PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)NAMEperlfaq5 - Files and Formats
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?
Perl does not support truly unbuffered output (except
insofar as you can "syswrite(OUT, $char, 1)"), although it
does support is "command buffering", in which a physical
write is performed after every output command.
The C standard I/O library (stdio) normally buffers charac-
ters sent to devices so that there isn't a system call for
each byte. In most stdio implementations, the type of output
buffering and the size of the buffer varies according to the
type of device. Perl's print() and write() functions nor-
mally buffer output, while syswrite() bypasses buffering all
together.
If you want your output to be sent immediately when you exe-
cute print() or write() (for instance, for some network pro-
tocols), you must set the handle's autoflush flag. This flag
is the Perl variable $| and when it is set to a true value,
Perl will flush the handle's buffer after each print() or
write(). Setting $| affects buffering only for the currently
selected default file handle. You choose this handle with
the one argument select() call (see "$|" in perlvar and
"select" in perlfunc).
Use select() to choose the desired handle, then set its
per-filehandle variables.
$old_fh = select(OUTPUT_HANDLE);
$| = 1;
select($old_fh);
Some idioms can handle this in a single statement:
select((select(OUTPUT_HANDLE), $| = 1)[0]);
$| = 1, select $_ for select OUTPUT_HANDLE;
Some modules offer object-oriented access to handles and
their variables, although they may be overkill if this is
the only thing you do with them. You can use IO::Handle:
perl v5.8.8 2006-06-30 1
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
use IO::Handle;
open(DEV, ">/dev/printer"); # but is this?
DEV->autoflush(1);
or IO::Socket:
use IO::Socket; # this one is kinda a pipe?
my $sock = IO::Socket::INET->new( 'www.example.com:80' );
$sock->autoflush();
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?
Use the Tie::File module, which is included in the standard
distribution since Perl 5.8.0.
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 perlop. 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 can I use Perl's "-i" option from within a program?
"-i" sets the value of Perl's $^I variable, which in turn
affects the behavior of "<>"; see perlrun for more details.
By modifying the appropriate variables directly, you can get
the same behavior within a larger program. For example:
perl v5.8.8 2006-06-30 2
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
# ...
{
local($^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 $.
}
}
# $^I and @ARGV return to their old values here
This block modifies all the ".c" files in the current direc-
tory, leaving a backup of the original data from each file
in a new ".c.orig" file.
How can I copy a file?
(contributed by brian d foy)
Use the File::Copy module. It comes with Perl and can do a
true copy across file systems, and it does its magic in a
portable fashion.
use File::Copy;
copy( $original, $new_copy ) or die "Copy failed: $!";
If you can't use File::Copy, you'll have to do the work
yourself: open the original file, open the destination file,
then print to the destination file as you read the original.
How do I make a temporary file name?
If you don't need to know the name of the file, you can use
"open()" with "undef" in place of the file name. The
"open()" function creates an anonymous temporary file.
open my $tmp, '+>', undef or die $!;
Otherwise, you can use the File::Temp module.
use File::Temp qw/ tempfile tempdir /;
$dir = tempdir( CLEANUP => 1 );
($fh, $filename) = tempfile( DIR => $dir );
# or if you don't need to know the filename
perl v5.8.8 2006-06-30 3
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
$fh = tempfile( DIR => $dir );
The File::Temp has been a standard module since Perl 5.6.1.
If you don't have a modern enough Perl installed, 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'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{TMPDIR} || $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;
# O_EXCL is required for security reasons.
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:
perl v5.8.8 2006-06-30 4
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
# sample input line:
# 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
my $PS_T = 'A6 A4 A7 A5 A*';
open my $ps, '-|', 'ps';
print scalar <$ps>;
my @fields = qw( pid tt stat time command );
while (<$ps>) {
my %process;
@process{@fields} = unpack($PS_T, $_);
for my $field ( @fields ) {
print "$field: <$process{$field}>\n";
}
print 'line=', pack($PS_T, @process{@fields} ), "\n";
}
We've used a hash slice in order to easily handle the fields
of each row. Storing the keys in an array means it's easy to
operate on them as a group or loop over them with for. It
also avoids polluting the program with global variables and
using symbolic references.
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?
As of perl5.6, open() autovivifies file and directory han-
dles as references if you pass it an uninitialized scalar
variable. You can then pass these references just like any
other scalar, and use them in the place of named handles.
open my $fh, $file_name;
open local $fh, $file_name;
print $fh "Hello World!\n";
process_file( $fh );
Before perl5.6, you had to deal with various typeglob idioms
which you may see in older code.
open FILE, "> $filename";
process_typeglob( *FILE );
process_reference( \*FILE );
sub process_typeglob { local *FH = shift; print FH "Typeglob!" }
sub process_reference { local $fh = shift; print $fh "Reference!" }
If you want to create many anonymous handles, you should
check out the Symbol or IO::Handle modules.
perl v5.8.8 2006-06-30 5
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
How can I use a filehandle indirectly?
An indirect filehandle is using something other than a sym-
bol 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 one of the IO::*
modules to create an anonymous filehandle, store that in a
scalar variable, and use it as though it were a normal
filehandle.
use IO::Handle; # 5.004 or higher
$fh = IO::Handle->new();
Then use any of those as you would a normal filehandle.
Anywhere that Perl is expecting a filehandle, an indirect
filehandle may be used instead. An indirect filehandle is
just a scalar variable that contains a filehandle. Func-
tions like "print", "open", "seek", or the "<FH>" diamond
operator will accept either a named filehandle or a scalar
variable containing one:
($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 $fh "Sending to indirect filehandle\n";
}
Or it can localize a typeglob and use the filehandle
directly:
sub accept_fh {
local *FH = shift;
print FH "Sending to localized filehandle\n";
}
Both styles work with either objects or typeglobs of real
filehandles. (They might also work with strings under some
circumstances, but this is risky.)
perl v5.8.8 2006-06-30 6
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
accept_fh(*STDOUT);
accept_fh($handle);
In the examples above, we assigned the filehandle to a
scalar variable before using it. That is because only sim-
ple 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 variable 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 read a record just as "<>"
does. Given the initialization shown above for @fd, this
would work, but only because readline() requires 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 fundamen-
tal operators. Playing the object game doesn't help you at
all here.
perl v5.8.8 2006-06-30 7
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
How can I set up a footer format to be used with write()?
There's no builtin way to do this, but perlform has a couple
of techniques to make it possible for the intrepid hacker.
How can I write() into a string?
See "Accessing Formatting Internals" in perlform for an
swrite() function.
How can I output my numbers with commas added?
(contributed by brian d foy and Benjamin Goldberg)
You can use Number::Format to separate places in a number.
It handles locale information for those of you who want to
insert full stops instead (or anything else that they want
to use, really).
This subroutine will add commas to your number:
sub commify {
local $_ = shift;
1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
return $_;
}
This regex from Benjamin Goldberg will add commas to
numbers:
s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
It is easier to see with comments:
s/(
^[-+]? # beginning of number.
\d+? # first digits before first comma
(?= # followed by, (but not included in the match) :
(?>(?:\d{3})+) # some positive multiple of three digits.
(?!\d) # an *exact* multiple, not x * 3 + 1 or whatever.
)
| # or:
\G\d{3} # after the last group, get three digits
(?=\d) # but they have to have more digits after them.
)/$1,/xg;
How can I translate tildes (~) in a filename?
Use the <> (glob()) operator, documented in perlfunc. Older
versions of Perl require that you have a shell installed
that groks tildes. Recent perl versions have this feature
built in. The File::KGlob module (available from CPAN) gives
perl v5.8.8 2006-06-30 8
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
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:
perl v5.8.8 2006-06-30 9
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
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, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
or die "can't open /foo/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 perlopentut 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.
perl v5.8.8 2006-06-30 10
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
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 File::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?
(contributed by Brian McCauley)
The special two argument form of Perl's open() function
ignores trailing blanks in filenames and infers the mode
from certain leading characters (or a trailing "|"). In
older versions of Perl this was the only version of open()
and so it is prevalent in old code and books.
Unless you have a particular reason to use the two argument
form you should use the three argument form of open() which
does not treat any charcters in the filename as special.
open FILE, "<", " file "; # filename is " file "
open FILE, ">", ">file"; # filename is ">file"
How can I reliably rename a file?
If your operating system supports a proper mv(1) utility or
its functional equivalent, this works:
rename($old, $new) or system("mv", $old, $new);
It may be more portable 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 rename(), which
preserves meta-information like permissions, timestamps,
inode info, etc.
Newer versions of File::Copy export a move() function.
How can I lock a file?
Perl's builtin flock() function (see perlfunc 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
perl v5.8.8 2006-06-30 11
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
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 perlfunc and
the INSTALL file in the source distribution for informa-
tion on building Perl to do this.
Two potentially non-obvious but traditional flock seman-
tics 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 "File
Locking" in perlopentut 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: $!";
perl v5.8.8 2006-06-30 12
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
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 use-
less? 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. :-)
All I want to do is append a small amount of text to the end
of a file. Do I still have to use locking?
If you are on a system that correctly implements flock() and
you use the example appending code from "perldoc -f flock"
everything will be OK even if the OS you are on doesn't
implement append mode correctly (if such a system exists.)
So if you are happy to restrict yourself to OSs that imple-
ment flock() (and that's not really much of a restriction)
then that is what you should do.
If you know you are only going to use a system that does
correctly implement appending (i.e. not Win32) then you can
omit the seek() from the above code.
If you know you are only writing code to run on an OS and
filesystem that does implement append mode correctly (a
local filesystem on a modern Unix for example), and you keep
the file in block-buffered mode and you write less than one
buffer-full of output between each manual flushing of the
perl v5.8.8 2006-06-30 13
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
buffer then each bufferload is almost guaranteed to be writ-
ten to the end of the file in one chunk without getting
intermingled with anyone else's output. You can also use the
syswrite() function which is simply a wrapper around your
systems write(2) system call.
There is still a small theoretical chance that a signal will
interrupt the system level write() operation before comple-
tion. There is also a possibility that some STDIO implemen-
tations may call multiple system level write()s even if the
buffer was empty to start. There may be some systems where
this probability is reduced to zero.
How do I randomly update a binary file?
If you're just trying to patch a binary, in many cases some-
thing 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 -A, -M, or -C file test operations as documented
in perlfunc. These retrieve the age of the file (measured
against the start-time of your program) in days as a float-
ing point number. Some platforms may not have all of these
times. See perlport for details. To retrieve the "raw" time
in seconds since the epoch, you would call the stat func-
tion, then use localtime(), gmtime(), or POSIX::strftime()
to convert this into human-readable form.
Here's an example:
perl v5.8.8 2006-06-30 14
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
$write_secs = (stat($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::strftime() approach has the benefit of being, in
theory, independent of the current locale. See perllocale
for details.
How do I set a file's timestamp in perl?
You use the utime() function documented in "utime" in perl-
func. 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.
The perldoc for utime also has an example that has the same
effect as touch(1) on files that already exist.
Certain file systems have a limited ability to store the
times on a file at the expected level of precision. For
example, the FAT and HPFS filesystem are unable to create
dates on files with a finer granularity than two seconds.
This is a limitation of the filesystems, not of utime().
How do I print to more than one file at once?
To connect one filehandle to several output filehandles, you
can use the IO::Tee or Tie::FileHandle::Multiplex modules.
If you only have to do this once, you can print individually
to each filehandle.
perl v5.8.8 2006-06-30 15
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
How can I read in an entire file all at once?
You can use the File::Slurp module to do it in one step.
use File::Slurp;
$all_of_it = read_file($filename); # entire file in scalar
@all_lines = read_file($filename); # one line perl element
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 Tie::File
module, or the 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.
You can read the entire filehandle contents into a scalar.
{
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> };
perl v5.8.8 2006-06-30 16
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
For ordinary files you can also use the read function.
read( INPUT, $var, -s INPUT );
The third argument tests the byte size of the data on the
INPUT filehandle and reads that many bytes into the buffer
$var.
How can I read in a file by paragraphs?
Use the $/ variable (see perlvar for details). You can
either set it to "" to eliminate empty paragraphs
("abc\n\n\n\ndef", for instance, gets treated as two para-
graphs 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 filehan-
dles, but it won't (easily) work on a terminal device. For
STDIN, either use the Term::ReadKey module from CPAN or use
the sample code in "getc" in perlfunc.
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();
perl v5.8.8 2006-06-30 17
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
$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 $key;
How can I tell whether there's a character waiting on a
filehandle?
The very first thing you should do is look into getting the
Term::ReadKey extension from CPAN. As we mentioned earlier,
it now even has limited support for non-portable (read: not
open systems, closed, proprietary, not POSIX, not Unix, etc)
systems.
You should also check out the Frequently Asked Questions
list in comp.unix.* for things like this: the answer is
essentially the same. It's very system dependent. Here's
perl v5.8.8 2006-06-30 18
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
one solution that works on BSD systems:
sub key_ready {
my($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", $size);
FIONREAD requires a filehandle connected to a stream, mean-
ing that sockets, pipes, and tty devices work, but not
files.
perl v5.8.8 2006-06-30 19
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
How do I do a "tail -f" in perl?
First try
seek(GWFILE, 0, 1);
The statement "seek(GWFILE, 0, 1)" doesn't change the
current position, but it does clear the end-of-file condi-
tion on the handle, so that the next <GWFILE> makes Perl try
again to read something.
If that doesn't work (it relies on features of your stdio
implementation), then you need something more like this:
for (;;) {
for ($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 "open" in perlfunc, you'll see that several of
the ways to call open() should do the trick. For example:
open(LOG, ">>/foo/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.
perl v5.8.8 2006-06-30 20
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
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? Why 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 "Quote and Quote-like Operators" in
perlop. 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 for-
ward slashes. Since all DOS and Windows versions since some-
thing 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?
perl v5.8.8 2006-06-30 21
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
This is elaborately and painstakingly described in the
file-dir-perms article in the "Far More Than You Ever Wanted
To Know" collection in
http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
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 hap-
pen 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 permis-
sions 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. You can find a proof of this method in The
Art of Computer Programming, Volume 2, Section 3.4.2, by
Donald E. Knuth.
You can use the File::Random module which provides a func-
tion for that algorithm:
use File::Random qw/random_line/;
my $line = random_line($filename);
Another way is to use the Tie::File module, which treats the
entire file as an array. Simply access a random array ele-
ment.
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:
perl v5.8.8 2006-06-30 22
PERLFAQ5(1) Perl Programmers Reference Guide PERLFAQ5(1)
little
fluffy
clouds
If your array contains lines, just print them:
print @lines;
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington,
and other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or
modify it under the same terms as Perl itself.
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 pro-
grams for fun or for profit as you see fit. A simple com-
ment in the code giving credit to the FAQ would be courteous
but is not required.
perl v5.8.8 2006-06-30 23
