Excesive string prefix to usage function output

[HarryMichal]

PR #1044 introduced in commit dd3684a a new prefix to the usage function. Example:

var rootCmd = &cobra.Command{
    Use: "cmd"
}

func usage(cmd *cobra.Command) error {
    err := fmt.Errorf("Custom usage message")
    return err
}

rootCmd.SetUsageFunc(usage)

Note that the example is very simplified. The main part is the use of a custom usage function.

Forcing the created binary to print out the usage (e.g., by passing a wrong flag) will yield the following output:

$ cmd --unknown-flag
Error: unknown flag: --unknown-flag
Error: Custom usage message

Note the Error: prefix before the usage text. The only way to bypass this behaviour is to set a template string instead.

The desired output would be:

$ cmd --unknown-flag
Error: unknown flag: --unknown-flag
Custom usage message

[github-actions]

This issue is being marked as stale due to a long period of inactivity

[HarryMichal]

This issue is still present in Cobra.

[hoshsadiq]

This seems like correct behaviour to me. If you return an error, it is explicitly communicated as an error. There are plenty of use-cases where the usage function could return an unexpected error (as opposed to an expected error). You should be using fmt.Println or a variation of it, combined returning nil for an error. I.e.:

var rootCmd = &cobra.Command{
    Use: "cmd"
}

func usage(cmd *cobra.Command) error {
    fmt.Println("Custom usage message")
    return nil
}

rootCmd.SetUsageFunc(usage)

[HarryMichal]

This behaviour should then be documented. It is highly perplexing when encountered at first. Also the reliance on side-effects in the function causes cmd.UsageString() to be s bit clunky to use when assembling custom messages.