perlfaq5
(1)
Name
perlfaq5 - Files and Formats
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLFAQ5(1)
NAME
perlfaq5 - 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?
(contributed by brian d foy)
You might like to read Mark Jason Dominus's "Suffering From
Buffering" at http://perl.plover.com/FAQs/Buffering.html .
Perl normally buffers output so it doesn't make a system
call for every bit of output. By saving up output, it makes
fewer expensive system calls. For instance, in this little
bit of code, you want to print a dot to the screen for every
line you process to watch the progress of your program.
Instead of seeing a dot for every line, Perl buffers the
output and you have a long wait before you see a row of 50
dots all at once:
# long wait, then row of dots all at once
while( <> ) {
print ".";
print "\n" unless ++$count % 50;
#... expensive line processing operations
}
To get around this, you have to unbuffer the output
filehandle, in this case, "STDOUT". You can set the special
variable $| to a true value (mnemonic: making your
filehandles "piping hot"):
$|++;
# dot shown immediately
while( <> ) {
print ".";
print "\n" unless ++$count % 50;
#... expensive line processing operations
}
The $| is one of the per-filehandle special variables, so
each filehandle has its own copy of its value. If you want
to merge standard output and standard error for instance,
you have to unbuffer each (although STDERR might be
unbuffered by default):
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{
my $previous_default = select(STDOUT); # save previous default
$|++; # autoflush STDOUT
select(STDERR);
$|++; # autoflush STDERR, to be sure
select($previous_default); # restore previous default
}
# now should alternate . and +
while( 1 )
{
sleep 1;
print STDOUT ".";
print STDERR "+";
print STDOUT "\n" unless ++$count % 25;
}
Besides the $| special variable, you can use "binmode" to
give your filehandle a ":unix" layer, which is unbuffered:
binmode( STDOUT, ":unix" );
while( 1 ) {
sleep 1;
print ".";
print "\n" unless ++$count % 50;
}
For more information on output layers, see the entries for
"binmode" and "open" in perlfunc, and the "PerlIO" module
documentation.
If you are using "IO::Handle" or one of its subclasses, you
can call the "autoflush" method to change the settings of
the filehandle:
use IO::Handle;
open my( $io_fh ), ">", "output.txt";
$io_fh->autoflush(1);
The "IO::Handle" objects also have a "flush" method. You can
flush the buffer any time you want without auto-buffering
$io_fh->flush;
How do I change, delete, or insert a line in a file, or append
to the beginning of a file?
(contributed by brian d foy)
The basic idea of inserting, changing, or deleting a line
from a text file involves reading and printing the file to
the point you want to make the change, making the change,
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then reading and printing the rest of the file. Perl doesn't
provide random access to lines (especially since the record
input separator, $/, is mutable), although modules such as
"Tie::File" can fake it.
A Perl program to do these tasks takes the basic form of
opening a file, printing its lines, then closing the file:
open my $in, '<', $file or die "Can't read old file: $!";
open my $out, '>', "$file.new" or die "Can't write new file: $!";
while( <$in> )
{
print $out $_;
}
close $out;
Within that basic form, add the parts that you need to
insert, change, or delete lines.
To prepend lines to the beginning, print those lines before
you enter the loop that prints the existing lines.
open my $in, '<', $file or die "Can't read old file: $!";
open my $out, '>', "$file.new" or die "Can't write new file: $!";
print $out "# Add this line to the top\n"; # <--- HERE'S THE MAGIC
while( <$in> )
{
print $out $_;
}
close $out;
To change existing lines, insert the code to modify the
lines inside the "while" loop. In this case, the code finds
all lowercased versions of "perl" and uppercases them. The
happens for every line, so be sure that you're supposed to
do that on every line!
open my $in, '<', $file or die "Can't read old file: $!";
open my $out, '>', "$file.new" or die "Can't write new file: $!";
print $out "# Add this line to the top\n";
while( <$in> )
{
s/\b(perl)\b/Perl/g;
print $out $_;
}
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close $out;
To change only a particular line, the input line number, $.,
is useful. First read and print the lines up to the one you
want to change. Next, read the single line you want to
change, change it, and print it. After that, read the rest
of the lines and print those:
while( <$in> ) # print the lines before the change
{
print $out $_;
last if $. == 4; # line number before change
}
my $line = <$in>;
$line =~ s/\b(perl)\b/Perl/g;
print $out $line;
while( <$in> ) # print the rest of the lines
{
print $out $_;
}
To skip lines, use the looping controls. The "next" in this
example skips comment lines, and the "last" stops all
processing once it encounters either "__END__" or
"__DATA__".
while( <$in> )
{
next if /^\s+#/; # skip comment lines
last if /^__(END|DATA)__$/; # stop at end of code marker
print $out $_;
}
Do the same sort of thing to delete a particular line by
using "next" to skip the lines you don't want to show up in
the output. This example skips every fifth line:
while( <$in> )
{
next unless $. % 5;
print $out $_;
}
If, for some odd reason, you really want to see the whole
file at once rather than processing line-by-line, you can
slurp it in (as long as you can fit the whole thing in
memory!):
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open my $in, '<', $file or die "Can't read old file: $!"
open my $out, '>', "$file.new" or die "Can't write new file: $!";
my @lines = do { local $/; <$in> }; # slurp!
# do your magic here
print $out @lines;
Modules such as "File::Slurp" and "Tie::File" can help with
that too. If you can, however, avoid reading the entire file
at once. Perl won't give that memory back to the operating
system until the process finishes.
You can also use Perl one-liners to modify a file in-place.
The following changes all 'Fred' to 'Barney' in inFile.txt,
overwriting the file with the new contents. With the "-p"
switch, Perl wraps a "while" loop around the code you
specify with "-e", and "-i" turns on in-place editing. The
current line is in $_. With "-p", Perl automatically prints
the value of $_ at the end of the loop. See perlrun for more
details.
perl -pi -e 's/Fred/Barney/' inFile.txt
To make a backup of "inFile.txt", give "-i" a file extension
to add:
perl -pi.bak -e 's/Fred/Barney/' inFile.txt
To change only the fifth line, you can add a test checking
$., the input line number, then only perform the operation
when the test passes:
perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt
To add lines before a certain line, you can add a line (or
lines!) before Perl prints $_:
perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt
You can even add a line to the beginning of a file, since
the current line prints at the end of the loop:
perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt
To insert a line after one already in the file, use the "-n"
switch. It's just like "-p" except that it doesn't print $_
at the end of the loop, so you have to do that yourself. In
this case, print $_ first, then print the line that you want
to add.
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perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt
To delete lines, only print the ones that you want.
perl -ni -e 'print unless /d/' inFile.txt
... or ...
perl -pi -e 'next unless /d/' inFile.txt
How do I count the number of lines in a file?
(contributed by brian d foy)
Conceptually, the easiest way to count the lines in a file
is to simply read them and count them:
my $count = 0;
while( <$fh> ) { $count++; }
You don't really have to count them yourself, though, since
Perl already does that with the $. variable, which is the
current line number from the last filehandle read:
1 while( <$fh> );
my $count = $.;
If you want to use $., you can reduce it to a simple one-
liner, like one of these:
% perl -lne '} print $.; {' file
% perl -lne 'END { print $. }' file
Those can be rather inefficient though. If they aren't fast
enough for you, you might just read chunks of data and count
the number of newlines:
my $lines = 0;
open my($fh), '<:raw', $filename or die "Can't open $filename: $!";
while( sysread $fh, $buffer, 4096 ) {
$lines += ( $buffer =~ tr/\n// );
}
close FILE;
However, that doesn't work if the line ending isn't a
newline. You might change that "tr///" to a "s///" so you
can count the number of times the input record separator,
$/, shows up:
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my $lines = 0;
open my($fh), '<:raw', $filename or die "Can't open $filename: $!";
while( sysread $fh, $buffer, 4096 ) {
$lines += ( $buffer =~ s|$/||g; );
}
close FILE;
If you don't mind shelling out, the "wc" command is usually
the fastest, even with the extra interprocess overhead.
Ensure that you have an untainted filename though:
#!perl -T
$ENV{PATH} = undef;
my $lines;
if( $filename =~ /^([0-9a-z_.]+)\z/ ) {
$lines = `/usr/bin/wc -l $1`
chomp $lines;
}
How do I delete the last N lines from a file?
(contributed by brian d foy)
The easiest conceptual solution is to count the lines in the
file then start at the beginning and print the number of
lines (minus the last N) to a new file.
Most often, the real question is how you can delete the last
N lines without making more than one pass over the file, or
how to do it without a lot of copying. The easy concept is
the hard reality when you might have millions of lines in
your file.
One trick is to use "File::ReadBackwards", which starts at
the end of the file. That module provides an object that
wraps the real filehandle to make it easy for you to move
around the file. Once you get to the spot you need, you can
get the actual filehandle and work with it as normal. In
this case, you get the file position at the end of the last
line you want to keep and truncate the file to that point:
use File::ReadBackwards;
my $filename = 'test.txt';
my $Lines_to_truncate = 2;
my $bw = File::ReadBackwards->new( $filename )
or die "Could not read backwards in [$filename]: $!";
my $lines_from_end = 0;
until( $bw->eof or $lines_from_end == $Lines_to_truncate )
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{
print "Got: ", $bw->readline;
$lines_from_end++;
}
truncate( $filename, $bw->tell );
The "File::ReadBackwards" module also has the advantage of
setting the input record separator to a regular expression.
You can also use the "Tie::File" module which lets you
access the lines through a tied array. You can use normal
array operations to modify your file, including setting the
last index and using "splice".
How can I use Perl's "-i" option from within a program?
"-i" sets the value of Perl's $^I variable, which in turn
affects the behavior of "<>"; see perlrun for more details.
By modifying the appropriate variables directly, you can get
the same behavior within a larger program. For example:
# ...
{
local($^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
directory, 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,
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then print to the destination file as you read the original.
You also have to remember to copy the permissions, owner,
and group to the new file.
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. In Perl 5.8
or later, 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 /;
my $dir = tempdir( CLEANUP => 1 );
($fh, $filename) = tempfile( DIR => $dir );
# or if you don't need to know the filename
my $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;
my $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 + $1)/e;
# O_EXCL is required for security reasons.
sysopen my($fh), $base_name, O_WRONLY|O_EXCL|O_CREAT;
}
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if( defined fileno($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
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
handles 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;
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print $fh "Hello World!\n";
process_file( $fh );
If you like, you can store these filehandles in an array or
a hash. If you access them directly, they aren't simple
scalars and you need to give "print" a little help by
placing the filehandle reference in braces. Perl can only
figure it out on its own when the filehandle reference is a
simple scalar.
my @fhs = ( $fh1, $fh2, $fh3 );
for( $i = 0; $i <= $#fhs; $i++ ) {
print {$fhs[$i]} "just another Perl answer, \n";
}
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.
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 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
my $fh = IO::Handle->new();
Then use any of those as you would a normal filehandle.
Anywhere that Perl is expecting a filehandle, an indirect
filehandle may be used instead. An indirect filehandle is
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just a scalar variable that contains a filehandle. Functions
like "print", "open", "seek", or the "<FH>" diamond operator
will accept either a named filehandle or a scalar variable
containing one:
($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
print $ofh "Type it: ";
my $got = <$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 variable as a filehandle is illegal and won't even
compile:
my @fd = (*STDIN, *STDOUT, *STDERR);
print $fd[1] "Type it: "; # WRONG
my $got = <$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:
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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:
my $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
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 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 open a filehandle to a string?
(contributed by Peter J. Holzer, hjp-usenet2@hjp.at)
Since Perl 5.8.0 a file handle referring to a string can be
created by calling open with a reference to that string
instead of the filename. This file handle can then be used
to read from or write to the string:
open(my $fh, '>', \$string) or die "Could not open string for writing";
print $fh "foo\n";
print $fh "bar\n"; # $string now contains "foo\nbar\n"
open(my $fh, '<', \$string) or die "Could not open string for reading";
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my $x = <$fh>; # $x now contains "foo\n"
With older versions of Perl, the "IO::String" module
provides similar functionality.
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.
)/$1,/xg;
How can I translate tildes (~) in a filename?
Use the <> ("glob()") operator, documented in perlfunc.
Versions of Perl older than 5.6 require that you have a
shell installed that groks tildes. Later versions of Perl
have this feature built in. The "File::KGlob" module
(available from CPAN) gives more portable glob
functionality.
Within Perl, you may use this directly:
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$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 my $fh, '+>', '/path/name'; # WRONG (almost always)
Whoops. You should instead use this, which will fail if the
file doesn't exist.
open my $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 my $fh, '<', $path or die $!;
sysopen my $fh, $path, O_RDONLY or die $!;
To open file for writing, create new file if needed or else
truncate old file:
open my $fh, '>', $path or die $!;
sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT or die $!;
sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666 or die $!;
To open file for writing, create new file, file must not
exist:
sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT or die $!;
sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT, 0666 or die $!;
To open file for appending, create if necessary:
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open my $fh, '>>' $path or die $!;
sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT or die $!;
sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT, 0666 or die $!;
To open file for appending, file must exist:
sysopen my $fh, $path, O_WRONLY|O_APPEND or die $!;
To open file for update, file must exist:
open my $fh, '+<', $path or die $!;
sysopen my $fh, $path, O_RDWR or die $!;
To open file for update, create file if necessary:
sysopen my $fh, $path, O_RDWR|O_CREAT or die $!;
sysopen my $fh, $path, O_RDWR|O_CREAT, 0666 or die $!;
To open file for update, file must not exist:
sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT or die $!;
sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT, 0666 or die $!;
To open a file without blocking, creating if necessary:
sysopen my $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.
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 File::KGlob, one that doesn't use the shell to
do globbing.
Is there a leak/bug in glob()?
(contributed by brian d foy)
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Starting with Perl 5.6.0, "glob" is implemented internally
rather than relying on an external resource. As such, memory
issues with "glob" aren't a problem in modern perls.
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 characters in the filename as special.
open my $fh, "<", " file "; # filename is " file "
open my $fh, ">", ">file"; # filename is ">file"
How can I reliably rename a file?
If your operating system supports a proper mv(1) utility or
its functional equivalent, this works:
rename($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.
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 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
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dubious at best. See the flock entry of perlfunc 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 "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 my $lock, '>', '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 my $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 less than desirable.
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.
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Anyway, this is what you can do if you can't help yourself.
use Fcntl qw(:DEFAULT :flock);
sysopen my $fh, "numfile", O_RDWR|O_CREAT or die "can't open numfile: $!";
flock $fh, LOCK_EX or die "can't flock numfile: $!";
my $num = <$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
implement "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 code in the previous answer.
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
buffer then each bufferload is almost guaranteed to be
written 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
system's write(2) system call.
There is still a small theoretical chance that a signal will
interrupt the system level "write()" operation before
completion. There is also a possibility that some STDIO
implementations 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, and this
is not a concern when using ":perlio" instead of your
system's STDIO.
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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 my $fh, '+<', 'somewhere' or die "can't update somewhere: $!";
seek $fh, $recno * $RECSIZE, 0;
read $fh, $record, $RECSIZE == $RECSIZE or 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
floating 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
function, then use "localtime()", "gmtime()", or
"POSIX::strftime()" to convert this into human-readable
form.
Here's an example:
my $write_secs = (stat($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;
my $date_string = ctime(stat($file)->mtime);
print "file $file updated at $date_string\n";
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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
perlfunc. 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";
}
my $timestamp = shift;
my($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.
for my $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
How can I read in an entire file all at once?
Are you sure you want to read the entire file and store it
in memory? If you mmap the file, you can virtually load the
entire file into a string without actually storing it in
memory:
use File::Map qw(map_file);
map_file my $string, $filename;
Once mapped, you can treat $string as you would any other
string. Since you don't actually load the data, mmap-ing is
very fast and does not increase your memory footprint.
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If you really want to load the entire file, you can use the
"File::Slurp" module to do it in one step.
use File::Slurp;
my $all_of_it = read_file($filename); # entire file in scalar
my @all_lines = read_file($filename); # one line per element
The customary Perl approach for processing all the lines in
a file is to do so one line at a time:
open my $input, '<', $file or die "can't open $file: $!";
while (<$input>) {
chomp;
# do something with $_
}
close $input or 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:
my @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 $/;
open my $fh, '<', $file or die "can't open $file: $!";
$var = <$fh>;
}
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 $/; <$fh> };
For ordinary files you can also use the read function.
read( $fh, $var, -s $fh );
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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
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 "getc" in perlfunc.
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) {
print "gimme: ";
my $got = getone();
print "--> $got\n";
}
exit;
BEGIN {
use POSIX qw(:termios_h);
my ($term, $oterm, $echo, $noecho, $fd_stdin);
my $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);
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$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 my $tty, '<', '/dev/tty';
print "Gimme a char: ";
ReadMode "raw";
my $key = ReadKey 0, $tty;
ReadMode "normal";
printf "\nYou said %s, char number %03d\n",
$key, ord $key;
How can I tell whether there's a character waiting on a
filehandle?
The very first thing you should do is look into getting the
Term::ReadKey extension from CPAN. As we mentioned earlier,
it now even has limited support for non-portable (read: not
open systems, closed, proprietary, not POSIX, not Unix,
etc.) systems.
You should also check out the Frequently Asked Questions
list in comp.unix.* for things like this: the answer is
essentially the same. It's very system dependent. Here's
one solution that works on BSD systems:
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sub key_ready {
my($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", $size);
FIONREAD requires a filehandle connected to a stream,
meaning that sockets, pipes, and tty devices work, but not
files.
How do I do a "tail -f" in perl?
First try
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seek(GWFILE, 0, 1);
The statement "seek(GWFILE, 0, 1)" doesn't change the
current position, but it does clear the end-of-file
condition on the handle, so that the next "<GWFILE>" makes
Perl try again to read something.
If that doesn't work (it relies on features of your stdio
implementation), then you need something more like this:
for (;;) {
for ($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 "clearerr" method
from "IO::Handle", which resets the error and end-of-file
states on the handle.
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 my $log, '>>', '/foo/logfile';
open STDERR, '>&LOG';
Or even with a literal numeric descriptor:
my $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?
If, for some reason, you have a file descriptor instead of a
filehandle (perhaps you used "POSIX::open"), you can use the
"close()" function from the "POSIX" module:
use POSIX ();
POSIX::close( $fd );
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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';
my $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()":
{
open my( $fh ), "<&=$fd" or die "Cannot reopen fd=$fd: $!";
close $fh;
}
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
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 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
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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?
Short of loading the file into a database or pre-indexing
the lines in the file, there are a couple of things that you
can do.
Here's a reservoir-sampling 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
function 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
element.
Why do I get weird spaces when I print an array of lines?
(contributed by brian d foy)
If you are seeing spaces between the elements of your array
when you print the array, you are probably interpolating the
array in double quotes:
my @animals = qw(camel llama alpaca vicuna);
print "animals are: @animals\n";
It's the double quotes, not the "print", doing this.
Whenever you interpolate an array in a double quote context,
Perl joins the elements with spaces (or whatever is in $",
which is a space by default):
animals are: camel llama alpaca vicuna
This is different than printing the array without the
interpolation:
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my @animals = qw(camel llama alpaca vicuna);
print "animals are: ", @animals, "\n";
Now the output doesn't have the spaces between the elements
because the elements of @animals simply become part of the
list to "print":
animals are: camelllamaalpacavicuna
You might notice this when each of the elements of @array
end with a newline. You expect to print one element per
line, but notice that every line after the first is
indented:
this is a line
this is another line
this is the third line
That extra space comes from the interpolation of the array.
If you don't want to put anything between your array
elements, don't use the array in double quotes. You can send
it to print without them:
print @lines;
How do I traverse a directory tree?
(contributed by brian d foy)
The "File::Find" module, which comes with Perl, does all of
the hard work to traverse a directory structure. It comes
with Perl. You simply call the "find" subroutine with a
callback subroutine and the directories you want to
traverse:
use File::Find;
find( \&wanted, @directories );
sub wanted {
# full path in $File::Find::name
# just filename in $_
... do whatever you want to do ...
}
The "File::Find::Closures", which you can download from
CPAN, provides many ready-to-use subroutines that you can
use with "File::Find".
The "File::Finder", which you can download from CPAN, can
help you create the callback subroutine using something
closer to the syntax of the "find" command-line utility:
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use File::Find;
use File::Finder;
my $deep_dirs = File::Finder->depth->type('d')->ls->exec('rmdir','{}');
find( $deep_dirs->as_options, @places );
The "File::Find::Rule" module, which you can download from
CPAN, has a similar interface, but does the traversal for
you too:
use File::Find::Rule;
my @files = File::Find::Rule->file()
->name( '*.pm' )
->in( @INC );
How do I delete a directory tree?
(contributed by brian d foy)
If you have an empty directory, you can use Perl's built-in
"rmdir". If the directory is not empty (so, no files or
subdirectories), you either have to empty it yourself (a lot
of work) or use a module to help you.
The "File::Path" module, which comes with Perl, has a
"remove_tree" which can take care of all of the hard work
for you:
use File::Path qw(remove_tree);
remove_tree( @directories );
The "File::Path" module also has a legacy interface to the
older "rmtree" subroutine.
How do I copy an entire directory?
(contributed by Shlomi Fish)
To do the equivalent of "cp -R" (i.e. copy an entire
directory tree recursively) in portable Perl, you'll either
need to write something yourself or find a good CPAN module
such as File::Copy::Recursive.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2010 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.
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Perl Programmers Reference Guide PERLFAQ5(1)
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.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
perl v5.12.5 Last change: 2012-11-03 31