filehandle\n";
}
Both styles work with either objects or typeglobs of real
filehandles. (They might also work with strings under
some circumstances, but this is risky.)
accept_fh(*STDOUT);
accept_fh($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 of
hashes or arrays, can be used with built-ins like "print",
"printf", or the diamond operator. Using something other
than a simple scalar varaible as a filehandle is illegal
and won't even compile:
@fd = (*STDIN, *STDOUT, *STDERR);
print $fd[1] "Type it: "; # WRONG
$got = <$fd[0]> # WRONG
print $fd[2] "What was that: $got"; # WRONG
With "print" and "printf", you get around this by using a
block and an expression where you would place the filehan
dle:
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 funda
mental 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 man
page has a couple of techniques to make it possible for
the intrepid hacker.
How can I write() into a string?
See the Accessing Formatting Internals entry in the perl
form 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 code commifies all numbers in a line
regardless of whether they have decimal portions, are pre
ceded 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 scalar reverse $input;
}
How can I translate tildes (~) in a filename?
Use the <> (glob()) operator, documented in the perlfunc
manpage. Older versions of Perl require that you have a
shell installed that groks tildes. Recent perl versions
have this feature built in. The Glob::KGlob module (avail
able 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.
open(FH, "+< /path/name"); # open for update
Using ">" always clobbers or creates. Using "<" never
does either. The "+" doesn't change this.
Here are examples of many kinds of file opens. Those
using sysopen() all assume
use Fcntl;
To open file for reading:
open(FH, "< $path") || die $!;
sysopen(FH, $path, O_RDONLY) || die $!;
To open file for writing, create new file if needed or
else truncate old file:
open(FH, "> $path") || die $!;
sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
To open file for writing, create new file, file must not
exist:
sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
To open file for appending, create if necessary:
open(FH, ">> $path") || die $!;
sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
To open file for appending, file must exist:
sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
To open file for update, file must exist:
open(FH, "+< $path") || die $!;
sysopen(FH, $path, O_RDWR) || die $!;
To open file for update, create file if necessary:
sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
To open file for update, file must not exist:
sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
To open a file without blocking, creating if necessary:
sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT)
or die "can't open /tmp/somefile: $!":
Be warned that neither creation nor deletion of files is
guaranteed to be an atomic operation over NFS. That is,
two processes might both successfully create or unlink the
same file! Therefore O_EXCL isn't as exclusive as you
might wish.
See also the new the perlopentut manpage if you have it
(new for 5.6).
Why do I sometimes get an "Argument list too long" when I
use <*>?
The "<>" operator performs a globbing operation (see
above). In Perl versions earlier than v5.6.0, the inter
nal glob() operator forks csh(1) to do the actual glob
expansion, but csh can't handle more than 127 items and so
gives the error message "Argument list too long". People
who installed tcsh as csh won't have this problem, but
their users may be surprised by it.
To get around this, either upgrade to Perl v5.6.0 or
later, do the glob yourself with readdir() and patterns,
or use a module like 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 sys
tems, when you use the glob() function or its angle-
bracket alias in a scalar context, you may cause a memory
leak and/or unpredictable behavior. It's best therefore
to use glob() only in list context.
How can I open a file with a leading ">" or trailing
blanks?
Normally perl ignores trailing blanks in filenames, and
interprets certain leading characters (or a trailing "|")
to mean something special. To avoid this, you might want
to use a routine like the one below. It turns incomplete
pathnames into explicit relative ones, and tacks a trail
ing null byte on the name to make perl leave it alone:
sub safe_filename {
local $_ = shift;
s#^([^./])#./$1#;
$_ .= "\0";
return $_;
}
$badpath = "<<<something really wicked ";
$fn = safe_filename($badpath");
open(FH, "> $fn") or "couldn't open $badpath: $!";
This assumes that you are using POSIX (portable operating
systems interface) paths. If you are on a closed, non-
portable, proprietary system, you may have to adjust the
""./"" above.
It would be a lot clearer to use sysopen(), though:
use Fcntl;
$badpath = "<<<something really wicked ";
sysopen (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
or die "can't open $badpath: $!";
For more information, see also the new the perlopentut
manpage if you have it (new for 5.6).
How can I reliably rename a file?
Well, usually you just use Perl's rename() function. That
may not work everywhere, though, particularly when renam
ing files across file systems. Some sub-Unix systems have
broken ports that corrupt the semantics of rename()--for
example, WinNT does this right, but Win95 and Win98 are
broken. (The last two parts are not surprising, but the
first is. :-)
If your operating system supports a proper mv(1) program
or its moral equivalent, this works:
rename($old, $new) or system("mv", $old, $new);
It may be more compelling to use the File::Copy module
instead. You just copy to the new file to the new name
(checking return values), then delete the old one. This
isn't really the same semantically as a real rename(),
though, which preserves metainformation like permissions,
timestamps, inode info, etc.
Newer versions of File::Copy exports a move() function.
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 net
work (e.g. on NFS file systems), so you'd need to
force the use of fcntl(2) when you build Perl. But
even this is dubious at best. See the flock entry of
the perlfunc manpage and the INSTALL file in the
source distribution for information on building Perl
to do this.
Two potentially non-obvious but traditional flock
semantics are that it waits indefinitely until the
lock is granted, and that its locks are merely advi_
sory. Such discretionary locks are more flexible, but
offer fewer guarantees. This means that files locked
with flock() may be modified by programs that do not
also use flock(). Cars that stop for red lights get
on well with each other, but not with cars that don't
stop for red lights. See the perlport manpage, your
port's specific documentation, or your system-specific
local manpages for details. It's best to assume tra
ditional behavior if you're writing portable programs.
(If you're not, you should as always feel perfectly
free to write for your own system's idiosyncrasies
(sometimes called "features"). Slavish adherence to
portability concerns shouldn't get in the way of your
getting your job done.)
For more information on file locking, see also the
File Locking entry in the perlopentut manpage if you
have it (new for 5.6).
Why 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
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 van
ity. It's better to pick a random number; they're more
realistic.
Anyway, this is what you can do if you can't help your
self.
use Fcntl qw(:DEFAULT :flock);
sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
flock(FH, LOCK_EX) or die "can't flock numfile: $!";
$num = <FH> || 0;
seek(FH, 0, 0) or die "can't rewind numfile: $!";
truncate(FH, 0) or die "can't truncate numfile: $!";
(print FH $num+1, "\n") or die "can't write numfile: $!";
close FH or die "can't close numfile: $!";
Here's a much better web-page hit counter:
$hits = int( (time() - 850_000_000) / rand(1_000) );
If the count doesn't impress your friends, then the code
might. :-)
How do I randomly update a binary file?
If you're just trying to patch a binary, in many cases
something as simple as this works:
perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
However, if you have fixed sized records, then you might
do something more like this:
$RECSIZE = 220; # size of record, in bytes
$recno = 37; # which record to update
open(FH, "+<somewhere") || die "can't update somewhere: $!";
seek(FH, $recno * $RECSIZE, 0);
read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
# munge the record
seek(FH, -$RECSIZE, 1);
print FH $record;
close FH;
Locking and error checking are left as an exercise for the
reader. Don't forget them or you'll be quite sorry.
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:\fIs0:strftime() to convert this into human-readable
form.
Here's an example:
$write_secs = (stat($file))[9];
printf "file %s updated at %s\n", $file,
scalar localtime($write_secs);
If you prefer something more legible, use the File::stat
module (part of the standard distribution in version 5.004
and later):
# error checking left as an exercise for reader.
use File::stat;
use Time::localtime;
$date_string = ctime(stat($file)->mtime);
print "file $file updated at $date_string\n";
The POSIX:\fIs0:strftime() approach has the benefit of
being, in theory, independent of the current locale. See
the perllocale manpage for details.
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 lit
tle program that copies the read and write times from its
first argument to all the rest of them.
if (@ARGV < 2) {
die "usage: cptimes timestamp_file other_files ...\n";
}
$timestamp = shift;
($atime, $mtime) = (stat($timestamp))[8,9];
utime $atime, $mtime, @ARGV;
Error checking is, as usual, left as an exercise for the
reader.
Note that utime() currently doesn't work correctly with
Win95/NT ports. A bug has been reported. Check it care
fully before using utime() 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 filehan
dles, 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 Chris
tiansen's, at
http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz ,
which is written in Perl and offers much greater function
ality than the stock version.
How can I read in an entire file all at once?
The customary Perl approach for processing all the lines
in a file is to do so one line at a time:
open (INPUT, $file) || die "can't open $file: $!";
while (<INPUT>) {
chomp;
# do something with $_
}
close(INPUT) || die "can't close $file: $!";
This is tremendously more efficient than reading the
entire file into memory as an array of lines and then pro
cessing it one element at a time, which is often--if not
almost always--the wrong approach. Whenever you see some
one do this:
@lines = <INPUT>;
you should think long and hard about why you need every
thing loaded at once. It's just not a scalable solution.
You might also find it more fun to use the standard
DB_File module's $DB_RECNO bindings, which allow you to
tie an array to a file so that accessing an element the
array actually accesses the corresponding line in the
file.
On very rare occasion, you may have an algorithm that
demands that the entire file be in memory at once as one
scalar. The simplest solution to that is
$var = `cat $file`;
Being in scalar context, you get the whole thing. In list
context, you'd get a list of all the lines:
@lines = `cat $file`;
This tiny but expedient solution is neat, clean, and
portable to all systems on which decent tools have been
installed. For those who prefer not to use the toolbox,
you can of course read the file manually, although this
makes for more complicated code.
{
local(*INPUT, $/);
open (INPUT, $file) || die "can't open $file: $!";
$var = <INPUT>;
}
That temporarily undefs your record separator, and will
automatically close the file at block exit. If the file
is already open, just use this:
$var = do { local $/; <INPUT> };
How can I read in a file by paragraphs?
Use the "$/" 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.
Note that a blank line must have no blanks in it. Thus
""fred\n \nstuff\n\n"" is one paragraph, but
""fred\n\nstuff\n\n"" is two.
How can I read a single character from a file? From the
keyboard?
You can use the builtin "getc()" function for most file
handles, 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 the portable operating system pro
gramming interface (POSIX), you can use the following
code, which you'll note turns off echo processing as well.
#!/usr/bin/perl -w
use strict;
$| = 1;
for (1..4) {
my $got;
print "gimme: ";
$got = getone();
print "--> $got\n";
}
exit;
BEGIN {
use POSIX qw(:termios_h);
my ($term, $oterm, $echo, $noecho, $fd_stdin);
$fd_stdin = fileno(STDIN);
$term = POSIX::Termios->new();
$term->getattr($fd_stdin);
$oterm = $term->getlflag();
$echo = ECHO | ECHOK | ICANON;
$noecho = $oterm & ~$echo;
sub cbreak {
$term->setlflag($noecho);
$term->setcc(VTIME, 1);
$term->setattr($fd_stdin, TCSANOW);
}
sub cooked {
$term->setlflag($oterm);
$term->setcc(VTIME, 0);
$term->setattr($fd_stdin, TCSANOW);
}
sub getone {
my $key = '';
cbreak();
sysread(STDIN, $key, 1);
cooked();
return $key;
}
}
END { cooked() }
The Term::ReadKey module from CPAN may be easier to use.
Recent versions include also support for non-portable sys
tems as well.
use Term::ReadKey;
open(TTY, "</dev/tty");
print "Gimme a char: ";
ReadMode "raw";
$key = ReadKey 0, *TTY;
ReadMode "normal";
printf "\nYou said %s, char number %03d\n",
$key, ord $key;
For legacy 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,$c,1)", and that value
tells you what combination it was according to this table:
# PC 2-byte keycodes = ^@ + the following:
# HEX KEYS
# -------
# 0F SHF TAB
# 10-19 ALT QWERTYUIOP
# 1E-26 ALT ASDFGHJKL
# 2C-32 ALT ZXCVBNM
# 3B-44 F1-F10
# 47-49 HOME,UP,PgUp
# 4B LEFT
# 4D RIGHT
# 4F-53 END,DOWN,PgDn,Ins,Del
# 54-5D SHF F1-F10
# 5E-67 CTR F1-F10
# 68-71 ALT F1-F10
# 73-77 CTR LEFT,RIGHT,END,PgDn,HOME
# 78-83 ALT 1234567890-=
# 84 CTR PgUp
This is all trial and error I did a long time ago; I hope
I'm reading the file that worked...
How can I tell whether there's a character waiting on a
filehandle?
The very first thing you should do is look into getting
the Term::ReadKey extension from CPAN. As we mentioned
earlier, it now even has limited support for non-portable
(read: not open systems, closed, proprietary, not POSIX,
not Unix, etc) systems.
You should also check out the Frequently Asked Questions
list in comp.unix.* for things like this: the answer is
essentially the same. It's very system dependent. Here's
one solution that works on BSD systems:
sub key_ready {
my($rin, $nfd);
vec($rin, fileno(STDIN), 1) = 1;
return $nfd = select($rin,undef,undef,0);
}
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.c
% ./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", $size);
FIONREAD requires a filehandle connected to a stream,
meaning that sockets, pipes, and tty devices work, but not
files.
How do I do a "tail -f" in perl?
First try
seek(GWFILE, 0, 1);
The statement "seek(GWFILE, 0, 1)" doesn't change the cur
rent position, but it does clear the end-of-file condition
on the handle, so that the next <GWFILE> makes Perl try
again to read something.
If that doesn't work (it relies on features of your stdio
implementation), then you need something more like this:
for (;;) {
for ($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.
There's also a File::Tail module from CPAN.
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() func
tion 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 $rc == -1;
Or, just use the fdopen(3S) feature of open():
{
local *F;
open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
close F;
}
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 file
name! Remember that within double quoted strings
("like\this"), the backslash is an escape character. The
full list of these is in the Quote and Quote-like Opera
tors entry 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 legacy DOS filesys
tem.
Either single-quote your strings, or (preferably) use for
ward slashes. Since all DOS and Windows versions since
something like MS-DOS 2.0 or so have treated "/" and "\"
the same in a path, you might as well use the one that
doesn't clash with Perl--or the POSIX shell, ANSI C and
C++, awk, Tcl, Java, or Python, just to mention a few.
POSIX paths are more portable, too.
Why doesn't glob("*.*") get all the files?
Because even on non-Unix ports, Perl's glob function fol
lows standard Unix globbing semantics. You'll need
"glob("*")" to get all (non-hidden) files. This makes
glob() portable even to legacy systems. Your port may
include proprietary globbing functions as well. Check its
documentation for details.
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 direc
tory, 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 the algorithm's correctness.
Why do I get weird spaces when I print an array of lines?
Saying
print "@lines\n";
joins together the elements of "@lines" with a space
between them. If "@lines" were "("little", "fluffy",
"clouds")" then the above statement would print
little fluffy clouds
but if each element of "@lines" was a line of text, ending
a newline character "("little\n", "fluffy\n", "clouds\n")"
then it would print:
little
fluffy
clouds
If your array contains lines, just print them:
print @lines;
AUTHOR AND COPYRIGHT
Copyright (c) 1997-1999 Tom Christiansen and Nathan
Torkington. All rights reserved.
When included as an integrated part of the Standard Dis
tribution of Perl or of its documentation (printed or oth
erwise), this works is covered under Perl's Artistic
License. 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 in the public domain. You are permitted and encour
aged to use this code and any derivatives thereof in your
own programs for fun or for profit as you see fit. A sim
ple comment in the code giving credit to the FAQ would be
courteous but is not required.
2001-04-07 perl v5.6.1 PERLFAQ5(1)
