save-output-to-file-01; working first version

main.go

@@ -23,6 +23,9 @@ func main() {
 		err := fmt.Errorf("error parsing arguments: %v", args.Err)
 		panic(err)
 	}
+	if args.FileOutput != "" {
+		defer tools.LogOutput(args.FileOutput)()
+	}
 	// var file *os.File
 	// if args.FileOutput != "" {
 	// 	_, fileString, _ := tools.IsValidPath(args.FileOutput)

tools/file-works.go

@@ -2,6 +2,8 @@ package tools
 
 import (
 	"fmt"
+	"io"
+	"log"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -66,3 +68,42 @@ func IsValidPath(path string) (isValid bool, absPath string, err error) {
 
 	return true, absPath, nil
 }
+
+func LogOutput(logfile string) func() {
+	// open file read/write | create if not exist | clear file at open if exists
+	f, _ := os.OpenFile(logfile, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0666)
+
+	// save existing stdout | MultiWriter writes to saved stdout and file
+	out := os.Stdout
+	mw := io.MultiWriter(out, f)
+
+	// get pipe reader and writer | writes to pipe writer come out pipe reader
+	r, w, _ := os.Pipe()
+
+	// replace stdout,stderr with pipe writer | all writes to stdout, stderr will go through pipe instead (fmt.print, log)
+	os.Stdout = w
+	os.Stderr = w
+
+	// writes with log.Print should also write to mw
+	log.SetOutput(mw)
+
+	//create channel to control exit | will block until all copies are finished
+	exit := make(chan bool)
+
+	go func() {
+		// copy all reads from pipe to multiwriter, which writes to stdout and file
+		_, _ = io.Copy(mw, r)
+		// when r or w is closed copy will finish and true will be sent to channel
+		exit <- true
+	}()
+
+	// function to be deferred in main until program exits
+	return func() {
+		// close writer then block on exit channel | this will let mw finish writing before the program exits
+		_ = w.Close()
+		<-exit
+		// close file after all writes have finished
+		_ = f.Close()
+	}
+
+}