Cobra Shell Completions Reference

Cobra generates shell completion scripts for bash, zsh, fish, and PowerShell automatically. Subcommand names and flag names are completed for free. You add completions for flag values and positional arguments.

Built-in completion command

Cobra registers a completion subcommand automatically:

myapp completion bash   # generate bash script
myapp completion zsh    # generate zsh script
myapp completion fish   # generate fish script
myapp completion powershell

# Install (example for zsh):
myapp completion zsh > "${fpath[1]}/_myapp"

ShellCompDirective

The ShellCompDirective controls shell behavior after your completion function returns:

Directive	Meaning
`ShellCompDirectiveDefault`	Fall back to file completion after your results
`ShellCompDirectiveNoFileComp`	Disable file completion fallback
`ShellCompDirectiveNoSpace`	Don't add a space after the completion
`ShellCompDirectiveFilterFileExt(exts)`	Only show files with given extensions
`ShellCompDirectiveFilterDirs(dirs)`	Only show directories
`ShellCompDirectiveError`	Signal an error (show no completions)

Combine with bitwise OR: cobra.ShellCompDirectiveNoFileComp | cobra.ShellCompDirectiveNoSpace.

Use ShellCompDirectiveNoFileComp whenever your list is exhaustive — it prevents the shell from appending irrelevant files.

Static arg completions

var getCmd = &cobra.Command{
    Use:       "get <resource>",
    ValidArgs: []string{"pod", "service", "deployment", "configmap"},
    Args:      cobra.OnlyValidArgs,
    RunE: func(cmd *cobra.Command, args []string) error { /* ... */ },
}

Dynamic arg completions

var getCmd = &cobra.Command{
    ValidArgsFunction: func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
        if len(args) > 0 {
            // first arg already provided — no more completions
            return nil, cobra.ShellCompDirectiveNoFileComp
        }
        resources, err := listResources(toComplete)
        if err != nil {
            return nil, cobra.ShellCompDirectiveError
        }
        return resources, cobra.ShellCompDirectiveNoFileComp
    },
}

toComplete is the prefix the user has typed so far — filter your results by it for responsive completions.

Flag value completions

func init() {
    rootCmd.RegisterFlagCompletionFunc("output", func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
        return []string{"json\tJSON output", "yaml\tYAML output", "table\tTable output"}, cobra.ShellCompDirectiveNoFileComp
    })

    rootCmd.RegisterFlagCompletionFunc("namespace", func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
        ns, err := listNamespaces()
        if err != nil {
            return nil, cobra.ShellCompDirectiveError
        }
        return ns, cobra.ShellCompDirectiveNoFileComp
    })
}

Descriptions after \t are shown in zsh and fish menus.

Completion annotations

Mark a flag to complete as a file or directory:

cmd.Flags().String("config", "", "config file")
cmd.MarkFlagFilename("config", "yaml", "yml", "json")  // only those extensions

cmd.Flags().String("dir", "", "output directory")
cmd.MarkFlagDirname("dir")

Testing completions

func TestCompletion(t *testing.T) {
    rootCmd.SetArgs([]string{"__complete", "get", ""})
    buf := new(bytes.Buffer)
    rootCmd.SetOut(buf)
    rootCmd.Execute()
    assert.Contains(t, buf.String(), "pod")
    assert.Contains(t, buf.String(), "service")
}

__complete is cobra's internal completion request verb. Pass the partial args as additional arguments.

Disabling the completion command

rootCmd.CompletionOptions.DisableDefaultCmd = true   // remove the completion subcommand
rootCmd.CompletionOptions.HiddenDefaultCmd = true    // keep it but hide from help

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