Custom error message formatter

[stevewi-niteflyte]

I would like to use a custom error message formatter that will handle urfave/cli errors (e.g. No help topic for 'foo') in a manner that adds colorization and some more informative formatting. I've looked through the errors.go file and didn't see anything apparent. I kinda new to Go so I may have overlooked something or what I need may be there but beyond my current level of comprehension.

Something like cli.HelpPrinter but for error messages...

Can anyone give me some pointers about how to do this?

[KevinAndrewDong]

You can use ExitErrHandler for this. It replaces the default error handler, so you can add your own formatting. Here's a small v3 (github.com/urfave/cli/v3) example:

package main

import (
    "context"
    "errors"
    "fmt"
    "os"

    "github.com/urfave/cli/v3"
)

func main() {
    cmd := &cli.Command{
        Name: "demo",
        ExitErrHandler: func(_ context.Context, cmd *cli.Command, err error) {
            if err == nil {
                return
            }
            code := 1
            var ec cli.ExitCoder
            if errors.As(err, &ec) {
                code = ec.ExitCode()
            }
            fmt.Fprintf(cmd.ErrWriter, "\x1b[31merror:\x1b[0m %v\n", err)
            os.Exit(code)
        },
    }
    if err := cmd.Run(context.Background(), os.Args); err != nil {
        fmt.Fprintln(os.Stderr, err)
        os.Exit(1)
    }
}

demo help foo reaches this handler. In v3.14.0, that error carries exit code 3, which this example preserves. The ANSI escapes are just a minimal color example; use your preferred color/terminal library there.

One thing to watch: the error is still returned to the caller if your handler returns. This example exits in the handler so it won't also get printed by the code after Run.

If you're using v2, the hook is on cli.App instead, with the signature func(c *cli.Context, err error); use c.App.ErrWriter in the handler. For flag-parsing messages such as Incorrect Usage, OnUsageError is the separate hook to look at; the official full API example shows how to customize that message.

[stevewi-niteflyte]

Thanks, this works perfectly! Calling cli.Exit(msg string, code uint8) invokes this handler from my own code.