fix: correct usage display to show [command] instead of [flags]

[niveshdandyan]

Summary

Fix the usage display when running nerdctl without any command.

Fixes #3185

Problem

When running nerdctl without any arguments, the usage text showed:

Usage: nerdctl [flags]

But it should show:

Usage: nerdctl [command]

Root Cause

The usage() function used c.Runnable() to decide whether to show [flags] or [command]. However, the root command has RunE set to helpers.UnknownSubcommandAction (for handling unknown subcommands), which makes it "runnable" from cobra's perspective.

Solution

Changed the logic to prioritize checking HasSubCommands() first. Commands with subcommands should always show [command] in their usage, regardless of whether they have a Run function.

// Before
if c.Runnable() {
    s += c.UseLine() + "\n"
} else {
    s += c.CommandPath() + " [command]\n"
}

// After
if c.HasSubCommands() {
    s += c.CommandPath() + " [command]\n"
} else if c.Runnable() {
    s += c.UseLine() + "\n"
} else {
    s += c.CommandPath() + " [command]\n"
}

Testing

• Code compiles successfully
• Logic is straightforward and follows the expected behavior

[ChengyuZhu6]

LGTM