Why log.Println("does not log into file")?

This is my first attempt to log into files with golang.

file, _ := os.Open("logfile")
log.SetOutput(file)
log.Println("foo")

these lines build, but are not working. Why?

os.Open calls OpenFile(name, O_RDONLY, 0). This does not have the flag O_CREATE to create the file if it does not exist. Therefore you need to call OpenFile with the O_CREATE flag. I have commented out the error code:

package main

import (
	"log"
	"os"
	"syscall"
)

const (
	O_RDONLY int = syscall.O_RDONLY // open the file read-only.
    O_RDWR   int = syscall.O_RDWR   // open the file read-write.
	O_CREATE int = syscall.O_CREAT  // create a new file if none exists.
    O_APPEND int = syscall.O_APPEND // append data to the file when writing.
)

func main() {
	/*f1, err := os.OpenFile("testlogfile1", O_RDONLY, 0) // Equivalent to os.Open("testlogfile1")
	if err != nil {
		log.Fatalf("error opening file1: %v", err)
	}
	// ** error opening file: open testlogfile1: no such file or directory exit status 1 **

	defer f1.Close()

	log.SetOutput(f1)
	log.Println("This is a test for log 1")*/

	f2, err := os.OpenFile("testlogfile2", O_RDWR | O_CREATE | O_APPEND, 0644)
    /* Note that you want:
     * O_RDWR to write to the file
     * O_CREATE to create file if it does not exist
     * O_APPEND to add additional information to existing file */

	if err != nil {
		log.Fatalf("error opening file2: %v", err)
	}

	defer f2.Close()

	log.SetOutput(f2)
	log.Println("This is a test for log 2")

}

Always check those errors!

> Why log.Println(“does not log into file”)?

The reason is because you didn't check the file is exist or not form your code.

file, _ := os.Open("logfile")

you're using _ and didn't check the error. This is important if you wanted to write something in to file. For example look at this code :

	f, err := os.OpenFile(filePath+fileName, os.O_RDWR|os.O_CREATE|os.O_APPEND, os.ModePerm)
	if err != nil {
		// if error then you need to create the file first
        err = os.MkdirAll(filePath, os.ModePerm)
	}

from above code you can see the error checking using if err != nil. if the file is not exist yet then create the file first. using os.MkdirAll().

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-html -->

file, err := os.Open("file.go") // For read access.
if err != nil {
	// do something
}

// and try to include the file path...

<!-- end snippet -->