How to efficiently read only last line of the text file

Question

Need to get just last line from big log file. What is the best way to do that?

score 14 · Accepted Answer · edited May 23 '17 at 12:10

14

You want to read the file backwards using ReverseLineReader:

How to read a text file reversely with iterator in C#

Then run .Take(1) on it.

var lines = new ReverseLineReader(filename);
var last = lines.Take(1);

You'll want to use Jon Skeet's library MiscUtil directly rather than copying/pasting the code.

edited May 23 '17 at 12:10

Community

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answered May 02 '12 at 08:13

yamen

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Nice! Thank you, will try it. – 0x49D1 May 02 '12 at 08:14
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Strange that the library is not in Nuget..Thanks! – 0x49D1 May 02 '12 at 08:17
See my answer in the possible duplicate http://stackoverflow.com/a/33907602/4821032 – Xan-Kun Clark-Davis Nov 25 '15 at 02:42
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I know this answer is from the past, but for anyone reading this, "You'll want to NOT use Jon Skeet's library directly" as per Jon Skeet himself, rather DO copy/paste, take what you want from the code. https://github.com/jpsingleton/ANCLAFS/issues/6 – joedotnot Feb 27 '21 at 02:38

score 2 · Answer 2 · answered Nov 22 '14 at 23:46

    String lastline="";
    String filedata;

    // Open file to read
    var fullfiledata = new FileStream(filepath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
    StreamReader sr = new StreamReader(fullfiledata);

    //long offset = sr.BaseStream.Length - ((sr.BaseStream.Length * lengthWeNeed) / 100);
    // Assuming a line doesnt have more than 500 characters, else use above formula
    long offset = sr.BaseStream.Length - 500;

    //directly move the last 500th position
    sr.BaseStream.Seek(offset, SeekOrigin.Begin);

    //From there read lines, not whole file
    while (!sr.EndOfStream)
    {
        filedata = sr.ReadLine();
        // Interate to see last line
        if (sr.Peek() == -1)
        {
            lastline = filedata;
        }
    }       

    return lastline;
}

score -2 · Answer 3 · answered May 18 '12 at 17:24

-2

Or you can do it two line (.Net 4 only)

var lines = File.ReadLines(path);
string line = lines.Last();

answered May 18 '12 at 17:24

RedFox

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While this can do the job, it's incredibly memory inefficient (as RJFalconer notes) and also has a great deal of latency - remember that reading a file is one of the slowest things you can do in a program. If you're only infrequently reading, this might work out, but this is _far_ from a strong, professional solution. – Andrew Gray Sep 18 '17 at 17:08

How to efficiently read only last line of the text file

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