Echo script

main.go

    package main

    import (
        "flag"
        "fmt"
        "io"
        "os"
        "strings"
    )

    var (
        n = flag.Bool("n", false, "omit trailing newline")
        s = flag.String("s", " ", "separator")
    )

    func main() {
        flag.Parse()
        var out io.Writer = os.Stdout // CREATE A LOCAL WRITER CLASS
        if err := echo(out, !*n, *s, flag.Args()); err != nil { // INJECT IT INTO THE ECHO FUNCTION
            fmt.Fprintf(os.Stderr, "echo: %v\n", err)
            os.Exit(1)
        }
    }

    // RECIEVE OUT AS PARAMETER
    func echo(out io.Writer, newline bool, sep string, args []string) error {
        fmt.Fprint(out, strings.Join(args, sep))
        if newline {
            fmt.Fprintln(out)
        }
        return nil
    }

main_test.go

    package main

    import (
        "bytes"
        "fmt"
        "testing"
    )

    func TestEcho(t *testing.T) {
        var tests = []struct {
            newline bool
            sep     string
            args    []string
            want    string
        }{
            {true, "", []string{}, "\n"},
            {false, "", []string{}, ""},
            {true, "\t", []string{"one", "two", "three"}, "one\ttwo\tthree\n"},
            {true, ",", []string{"a", "b", "c"}, "a,b,c\n"},
            {false, ":", []string{"1", "2", "3"}, "1:2:3"},
        }
        for _, test := range tests {
            descr := fmt.Sprintf("echo(%v, %q, %q)", test.newline, test.sep, test.args)
            var out = new(bytes.Buffer) // CREATE A WRITER LIKE CLASS JUST FOR THIS TEST
            if err := echo(out, test.newline, test.sep, test.args); err != nil { // INJECT INTO ECHO FUNCTION
                t.Errorf("%s failed: %v", descr, err)
                continue
            }
            got := out.String() // READ OUTPUT
            if got != test.want {
                t.Errorf("%s = %q, want %q", descr, got, test.want)
            }
        }
    }