https://docs.google.com/presentation/d/1KiU14z2owLUoiTYz5pKo-gP8RnP2BINmucVYJ6DfxTs/edit#slide=id.g4cc8086b3f_0_541

cat.go

package main

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

var showLineNumber = flag.Bool("n", false, "Line number")

func main() {
	flag.Parse()
	allLines := make([]string, 0, 0)
	for _, filename := range flag.Args() {
		lines, err := read(filename)
		if err != nil {
			log.Fatal("owata")
		}
		allLines = append(allLines, lines...)
	}
	show(&allLines)
}

func read(src string) ([]string, error) {
	sf, err := os.Open(src)
	if err != nil {
		return nil, err
	}
	defer sf.Close()

	scanner := bufio.NewScanner(sf)
	lines := make([]string, 0, 0)
	for scanner.Scan() {
		lines = append(lines, scanner.Text())
	}

	if err := scanner.Err(); err != nil {
		fmt.Fprintln(os.Stderr, "読み込みに失敗しました:", err)
	}
	return lines, nil
}

func show(allLines *[]string) {
	for i, st := range *allLines {
		if *showLineNumber {
			fmt.Fprint(os.Stdout, i+1)
			fmt.Fprint(os.Stdout, ": ")
		}

		fmt.Fprintln(os.Stdout, st)
	}
}