Wed, 13 Jun 2007

Atomic in-place rewriting of files with backup in perl

In my article on Perl's IO::Handle objects I talked briefly about IO::AtomicFile and IO::Digest. I've just had reason to use these very useful modules to create a script which edits a file in place. These modules allowed me to do the rewrite atomically and optionally make a backup if the contents have changed. The example assumes you have a function called perform_rewrite that takes two file handles as the first two parameters.

use File::Copy;
use IO::File;
use IO::AtomicFile;
use IO::Digest;

sub rewrite_file {
   my $file = shift;
   my $sub = shift;
   my $input = new IO::File($file,'r');
   my $input_md5 = new IO::Digest($input, 'MD5');
   my $output = new IO::AtomicFile($file,'w');
   my $output_md5 = new IO::Digest($output, 'MD5');

   $sub->($input, $output, @_);

   if ($input_md5->hexdigest ne $output_md5->hexdigest) {
           copy ("$file", "$file.bak");
           $output->close();
   } else {
           # we haven't changed so don't bother updating
           $output->delete();
   }
   $input->close();
}

rewrite_file("/foo/bar", \&perform_rewrite, $baz, $quux);
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