Check if a file has a particular set of bytes in it, and replace it with other bytes.

app.go

package main

import (
	"bytes"
	"io/ioutil"
	"log"
	"os"
)

//Open a file, read the bytes, check if the bytes have a sequence of bytes, then replace those bytes with other ones

//to test, make a file called "test.txt" and write something in it that has "hello" (lower case). eg, "hello, world"
func main() {

	hello := []byte("hello") //The word to compare against, as bytes
	holla := []byte("holla") //The word to change to, as bytes

	//Open the test file with read/write rights ( my file said, "Go says hello world")
	f, err := os.OpenFile("test.txt", os.O_APPEND|os.O_RDWR, os.ModeAppend)
	if err != nil {
		log.Fatalln(err.Error())
	}
	defer f.Close() //Be nice and close the file at the end of main

	bs, err := ioutil.ReadAll(f) //Use a utility to read the entire stream of bytes in from the file
	if err != nil {
		log.Fatalln(err.Error())
	}

	//compare the bytes to the hello byte slice to see if it's there. If so, the index is honest. If not, kill the app
	hasWord, start := compare(bs, hello)
	if !hasWord {
		log.Fatalf("File does not have %s\n", string(hello))
	}

	for i := range holla {
		bs[start+i] = holla[i] //for each byte in bs that correlates to hello, change it to the holla one.
	}

	_, err = f.WriteAt(bs, 0) //write the new slice of bytes to the file and check for errors. Ignore the count of bytes written.
	if err != nil {
		log.Fatalln(err.Error())
	}

	log.Println("Done")

}

//comprae compares a source slice against a compare-to slice to see if the source has a particular slice of bytes
//returns true or false, and possibly an offset/location
func compare(src, compare []byte) (bool, int) {
	for i := range src {
		if src[i] == compare[0] {
			return bytes.Equal(src[i:i+len(compare)], compare), i
		}
	}
	return false, 0
}