Request for comments: In-built autocompletion for shell

[amitsaha]

Hi all, since a cobra CLI application already knows what commands/sub-commands and flags it can handle, why not use the cmd API to implement auto-completion itself? That is, say for BASH, we register $complete -C mycli mycli - that is mycli is invoked whenever I type in mycli <TAB> and then I have logic in mycli itself to hint to bash what are the possible options for auto-completion. Here's how I implemented it in my main.go of a standard cobra CLI app:

func main() {
	// handle bash autocomplete
	if len(os.Getenv("COMP_LINE")) != 0 {
		if len(os.Getenv("COMP_DEBUG")) != 0 {
			fmt.Printf("%#v\n", os.Getenv("COMP_LINE"))
		}
		compLine := strings.Split(os.Getenv("COMP_LINE"), " ")

		// we only handle auto complete, if we are the command
		// being invoked
		if compLine[0] == cmd.RootCmd.Name() {
			var cmdArgs []string
			if len(compLine) > 1 {
				cmdArgs = compLine[1:]
			} else {
				cmdArgs = compLine[0:]
			}
			c, _, _ := cmd.RootCmd.Find(cmdArgs)
			suggestions := c.SuggestionsFor(cmdArgs[len(cmdArgs)-1])
			for _, s := range suggestions {
				fmt.Printf("%s\n", s)
			}

			localNonPersistentFlags := c.LocalNonPersistentFlags()
			c.NonInheritedFlags().VisitAll(func(flag *pflag.Flag) {
				if nonCompletableFlag(flag) {
					return
				}
				if strings.HasPrefix(fmt.Sprintf("--%s", flag.Name), cmdArgs[len(cmdArgs)-1]) {
					fmt.Printf("--%s\n", flag.Name)
				}
				if localNonPersistentFlags.Lookup(flag.Name) != nil {
					if strings.HasPrefix(fmt.Sprintf("--%s", flag.Name), cmdArgs[len(cmdArgs)-1]) {
						fmt.Printf("--%s\n", flag.Name)
					}
				}
			})

			c.InheritedFlags().VisitAll(func(flag *pflag.Flag) {
				if nonCompletableFlag(flag) {
					return
				}
				if strings.HasPrefix(flag.Name, cmdArgs[len(cmdArgs)-1]) {
					fmt.Printf("--%s\n", flag.Name)
				}
				if len(flag.Shorthand) > 0 {
					if strings.HasPrefix(flag.Name, cmdArgs[len(cmdArgs)-1]) {
						fmt.Printf("--%s\n", flag.Name)
					}
				}
			})

		}

	} else {
		cmd.Execute()
	}
}

It seems to be working for me.

Thoughts on this?

[github-actions]

This issue is being marked as stale due to a long period of inactivity

[marckhouzam]

@amitsaha I just fell upon this suggestion. You had a great idea! Coincidentally, Cobra has actually started using this technique to provide Fish shell completion (#1048). The same is being applied for zsh completion in #1070.

A hidden command was used to achieve this. So you can do things like:
myprog __complete sub<ENTER>
to obtain the completion.

[marckhouzam]

This suggestion is actually being used by Cobra. I believe this issue can be closed . /cc @jpmcb