Why don't I see fmt logs in my terminal when running go app locally?

Question

I'm trying to debug my golang app. Currently, I have an API request that isn't working which has this line of code: fmt.Errorf("Object(%q).CopierFrom(%q).Run: %v", dstName, object, err)

How can I view the output of this error log? And if it's not possible what are some other ways to debug in go? (runtime invocation would be nice)

If you want to log an error, you have to log it. `fmt.Errorf` just creates an error object. An error has no output. — Jonathan Hall, Dec 29 '21 at 16:11
If you'll update your question with some actual code, it will be easier to provide a concrete suggestion. But maybe you want to call `fmt.Printf` or `log.Error` or something like that instead. — Jonathan Hall, Dec 29 '21 at 16:12

xgord · Answer 1 · 2021-12-29T16:18:11.460

fmt.Errorf creates an error object; it does not print.

From the docs for fmt.Errorf:

func Errorf(format string, a ...interface{}) error

If you're just trying to print the message to stdout:

fmt.Printf("Object(%q).CopierFrom(%q).Run: %v\n", dstName, object, err)

If you want to write to an error log, I recommend looking at the log package. For example, if you were looking to write to stderr:

logger := log.New(os.Stderr, "my-app", 0)
logger.Printf("Object(%q).CopierFrom(%q).Run: %v", dstName, object, err)

score 2 · Accepted Answer · answered Dec 29 '21 at 16:39

fmt.Errorf() creates an error object. but not print.doc

If you're just trying to print the message to stdout: run

package main

import (
    "fmt"
)

func main() {
    const name, id = "bueller", 17
    err := fmt.Errorf("user %q (id %d) not found", name, id)
    fmt.Println(err.Error())

}

out:

user "bueller" (id 17) not found

if you want debug golang code, I recommend use log packages for example: zerolog


package main

import (
    "errors"

    "github.com/rs/zerolog"
    "github.com/rs/zerolog/log"
)

func main() {
    // UNIX Time is faster and smaller than most timestamps
    zerolog.TimeFieldFormat = zerolog.TimeFormatUnix

    err := errors.New("seems we have an error here")
    log.Error().Err(err).Msg("this is an error")
}

out:

{"level":"error","error":"seems we have an error here","time":1640795128,"message":"this is an error"}

score 1 · Answer 3 · answered Dec 29 '21 at 16:16

1

fmt.Errorf creates an error - ideal for function returns - but it's not implicitly logged.

If you want to log an error simply:

log.Printf("api X: error %v", err)

answered Dec 29 '21 at 16:16

colm.anseo

19,337
4
43
52

score 0 · Answer 4 · answered Dec 30 '21 at 08:43

It is better to read the function signature and comments before you use any functions.

// Errorf formats according to a format specifier and returns the string as a
// value that satisfies error.
//
// If the format specifier includes a %w verb with an error operand,
// the returned error will implement an Unwrap method returning the operand. It is
// invalid to include more than one %w verb or to supply it with an operand
// that does not implement the error interface. The %w verb is otherwise
// a synonym for %v.
func Errorf(format string, a ...interface{}) error

Why don't I see fmt logs in my terminal when running go app locally?

4 Answers4