Learn to create command line tools with Go in one hour
Foreword
Go has been written for a period of time recently because of the project. Compared with Java, the syntax is simple and it has some syntactic sugar like Python, which makes people shout "really fragrant".

Learn to create command line tools with Go in one hour
But at this stage, relatively speaking, I have written more in Python, and occasionally I have to go back and write some Java; naturally, I am not familiar with Go.

Therefore, it is convenient to make a small project on the weekend to deepen some experience. So I thought of a blog gadget written in Java before.

At that time, a large number of pictures on Weibo Tubed were forbidden to be linked, which caused many pictures in personal blogs to be unavailable for viewing. This tool can back up the pictures in the article to the local, and can also directly replace the pictures to other pictures.

Learn to create command line tools with Go in one hour

I personally use it all the time now, usually using iPic and other tools to upload pictures to Weibo Tubed (mainly convenient + free) when codewords. After writing, use this tool to switch to a paid image bed such as SM.MS with one click , and the image will be backed up to the local disk at the same time.

The effect of using Go to rewrite as cli tool is as follows:

Learn to create command line tools with Go in one hour
3-min.gif

What skills need to be mastered The
reason why I chose this tool to rewrite it with Go; one is that the function is relatively simple, but it can also take advantage of some of the characteristics of Go, such as network IO, coroutine synchronization, etc.

At the same time, after modifying it to a command line tool, does it feel more geeky?

Before we start, let's introduce some knowledge points for Javaers who are not familiar with Go:

Use and manage third-party dependency packages (go mod)
The use of coroutines.
Multi-platform packaging.
Let's start with the specific operation. I think even friends who haven't been in touch with Go can quickly get started to implement a small tool after reading it.

Use and manage third-party dependencies

If you haven't installed Go, please refer to the official website to install it yourself.
First, let me introduce Go's dependency management. After version 1.11, the official dependency management module has been included, so it is strongly recommended in the current latest version 1.15.

Its purpose and function are similar to maven in Java and pip in Python, but it is much simpler to use than maven.

Learn to create command line tools with Go in one hour
According to its use reference, you need to execute go mod init in the project directory to initialize a go.mod file. Of course, if you are using an IDE like GoLang, it will automatically create a directory structure for us when creating a new project. Of course Also include the go.mod file.

In this file we introduce the third-party packages we need:

module btb

go 1.15

require (
 github.com/cheggaaa/pb/v3 v3.0.5
 github.com/fatih/color v1.10.0
 github.com/urfave/cli/v2 v2.3.0
)

I used three packages here, namely:

pb: progress bar, used to output a progress bar on the console.
color: Used to output text in different colors on the console.
cli: Command line tool development kit.

import (
"btb/constants"
"btb/service"
"github.com/urfave/cli/v2"
"log"
"os"
)

func main() {
var model string
downloadPath := constants.DownloadPath
markdownPath := constants.MarkdownPath

app := &cli.App{
Flags: []cli.Flag{
&cli.StringFlag{
Name: "model",
Usage: "operating mode; r:replace, b:backup",
DefaultText: "b",
Aliases: []string{"m"},
Required: true,
Destination: &model,
},
&cli.StringFlag{
Name: "download-path",
Usage: "The path where the image is stored",
Aliases: []string{"dp"},
Destination: &downloadPath,
Required: true,
Value: constants.DownloadPath,
},
&cli.StringFlag{
Name: "markdown-path",
Usage: "The path where the markdown file is stored",
Aliases: []string{"mp"},
Destination: &markdownPath,
Required: true,
Value: constants.MarkdownPath,
},
},
Action: func(c *cli.Context) error {
service.DownLoadPic(markdownPath, downloadPath)

return nil
},
Name: "btb",
Usage: "Help you backup and replace your blog's images",
}

err := app.Run(os.Args)
if err != nil {
log.Fatal(err)
}
}


代码非常简单，无非就是使用了 cli 所提供的 api 创建了几个命令，将用户输入的 -dp、-mp 参数映射到 downloadPath、markdownPath 变量中。

之后便利用这两个数据扫描所有的图片，以及将图片下载到对应的目录中。

更多使用指南可以直接参考官方文档。

可以看到部分语法与 Java 完全不同，比如：

* 申明变量时类型是放在后边，先定义变量名称；方法参数类似。
* 类型推导，可以不指定变量类型（新版本的 Java 也支持）
* 方法支持同时返回多个值，这点非常好用。
* 公共、私用函数利用首字母大小写来区分。
* 还有其他的就不一一列举了。
**协程**
紧接着命令执行处调用了 service.DownLoadPic(markdownPath, downloadPath) 处理业务逻辑。

这里包含的文件扫描、图片下载之类的代码就不分析了；官方 SDK 写的很清楚，也比较简单。

重点看看 Go 里的 goroutime 也就是协程。

我这里使用的场景是每扫描到一个文件就利用一个协程去解析和下载图片，从而可以提高整体的运行效率。

func DownLoadPic(markdownPath, downloadPath string) {
wg := sync.WaitGroup{}
allFile, err := util.GetAllFile(markdownPath)
wg.Add(len(*allFile))

if err != nil {
log.Fatal("read file error")
}

for _, filePath := range *allFile {

go func(filePath string) {
allLine, err := util.ReadFileLine(filePath)
if err != nil {
log.Fatal(err)
}
availableImgs := util.MatchAvailableImg(allLine)
bar := pb.ProgressBarTemplate(constants.PbTmpl).Start(len(*availableImgs))
bar.Set("fileName", filePath).
SetWidth(120)

for _, url := range availableImgs {
if err != nil {
log.Fatal(err)
}
err := util.DownloadFile(url, genFullFileName(downloadPath, filePath, &url))
if err != nil {
log.Fatal(err)
}
bar.Increment()

}
bar.Finish()
wg.Done()

}(filePath)
}
wg.Wait()
color.Green("Successful handling of [%v] files.\n", len(*allFile))

if err != nil {
log.Fatal(err)
}
}


就代码使用层面看起来是不是要比 Java 简洁许多，我们不用像 Java 那样需要维护一个 executorService，也不需要考虑这个线程池的大小，一切都交给 Go 自己去调度。

使用时只需要在调用函数之前加上 go 关键字，只不过这里是一个匿名函数。

而且由于 goroutime 非常轻量，与 Java 中的 thread 相比占用非常少的内存，所以我们也不需要精准的控制创建数量。

不过这里也用到了一个和 Java 非常类似的东西：WaitGroup。

它的用法与作用都与 Java 中的 CountDownLatch 非常相似；主要用于等待所有的 goroutime 执行完毕，在这里自然是等待所有的图片都下载完毕然后退出程序。

使用起来主要分为三步：

* 创建和初始化 goruntime 的数量：wg.Add(len(number)
* 每当一个 goruntime 执行完毕调用 wg.Done() 让计数减一。
* 最终调用 wg.Wait() 等待WaitGroup 的数量减为0。
对于协程 Go 推荐使用 chanel 来互相通信，这点今后有机会再讨论。

**打包**
核心逻辑也就这么多，下面来讲讲打包与运行；这点和 Java 的区别就比较大了。

众所周知，Java 有一句名言：write once run anywhere

这是因为有了 JVM 虚拟机，所以我们不管代码最终运行于哪个平台都只需要打出一个包；但 Go 没有虚拟机它是怎么做到在个各平台运行呢。

简单来说 Go 可以针对不同平台打包出不同的二进制文件，这个文件包含了所有运行所需要的依赖，甚至都不需要在目标平台安装 Go 环境。

* 虽说 Java 最终只需要打一个包，但也得在各个平台安装兼容的 Java 运行环境。
我在这里编写了一个 Makefile 用于执行打包：make release

Binary name

BINARY=btb
GOBUILD=go build -ldflags "-s -w" -o ${BINARY}
GOCLEAN=go clean
RMTARGZ=rm -rf *.gz
VERSION=0.0.1

release:

Clean

$(GOCLEAN)
$(RMTARGZ)

Build for mac

CGO_ENABLED=0 GOOS=darwin GOARCH=amd64 $(GOBUILD)
tar czvf ${BINARY}-mac64-${VERSION}.tar.gz ./${BINARY}

Build for arm

$(GOCLEAN)
CGO_ENABLED=0 GOOS=linux GOARCH=arm64 $(GOBUILD)
tar czvf ${BINARY}-arm64-${VERSION}.tar.gz ./${BINARY}

Build for linux

$(GOCLEAN)
CGO_ENABLED=0 GOOS=linux GOARCH=amd64 $(GOBUILD)
tar czvf ${BINARY}-linux64-${VERSION}.tar.gz ./${BINARY}

Build for win

$(GOCLEAN)
CGO_ENABLED=0 GOOS=windows GOARCH=amd64 $(GOBUILD).exe
tar czvf ${BINARY}-win64-${VERSION}.tar.gz ./${BINARY}.exe
$(GOCLEAN)


可以看到我们只需要在 go build 之前指定系统变量即可打出不同平台的包，比如我们为 Linux 系统的 arm64 架构打包文件：

CGO_ENABLED=0 GOOS=linux GOARCH=arm64 go build main.go -o btb

便可以直接在目标平台执行 ./btb  运行程序。

**总结**
本文所有代码都已上传 Github: https://github.com/crossoverJie/btb

感兴趣的也可以直接运行安装脚本体验。

curl -fsSL https://raw.githubusercontent.com/crossoverJie/btb/master/install.sh | bash
目前这个版本只实现了图片下载备份，后续会完善图床替换及其他功能。
这段时间接触 Go 之后给我的感触颇深，对于年纪 25 岁的 Java 来说，Go 确实是后生可畏，更气人的是还赶上了云原生这个浪潮，就更惹不起了。

一些以前看来不那么重要的小毛病也被重点放大，比如启动慢、占用内存多、语法啰嗦等；不过我依然对这位赏饭吃的祖师爷保持期待，从新版本的 Java 可以看出也在积极改变，更不用说它还有无人撼动的庞大生态。