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PERLFAQ5(1)
NAME
perlfaq5 - Files and Formats ($Revision: 1.38 $, $Date: 1999/05/23 16:08:30
$)
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 perlvar/$ 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: $document\n";
Note the bizarrely hardcoded carriage return and newline in their octal
equivalents. This is the ONLY way (currently) to assure a proper flush on
all platforms, including Macintosh. That's the way things work in network
programming: you really should specify the exact bit pattern on the network
line terminator. In practice, ""\n\n"" often works, but this is not
portable.
See the perlfaq9 manpage for other examples of fetching URLs over the web.
How do I change one line in a file/delete a line in a file/insert a line in
the middle of a file/append to the beginning of a file?
Those are operations of a text editor. Perl is not a text editor. Perl is
a programming language. You have to decompose the problem into low-level
calls to read, write, open, close, and seek.
Although humans have an easy time thinking of a text file as being a
sequence of lines that operates much like a stack of playing cards--or
punch cards--computers usually see the text file as a sequence of bytes.
In general, there's no direct way for Perl to seek to a particular line of
a file, insert text into a file, or remove text from a file.
(There are exceptions in special circumstances. You can add or remove data
at the very end of the file. A sequence of bytes can be replaced with
another sequence of the same length. The "$DB_RECNO" array bindings as
documented 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 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 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 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 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
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 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
$fh = IO::Handle->new();
Then use any of those as you would a normal filehandle. Anywhere that Perl
is expecting a filehandle, an indirect filehandle may be used instead. An
indirect filehandle is just a scalar variable that contains a filehandle.
Functions like "print", "open", "seek", or the "<FH>" diamond operator will
accept either a read filehandle or a scalar variable containing one:
($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 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 Accessing Formatting Internals entry in 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 code 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 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
(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.
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 internal glob() operator forks csh(1) to
do the actual glob expansion, but csh can't handle more than 127 items and
so gives the error message "Argument list too long". People who installed
tcsh as csh won't have this problem, but their users may be surprised by
it.
To get around this, either upgrade to Perl v5.6.0 or later, do the glob
yourself with readdir() and patterns, or use a module like 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 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 trailing 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: $!";
For more information, see also the new the perlopentut manpage if you have
it (new for 5.6).
How can I reliably rename a file?
Well, usually you just use Perl's rename() function. That may not work
everywhere, though, particularly when renaming files across file systems.
Some sub-Unix systems have broken ports that corrupt the semantics of
rename()--for example, WinNT does this right, but Win95 and Win98 are
broken. (The last two parts are not surprising, but the first is. :-)
If your operating system supports a proper mv(1) program or its moral
equivalent, this works:
rename($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 network (e.g. on NFS
file systems), so you'd need to force the use of fcntl(2) when you
build Perl. But even this is dubious at best. See the flock entry of
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 advisory. Such discretionary locks are more flexible, but offer
fewer guarantees. This means that files locked with flock() may be
modified by programs that do not also use flock(). Cars that stop for
red lights get on well with each other, but not with cars that don't
stop for red lights. See the perlport manpage, your port's specific
documentation, or your system-specific local manpages for details.
It's best to assume traditional behavior if you're writing portable
programs. (If you're not, you should as always feel perfectly free to
write for your own system's idiosyncrasies (sometimes called
"features"). Slavish adherence to portability concerns shouldn't get
in the way of your getting your job done.)
For more information on file locking, see also 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 vanity. It's better to pick a random number; they're
more realistic.
Anyway, this is what you can do if you can't help yourself.
use Fcntl qw(:DEFAULT :flock);
sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
flock(FH, LOCK_EX) or die "can't flock numfile: $!";
$num = <FH> || 0;
seek(FH, 0, 0) or die "can't rewind numfile: $!";
truncate(FH, 0) or die "can't truncate numfile: $!";
(print FH $num+1, "\n") or die "can't write numfile: $!";
close FH or die "can't close numfile: $!";
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 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, as usual, left as an exercise for the reader.
Note that utime() currently doesn't work correctly with Win95/NT ports. A
bug has been reported. Check it carefully before using utime() on those
platforms.
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 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 processing it one element at a time,
which is often--if not almost always--the wrong approach. Whenever you see
someone do this:
@lines = <INPUT>;
you should think long and hard about why you need everything loaded at
once. It's just not a scalable solution. You might also find it more fun
to use the standard 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 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 the portable operating system programming interface
(POSIX), you can use the following code, which you'll note turns off echo
processing as well.
#!/usr/bin/perl -w
use strict;
$| = 1;
for (1..4) {
my $got;
print "gimme: ";
$got = getone();
print "--> $got\n";
}
exit;
BEGIN {
use POSIX qw(:termios_h);
my ($term, $oterm, $echo, $noecho, $fd_stdin);
$fd_stdin = fileno(STDIN);
$term = POSIX::Termios->new();
$term->getattr($fd_stdin);
$oterm = $term->getlflag();
$echo = ECHO | ECHOK | ICANON;
$noecho = $oterm & ~$echo;
sub cbreak {
$term->setlflag($noecho);
$term->setcc(VTIME, 1);
$term->setattr($fd_stdin, TCSANOW);
}
sub cooked {
$term->setlflag($oterm);
$term->setcc(VTIME, 0);
$term->setattr($fd_stdin, TCSANOW);
}
sub getone {
my $key = '';
cbreak();
sysread(STDIN, $key, 1);
cooked();
return $key;
}
}
END { cooked() }
The Term::ReadKey module from CPAN may be easier to use. Recent versions
include also support for non-portable systems as well.
use Term::ReadKey;
open(TTY, "</dev/tty");
print "Gimme a char: ";
ReadMode "raw";
$key = ReadKey 0, *TTY;
ReadMode "normal";
printf "\nYou said %s, char number %03d\n",
$key, ord $key;
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", $size);
FIONREAD requires a filehandle connected to a stream, meaning that sockets,
pipes, and tty devices work, but not files.
How do I do a "tail -f" in perl?
First try
seek(GWFILE, 0, 1);
The statement "seek(GWFILE, 0, 1)" doesn't change the current position, but
it does clear the end-of-file condition on the handle, so that the next
<GWFILE> makes Perl try again to read something.
If that doesn't work (it relies on features of your stdio implementation),
then you need something more like this:
for (;;) {
for ($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.
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() 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 $rc == -1;
Or, just use the fdopen(3S) feature of open():
{
local *F;
open F, "<&=$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 filename! 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 Operators
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
filesystem.
Either single-quote your strings, or (preferably) use forward slashes.
Since all DOS and Windows versions since something like MS-DOS 2.0 or so
have treated "/" and "\" the same in a path, you might as well use the one
that doesn't clash with Perl--or the POSIX shell, ANSI C and C++, awk, Tcl,
Java, or Python, just to mention a few. POSIX paths are more portable,
too.
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 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 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 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 Distribution of Perl or
of its documentation (printed or otherwise), 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 encouraged to use this code and any
derivatives thereof in your own programs for fun or for profit as you see
fit. A simple comment in the code giving credit to the FAQ would be
courteous but is not required.
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