What is the best way to slurp a file into a string in Perl?

Question

Yes, There's More Than One Way To Do It but there must be a canonical or most efficient or most concise way. I'll add answers I know of and see what percolates to the top.

To be clear, the question is how best to read the contents of a file into a string. One solution per answer.

Solution 1

How about this:

use File::Slurp;
my $text = read_file($filename);

ETA: note Bug #83126 for File-Slurp: Security hole with encoding(UTF-8). I now recommend using File::Slurper (disclaimer: I wrote it), also because it has better defaults around encodings:

use File::Slurper 'read_text';
my $text = read_text($filename);

or Path::Tiny:

use Path::Tiny;
path($filename)->slurp_utf8;

Solution 2

I like doing this with a do block in which I localize @ARGV so I can use the diamond operator to do the file magic for me.

 my $contents = do { local(@ARGV, $/) = $file; <> };

If you need this to be a bit more robust, you can easily turn this into a subroutine.

If you need something really robust that handles all sorts of special cases, use File::Slurp. Even if you aren't going to use it, take a look at the source to see all the wacky situations it has to handle. File::Slurp has a big security problem that doesn't look to have a solution. Part of this is its failure to properly handle encodings. Even my quick answer has that problem. If you need to handle the encoding (maybe because you don't make everything UTF-8 by default), this expands to:

my $contents = do {
    open my $fh, '<:encoding(UTF-8)', $file or die '...';
    local $/;
    <$fh>;
    };

If you don't need to change the file, you might be able to use File::Map.

Solution 3

In writing File::Slurp (which is the best way), Uri Guttman did a lot of research in the many ways of slurping and which is most efficient. He wrote down his findings here and incorporated them info File::Slurp.

Solution 4

open(my $f, '<', $filename) or die "OPENING $filename: $!\n";
$string = do { local($/); <$f> };
close($f);

Solution 5

Things to think about (especially when compared with other solutions):

1. Lexical filehandles
2. Reduce scope
3. Reduce magic

So I get:

my $contents = do {
  local $/;
  open my $fh, $filename or die "Can't open $filename: $!";
  <$fh>
};

I'm not a big fan of magic <> except when actually using magic <>. Instead of faking it out, why not just use the open call directly? It's not much more work, and is explicit. (True magic <>, especially when handling "-", is far more work to perfectly emulate, but we aren't using it here anyway.)

View more solutions

Comments

[user2522201]

This is probably the most inefficient way I can think of, especially for large files. Now you have two copies of the same data and you have processed it twice just to load it into a scalar.

[dland]

And in case it's not obvious to those following along at home, at the end of the curly block, $fh goes out of scope and the file handle is closed automatically.

[ephemient]

I'm lazy and write my $contents = do {local (@ARGV,$/) = $file; <>};, which is the exact same thing in less characters :)

[Mr.Ree]

It's all situational. For a small file or a run-only-once quickie script, where "$string=cat $filename" is not available, this is perfectly reasonable. Inefficient yes! But that's not necessarily the only consideration.

[Powerlord]

I'm wondering why local @ARGV = $file; <> would be any different than <$file>.

[brian d foy]

@Bemrose: because $file is not a filehandle.

[Kip]

this does have the disadvantage that it is not included in out-of-the-box perl. at least not my ActiveState perl for windows (v5.10.0).

[unixman83]

This answer doesn't deserve a negative rating. Bunch of script kiddies that don't understand or care about what perl means by <FILEHANDLE>. It's an array silly. No worse performance than some of the other answers on this page. Very informative on how to think about Perl filehandles and slurping, as an array.

[Leon Timmermans]

Actually, File::Map (disclaimer: written by me) would be a better choice nowadays. It's far more portable (it works on both Unix and Windows), but also easier to use («map_file my $str, $file_name;»).

[brian d foy]

Note that File::Slurp has recently been discovered to be a huge security problem: rt.cpan.org/Ticket/Display.html?id=83126

[brian d foy]

Note that File::Slurp has recently been discovered to be a huge security problem: rt.cpan.org/Ticket/Display.html?id=83126

[Adam Millerchip]

I got shot in the foot adding this method to a file that further down was already using <>, expecting it to read from STDIN. The behaviour of <> differs from the first call to subsequent calls, and since I changed the first call, I altered the behaviour of the existing call too (which was expecting the <STDIN> behaviour of <>).

[stenlytw]

Hi, I got Undefined subroutine &main::read_text. It should be use File::Slurper 'read_text';. metacpan.org/pod/File::Slurper

[Britton Kerin]

Makes for a nice way to check that an attempted line substitution in a file actually happens: perl -p -i -0 -e 's/^old_line/new_line/m or (print and die)' some_file, or probably could use /mg to do all matching lines if many expected.

[Freon Sandoz]

File::Slurp isn't a portable solution. On Windows, it will barf on all non-ANSI filenames. I haven't tested Path::Tiny, but given Perl's general level of contempt for Windows, I'll bet that it's the same. It may be possible to open a file with Win32::LongPath and pass the filehandle instead of the filepath. (That's something that Perl really should support in all file I/O,)