bash – 从12GB文件中删除特定行

>这太疯狂了.
>它可以消耗更多的内存,而不仅仅是将整个文件放入内存,即使你限制其缓冲区的大小.

你遇到了这两个问题.您遇到文件的每一行,因此Tie :: File将读取整个文件并将每行的索引存储在内存中.这在64位构建的Perl上每行占用28个字节(不计算内存分配器中的任何开销).

要删除文件的最后9行,可以使用以下命令：

use File::ReadBackwards qw( );

my $qfn = '...';

my $pos;
{
   my $bw = File::ReadBackwards->new($qfn)
      or die("Can't open "$qfn": $!n");

   for (1..9) {
      defined( my $line = $bw->readline() )
         or last;
   }

   $pos = $bw->tell();
}

# Can't use $bw->get_handle because it's a read-only handle.
truncate($qfn,$pos)
   or die("Can't truncate "$qfn": $!n");

要删除任意行,您可以使用以下内容：

my $qfn = '...';

open(my $fh_src,'<:raw',$qfn)
   or die("Can't open "$qfn": $!n");    
open(my $fh_dst,'+<:raw',$qfn)
   or die("Can't open "$qfn": $!n");

while (<$fh_src>) {
   next if $. == 9;  # Or "if /keyword/",or whatever condition you want.

   print($fh_dst $_)
      or die($!);
}

truncate($fh_dst,tell($fh_dst))
   or die($!);

以下优化版本假设只有一行(或行块)要删除：

use Fcntl qw( SEEK_CUR SEEK_SET );

use constant BLOCK_SIZE => 4*1024*1024;

my $qfn = 'file';

open(my $fh_src,$qfn)
   or die("Can't open "$qfn": $!n");
open(my $fh_dst,$qfn)
   or die("Can't open "$qfn": $!n");

my $dst_pos;
while (1) {
   $dst_pos = tell($fh_src);
   defined( my $line = <$fh_src> )
      or do {
         $dst_pos = undef;
         last;
      };

   last if $. == 9;  # Or "if /keyword/",or whatever condition you want.
}

if (defined($dst_pos)) {
   # We're switching from buffered I/O to unbuffered I/O,# so we need to move the system file pointer from where the
   # buffered read left off to where we actually finished reading.
   sysseek($fh_src,tell($fh_src),SEEK_SET)
      or die($!);

   sysseek($fh_dst,$dst_pos,SEEK_SET)
      or die($!);

   while (1) {
      my $rv = sysread($fh_src,my $buf,BLOCK_SIZE);
      die($!) if !defined($rv);
      last if !$rv;

      my $written = 0;
      while ($written < length($buf)) {
         my $rv = syswrite($fh_dst,$buf,length($buf)-$written,$written);
         die($!) if !defined($rv);
         $written += $rv;
      }
   }

   # Must use sysseek instead of tell with sysread/syswrite.    
   truncate($fh_dst,sysseek($fh_dst,SEEK_CUR))
      or die($!);
}