Learning Go – Day 5

• Finishing going through https://golangbot.com/learn-golang-series/
• Continuing to watch some of Golang University 101
  • GopherCon 2019: Carolyn Van Slyck – Design Command-Line Tools People Love
  • Google I/O 2012 – Go Concurrency Patterns

Reading files

• Reading the entire file into memory.

import "io/ioutil"

data, err := ioutil.ReadFile("lorem.txt")
if err != nil {
	fmt.Println("Failed to read the file. Error: ", err)
	return
}
//NOTE: ReadFile returns a byte slice
fmt.Printf("Contents of file:\n%s\n", string(data))

• You can pack static files into the binary using a tool like packr. Might and might not be of use to me in the future.
• Reading a file in chunks using bufio package.

package main

import (
	"bufio"
	"fmt"
	"log"
	"os"
)

func main() {
	// Open the file
	f, err := os.Open("lorem.txt")
	if err != nil {
		log.Fatal(err)
	}

	// Ensure the file is closed when we are done
	defer func() {
		if err = f.Close(); err != nil {
			log.Fatal(err)
		}
	}()

	// Read from the file (f) into a buffer (b) 128 bytes at a time
	r := bufio.NewReader(f)
	b := make([]byte, 128)
	for {
		n, err := r.Read(b)
		if err != nil {
			fmt.Println("Error reading file:", err)
			break
		}
		fmt.Println(string(b[0:n]))
	}
}

• Reading a text file line by line.

s := bufio.NewScanner(f)
for s.Scan() {
    fmt.Println(s.Text())
}
// Check if any errors occurred (other than EOF)
err = s.Err()
if err != nil {
    log.Fatal(err)
}

Writing files

• Writing strings to a file.

package main

import (
	"fmt"
	"log"
	"os"
)

func main() {
	// Create a new file
	f, err := os.Create("test.txt")
	if err != nil {
		log.Fatal(err)
		return
	}

	// Ensure the file is closed when we are done
	defer func() {
		if err = f.Close(); err != nil {
			log.Fatal(err)
		}
	}()

  // Write a single string
	bc, err := f.WriteString("The quick brown fox ...")
	if err != nil {
		log.Fatal(err)
		return
	}

	fmt.Println(bc, "bytes written successfully")
}

• Writing bytes to file.

data := []byte{0x41, 0x6E, 0x64, 0x72, 0x65}
bc, err := f.Write(data)

• The fmt package also have some nice utility functions like FPrintf & FPrintln
• Open a file for appending.

f, err := os.OpenFile("/path/to/file", os.O_APPEND | os.O_WRONLY, 0644)
// Flags are append and "write-only"
// Also passing unix permissions 0644

What’s next?

I will be working through Alex Ellis’ Everyday Golang book.

I already know of at least 2 small little programs that I want to write that deals with my Linux server and to help me organise some of my data. More details will follow once I actually write the apps.