简介

Cobra既是一个用来创建强大的现代CLI命令行的golang库，也是一个生成程序应用和命令行文件的程序。下面是Cobra使用的一个演示：

Cobra提供的功能

• 简易的子命令行模式，如 app server， app fetch等等
• 完全兼容posix命令行模式
• 嵌套子命令subcommand
• 支持全局，局部，串联flags
• 使用Cobra很容易的生成应用程序和命令，使用cobra create appname和cobra add cmdname
• 如果命令输入错误，将提供智能建议，如 app srver，将提示srver没有，是否是app server
• 自动生成commands和flags的帮助信息
• 自动生成详细的help信息，如app help
• 自动识别-h，--help帮助flag
• 自动生成应用程序在bash下命令自动完成功能
• 自动生成应用程序的man手册
• 命令行别名
• 自定义help和usage信息
• 可选的紧密集成的viper apps

如何使用

上面所有列出的功能我没有一一去使用，下面我来简单介绍一下如何使用Cobra，基本能够满足一般命令行程序的需求，如果需要更多功能，可以研究一下源码github。

安装cobra

Cobra是非常容易使用的，使用go get来安装最新版本的库。当然这个库还是相对比较大的，可能需要安装它可能需要相当长的时间，这取决于你的速网。安装完成后，打开GOPATH目录，bin目录下应该有已经编译好的cobra.exe程序，当然你也可以使用源代码自己生成一个最新的cobra程序。

> go get -v github.com/spf13/cobra/cobra

使用cobra生成应用程序

假设现在我们要开发一个基于CLIs的命令程序，名字为demo。首先打开CMD，切换到GOPATH的src目录下[^1]，执行如下指令：
[^1]:cobra.exe只能在GOPATH目录下执行

src> ..\bin\cobra.exe init demo 
Your Cobra application is ready at
C:\Users\liubo5\Desktop\transcoding_tool\src\demo
Give it a try by going there and running `go run main.go`
Add commands to it by running `cobra add [cmdname]`

在src目录下会生成一个demo的文件夹，如下：

▾ demo
    ▾ cmd/
        root.go
    main.go

如果你的demo程序没有subcommands，那么cobra生成应用程序的操作就结束了。

如何实现没有子命令的CLIs程序

接下来就是可以继续demo的功能设计了。例如我在demo下面新建一个包，名称为imp。如下：

▾ demo
    ▾ cmd/
        root.go
    ▾ imp/
        imp.go
        imp_test.go
    main.go

imp.go文件的代码如下：

package imp

import(
    "fmt"
)

func Show(name string, age int) {
    fmt.Printf("My Name is %s, My age is %d\n", name, age)
}

demo程序成命令行接收两个参数name和age，然后打印出来。打开cobra自动生成的main.go文件查看：

// Copyright © 2016 NAME HERE <EMAIL ADDRESS>
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package main

import "demo/cmd"

func main() {
    cmd.Execute()
}

可以看出main函数执行cmd包，所以我们只需要在cmd包内调用imp包就能实现demo程序的需求。接着打开root.go文件查看：

// Copyright © 2016 NAME HERE <EMAIL ADDRESS>
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package cmd

import (
    "fmt"
    "os"

    "github.com/spf13/cobra"
    "github.com/spf13/viper"
)

var cfgFile string

// RootCmd represents the base command when called without any subcommands
var RootCmd = &cobra.Command{
    Use:   "demo",
    Short: "A brief description of your application",
    Long: `A longer description that spans multiple lines and likely contains
examples and usage of using your application. For example:

Cobra is a CLI library for Go that empowers applications.
This application is a tool to generate the needed files
to quickly create a Cobra application.`,
// Uncomment the following line if your bare application
// has an action associated with it:
//  Run: func(cmd *cobra.Command, args []string) { },
}

// Execute adds all child commands to the root command sets flags appropriately.
// This is called by main.main(). It only needs to happen once to the rootCmd.
func Execute() {
    if err := RootCmd.Execute(); err != nil {
        fmt.Println(err)
        os.Exit(-1)
    }
}

func init() {
    cobra.OnInitialize(initConfig)

    // Here you will define your flags and configuration settings.
    // Cobra supports Persistent Flags, which, if defined here,
    // will be global for your application.

    RootCmd.PersistentFlags().StringVar(&cfgFile, "config", "", "config file (default is $HOME/.demo.yaml)")
    // Cobra also supports local flags, which will only run
    // when this action is called directly.
    RootCmd.Flags().BoolP("toggle", "t", false, "Help message for toggle")
}

// initConfig reads in config file and ENV variables if set.
func initConfig() {
    if cfgFile != "" { // enable ability to specify config file via flag
        viper.SetConfigFile(cfgFile)
    }

    viper.SetConfigName(".demo") // name of config file (without extension)
    viper.AddConfigPath("$HOME")  // adding home directory as first search path
    viper.AutomaticEnv()          // read in environment variables that match

    // If a config file is found, read it in.
    if err := viper.ReadInConfig(); err == nil {
        fmt.Println("Using config file:", viper.ConfigFileUsed())
    }
}

从源代码来看cmd包进行了一些初始化操作并提供了Execute接口。十分简单，其中viper是cobra集成的配置文件读取的库，这里不需要使用，我们可以注释掉（不注释可能生成的应用程序很大约10M，这里没哟用到最好是注释掉）。cobra的所有命令都是通过cobra.Command这个结构体实现的。为了实现demo功能，显然我们需要修改RootCmd。修改后的代码如下：

// Copyright © 2016 NAME HERE <EMAIL ADDRESS>
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package cmd

import (
    "fmt"
    "os"

    "github.com/spf13/cobra"
    //  "github.com/spf13/viper"
    "demo/imp"
)

//var cfgFile string
var name string
var age int

// RootCmd represents the base command when called without any subcommands
var RootCmd = &cobra.Command{
    Use:   "demo",
    Short: "A test demo",
    Long:  `Demo is a test appcation for print things`,
    // Uncomment the following line if your bare application
    // has an action associated with it:
    Run: func(cmd *cobra.Command, args []string) {
        if len(name) == 0 {
            cmd.Help()
            return
        }
        imp.Show(name, age)
    },
}

// Execute adds all child commands to the root command sets flags appropriately.
// This is called by main.main(). It only needs to happen once to the rootCmd.
func Execute() {
    if err := RootCmd.Execute(); err != nil {
        fmt.Println(err)
        os.Exit(-1)
    }
}

func init() {
    //  cobra.OnInitialize(initConfig)

    // Here you will define your flags and configuration settings.
    // Cobra supports Persistent Flags, which, if defined here,
    // will be global for your application.

    //  RootCmd.PersistentFlags().StringVar(&cfgFile, "config", "", "config file (default is $HOME/.demo.yaml)")
    // Cobra also supports local flags, which will only run
    // when this action is called directly.
    //  RootCmd.Flags().BoolP("toggle", "t", false, "Help message for toggle")
    RootCmd.Flags().StringVarP(&name, "name", "n", "", "person's name")
    RootCmd.Flags().IntVarP(&age, "age", "a", 0, "person's age")
}

// initConfig reads in config file and ENV variables if set.
//func initConfig() {
//  if cfgFile != "" { // enable ability to specify config file via flag
//      viper.SetConfigFile(cfgFile)
//  }

//  viper.SetConfigName(".demo") // name of config file (without extension)
//  viper.AddConfigPath("$HOME") // adding home directory as first search path
//  viper.AutomaticEnv()         // read in environment variables that match

//  // If a config file is found, read it in.
//  if err := viper.ReadInConfig(); err == nil {
//      fmt.Println("Using config file:", viper.ConfigFileUsed())
//  }
//}

到此demo的功能已经实现了，我们编译运行一下看看实际效果：

>demo.exe
Demo is a test appcation for print things

Usage:
  demo [flags]

Flags:
  -a, --age int       person's age
  -h, --help          help for demo
  -n, --name string   person's name

>demo -n borey --age 26
My Name is borey, My age is 26

如何实现带有子命令的CLIs程序

在执行cobra.exe init demo之后，继续使用cobra为demo添加子命令test：

src\demo>..\..\bin\cobra add test
test created at C:\Users\liubo5\Desktop\transcoding_tool\src\demo\cmd\test.go

在src目录下demo的文件夹下生成了一个cmd\test.go文件，如下：

▾ demo
    ▾ cmd/
        root.go
        test.go
    main.go

接下来的操作就和上面修改root.go文件一样去配置test子命令。效果如下：

src\demo>demo
Demo is a test appcation for print things

Usage:
  demo [flags]
  demo [command]

Available Commands:
  test        A brief description of your command

Flags:
  -a, --age int       person's age
  -h, --help          help for demo
  -n, --name string   person's name

Use "demo [command] --help" for more information about a command.

可以看出demo既支持直接使用标记flag，又能使用子命令

src\demo>demo test -h
A longer description that spans multiple lines and likely contains examples
and usage of using your command. For example:

Cobra is a CLI library for Go that empowers applications.
This application is a tool to generate the needed files
to quickly create a Cobra application.

Usage:
  demo test [flags]

调用test命令输出信息，这里没有对默认信息进行修改。

src\demo>demo tst
Error: unknown command "tst" for "demo"

Did you mean this?
        test

Run 'demo --help' for usage.
unknown command "tst" for "demo"

Did you mean this?
        test

这是错误命令提示功能

OVER

Cobra的使用就介绍到这里，更新细节可去github详细研究一下。这里只是一个简单的使用入门介绍，如果有错误之处，敬请指出，谢谢～

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