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