go-coreutils/head/head.go

package main

import (
	"bufio"
	"fmt"
	"io"
	"os"

	"github.com/alexflint/go-arg"
	common "source.heropunch.io/tomo/go-coreutils"
)

func head(num uint, reader io.Reader) {
	scanner := bufio.NewScanner(reader)
	for scanner.Scan() && num > 0 {
		fmt.Println(scanner.Text())
		num = num - 1
	}
	common.ExitIfError(scanner.Err())
}

func main() {
	var args struct {
		Lines uint     `arg:"-n"`
		Files []string `arg:"positional"`
	}
	args.Lines = 10
	arg.MustParse(&args)

	if len(args.Files) == 0 {
		head(args.Lines, os.Stdin)
		os.Exit(0)
	}
	for _, filename := range args.Files {
		file, err := os.Open(filename)
		common.ExitIfError(err)

		head(args.Lines, file)

		err = file.Close()
		common.ExitIfError(err)
	}
}