Append to an existing file in Go (Golang)

os.OpenFile() function of the os package can be used to open to a file in an append mode and then write to it

Let’s see an example. In the below program:

• First, write to a file using ioutil package
• Open the file again in Append mode and write the second line to it
• Read the file to verify the content.

package main

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

func main() {
    //Write first line
    err := ioutil.WriteFile("temp.txt", []byte("first line\n"), 0644)
    if err != nil {
        log.Fatal(err)
    }
    
    //Append second line
    file, err := os.OpenFile("temp.txt", os.O_APPEND|os.O_WRONLY, 0644)
    if err != nil {
        log.Println(err)
    }
    defer file.Close()
    if _, err := file.WriteString("second line"); err != nil {
        log.Fatal(err)
    }
    
    //Print the contents of the file
    data, err := ioutil.ReadFile("temp.txt")
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(string(data))
}

Output:

first line
second line