Print & Logs in Go

fmt.Print

• Go fmt package provides us with three different functions to print to the stdout.

Print in Go

• fmt.Print
• fmt.Println: print with "\n"
• fmt.Printf: formatted print. print some variables too.

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fmt.Printf format specifiers

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Logs in Go

• The log functions print to stderr by default and can be directed to an arbitrary writer.
• The fmt.Printf function prints to stdout.
• The log functions can print timestamp, source code location and other info.
• The log functions and fmt.Printf are both thread safe, but concurrent writes by fmt.Printf above an OS dependent size can be interleaved.

logs in Go

• log.Print
• log.Println: print with "\n"
• log.Printf: formatted print. print some variables too.
• log.Fatal: log and then os.Exit(1)
• log.Fatalln: log with "\n".
• log.Fatalf: formatted log

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Note 👇🏻

Remember

• fmt outputs to stdout
• log outputs to stderr by default but can be configured to any other writer.

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Logging to a file

• Use log.SetOutput(file) to set logs output to a file.

    package main

    import (
        "log"
        "os"
        "time"
    )

    func main() {
        log.Println("Hello from log.Println") // Log message with timestamp

        // Open a file in append mode, create it if it doesn't exist
        file, err := os.OpenFile("logfile.log", os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
        if err != nil {
            log.Fatal(err) // Logs to stderr by default
        }
        defer file.Close()

        // Redirect log output to the file
        log.SetOutput(file)

        i := 0

        for {
            // This will now write to logfile.log instead of stderr
            log.Printf("This is a log message with count: %d", i)
            i++
            time.Sleep(1 * time.Second)
        }
    }