spf13--cobra/site/content/completions/bash.md

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Bash completions

Dependencies

The bash completion script generated by Cobra requires the bash_completion package. You should update the help text of your completion command to show how to install the bash_completion package (Kubectl docs)

Aliases

You can also configure bash aliases for your program and they will also support completions.

alias aliasname=origcommand
complete -o default -F __start_origcommand aliasname

# and now when you run `aliasname` completion will make
# suggestions as it did for `origcommand`.

$ aliasname <tab><tab>
completion     firstcommand   secondcommand

Bash legacy dynamic completions

For backward compatibility, Cobra still supports its bash legacy dynamic completion solution. Please refer to Bash Completions for details.

Bash completion V2

Cobra provides two versions for bash completion. The original bash completion (which started it all!) can be used by calling GenBashCompletion() or GenBashCompletionFile().

A new V2 bash completion version is also available. This version can be used by calling GenBashCompletionV2() or GenBashCompletionFileV2(). The V2 version does not support the legacy dynamic completion (see Bash Completions) but instead works only with the Go dynamic completion solution described in this document. Unless your program already uses the legacy dynamic completion solution, it is recommended that you use the bash completion V2 solution which provides the following extra features:

• Supports completion descriptions (like the other shells)
• Small completion script of less than 300 lines (v1 generates scripts of thousands of lines; kubectl for example has a bash v1 completion script of over 13K lines)
• Streamlined user experience thanks to a completion behavior aligned with the other shells

Bash completion V2 supports descriptions for completions. When calling GenBashCompletionV2() or GenBashCompletionFileV2() you must provide these functions with a parameter indicating if the completions should be annotated with a description; Cobra will provide the description automatically based on usage information. You can choose to make this option configurable by your users.

# With descriptions
$ helm s[tab][tab]
search  (search for a keyword in charts)           status  (display the status of the named release)
show    (show information of a chart)

# Without descriptions
$ helm s[tab][tab]
search  show  status

Note: Cobra's default completion command uses bash completion V2. If for some reason you need to use bash completion V1, you will need to implement your own completion command.

Bash legacy dynamic completions

For backward compatibility, Cobra still supports its legacy dynamic completion solution (described below). Unlike the ValidArgsFunction solution, the legacy solution will only work for Bash shell-completion and not for other shells. This legacy solution can be used along-side ValidArgsFunction and RegisterFlagCompletionFunc(), as long as both solutions are not used for the same command. This provides a path to gradually migrate from the legacy solution to the new solution.

Note: Cobra's default completion command uses bash completion V2. If you are currently using Cobra's legacy dynamic completion solution, you should not use the default completion command but continue using your own.

The legacy solution allows you to inject bash functions into the bash completion script. Those bash functions are responsible for providing the completion choices for your own completions.

Some code that works in kubernetes:

const (
        bash_completion_func = `__kubectl_parse_get()
{
    local kubectl_output out
    if kubectl_output=$(kubectl get --no-headers "$1" 2>/dev/null); then
        out=($(echo "${kubectl_output}" | awk '{print $1}'))
        COMPREPLY=( $( compgen -W "${out[*]}" -- "$cur" ) )
    fi
}

__kubectl_get_resource()
{
    if [[ ${#nouns[@]} -eq 0 ]]; then
        return 1
    fi
    __kubectl_parse_get ${nouns[${#nouns[@]} -1]}
    if [[ $? -eq 0 ]]; then
        return 0
    fi
}

__kubectl_custom_func() {
    case ${last_command} in
        kubectl_get | kubectl_describe | kubectl_delete | kubectl_stop)
            __kubectl_get_resource
            return
            ;;
        *)
            ;;
    esac
}
`)

And then I set that in my command definition:

cmds := &cobra.Command{
	Use:   "kubectl",
	Short: "kubectl controls the Kubernetes cluster manager",
	Long: `kubectl controls the Kubernetes cluster manager.

Find more information at https://github.com/GoogleCloudPlatform/kubernetes.`,
	Run: runHelp,
	BashCompletionFunction: bash_completion_func,
}

The BashCompletionFunction option is really only valid/useful on the root command. Doing the above will cause __kubectl_custom_func() (__<command-use>_custom_func()) to be called when the built in processor was unable to find a solution. In the case of kubernetes a valid command might look something like kubectl get pod [mypod]. If you type kubectl get pod [tab][tab] the __kubectl_customc_func() will run because the cobra.Command only understood "kubectl" and "get." __kubectl_custom_func() will see that the cobra.Command is "kubectl_get" and will thus call another helper __kubectl_get_resource(). __kubectl_get_resource will look at the 'nouns' collected. In our example the only noun will be pod. So it will call __kubectl_parse_get pod. __kubectl_parse_get will actually call out to kubernetes and get any pods. It will then set COMPREPLY to valid pods!

Similarly, for flags:

	annotation := make(map[string][]string)
	annotation[cobra.BashCompCustom] = []string{"__kubectl_get_namespaces"}

	flag := &pflag.Flag{
		Name:        "namespace",
		Usage:       usage,
		Annotations: annotation,
	}
	cmd.Flags().AddFlag(flag)

In addition add the __kubectl_get_namespaces implementation in the BashCompletionFunction value, e.g.:

__kubectl_get_namespaces()
{
    local template
    template="{{ range .items  }}{{ .metadata.name }} {{ end }}"
    local kubectl_out
    if kubectl_out=$(kubectl get -o template --template="${template}" namespace 2>/dev/null); then
        COMPREPLY=( $( compgen -W "${kubectl_out}[*]" -- "$cur" ) )
    fi
}