What You Will Learn

By the end of this chapter, you will know how to:

• Read and write files using the os and io packages
• Use buffered I/O for performance
• Marshal and unmarshal JSON
• Work with encoding formats: JSON, XML, and CSV
• Make HTTP requests with net/http
• Access environment variables and interact with the OS

This chapter prepares you to handle real-world data processing.

10.1 Reading and Writing Files

Go provides the os package for filesystem access.

a. Write to a File

package main

import (
 "os"
)
func main() {
 content := []byte("Hello, Go file!")
 err := os.WriteFile("example.txt", content, 0644)
 if err != nil {
  panic(err)
 }
}

b. Read a File

data, err := os.ReadFile("example.txt")
if err != nil {
	panic(err)
}
fmt.Println(string(data))

c. Writing with Append Mode

f, err := os.OpenFile("example.txt", os.O_APPEND|os.O_WRONLY, 0644)
if err != nil {
	panic(err)
}
defer f.Close()

f.WriteString("\nAppending text!")

10.2 Buffered I/O (io & bufio)

Buffered I/O improves performance by reducing system calls.

a. Writing Buffered Output

f, _ := os.Create("buffer.txt")
w := bufio.NewWriter(f)

w.WriteString("Buffered write example!")
w.Flush()

b. Reading Buffered Input

file, _ := os.Open("buffer.txt")
r := bufio.NewReader(file)

line, _ := r.ReadString('\n')
fmt.Println(line)

10.3 Working With JSON

Go uses encoding/json.

a. Struct → JSON (Marshalling)

type User struct {
	Name string `json:"name"`
	Age  int    `json:"age"`
}

user := User{"Alice", 30}
jsonData, _ := json.MarshalIndent(user, "", "  ")
fmt.Println(string(jsonData))

b. JSON → Struct (Unmarshalling)

var u User
json.Unmarshal(jsonData, &u)
fmt.Println(u.Name, u.Age)

10.4 Encoding Formats: JSON, XML, CSV

✔ XML

type Person struct {
	XMLName xml.Name `xml:"person"`
	Name    string   `xml:"name"`
	Age     int      `xml:"age"`
}

p := Person{"John", 25}
out, _ := xml.MarshalIndent(p, "", "  ")
fmt.Println(string(out))

✔ CSV

file, _ := os.Create("data.csv")
writer := csv.NewWriter(file)

writer.Write([]string{"Name", "Age"})
writer.Write([]string{"Alice", "30"})
writer.Flush()

Read CSV:

file, _ := os.Open("data.csv")
reader := csv.NewReader(file)


records, _ := reader.ReadAll()
fmt.Println(records)

10.5 HTTP Requests (net/http)

a. GET Request

resp, _ := http.Get("https://api.github.com")
defer resp.Body.Close()


body, _ := io.ReadAll(resp.Body)
fmt.Println(string(body))

b. POST Request (JSON)

payload := strings.NewReader(`{"name":"test"}`)


resp, _ := http.Post("https://httpbin.org/post", "application/json", payload)
defer resp.Body.Close()
body, _ := io.ReadAll(resp.Body)
fmt.Println(string(body))

10.6 Environment Variables & OS Package

a. Get Environment Variable

value := os.Getenv("HOME")
fmt.Println("Home directory:", value)

b. Set an Environment Variable (Temporary)

os.Setenv("APP_ENV", "production")
fmt.Println(os.Getenv("APP_ENV"))

c. Listing System Files

entries, _ := os.ReadDir(".")
for _, e := range entries {
	fmt.Println(e.Name())
}

Summary of Chapter 10

You now understand how to:

✔ Read & write files with os and io ✔ Use buffered I/O to improve performance ✔ Encode and decode JSON, XML, and CSV ✔ Make HTTP requests and consume APIs ✔ Read environment variables and interact with the operating system