Why Aren't My Go Log Messages Writing to the File?

Writing Log Messages to a File in Go

In Go, logging to a file can be a straightforward task. However, certain approaches may encounter errors, leaving you wondering why your log messages are not being written to the file.

Failed Approaches

The provided code shows three unsuccessful attempts at logging to a file:

func TestLogging(t *testing.T) {
    // ...

    // Attempt #1
    log.SetOutput(io.MultiWriter(os.Stderr, f))
    log.Println("hello, logfile")

    // Attempt #2
    log.SetOutput(io.Writer(f))
    log.Println("hello, logfile")

    // Attempt #3
    log.SetOutput(f)
    log.Println("hello, logfile")
}

These attempts fail because os.Open(), used to open the log file, opens the file for reading only, which is incompatible with the writing required by log.SetOutput().

Solution

To successfully log to a file, you should use os.OpenFile() instead:

f, err := os.OpenFile("testlogfile", os.O_RDWR | os.O_CREATE | os.O_APPEND, 0666)
if err != nil {
    log.Fatalf("error opening file: %v", err)
}
defer f.Close()

log.SetOutput(f)
log.Println("This is a test log entry")

os.OpenFile() opens the file with read, write, create, and append permissions, allowing you to successfully write log messages to the file. Remember to close the file after logging is complete using defer f.Close().

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