cat.go

package main

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

func main() {
    argsWithoutProg := os.Args[1:]

    if len(argsWithoutProg) == 0 {

        s := bufio.NewScanner(os.Stdin)

        for s.Scan() {
            fmt.Printf("%s\n", s.Text())
        }

    } else {

        for _, path := range argsWithoutProg {

            f, err := os.Open(path)

            if err != nil {
                os.Stderr.WriteString(fmt.Sprintf("cat: failed to open with err: %v", err))
                continue
            }

            if isDir(path) {
                os.Stderr.WriteString(fmt.Sprintf("cat: %s: Is a directory", path))
                continue
            }

            s := bufio.NewScanner(f)

            for s.Scan() {
                fmt.Printf("%s\n", s.Text())
            }
        }
    }
}

func isDir(path string) bool {
    info, err := os.Stat(path)
    if err != nil {
        log.Panic("failed to Stat")
    }

    return info.Mode().IsDir()
}