file.go

package main

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

func main() {
	writing()
	reading()
	deleting()
}

// https://gobyexample.com/reading-files
func reading() {
	fmt.Println("=== file.reading ===")

	// Perhaps the most basic file reading task is slurping a file's entire contents into memory.
	dat, err := ioutil.ReadFile("/tmp/dat")
	check(err)
	fmt.Print(string(dat))

	// You'll often want more control over how and what parts of a file are read. For these tasks, start by Opening a file to obtain an os.File value.
	f, err := os.Open("/tmp/dat")
	check(err)

	// Read some bytes from the beginning of the file. Allow up to 5 to be read but also note how many actually were read.
	b1 := make([]byte, 5)
	n1, err := f.Read(b1)
	check(err)
	fmt.Printf("%d bytes: %s\n", n1, string(b1))

	// You can also Seek to a known location in the file and Read from there.
	o2, err := f.Seek(6, 0)
	check(err)
	b2 := make([]byte, 2)
	n2, err := f.Read(b2)
	check(err)
	fmt.Printf("%d bytes @ %d: %s\n", n2, o2, string(b2))

	//The io package provides some functions that may be helpful for file reading. For example, reads like the ones above can be more robustly implemented with ReadAtLeast.
	o3, err := f.Seek(6, 0)
	check(err)
	b3 := make([]byte, 2)
	n3, err := io.ReadAtLeast(f, b3, 2)
	check(err)
	fmt.Printf("%d bytes @ %d: %s\n", n3, o3, string(b3))

	// There is no built-in rewind, but Seek(0, 0) accomplishes this.
	_, err = f.Seek(0, 0)
	check(err)

	// The bufio package implements a buffered reader that may be useful both for its efficiency with many small reads and because of the additional reading methods it provides.
	r4 := bufio.NewReader(f)
	b4, err := r4.Peek(5)
	check(err)
	fmt.Printf("5 bytes: %s\n", string(b4))

	// Close the file when you're done (usually this would be scheduled immediately after Opening with defer).
	f.Close()
}

// https://gobyexample.com/writing-files
func writing() {
	fmt.Println("=== file.writing ===")

	// Here’s how to dump a string (or just bytes) into a file.
	f1 := []byte("Hi\nMom\nand Dad.\nI'm on TV!")
	err := ioutil.WriteFile("/tmp/dat", f1, 0644)
	check(err)

	// For more granular writes, open a file for writing.
	f, err := os.Create("/tmp/dat")
	// It’s idiomatic to defer a Close immediately after opening a file.
	defer f.Close()

	// You can Write byte slices as you'd expect.
	d2 := []byte{115, 111, 109, 101, 10}
	n2, err := f.Write(d2)
	check(err)
	fmt.Printf("wrote %d bytes\n", n2)

	// A WriteString is also available.
	n3, err := f.WriteString("writes\n")
	fmt.Printf("wrote %d bytes\n", n3)

	// Issue a Sync to flush writes to stable storage.
	f.Sync()

	// bufio provides buffered writers in addition to buffered readers
	w := bufio.NewWriter(f)
	n4, err := w.WriteString("buffered\n")
	fmt.Printf("wrote %d bytes\n", n4)

	// Use Flush to ensure all buffered operations have been applied to the underlying writer.
	w.Flush()
}

// deleting show how to delete a file
func deleting() {
	fmt.Println("=== file.deleting ===")
	err := os.Remove("/tmp/dat")
	check(err)
}

func check(e error) {
	if e != nil {
		panic(e)
	}
}