package main

import (
    "bufio"
    "fmt"
    "os"
    "path/filepath"
)

func check(e error) {
    if e != nil {
        panic(e)
    }
}

func main() {

    d1 := []byte("hello\ngo\n")
    path1 := filepath.Join(os.TempDir(), "dat1")
    err := os.WriteFile(path1, d1, 0644)
    check(err)

    path2 := filepath.Join(os.TempDir(), "dat2")
    f, err := os.Create(path2)
    check(err)

    defer f.Close()

    d2 := []byte{115, 111, 109, 101, 10}
    n2, err := f.Write(d2)
    check(err)
    fmt.Printf("wrote %d bytes\n", n2)

    n3, err := f.WriteString("writes\n")
    check(err)
    fmt.Printf("wrote %d bytes\n", n3)

    f.Sync()

    w := bufio.NewWriter(f)
    n4, err := w.WriteString("buffered\n")
    check(err)
    fmt.Printf("wrote %d bytes\n", n4)

    w.Flush()

$ go run writing-files.go 
wrote 5 bytes
wrote 7 bytes
wrote 9 bytes

$ cat /tmp/dat1
hello
go
$ cat /tmp/dat2
some
writes
buffered