# 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:

```go

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](https://debian-administration.org/article/316/An_introduction_to_bash_completion_part_1) 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:

```bash

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

}

    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:

```go

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,

}

```

## 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:

```go

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

```bash

# 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`:

```go

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:

```bash

# 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.

```go

cmd.MarkFlagRequired("pod")

cmd.MarkFlagRequired("container")

```

and you'll get something like

```bash

# 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.

```go

	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

```bash

# 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:

```go

	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.:

```bash

__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.

```bash

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

```