Generating Bash Completions For Your Own cobra.Command

Generating bash completions from a cobra command is incredibly easy. An actual program which does so for the kubernetes kubectl binary is as follows:

package main

import (
	"io/ioutil"
	"os"

	"k8s.io/kubernetes/pkg/kubectl/cmd"
	"k8s.io/kubernetes/pkg/kubectl/cmd/util"
)

func main() {
	kubectl := cmd.NewKubectlCommand(util.NewFactory(nil), os.Stdin, ioutil.Discard, ioutil.Discard)
	kubectl.GenBashCompletionFile("out.sh")
}

out.sh will get you completions of subcommands and flags. Copy it to /etc/bash_completion.d/ as described here and reset your terminal to use autocompletion. If you make additional annotations to your code, you can get even more intelligent and flexible behavior.

Creating your own custom functions

Some more actual 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
}

__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 __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 __customc_func() will run because the cobra.Command only understood "kubectl" and "get." __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!

Have the completions code complete your 'nouns'

In the above example "pod" was assumed to already be typed. But if you want kubectl get [tab][tab] to show a list of valid "nouns" you have to set them. Simplified code from kubectl get looks like:

validArgs []string = { "pod", "node", "service", "replicationcontroller" }

cmd := &cobra.Command{
	Use:     "get [(-o|--output=)json|yaml|template|...] (RESOURCE [NAME] | RESOURCE/NAME ...)",
	Short:   "Display one or many resources",
	Long:    get_long,
	Example: get_example,
	Run: func(cmd *cobra.Command, args []string) {
		err := RunGet(f, out, cmd, args)
		util.CheckErr(err)
	},
	ValidArgs: validArgs,
}

Notice we put the "ValidArgs" on the "get" subcommand. Doing so will give results like

# kubectl get [tab][tab]
node                 pod                    replicationcontroller  service

Plural form and shortcuts for nouns

If your nouns have a number of aliases, you can define them alongside ValidArgs using ArgAliases:

argAliases []string = { "pods", "nodes", "services", "svc", "replicationcontrollers", "rc" }

cmd := &cobra.Command{
    ...
	ValidArgs:  validArgs,
	ArgAliases: argAliases
}

The aliases are not shown to the user on tab completion, but they are accepted as valid nouns by
the completion algorithm if entered manually, e.g. in:

# kubectl get rc [tab][tab]
backend        frontend       database

Note that without declaring rc as an alias, the completion algorithm would show the list of nouns
in this example again instead of the replication controllers.

Mark flags as required

Most of the time completions will only show subcommands. But if a flag is required to make a subcommand work, you probably want it to show up when the user types [tab][tab]. Marking a flag as 'Required' is incredibly easy.

cmd.MarkFlagRequired("pod")
cmd.MarkFlagRequired("container")

and you'll get something like

# kubectl exec [tab][tab][tab]
-c            --container=  -p            --pod=

Specify valid filename extensions for flags that take a filename

In this example we use --filename= and expect to get a json or yaml file as the argument. To make this easier we annotate the --filename flag with valid filename extensions.

	annotations := []string{"json", "yaml", "yml"}
	annotation := make(map[string][]string)
	annotation[cobra.BashCompFilenameExt] = annotations

	flag := &pflag.Flag{
		Name:        "filename",
		Shorthand:   "f",
		Usage:       usage,
		Value:       value,
		DefValue:    value.String(),
		Annotations: annotation,
	}
	cmd.Flags().AddFlag(flag)

Now when you run a command with this filename flag you'll get something like

# kubectl create -f 
test/                         example/                      rpmbuild/
hello.yml                     test.json

So while there are many other files in the CWD it only shows me subdirs and those with valid extensions.

Specify custom flag completion

Similar to the filename completion and filtering using cobra.BashCompFilenameExt, you can specify
a custom flag completion function with cobra.BashCompCustom:

	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 __handle_namespace_flag 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
}

Using bash aliases for commands

You can also configure the bash aliases for the commands 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

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