如何在 Go 中將訊息記錄到控制台和檔案？

How to Log Messages to Multiple Destinations in Go

When logging messages in Go, it may be necessary to output logs to both the console and a file simultaneously. While it's straightforward to direct messages solely to a file using log.SetOutput(logFile), there's a way to enable logging in both terminals and target files.

Implementing Multiple Destinations

To achieve logging in multiple destinations, utilize the io.MultiWriter. As implied by its name, MultiWriter facilitates writing to multiple writers concurrently, similar to the Unix tee(1) command.

Here's how to implement this using MultiWriter:

import (
  "os"
  "io"
  "log"
)

func main() {
  logFile, err := os.OpenFile("log.txt", os.O_CREATE|os.O_APPEND|os.O_RDWR, 0666)
  if err != nil {
    panic(err)
  }
  
  // Initialize io.MultiWriter to write to both file and console
  mw := io.MultiWriter(os.Stdout, logFile)
  
  // Set output to MultiWriter
  log.SetOutput(mw)
}

In this example, mw is an io.MultiWriter instance that writes to both os.Stdout (console) and the logFile. By setting log.SetOutput(mw), all subsequent log messages will be printed in the console and logged to the specified file.

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