Gotac prints lines from standard input or files in command-line in reverse order

gotac.go

// Gotac prints lines from standard input or files in command-line in reverse order.
package main

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

func main() {
	args := os.Args[1:]
	if len(args) == 0 {
		tac(os.Stdin, "stdin")
	} else {
		for _, path := range args {
			var f *os.File
			if path == "-" {
				f = os.Stdin
				path = "stdin"
			} else {
				var err error
				f, err = os.OpenFile(path, os.O_RDONLY, 0)
				if err != nil {
					fmt.Fprintf(os.Stderr, "cannot open %v: %v\n", path, err)
					continue
				}
				defer f.Close()
			}
			tac(f, path)
		}
	}
}

// tac reads f line by line and prints lines in reverse order.
// path is used for error handling.
func tac(f *os.File, path string) {
	lines, err := readLines(f)
	printReverse(lines)
	if err != nil {
		fmt.Fprintf(os.Stderr, "error while reading %v: %v\n", path, err)
	}
}

// readLines returns a slice containing all lines in f without end-of-line.
// If an error occurs, lines contains all lines accumulated before the error.
func readLines(f *os.File) ([]string, error) {
	lines := make([]string, 0, 16)
	scanner := bufio.NewScanner(f)
	for scanner.Scan() {
		lines = append(lines, scanner.Text())
	}
	return lines, scanner.Err()
}

// printReverse prints lines from end to beginning.
func printReverse(lines []string) {
	for i := len(lines) - 1; i >= 0; i-- {
		fmt.Println(lines[i])
	}
}