Go: Write a command-line (CLI) application

This example is a simplified version of the Unix grep command. The program searches the input file for lines containing the given pattern and prints these lines.

func main() {
        log.SetPrefix("grep: ")
        log.SetFlags(0) // no extra info in log messages

        if len(os.Args) != 3 {
                fmt.Printf("Usage: %v PATTERN FILE\n", os.Args[0])
                return
        }

        pattern, err := regexp.Compile(os.Args[1])
        if err != nil {
                log.Fatalln(err)
        }

        file, err := os.Open(os.Args[2])
        if err != nil {
                log.Fatalln(err)
        }
        defer file.Close()

        scanner := bufio.NewScanner(file)
        for scanner.Scan() {
                line := scanner.Text()
                if pattern.MatchString(line) {
                        fmt.Println(line)
                }
        }
        if err := scanner.Err(); err != nil {
                log.Println(err)
        }
}