How to Redirect Command Output to Console and Log File Simultaneously in Real Time?

How to Redirect Command Output to Console and Log File Simultaneously in Real Time

The provided code prints the stdout and stderr of a command to the console but lacks the ability to log it to a file in real time. This article presents a solution using MultiWriter to achieve simultaneous logging and console output while adhering to the following criteria:

• Real-time output to console
• Exact replication of output in the log file
• Respect for write order
• No modification of the executed command

Previous attempts using scanners, io.Copy, and other techniques failed to maintain the order of output due to potential race conditions between goroutines.

Solution: Using MultiWriter

The solution leverages the io.MultiWriter type, which allows writing to multiple destinations simultaneously. Here's the revised code:

<code class="go">package main

import (
    "io"
    "log"
    "os"
    "os/exec"
)

func main() {
    // Logging capability
    f, err := os.OpenFile("log.log", os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
    if err != nil {
        log.Fatalf("Error opening file: %v", err)
    }
    defer f.Close()
    mwriter := io.MultiWriter(f, os.Stdout)
    cmd := exec.Command("ls")
    cmd.Stderr = mwriter
    cmd.Stdout = mwriter
    err = cmd.Run() //blocks until sub process is complete
    if err != nil {
        panic(err)
    }
}</code>

The MultiWriter instance mwriter combines the log file and standard output. When specifying Stdout and Stderr for the command, use mwriter to ensure that both console output and logging occur simultaneously.

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