Working with Files

Reading File

You can read a complete file in using ioutil.ReadFile. The function returns two variables, the first the content of the file. The second variable is the error if one occurred. If no error, than err will be nil

filename := "./extras/rabbits.txt"

content, err := ioutil.ReadFile(filename)
if err != nil {
	log.Fatalln("Error reading file", filename)

The content is returned as a []byte and not a string. You need to cast to a string to use as such, for example to display. Use string() to cast a []byte to string type.


Check if File Exists

One of the above errors can be if the file does not exist. You can use os.Stat to check explicitly if file exists without trying to open.

if _, err := os.Stat(""); os.IsNotExist(err) {
	fmt.Println("File: does not exist")

Write to a new File

Use ioutil.WriteFile to write a file out. The function takes three variables, the filename, the content (as a []byte) and the file system mode.

outfile := "output.txt"
err = ioutil.WriteFile(outfile, content, 0644)
if err != nil {
	fmt.Println("Error writing file: ", err)
} else {
	fmt.Println("Created: ", outfile)

Append to an existing File

You can write out to an existing file appending the content using the following.

af, err := os.OpenFile(outfile, os.O_APPEND|os.O_WRONLY, 0644)
if err != nil {
	fmt.Println("Error appending to file:", err)
defer af.Close()
if _, err = af.WriteString("Appending this text"); err != nil {
	fmt.Println("Error writing to file:", err)

The defer statement defers the execution until the surrounding function (or overall program) completes. You should always use defer for something that needs to be closed, or cleaned up.

Using Filepath

Use the filepath package for working with cross-platform paths properly. For example, use filepath.Join for creating a path with directory.

package main

import (

func main() {
    fmt.Println(filepath.Join("a", "b", "file.ext"))

See filepath package documentation for additional function, including splitting paths, checking filename extension, base and more.