io.Copy() erase the Reader content

	package main

	import (
		"fmt"
		"io"
		"io/ioutil"
		"os"
	)

	func main() {
		file, err := os.Open("HelloWorld")
		if nil != err {
			fmt.Println(err)
		}
		defer file.Close()

		fileTo, err := os.Create("fileTo")
		if nil != err {
			fmt.Println(err)
		}
		defer file.Close()

		_, err = io.Copy(fileTo, file)
		if nil != err {
			fmt.Println(err)
		}

		fileByteOne, err := ioutil.ReadAll(file)
		if nil != err {
			fmt.Println(err)
		}

		fmt.Println(fileByteOne)
	}

io.Copy() will erase the file content, the output is :

[]

Copy(dst Writer, src Reader) copies from src to dst, it will erase the src content. Is there
any way to avoid erasing?

<!-- language-all: go -->

> io.Copy(fileTo, file) will erase the file content

It won't. But it will move the read position to EOF, meaning the next ioutil.ReadAll() will start at ... EOF.

You could close it and re-open 'file'before your ioutil.ReadAll().

By the way, you have two defer file.Close() instances: the second one should be defer fileTo.Close().
Or, simpler, reset it with a SectionReader.Seek(), as suggested by PeterSO's answer.

 _, err = file.Seek(0, io.SeekStart)

It is also illustrated in GoByExamples Reading Files:

> There is no built-in rewind, but Seek(0, 0) accomplishes this.

(os.SEEK_SET is define in os constants, as 0)

const SEEK_SET int = 0 // seek relative to the origin of the file

Now (2020) deprecated and replaced with io.SeekStart.
See also "Golang, a proper way to rewind file pointer".

Reset from the end of the file to the start of the file with a seek. For example.

package main

import (
	"fmt"
	"io"
	"io/ioutil"
	"os"
)

func main() {
	file, err := os.Open("HelloWorld")
	if err != nil {
		fmt.Println(err)
	}
	defer file.Close()

	fileTo, err := os.Create("fileTo")
	if err != nil {
		fmt.Println(err)
	}
	defer fileTo.Close()

	_, err = io.Copy(fileTo, file)
	if err != nil {
		fmt.Println(err)
	}

	_, err = file.Seek(0, os.SEEK_SET) // start of file
	if err != nil {
		fmt.Println(err)
	}
	fileByteOne, err := ioutil.ReadAll(file)
	if err != nil {
		fmt.Println(err)
	}

	fmt.Println(fileByteOne)
}

Output:

[72 101 108 108 111 44 32 87 111 114 108 100 33 10]