Golang : Replace a parameter's value inside a configuration file example

Alright, putting this here for my own future reference. In this tutorial, we will explore how to read in a configuration(text) file, replace certain value of a parameter and write it back(update) into the file.

Basically, what this simple program does is to convert the entire configuration(text) file into a slice, scan for the parameter that we want to alter, update the parameter's value and write back to file.

Here you go!

 package main

 import (
  "fmt"
  "strings"
 )

 func main() {

  input, err := ioutil.ReadFile(configFilename)
  if err != nil {
 log.Println(err)
  }

  // split into a slice
  lines := strings.Split(string(input), "\n")

  fmt.Println("before : ", lines)

  // assuming that we want to change the value of thirdItem from 3 to 100
  replacementText := "thirdItem = 100"

  for i, line := range lines {
 if equal := strings.Index(line, "thirdItem"); equal == 0 {
 // update to the new value
 lines[i] = replacementText
 }
  }

  fmt.Println("after : ", lines)

  // join back before writing into the file
  linesBytes := []byte(strings.Join(lines, "\n"))

  //the value will be UPDATED!!!
  if err = ioutil.WriteFile(configFilename, linesBytes, 0666); err != nil {
 log.Println(err)
  }
 }

You can play with a demo without modifying any file at https://play.golang.org/p/G8RKWdfh5hu

Hope this helps and happy coding!

References :

https://www.socketloop.com/references/golang-io-ioutil-writefile-function-example

https://www.socketloop.com/tutorials/golang-convert-string-to-byte-examples