How to Read last lines from a big file with Go every 10 secs

Question

how can I read the last two lines from a big log file without load it into memory completely?

I need read it every 10 secs(On a Win machine)...and I'm stuck trying to read the last lines..

package main  import (     "fmt"     "time"     "os" )  const MYFILE = "logfile.log"  func main() {     c := time.Tick(10 * time.Second)     for now := range c {         readFile(MYFILE)     } }  func readFile(fname string){     file, err:=os.Open(fname)     if err!=nil{         panic(err)     }     buf:=make([]byte, 32)     c, err:=file.ReadAt(32, ????)     fmt.Printf("%s\n", c)   } 

The log file is something like:

07/25/2013 11:55:42.400, 0.559 07/25/2013 11:55:52.200, 0.477 07/25/2013 11:56:02.000, 0.463 07/25/2013 11:56:11.800, 0.454 07/25/2013 11:56:21.600, 0.424 07/25/2013 11:56:31.400, 0.382 07/25/2013 11:56:41.200, 0.353 07/25/2013 11:56:51.000, 0.384 07/25/2013 11:57:00.800, 0.393 07/25/2013 11:57:10.600, 0.456 

Thanks!

Answer 1

You can use file.Seek() or file.ReadAt() to almost the end and then Reading forward. You can only estimate where to start seeking unless you can know that 2 lines = x bytes.

You can get the File length by using the os.Stat(name)

Here is an example based on ReadAt, Stat, and your sample log file:

package main  import (     "fmt"     "os"     "time" )  const MYFILE = "logfile.log"  func main() {     c := time.Tick(10 * time.Second)     for _ = range c {         readFile(MYFILE)     } }  func readFile(fname string) {     file, err := os.Open(fname)     if err != nil {         panic(err)     }     defer file.Close()      buf := make([]byte, 62)     stat, err := os.Stat(fname)     start := stat.Size() - 62     _, err = file.ReadAt(buf, start)     if err == nil {         fmt.Printf("%s\n", buf)     }  }