How do I print message to stderr in Go?

Question

I want to print some logs for debugging and testing, but existing logs are massive, so I want to print my own logs to stderr:

go run main.go 1>/dev/null
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Answer 1

The log package by default prints to os.Stderr.

You can also use os.Stderr directly (it's an os.File).

Answer 2

There are multiple methods to send a message to stderr:

1. Creating a new log.Logger:

   l := log.New(os.Stderr, "", 1)
   l.Println("log message")
   

2. Using fmt.Fprintf:

   fmt.Fprintf(os.Stderr, "log message: %s", str)
   

3. Directly writing to os.Stderr using os.Stderr.WriteString:

   os.Stderr.WriteString("log message")
   

Answer 3

On Linux systems, you could also use syslog(3) facilities. By default, the underlying openlog(3) does not go to stderr, but this can be changed.

See Go package log/syslog.