Go Standard Library

1. os (File Operations, Environment Variables)

File Operations:

• os.Open(name string) (*os.File, error) – Opens a file for reading.
• os.Create(name string) (*os.File, error) – Creates a new file for writing.
• file.Close() – Closes the opened file.
• ioutil.ReadFile(filename string) ([]byte, error) – Reads the entire content of the file into a byte slice.
• os.Remove(name string) error – Deletes the specified file.
• os.Stat(name string) (os.FileInfo, error) – Retrieves file information like size, permissions, etc.

Environment Variables:

• os.Getenv(key string) string – Gets the value of the environment variable.
• os.Setenv(key, value string) error – Sets the value of the environment variable.
• os.Environ() – Returns a list of all environment variables.

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2. net/http (Building HTTP Servers, Clients)

HTTP Server:

• http.HandleFunc(pattern string, handler func(http.ResponseWriter, *http.Request)) – Register a handler function for the given pattern.
• http.ListenAndServe(addr string, handler http.Handler) error – Starts an HTTP server at the specified address.
• Example:

  http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "Hello, world!")
  })
  log.Fatal(http.ListenAndServe(":8080", nil))
  

HTTP Client:

• http.Get(url string) (*http.Response, error) – Sends a GET request.
• http.Post(url, contentType string, body io.Reader) (*http.Response, error) – Sends a POST request.
• http.NewRequest(method, url string, body io.Reader) (*http.Request, error) – Creates a new HTTP request (useful for custom headers, etc.).
• Example:

  resp, err := http.Get("https://example.com")
  if err != nil {
      log.Fatal(err)
  }
  defer resp.Body.Close()
  

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3. encoding/json (JSON Marshalling/Unmarshalling)

Marshalling (Go struct to JSON):

• json.Marshal(v interface{}) ([]byte, error) – Converts Go values to JSON.
• Example:

  person := struct {
      Name string
      Age  int
  }{"Alice", 30}
  jsonData, _ := json.Marshal(person)
  

Unmarshalling (JSON to Go struct):

• json.Unmarshal(data []byte, v interface{}) error – Converts JSON back to Go values.
• Example:

  jsonStr := `{"Name":"Alice","Age":30}`
  var person struct {
      Name string
      Age  int
  }
  json.Unmarshal([]byte(jsonStr), &person)
  

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4. strconv (Type Conversions)

String to Int/Float:

• strconv.Atoi(s string) (int, error) – Converts string to integer.
• strconv.ParseInt(s string, base, bitSize int) (int64, error) – Converts string to integer with specified base and bit size.
• strconv.ParseFloat(s string, bitSize int) (float64, error) – Converts string to float.
• Example:

  num, _ := strconv.Atoi("42")
  

Int/Float to String:

• strconv.Itoa(i int) string – Converts integer to string.
• strconv.FormatFloat(f float64, fmt byte, prec, bitSize int) string – Converts float to string.
• Example:

  numStr := strconv.Itoa(42)
  

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5. time (Time Manipulation)

Current Time:

• time.Now() – Returns the current local time.
• Example:

  t := time.Now()
  fmt.Println(t)
  

Formatting Time:

• t.Format(layout string) string – Formats the time based on layout.
• Layout example: 2006-01-02 15:04:05
• Example:

  fmt.Println(t.Format("2006-01-02 15:04:05"))
  

Sleeping:

• time.Sleep(duration time.Duration) – Pauses execution for the specified duration.
• Example:

  time.Sleep(2 * time.Second)
  

Time Parsing:

• time.Parse(layout, value string) (Time, error) – Parses a string into time.
• Example:

  parsedTime, _ := time.Parse("2006-01-02", "2024-11-17")
  

Duration:

• time.Duration – Represents elapsed time.
• time.Second, time.Minute etc., are constants representing durations.