field/main.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

func main() {
	if len(os.Args) < 2 {
		fmt.Fprintf(os.Stderr, "usage: %s <field-number>\n", os.Args[0])
		os.Exit(1)
	}

	fieldNum, err := strconv.Atoi(os.Args[1])
	if err != nil || fieldNum < 1 {
		fmt.Fprintln(os.Stderr, "error: invalid field number")
		os.Exit(1)
	}

	scanner := bufio.NewScanner(os.Stdin)
	for scanner.Scan() {
		line := scanner.Text()
		fields := strings.Fields(line)
		if len(fields) >= fieldNum {
			fmt.Println(fields[fieldNum-1])
		}
	}
	if err := scanner.Err(); err != nil {
		fmt.Fprintln(os.Stderr, "error reading input:", err)
	}
}