Go: Goの実践：CLIツールの開発 — フラグの解析、Cobraのサブコマンド、パスワードマネージャー

# Desired CLI
$ passgen generate --length 16 --special
Generated: Kd9#mP2$xL7qR!vB

$ passgen save mydb -p Kd9#mP2$xL7qR!vB
Saved: mydb

$ passgen list
mydb    (created: 2026-07-08)
admin   (created: 2026-07-05)
api-key (created: 2026-07-01)

$ passgen get mydb
mydb: Kd9#mP2$xL7qR!vB

// passgen.go — Version 1: Basic password generation
package main

import (
    "crypto/rand"
    "flag"
    "fmt"
    "math/big"
)

var (
    length  = flag.Int("length", 12, "password length")
    special = flag.Bool("special", false, "include special characters")
    count   = flag.Int("count", 1, "number of passwords to generate")
)

const (
    lowerChars   = "abcdefghijklmnopqrstuvwxyz"
    upperChars   = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    digitChars   = "0123456789"
    specialChars = "!@#$%^&*()-_=+[]{}|;:,.<>?"
)

func generatePassword(length int, includeSpecial bool) string {
    charset := lowerChars + upperChars + digitChars
    if includeSpecial {
        charset += specialChars
    }

    pwd := make([]byte, length)
    for i := range pwd {
        n, _ := rand.Int(rand.Reader, big.NewInt(int64(len(charset))))
        pwd[i] = charset[n.Int64()]
    }
    return string(pwd)
}

func main() {
    flag.Parse()

    for i := 0; i < *count; i++ {
        fmt.Println(generatePassword(*length, *special))
    }
}

$ go run passgen.go --length 16 --special
Kd9#mP2$xL7qR!vB

$ go run passgen.go --length 8
aB3xK9mP

package main

import (
    "flag"
    "fmt"
)

func main() {
    // Register parameters (returns pointers)
    name := flag.String("name", "World", "name to greet")
    age := flag.Int("age", 0, "age")
    verbose := flag.Bool("verbose", false, "verbose output")

    // Parse (must be called before reading values)
    flag.Parse()

    if *verbose {
        fmt.Printf("name=%s, age=%d\n", *name, *age)
    }
    fmt.Printf("Hello, %s!\n", *name)
}

$ go run main.go --name Alice --age 28 --verbose
name=Alice, age=28
Hello, Alice!

$ go run main.go
Hello, World!

package main

import (
    "flag"
    "fmt"
    "os"
)

func main() {
    // Custom Usage message
    flag.Usage = func() {
        fmt.Fprintf(os.Stderr, "Usage: mytool [options] <file path>\n\n")
        fmt.Fprintf(os.Stderr, "Options:\n")
        flag.PrintDefaults()
    }

    // Register parameters
    output := flag.String("o", "output.txt", "output file path")
    limit := flag.Int("n", 0, "lines to process (0=all)")
    debug := flag.Bool("debug", false, "enable debug mode")

    // Parse
    flag.Parse()

    // Access non-flag arguments (remaining args)
    args := flag.Args()
    if len(args) < 1 {
        flag.Usage()
        os.Exit(1)
    }

    filePath := args[0]
    fmt.Printf("Processing file: %s\n", filePath)
    fmt.Printf("Output: %s\n", *output)
    fmt.Printf("Lines: %d\n", *limit)
    fmt.Printf("Debug: %v\n", *debug)

    // Supports both -o and --o forms
    // Supports both -o=file and -o file syntax
}

$ go run main.go -o result.txt -n 100 data.csv
Processing file: data.csv
Output: result.txt
Lines: 100
Debug: false

package main

import (
    "fmt"
    "os"
)

func main() {
    fmt.Printf("Program: %s\n", os.Args[0])
    fmt.Printf("Arg count: %d\n", len(os.Args)-1)

    for i, arg := range os.Args[1:] {
        fmt.Printf("Arg %d: %s\n", i+1, arg)
    }
}

$ go run main.go hello world --debug
Program: /tmp/main
Arg count: 3
Arg 1: hello
Arg 2: world
Arg 3: --debug

package main

import (
    "flag"
    "fmt"
)

func main() {
    // flag.Int returns a pointer vs flag.IntVar binds to an existing variable
    var (
        name   string
        port   int
        debug  bool
    )

    flag.StringVar(&name, "name", "World", "name")
    flag.IntVar(&port, "port", 8080, "port")
    flag.BoolVar(&debug, "debug", false, "debug mode")

    flag.Parse()

    fmt.Printf("name=%s, port=%d, debug=%v\n", name, port, debug)
}

package main

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

func main() {
    stat, _ := os.Stdin.Stat()
    hasStdin := (stat.Mode() & os.ModeCharDevice) == 0

    if hasStdin {
        // Read from pipe (echo "hello" | go run main.go)
        scanner := bufio.NewScanner(os.Stdin)
        for scanner.Scan() {
            line := strings.ToUpper(scanner.Text())
            fmt.Println(line)
        }
    } else {
        // Read from arguments (go run main.go hello)
        if len(os.Args) > 1 {
            fmt.Println(strings.ToUpper(strings.Join(os.Args[1:], " ")))
        } else {
            fmt.Println("Please provide input (pipe or arguments)")
        }
    }
}

$ echo "hello world" | go run main.go
HELLO WORLD

$ go run main.go hello world
HELLO WORLD

package main

import "fmt"

func main() {
    var name string
    var age int

    fmt.Print("Enter name: ")
    fmt.Scanf("%s", &name)

    fmt.Print("Enter age: ")
    fmt.Scanf("%d", &age)

    fmt.Printf("Hello %s, you are %d years old\n", name, age)
}

package main

import (
    "fmt"
    "os"
)

func main() {
    // Write directly to stdout
    fmt.Fprintln(os.Stdout, "Standard output")
    fmt.Fprintln(os.Stderr, "Error output")

    // Redirect to file
    file, _ := os.Create("output.log")
    defer file.Close()

    // fmt.Fprintln can write to any io.Writer
    fmt.Fprintln(file, "This line was written to the file")
    fmt.Fprintln(file, "Second line")
}

# Install cobra-cli
$ go install github.com/spf13/cobra-cli@latest

# Initialize project
$ cobra-cli init passgen
$ cobra-cli add generate
$ cobra-cli add save
$ cobra-cli add list

// cmd/root.go
package cmd

import (
    "fmt"
    "os"

    "github.com/spf13/cobra"
)

var rootCmd = &cobra.Command{
    Use:   "passgen",
    Short: "Password Manager",
    Long:  `A CLI tool for generating, storing, and managing passwords.`,
}

func Execute() {
    if err := rootCmd.Execute(); err != nil {
        fmt.Fprintln(os.Stderr, err)
        os.Exit(1)
    }
}

// cmd/generate.go
package cmd

import (
    "fmt"
    "crypto/rand"
    "math/big"

    "github.com/spf13/cobra"
)

var (
    genLength  int
    genSpecial bool
    genCount   int
)

var generateCmd = &cobra.Command{
    Use:   "generate",
    Short: "Generate a random password",
    Long:  `Generate a random password with specified length and complexity.`,
    Run: func(cmd *cobra.Command, args []string) {
        for i := 0; i < genCount; i++ {
            fmt.Println(generatePassword(genLength, genSpecial))
        }
    },
}

func init() {
    rootCmd.AddCommand(generateCmd)
    generateCmd.Flags().IntVarP(&genLength, "length", "l", 12, "password length")
    generateCmd.Flags().BoolVarP(&genSpecial, "special", "s", false, "include special characters")
    generateCmd.Flags().IntVarP(&genCount, "count", "c", 1, "number to generate")
}

func generatePassword(length int, special bool) string {
    charset := "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
    if special {
        charset += "!@#$%^&*()-_=+[]{}|;:,.<>?"
    }
    pwd := make([]byte, length)
    for i := range pwd {
        n, _ := rand.Int(rand.Reader, big.NewInt(int64(len(charset))))
        pwd[i] = charset[n.Int64()]
    }
    return string(pwd)
}

// main.go
package main

import "passgen/cmd"

func main() {
    cmd.Execute()
}

$ go build -o passgen
$ ./passgen generate --length 16 --special
Kd9#mP2$xL7qR!vB
$ ./passgen generate -l 8
aB3xK9mP

// main.go
package main

import (
    "bufio"
    "crypto/rand"
    "encoding/json"
    "flag"
    "fmt"
    "math/big"
    "os"
    "strings"
    "time"
)

// ---------- Data Structures ----------

type PasswordEntry struct {
    Name      string    `json:"name"`
    Password  string    `json:"password"`
    CreatedAt time.Time `json:"created_at"`
    UpdatedAt time.Time `json:"updated_at,omitempty"`
}

type PasswordStore struct {
    entries  map[string]PasswordEntry
    filePath string
}

const defaultStoreFile = "passwords.json"

// ---------- Storage Layer ----------

func NewPasswordStore(filePath string) *PasswordStore {
    store := &PasswordStore{
        entries:  make(map[string]PasswordEntry),
        filePath: filePath,
    }
    store.load()
    return store
}

func (s *PasswordStore) load() {
    data, err := os.ReadFile(s.filePath)
    if err != nil {
        return
    }
    json.Unmarshal(data, &s.entries)
}

func (s *PasswordStore) save() error {
    data, err := json.MarshalIndent(s.entries, "", "  ")
    if err != nil {
        return err
    }
    return os.WriteFile(s.filePath, data, 0600)
}

func (s *PasswordStore) Add(name, password string) {
    now := time.Now()
    if existing, ok := s.entries[name]; ok {
        existing.Password = password
        existing.UpdatedAt = now
        s.entries[name] = existing
    } else {
        s.entries[name] = PasswordEntry{
            Name:      name,
            Password:  password,
            CreatedAt: now,
        }
    }
    s.save()
}

func (s *PasswordStore) Get(name string) (PasswordEntry, bool) {
    entry, ok := s.entries[name]
    return entry, ok
}

func (s *PasswordStore) Delete(name string) bool {
    _, ok := s.entries[name]
    if ok {
        delete(s.entries, name)
        s.save()
    }
    return ok
}

func (s *PasswordStore) List() []PasswordEntry {
    result := make([]PasswordEntry, 0, len(s.entries))
    for _, entry := range s.entries {
        result = append(result, entry)
    }
    return result
}

// ---------- Password Generation ----------

const (
    letters  = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
    digits   = "0123456789"
    specials = "!@#$%^&*()-_=+[]{}|;:,.<>?"
    noAmbigu = "ABCDEFGHJKLMNPQRSTUVWXYZabcdefghjkmnpqrstuvwxyz23456789"
)

func generatePassword(length int, useSpecial bool) (string, error) {
    if length < 4 {
        return "", fmt.Errorf("password length must be at least 4")
    }

    charset := letters + digits
    if useSpecial {
        charset += specials
    }

    pwd := make([]byte, length)
    for i := range pwd {
        n, err := rand.Int(rand.Reader, big.NewInt(int64(len(charset))))
        if err != nil {
            return "", err
        }
        pwd[i] = charset[n.Int64()]
    }
    return string(pwd), nil
}

// ---------- CLI Commands ----------

func cmdGenerate(args []string) {
    cmd := flag.NewFlagSet("generate", flag.ExitOnError)
    length := cmd.Int("length", 12, "password length")
    special := cmd.Bool("special", false, "include special characters")
    count := cmd.Int("count", 1, "number to generate")
    _ = cmd.Bool("no-ambiguous", false, "exclude ambiguous characters (0OIl1)")
    cmd.Parse(args)

    for i := 0; i < *count; i++ {
        pwd, err := generatePassword(*length, *special)
        if err != nil {
            fmt.Fprintf(os.Stderr, "Error: %v\n", err)
            os.Exit(1)
        }
        fmt.Println(pwd)
    }
}

func cmdSave(args []string, store *PasswordStore) {
    cmd := flag.NewFlagSet("save", flag.ExitOnError)
    password := cmd.String("password", "", "password (leave empty to auto-generate)")
    length := cmd.Int("length", 16, "auto-generated password length")
    cmd.Parse(args)

    name := cmd.Arg(0)
    if name == "" {
        fmt.Fprintln(os.Stderr, "Usage: passgen save <name> [--password <pwd>]")
        os.Exit(1)
    }

    pwd := *password
    if pwd == "" {
        var err error
        pwd, err = generatePassword(*length, true)
        if err != nil {
            fmt.Fprintf(os.Stderr, "Error: %v\n", err)
            os.Exit(1)
        }
    }

    store.Add(name, pwd)
    fmt.Printf("Saved: %s = %s\n", name, pwd)
}

func cmdGet(args []string, store *PasswordStore) {
    if len(args) < 1 {
        fmt.Fprintln(os.Stderr, "Usage: passgen get <name>")
        os.Exit(1)
    }

    entry, ok := store.Get(args[0])
    if !ok {
        fmt.Fprintf(os.Stderr, "Not found: %s\n", args[0])
        os.Exit(1)
    }
    fmt.Printf("%s: %s\n", entry.Name, entry.Password)
}

func cmdList(args []string, store *PasswordStore) {
    entries := store.List()
    if len(entries) == 0 {
        fmt.Println("(no saved passwords)")
        return
    }

    fmt.Printf("%-20s %-20s %s\n", "Name", "Password", "Created")
    fmt.Println(strings.Repeat("-", 60))
    for _, e := range entries {
        pwd := e.Password
        if len(pwd) > 16 {
            pwd = pwd[:16] + "..."
        }
        fmt.Printf("%-20s %-20s %s\n",
            e.Name, pwd, e.CreatedAt.Format("2006-01-02"))
    }
}

func cmdDelete(args []string, store *PasswordStore) {
    if len(args) < 1 {
        fmt.Fprintln(os.Stderr, "Usage: passgen delete <name>")
        os.Exit(1)
    }

    if store.Delete(args[0]) {
        fmt.Printf("Deleted: %s\n", args[0])
    } else {
        fmt.Fprintf(os.Stderr, "Not found: %s\n", args[0])
    }
}

func cmdExport(args []string, store *PasswordStore) {
    output := "passwords_export.json"
    if len(args) > 0 {
        output = args[0]
    }

    data, _ := json.MarshalIndent(store.List(), "", "  ")
    os.WriteFile(output, data, 0600)
    fmt.Printf("Exported to: %s\n", output)
}

// ---------- Main Function ----------

func printUsage() {
    fmt.Println(`Password Manager CLI

Usage:
  passgen <command> [options]

Commands:
  generate    Generate a random password
  save        Save a password
  get         Retrieve a password
  list        List all passwords
  delete      Delete a password
  export      Export passwords to a file

Options:
  --store     Specify storage file path (default: passwords.json)

Examples:
  passgen generate --length 16 --special
  passgen save mydb --length 20
  passgen get mydb
  passgen list
  passgen delete mydb`)
}

func main() {
    // Global --store parameter
    storeFile := defaultStoreFile
    if len(os.Args) > 1 {
        for i := 1; i < len(os.Args); i++ {
            if os.Args[i] == "--store" && i+1 < len(os.Args) {
                storeFile = os.Args[i+1]
                // Remove the store parameter
                os.Args = append(os.Args[:i], os.Args[i+2:]...)
                break
            }
        }
    }

    store := NewPasswordStore(storeFile)

    if len(os.Args) < 2 {
        printUsage()
        return
    }

    command := os.Args[1]
    args := os.Args[2:]

    switch command {
    case "generate":
        cmdGenerate(args)
    case "save":
        cmdSave(args, store)
    case "get":
        cmdGet(args, store)
    case "list":
        cmdList(args, store)
    case "delete":
        cmdDelete(args, store)
    case "export":
        cmdExport(args, store)
    case "help", "--help", "-h":
        printUsage()
    default:
        fmt.Fprintf(os.Stderr, "Unknown command: %s\n", command)
        fmt.Fprintf(os.Stderr, "Run 'passgen help' to see available commands\n")
        os.Exit(1)
    }
}

# Generate password
$ go run main.go generate --length 16 --special
Kd9#mP2$xL7qR!vB

# Save password (auto-generated)
$ go run main.go save mydb --length 20
Saved: mydb = aB3xK9mP$L7qR!vD2nFg

# Save password (manually specified)
$ go run main.go save admin --password "MyStr0ng!Pass"
Saved: admin = MyStr0ng!Pass

# Retrieve password
$ go run main.go get mydb
mydb: aB3xK9mP$L7qR!vD2nFg

# List
$ go run main.go list
Name                 Password              Created
------------------------------------------------------------
mydb                 aB3xK9mP$L7qR!v...   2026-07-08
admin                MyStr0ng!Pass         2026-07-08

# Export
$ go run main.go export
Exported to: passwords_export.json

# Specify storage file
$ go run main.go --store vault.json generate -l 10