File I/O Tutorial - Read, Write, Scanner & Append | Go Tutorial #22

File I/O in Go: Reading, Writing, Scanning & Appending

Working with files is essential for many Go applications, and this tutorial demonstrates how to read, write, scan, and append to files efficiently. You'll learn to open files, read their contents fully or line-by-line, write new data, and append to existing files using Go's standard libraries.

Code

package main

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

func main() {
  // Reading a file fully
  file, err := os.Open("example.txt")
  if err != nil {
    fmt.Println("Error opening file:", err)
    return
  }
  defer file.Close()

  content, err := io.ReadAll(file)
  if err != nil {
    fmt.Println("Error reading file:", err)
    return
  }
  fmt.Println("Full file content:")
  fmt.Println(string(content))

  // Getting file info
  info, err := os.Stat("example.txt")
  if err == nil {
    fmt.Printf("File size: %d bytes\n", info.Size())
  }

  // Writing to a new file
  newFile, err := os.Create("output.txt")
  if err != nil {
    fmt.Println("Error creating file:", err)
    return
  }
  defer newFile.Close()

  _, err = newFile.WriteString("Hello, Go file writing!\n")
  if err != nil {
    fmt.Println("Error writing to file:", err)
    return
  }

  // Reading file line-by-line with Scanner
  file.Seek(0, io.SeekStart) // Reset file pointer to start
  scanner := bufio.NewScanner(file)
  fmt.Println("Reading file line-by-line:")
  for scanner.Scan() {
    fmt.Println(scanner.Text())
  }
  if err := scanner.Err(); err != nil {
    fmt.Println("Scanner error:", err)
  }

  // Appending to a file
  appendFile, err := os.OpenFile("output.txt", os.O_APPEND|os.O_WRONLY, 0644)
  if err != nil {
    fmt.Println("Error opening file for appending:", err)
    return
  }
  defer appendFile.Close()

  _, err = appendFile.WriteString("Appended line.\n")
  if err != nil {
    fmt.Println("Error appending to file:", err)
  }
}

Key Points

• Use os.Open to open files for reading and defer Close to release resources.
• io.ReadAll reads the entire file content into memory for easy processing.
• bufio.Scanner enables efficient line-by-line reading without loading the whole file.
• os.Create creates or truncates files for writing, while os.OpenFile with flags allows appending.
• Always handle errors and use defer to ensure files are properly closed after operations.