When it comes to CLI applications, there are many different ways to provide configuration and options to them. Some of the ways we've already seen include adding in CLI arguments or passing in data through files or the standard input stream.

However, another great way to auto-control data is through the use of CLI flags, which are arguments that are passed into a command with a dash prefix.

For example, with the wc command:

wc words.txt

This will print out the lines, words, bytes, and the file's name.

If we want to constrain the output:

wc -w words.txt

It only prints the number of words. This CLI argument with a dash prefix is known as a flag, and allows you to configure the behavior of your application.

Additional flags:

• -l — display number of lines
• -c — display number of bytes

Flags can be combined:

wc -l -c words.txt

Other CLI apps may support flags like:

• --name=alice
• --limit=42

---

Adding Flags to Our Go CLI

To add flags in Go, we use the flag package.

It allows parsing in a Go-standard format:

• Single dash (-w)
• Double dash (--w) — also valid

First, import the flag package.

Define a boolean flag:

showWords := flag.Bool("w", false, "Used to toggle whether or not to show the word count")

Note: This returns a *bool, not a bool.

To access its value:

fmt.Println(*showWords)

Run with:

go run main.go -w words.txt

---

⚠ Problem: Flag Not Working

The flag value shows as false.

Why? We forgot to call:

flag.Parse()

This must be called after all flags are defined.

After calling flag.Parse(), now:

go run main.go -w words.txt

✅ showWords is true.

---

⚠ Problem: -w is Treated as a Filename

We're using:

os.Args[1:]

This includes flags and file names.

Fix: Replace with flag.Args() to get only non-flag arguments.

---

Better Flag Handling with BoolVar

Instead of using the pointer-returning flag.Bool, we can do:

var showWords bool
flag.BoolVar(&showWords, "W", false, "Used to toggle whether or not to show the word count")

Now showWords is a bool, not a pointer.

---

Adding More Flags

We'll need:

• -w → showWords
• -l → showLines
• -c → showBytes

To manage this cleanly, define a DisplayOptions struct:

type DisplayOptions struct {
    ShowWords bool
    ShowBytes bool
    ShowLines bool
}

Then:

var opts DisplayOptions

flag.BoolVar(&opts.ShowWords, "w", false, "Toggle word count")
flag.BoolVar(&opts.ShowBytes, "c", false, "Toggle byte count")
flag.BoolVar(&opts.ShowLines, "l", false, "Toggle line count")

---

Built-in Help

You can run:

go run main.go -h

It prints available flags with their usage descriptions.

---

Testing Combinations

go run main.go -w words.txt
go run main.go -c -l words.txt

You should see toggled combinations of output.

---

Using DisplayOptions in Print Method

Update Print method to accept opts DisplayOptions:

func (c Counts) Print(w io.Writer, opts DisplayOptions, filenames ...string)

Note: Variadic parameters must go last.

---

Testing DisplayOptions

In count_test.go, add test cases with DisplayOptions:

{
    name: "show only lines",
    input: Inputs{
        Counts: Counts{Lines: 1, Words: 5, Bytes: 24},
        FileNames: []string{"words.txt"},
        Options: DisplayOptions{ShowLines: true},
    },
    Want: "1 words.txt\n",
}

Use t.Run(...) and assert the string output using a bytes.Buffer.

---

Conditionally Formatting Output

Build a string slice and use strings.Join:

var xs []string

if opts.ShowLines {
    xs = append(xs, strconv.Itoa(c.Lines))
}
if opts.ShowWords {
    xs = append(xs, strconv.Itoa(c.Words))
}
if opts.ShowBytes {
    xs = append(xs, strconv.Itoa(c.Bytes))
}

xs = append(xs, filenames...) // Add suffixes

fmt.Fprintln(w, strings.Join(xs, " "))

---

Bug: No Output Without Flags

If no flags are passed, nothing prints.

Fix: Add helper method on DisplayOptions:

func (d DisplayOptions) ShouldShowBytes() bool {
    return !d.ShowBytes && !d.ShowWords && !d.ShowLines || d.ShowBytes
}

Repeat similarly for ShouldShowWords() and ShouldShowLines().

Update Print method to use those instead.

---

Tests Pass ✅

Test the behavior with and without flags. It should default to showing all values if no flags are set.

---

One Final Bug

Standard input with piped input (e.g. echo foo | go run main.go) fails because it can’t seek back to the beginning.

We’ll fix this in the next lesson by changing the logic to count all properties in a single pass.

---

Commit Your Changes

git add main.go count.go count_test.go
git commit -m "Added CLI flags and display options"

With that, we’re ready to move on and fix the final issue. See you in the next lesson!