# 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
}
__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:
```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,
}
```
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:
```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
```
