Get last 10 lines of very large text file > 10GB

What is the most efficient way to display the last 10 lines of a very large text file (this particular file is over 10GB). I was thinking of just writing a simple C# app but I'm not sure how to do this effectively.

Read to the end of the file, then seek backwards until you find ten newlines, and then read forward to the end taking into consideration various encodings. Be sure to handle cases where the number of lines in the file is less than ten. Below is an implementation (in C# as you tagged this), generalized to find the last numberOfTokens in the file located at path encoded in encoding where the token separator is represented by tokenSeparator; the result is returned as a string (this could be improved by returning an IEnumerable<string> that enumerates the tokens).

public static string ReadEndTokens(string path, Int64 numberOfTokens, Encoding encoding, string tokenSeparator) {

    int sizeOfChar = encoding.GetByteCount("\n");
    byte[] buffer = encoding.GetBytes(tokenSeparator);


    using (FileStream fs = new FileStream(path, FileMode.Open)) {
        Int64 tokenCount = 0;
        Int64 endPosition = fs.Length / sizeOfChar;

        for (Int64 position = sizeOfChar; position < endPosition; position += sizeOfChar) {
            fs.Seek(-position, SeekOrigin.End);
            fs.Read(buffer, 0, buffer.Length);

            if (encoding.GetString(buffer) == tokenSeparator) {
                tokenCount++;
                if (tokenCount == numberOfTokens) {
                    byte[] returnBuffer = new byte[fs.Length - fs.Position];
                    fs.Read(returnBuffer, 0, returnBuffer.Length);
                    return encoding.GetString(returnBuffer);
                }
            }
        }

        // handle case where number of tokens in file is less than numberOfTokens
        fs.Seek(0, SeekOrigin.Begin);
        buffer = new byte[fs.Length];
        fs.Read(buffer, 0, buffer.Length);
        return encoding.GetString(buffer);
    }
}

I'd likely just open it as a binary stream, seek to the end, then back up looking for line breaks. Back up 10 (or 11 depending on that last line) to find your 10 lines, then just read to the end and use Encoding.GetString on what you read to get it into a string format. Split as desired.

Tail? Tail is a unix command that will display the last few lines of a file. There is a Windows version in the Windows 2003 Server resource kit.

As the others have suggested, you can go to the end of the file and read backwards, effectively. However, it's slightly tricky - particularly because if you have a variable-length encoding (such as UTF-8) you need to be cunning about making sure you get "whole" characters.

You should be able to use FileStream.Seek() to move to the end of the file, then work your way backwards, looking for \n until you have enough lines.