Read a Whole File into Memory in Go with os.ReadFile

A Go program needs the complete contents of a small file in one call.

package main

import (
	"errors"
	"fmt"
	"io/fs"
	"os"
)

func main() {
	os.WriteFile("hello.txt", []byte("hello\nworld\n"), 0o644)

	data, err := os.ReadFile("hello.txt")
	fmt.Printf("%q %v\n", data, err)

	_, err = os.ReadFile("missing.txt")
	fmt.Println(err)
	fmt.Println("not exist:", errors.Is(err, fs.ErrNotExist))

	_, err = os.ReadFile("/tmp")
	fmt.Println(err)
}

"hello\nworld\n" <nil>
open missing.txt: no such file or directory
not exist: true
read /tmp: is a directory

os.ReadFile(name) opens the file, reads to the end and closes it, returning []byte. The errors keep the operation and the path, as in open missing.txt: no such file or directory, and they wrap the underlying error, so errors.Is(err, fs.ErrNotExist) is true for a missing file. Reading a directory fails with is a directory. See os.ReadFile.

The whole file is held in memory, so use bufio.Scanner or io.Copy for large files or when you can process a line at a time. ioutil.ReadFile still works but is deprecated. The run shows %q output because file contents are bytes and may hold anything. This was run with Go 1.19.