5 ms·

The dd approach is interesting. Editing a file like that in place cowboy style with dd would fill me with anxiety however

• tumetab1 6y ago
  Can anyone explain how dd is able to read and write a file at the same time without using a temporary file? :\
  • arbitrage 6y ago
    it opens up two file handles, reads from the first and writes to the second.
    • tumetab1 6y ago
      That doesn't fully explain it but I think I got it, each line is overwritten with what it's place on the 300th line after. Something like readLineOffset = 301 writeLineOffset = 1 While not read eof line = Read(readLineOffset) Write(WriteLineOffset, line) readLineOffset ++ writeLineOffset ++
      • ComodoHacker 6y ago
        dd reads and writes in blocks of bytes, not lines. You have to figure out where 300th line starts beforehand.
  • netmonk 6y ago
    More over, how dd deal with EOL ? and counting them ?
    • jrochkind1 6y ago
      I believe it doesn't, and the solution in the OP first counts EOLs to get the byte offset you want, then tells dd what to do just in terms of byte offsets.
  • tyingq 6y ago
    You can do it with a single filehandle, reading the data then seek()ing backwards before you do the writes. A Perl example that reads a file with the numbers 1 to 100 then replaces the digits with X's: #!/usr/bin/perl open(my $fh,">foo") or die($!); for (1..100) {print $fh "$_\n"}; close($fh); open($fh,'+<','foo') or die($!); while(<$fh>) { my $spot=tell($fh)-length($_); seek($fh,$spot,SEEK_SET) or die($!); s/\d/X/g;#replace digits with X's print $fh "$_" or die($!); } close($fh); If his file has lines of equal size, this would be fairly easy to do in place. Would be a little more complicated if not.
    • yencabulator 6y ago
      Number of file handles is irrelevant, all handles for the one file are accessing the same data.
      • tyingq 6y ago
        Just pointing out it's not needed. Why is that irrelevant?
  • seppel 6y ago
    There's nothing inherently wrong with reading and writing to the same file (as least if it is a binary file). And if you read at offset x and write at offset x-y you can easily move contents around with only a very small buffer. Just be sure that you move in the correct direction, so that you dont override stuff you need later. That people normally use temporary files, has two reasons: The first is that it is much safer if something goes wrong. You then still have the original and not some half-way processed file. The other reason is that reading and writing in text mode does not really work. But you can of course process text files as binary files and deal with line endings yourself.