4 cases of reading and writing files in Go

1. How to create empty file in Go

package main
import "os"

func main() {
  file, _ := os.Create("/tmp/go.txt")
  defer file.Close()
}

• /tmp/go.txt - path to file to create

2. How to create new or overwrite existing file with new content in Go

package main
import "os"

func main() {
  f, _ := os.OpenFile("/tmp/go.txt", os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
  defer f.Close()
  f.WriteString("new content\n")
}

• /tmp/go.txt path to file to create/overwrite
• os.O_RDWR|os.O_CREATE|os.O_TRUNC means we will either create new or truncate existing file before writing content
• new content\n new content to write to file

3. How to append line to existing file in Go

package main
import "os"

func main() {
  f, _ := os.OpenFile("/tmp/go.txt", os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
  defer f.Close()
  f.WriteString("new line\n")
}

Here, we're appending "new line\n" text (with new line symbol at the end) to /tmp/go.txt file. In order to make that right, we need to set os.O_APPEND|os.O_CREATE|os.O_WRONLY permissions.

4. How to read file contents to a string variable:

package main
import "os"

func main() {
  t, _ := os.ReadFile("/tmp/go.txt")
  str := string(t)
}

Here we read everything from /tmp/go.txt file to str variable. Note we're converting read bytes to string using string() function.

More useful Go solutions to work with files

• How to rename a file
• How to remove a file
• How to check if file exists
• How to read big file line by line