Building a CLI using urfave/cli

I’ve been using Cobra for a really long time. Whenever I start a project that needs to handle more than one command add Cobra. For a small toolbox for datasphere I wanted to try something different: urfave/cli.

Compared to Cobra, urfave/cli is much more lightweight by not integrating any kind of configuration management, fewer hooks, and also handling flags differently, but for that small project I don’t need that anyway 😅

Right when you start with this library you run into the first API difference: While in Cobra you always work with cobra.Command objects, in urfave/cli you start off with a cli.App and only work with cli.Command instances if you’re dealing with sub-commands:

package main

import (
  "fmt"

  "github.com/urfave/cli"
)

func main() {
  app := cli.App{
    Action: function(ctx *cli.Context) error {
      fmt.Println("hello world")
    },
  }

  app.Run(os.Args)
}

Flags

If you need any flags, you add them to an array that is attached to the App instance:

cli.App{
  ...

  Flags: []cli.Flag{
    &cli.BoolFlag{
      Name: "verbose",
      Aliases: []string{"v"},
    },
  },

  ...
}

Flag-names longer than one character are rendered with two dashes in the usage message, those with just one character receive only a single dash:

   --verbose, -v           Verbose logging (default: false)

As you saw in the example above, the action of an app receives only a single argument: a cli.Context instance. This is also how you gain access to the parsed flags using functions like GetBool(name).

Don’t let that name confuse you, though: cli.Context doesn’t implement the context.Context interface but you can still work with those thanks to cli.Context.Context 😬

Sub-commands

Sub-commands work pretty much like Apps but can be nested:

app := cli.App{}
app.Commands = []*cli.Command{
	&cli.Command{
		Name: "subcommand",
		Action: func(ctx *cli.Context) error {
			return nil
		},
		Subcommands: []*cli.Command{
			&cli.Command{
				Name: "subsubcommand",
				Action: func(ctx *cli.Context) error {
					return nil
				},
			},
		},
	},
}

One thing to keep in mind with sub-commands, though, is that you have to pass the flags at the same level as the one where it has been declared. If a flag is defined for the application, you cannot specify it after the sub-command.

That’s a bit counter-intuitive coming from Cobra, but it’s no show-stopper for me. And that’s as far as I’ve come so far. It seems to get the job done while not getting in my way (similar to Cobra) 👍