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Build a CLI in Golang with the Cobra library

Deven Rathore 6 min read
Build a CLI in Golang with the Cobra library

In this tutorial, we will create color CLI in Golang using the Cobra library.

CLI is a standard interface between software and a user.

The objective of CLI in Golang

The objective of the colorcli is to return the name of the hexcode of the color.

If the hexcode doesn’t exist then colorcli can add the color.

Pre-requisites

  1. Golang
  2. Cobra library go get -u github.com/spf13/cobra/cobra

Cobra

Cobra is a library providing a simple interface and application scaffolding to create a CLI interface.
Cobra is built on a structure of commands, arguments & flags. Learn more.

  • Commands are Verb
  • Arguments are Noun
  • Flags are Adjective

Example:

App Command -Flag Argument
go get -u github.com/spf13/cobra/cobra

Project Setup

Create a colorcli project using Cobra cli.

cobra init colorcli --pkg-name colorcli

This will create a colorcli scaffolding.

colorcli
    |- cmd
        |- root.go
    |- main.go

The main.go will execute the root command from the cmd/root.go.

package main
 
import "colorcli/cmd"
 
func main() {
  cmd.Execute()
}

The root command is the cli name or app name. In this case, it is colorcli.

Let’s build the cli and run the colorcli. 

go install colorcli

We’ll get an error.

can't load package: package colorcli: cannot find package "colorcli" in any of:
        C:\Go\src\colorcli (from $GOROOT)
        C:\Users\user\go\src\colorcli (from $GOPATH)

You’ll not encounter this error if the project is inside the GOPATH.

By default go find the packages inside the GOROOT and GOPATH.
We have to initiate the Go modules inside the project to tell go to find the package in the project.

Go Modules is a package manager in the Golang. The go module tracks all the packages and their version used in the project.

You can consider it similar to node modules in the nodejs. Learn more.

Initiate the go modules

go mod init colorcli

This will create a new file go.mod.

Now, try again to build the binary. 

go install colorcli

The binary is generated and saved in the $GOPATH/bin/. Now, you can directly run the colorcli command without setting the environment variable.

colorcli

Output

$ colorcli
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.
 
subcommand is required

Hexcode Color names

We are going to use the meodai/color-names git repo. This repo has a handpicked list of 18376 unique color names from various sources and thousands of curated user submissions.

Download the hexcode to color name Json file

Open the terminal in the project directory.

curl -O https://unpkg.com/[email protected]/dist/colornames.min.json

You can download it in other formats. Link

Create a name command in the colorcli

To add a command, we are going to use add command of the cobra.

cobra add name

This will create a new file name.go in the cmd folder.

Build the binary.

go install colorcli

Run the name command.

colorcli name

Output

$ colorcli name
name called

Open the name.go and check the init function.

rootCmd.AddCommand(nameCmd)

The AddCommand is adding the nameCmd to the rootCmd. In this case it is colorcli.

Add a new function hexToName.

import (
    "encoding/json"
    "fmt"
    "io/ioutil"
 
    "github.com/spf13/cobra"
)
 
...
 
func hexToName(args []string) {
    var hexMap map[string]string
 
    // read the color.min.json
    content, err := ioutil.ReadFile("colornames.min.json")
 
    if err != nil {
        fmt.Printf("Error while reading the file %v", err)
    }
    _ = json.Unmarshal(content, &hexMap)
 
    name, ok := hexMap[args[0]]
    if ok {
        fmt.Printf("Name: %s, Hex: %s\n", name, args[0])
    } else {
 
        fmt.Println("Color name not found")
    }
}

We are using ioutil package to read the file. The file is in json format. We are using encoding/json package to convert the json into struct to read it.

Update the nameCmd.

var nameCmd = &cobra.Command{
    Use:   "name",
    Short: "A brief description of your command",
    Long: `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.`,
    Run: func(cmd *cobra.Command, args []string) {
        // call hexToName function
        hexToName(args)
    },
}

Build the binary to reflect the changes.

go install colorcli

Run the cli.

colorcli name ffffff
 
colorcli name fffeee

Output

Name: White, Hex: ffffff
 
Color name not found

Add a color name to the colornames.min.json

Add an addcolor command, to add custom or extra color to the colornames.min.json file.

cobra add addcolor

Open the addcolor.go and add the addColor function.

import (
    "encoding/json"
    "fmt"
    "io/ioutil"
 
    "github.com/spf13/cobra"
)
...
 
func addColor(args []string) {
 
    hex := args[0]
    colorName := args[1]
    // Read the file
    content, err := ioutil.ReadFile("colornames.min.json")
 
    if err != nil {
        fmt.Printf("Error while reading a file %v", err)
    }
 
    // map hex value to color name
    var hexMap map[string]string
 
    // unmarshal to hexMap
    _ = json.Unmarshal(content, &hexMap)
 
    // check if hex exist
    name, ok := hexMap[hex]
 
    if ok {
        fmt.Printf("Hex already exist. Color Name is :%s\n", name)
    } else {
        hexMap[hex] = colorName
 
        // marshal to json or convert to json
        hexJSON, _ := json.Marshal(hexMap)
 
        // write to colornames.min.json
        err = ioutil.WriteFile("colornames.min.json", hexJSON, 0777)
 
        if err != nil {
            fmt.Printf("Error while writing a file %v", err)
        }
 
        fmt.Println("Hex to color added successfully!")
 
    }
}

We are using ioutil package for updating the file. It is creating a new file instead of just updating the existing one. Learn more.

Update the addcolorCmd.

// addcolorCmd represents the addcolor command
var addcolorCmd = &cobra.Command{
    Use:   "addcolor",
    Short: "A brief description of your command",
    Long: `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.`,
    Run: func(cmd *cobra.Command, args []string) {
        addColor(args)
    },
}

Build the binary.

go install colorcli

Add a new color. The first argument is hexcode and second is color name.

colorcli addcolor fffeee lightcream

Output

Hex to color added successfully!

Check the color.

colorcli name fffeee

Output

Name: lightcream, Hex: fffeee

In cobra library, all the arguments are in string format. You have to explicitly convert the string into the respective type.

Conclusion

Congratulations. We just created a colorcli in Golang using the Cobra library. There are many features you can add in it, like creating a flag to return the color in RGB format. The colorcli has a dependency on the colornames.min.json file. There are many free apis you can utilize to remove the dependencies.
Any suggestion is most welcome. Keep coding.

The complete code is saved in GitHub.

Further Reading