Replacing a line within a file with Golang


I’m new to Golang, starting out with some examples. Currently, what I’m trying to do is reading a file line by line and replace it with another string in case it meets a certain condition.
The file is use for testing purposes contains four lines:


The code working on that file looks like this:

 func main() {

     file, err := os.OpenFile("test.txt", os.O_RDWR, 0666)

     if err != nil {

     reader := bufio.NewReader(file)

      for {

         fmt.Print("Try to read ...\n")

         pos,_ := file.Seek(0, 1)
         log.Printf("Position in file is: %d", pos)
         bytes, _, _ := reader.ReadLine()

         if (len(bytes) == 0) {

         lineString := string(bytes)

         if(lineString == "two") {
             file.Seek(int64(-(len(lineString))), 1)
             file.WriteString("This is a test.")

         fmt.Printf(lineString + "\n")


As you can see in the code snippet, I want to replace the string “two” with “This is a test” as soon as this string is read from the file.
In order to get the current position within the file I use Go’s Seek method.
However, what happens is that always the last line gets replaced by This is a test, making the file looking like this:

This is a test

Examining the output of the print statement which writes the current file position to the terminal, I get that kind of output after the first line has been read:

2016/12/28 21:10:31 Try to read ...
2016/12/28 21:10:31 Position in file is: 19

So after the first read, the position cursor already points to the end of my file, which explains why the new string gets appended to the end. Does anyone understand what is happening here or rather what is causing that behavior?


The Reader is not controller by the file.Seek. You have declared the reader as: reader := bufio.NewReader(file) and then you read one line at a time bytes, _, _ := reader.ReadLine() however the file.Seek does not change the position that the reader is reading.

Suggest you read about the ReadSeeker in the docs and switch over to using that. Also there is an example using the SectionReader.

Answered By – miltonb

Answer Checked By – Clifford M. (GoLangFix Volunteer)

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