Read a text file, replace its words, output to another text file

So I am trying to make a program in GO to take a text file full of code and convert that into GO code and then save that file into a GO file or text file. I have been trying to figure out how to save the changes I made to the text file, but the only way I can see the changes is through a println statement because I am using strings.replace to search the string array that the text file is stored in and change each occurrence of a word that needs to be changed (ex. BEGIN -> { and END -> }). So is there any other way of searching and replacing in GO I don't know about or is there a way to edit a text file that I don't know about or is this impossible?

Thanks

Here is the code I have so far.

package main

import (
    "os"
    "bufio"
    "bytes"
    "io"
    "fmt"
    "strings"
)


func readLines(path string) (lines []string, errr error) {
    var (
        file *os.File
        part []byte
        prefix bool
    )
    if file, errr = os.Open(path); errr != nil {
        return
    }
    defer file.Close()

    reader := bufio.NewReader(file)
    buffer := bytes.NewBuffer(make([]byte, 0))
    for {
        if part, prefix, errr = reader.ReadLine(); errr != nil {
            break
        }
    buffer.Write(part)
        if !prefix {
            lines = append(lines, buffer.String())
            buffer.Reset()
        }
    }
    if errr == io.EOF {
        errr = nil
    }
    return
}

func writeLines(lines []string, path string) (errr error) {
    var (
        file *os.File
    )

    if file, errr = os.Create(path); errr != nil {
        return
    }
    defer file.Close()


    for _,item := range lines {

        _, errr := file.WriteString(strings.TrimSpace(item) + "\n");

        if errr != nil {

            fmt.Println(errr)
            break
        }
    }

    return
}

func FixBegin(lines []string) (errr error) {
    var(
    a string
    
    )
for i := 0; ; i++ {
        a = lines[i];
      
        fmt.Println(strings.Replace(a, "BEGIN", "{", -1))
    }

    return
}

func FixEnd(lines []string) (errr error) {
    var(
    a string
    
    )
for i := 0; ; i++ {
        a = lines[i];
      
        fmt.Println(strings.Replace(a, "END", "}", -1))
    }
    return
}

func main() {
    lines, errr := readLines("foo.txt")
    if errr != nil {
        fmt.Println("Error: %s\n", errr)
        return
    }
    for _, line := range lines {
        fmt.Println(line)
    }


    errr = FixBegin(lines)
   
    errr = writeLines(lines, "beer2.txt")
    fmt.Println(errr)

    errr = FixEnd(lines)
    lines, errr = readLines("beer2.txt")
    if errr != nil {
        fmt.Println("Error: %s\n", errr)
        return
    }
    errr = writeLines(lines, "beer2.txt")
    fmt.Println(errr)
}

jnml@fsc-r630:~/src/tmp/SO/13789882$ ls
foo.txt  main.go
jnml@fsc-r630:~/src/tmp/SO/13789882$ cat main.go 
package main

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

func main() {
        src, err := ioutil.ReadFile("foo.txt")
        if err != nil {
                log.Fatal(err)
        }

        src = bytes.Replace(src, []byte("BEGIN"), []byte("{"), -1)
        src = bytes.Replace(src, []byte("END"), []byte("}"), -1)
        if err = ioutil.WriteFile("beer2.txt", src, 0666); err != nil {
                log.Fatal(err)
        }
}
jnml@fsc-r630:~/src/tmp/SO/13789882$ cat foo.txt 
BEGIN
  FILE F(KIND=REMOTE);
  EBCDIC ARRAY E[0:11];
  REPLACE E BY "HELLO WORLD!";
  WRITE(F, *, E);
END.
jnml@fsc-r630:~/src/tmp/SO/13789882$ go run main.go 
jnml@fsc-r630:~/src/tmp/SO/13789882$ cat beer2.txt 
{
  FILE F(KIND=REMOTE);
  EBCDIC ARRAY E[0:11];
  REPLACE E BY "HELLO WORLD!";
  WRITE(F, *, E);
}.
jnml@fsc-r630:~/src/tmp/SO/13789882$ 

I agree with @jnml wrt using ioutil to slurp the file and to write it back. But I think that the replacing shouldn't be done by multiple passes over []byte. Code and data are strings/text and should be treated as such (even if dealing with non ascii/utf8 encodings requires estra work); a one pass replacement (of all placeholders 'at once') avoids the risk of replacing results of previous changes (even if my regexp proposal must be improved to handle non-trivial tasks).

package main

import(
    "fmt"
    "io/ioutil"
    "log"
    "regexp"
    "strings"
)

func main() {
    // (1) slurp the file
    data, err := ioutil.ReadFile("../tmpl/xpl.go")
    if err != nil {
        log.Fatal("ioutil.ReadFile: ", err)
    }
    s := string(data)
    fmt.Printf("----\n%s----\n", s)
    // => function that works for files of (known) other encodings that ascii or utf8

    // (2) create a map that maps placeholder to be replaced to the replacements
    x := map[string]string {
        "BEGIN" : "{",
        "END" : "}"}
    ks := make([]string, 0, len(x))
    for k := range x {
        ks = append(ks, k)
    }
    // => function(s) that gets the keys from maps

    // (3) create a regexp that finds the placeholder to be replaced
    p := strings.Join(ks, "|")
    fmt.Printf("/%s/\n", p)
    r := regexp.MustCompile(p)
    // => funny letters & order need more consideration

    // (4) create a callback function for ..ReplaceAllStringFunc that knows
    // about the map x
    f := func(s string) string {
        fmt.Printf("*** '%s'\n", s)
        return x
展开收缩
    }
    // => function (?) to do Step (2) .. (4) in a reusable way

    // (5) do the replacing (s will be overwritten with the result)
    s = r.ReplaceAllStringFunc(s, f)
    fmt.Printf("----\n%s----\n", s)

    // (6) write back
    err = ioutil.WriteFile("result.go", []byte(s), 0644)
    if err != nil {
        log.Fatal("ioutil.WriteFile: ", err)
    }
    // => function that works for files of (known) other encodings that ascii or utf8
}

output:

go run 13789882.go
----
func main() BEGIN
END
----
/BEGIN|END/
*** 'BEGIN'
*** 'END'
----
func main() {
}
----

If your file size is huge, reading everything in memory might not be possible nor advised. Give BytesReplacingReader a try as it is done replacement in streaming fashion. And it's reasonably performant. If you want to replace two strings (such as BEGIN -> { and END -> }), just need to wrap two BytesReplacingReader over original reader, one for BEGIN and one for END:

r := NewBytesReplacingReader(
    NewBytesReplacingReader(inputReader, []byte("BEGIN"), []byte("{"),
    []byte("END"), []byte("}")
// use r normally and all non-overlapping occurrences of
// "BEGIN" and "END" will be replaced with "{" and "}"