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Package ioutil

import "io/ioutil"
Overview
Index
Examples

Overview ▾

Package ioutil implements some I/O utility functions.

As of Go 1.16, the same functionality is now provided by package io or package os, and those implementations should be preferred in new code. See the specific function documentation for details.

Variables

Discard is an io.Writer on which all Write calls succeed without doing anything.

As of Go 1.16, this value is simply io.Discard. 

var Discard io.Writer = io.Discard

func NopCloser 

func NopCloser(r io.Reader) io.ReadCloser

NopCloser returns a ReadCloser with a no-op Close method wrapping the provided Reader r.

As of Go 1.16, this function simply calls io.NopCloser.

func ReadAll 

func ReadAll(r io.Reader) ([]byte, error)

ReadAll reads from r until an error or EOF and returns the data it read. A successful call returns err == nil, not err == EOF. Because ReadAll is defined to read from src until EOF, it does not treat an EOF from Read as an error to be reported.

As of Go 1.16, this function simply calls io.ReadAll.

Example

Code:

r := strings.NewReader("Go is a general-purpose language designed with systems programming in mind.")

b, err := ioutil.ReadAll(r)
if err != nil {
		log.Fatal(err)
}

fmt.Printf("%s", b)

Output:

Go is a general-purpose language designed with systems programming in mind.

func ReadDir 

func ReadDir(dirname string) ([]fs.FileInfo, error)

ReadDir reads the directory named by dirname and returns a list of fs.FileInfo for the directory's contents, sorted by filename. If an error occurs reading the directory, ReadDir returns no directory entries along with the error.

As of Go 1.16, os.ReadDir is a more efficient and correct choice: it returns a list of fs.DirEntry instead of fs.FileInfo, and it returns partial results in the case of an error midway through reading a directory.

Example

Code:

files, err := ioutil.ReadDir(".")
if err != nil {
		log.Fatal(err)
}

for _, file := range files {
		fmt.Println(file.Name())
}

func ReadFile 

func ReadFile(filename string) ([]byte, error)

ReadFile reads the file named by filename and returns the contents. A successful call returns err == nil, not err == EOF. Because ReadFile reads the whole file, it does not treat an EOF from Read as an error to be reported.

As of Go 1.16, this function simply calls os.ReadFile.

Example

Code:

content, err := ioutil.ReadFile("testdata/hello")
if err != nil {
		log.Fatal(err)
}

fmt.Printf("File contents: %s", content)

Output:

File contents: Hello, Gophers!

func TempDir 

func TempDir(dir, pattern string) (name string, err error)

TempDir creates a new temporary directory in the directory dir. The directory name is generated by taking pattern and applying a random string to the end. If pattern includes a "*", the random string replaces the last "*". TempDir returns the name of the new directory. If dir is the empty string, TempDir uses the default directory for temporary files (see os.TempDir). Multiple programs calling TempDir simultaneously will not choose the same directory. It is the caller's responsibility to remove the directory when no longer needed.

As of Go 1.17, this function simply calls os.MkdirTemp.

Example

Code:

content := []byte("temporary file's content")
dir, err := ioutil.TempDir("", "example")
if err != nil {
		log.Fatal(err)
}

defer os.RemoveAll(dir) // clean up

tmpfn := filepath.Join(dir, "tmpfile")
if err := ioutil.WriteFile(tmpfn, content, 0666); err != nil {
		log.Fatal(err)
}
Example (Suffix)

Code:

parentDir := os.TempDir()
logsDir, err := ioutil.TempDir(parentDir, "*-logs")
if err != nil {
		log.Fatal(err)
}
defer os.RemoveAll(logsDir) // clean up

// Logs can be cleaned out earlier if needed by searching
// for all directories whose suffix ends in *-logs.
globPattern := filepath.Join(parentDir, "*-logs")
matches, err := filepath.Glob(globPattern)
if err != nil {
		log.Fatalf("Failed to match %q: %v", globPattern, err)
}

for _, match := range matches {
		if err := os.RemoveAll(match); err != nil {
				log.Printf("Failed to remove %q: %v", match, err)
		}
}

func TempFile 

func TempFile(dir, pattern string) (f *os.File, err error)

TempFile creates a new temporary file in the directory dir, opens the file for reading and writing, and returns the resulting *os.File. The filename is generated by taking pattern and adding a random string to the end. If pattern includes a "*", the random string replaces the last "*". If dir is the empty string, TempFile uses the default directory for temporary files (see os.TempDir). Multiple programs calling TempFile simultaneously will not choose the same file. The caller can use f.Name() to find the pathname of the file. It is the caller's responsibility to remove the file when no longer needed.

As of Go 1.17, this function simply calls os.CreateTemp.

Example

Code:

content := []byte("temporary file's content")
tmpfile, err := ioutil.TempFile("", "example")
if err != nil {
		log.Fatal(err)
}

defer os.Remove(tmpfile.Name()) // clean up

if _, err := tmpfile.Write(content); err != nil {
		log.Fatal(err)
}
if err := tmpfile.Close(); err != nil {
		log.Fatal(err)
}
Example (Suffix)

Code:

content := []byte("temporary file's content")
tmpfile, err := ioutil.TempFile("", "example.*.txt")
if err != nil {
		log.Fatal(err)
}

defer os.Remove(tmpfile.Name()) // clean up

if _, err := tmpfile.Write(content); err != nil {
		tmpfile.Close()
		log.Fatal(err)
}
if err := tmpfile.Close(); err != nil {
		log.Fatal(err)
}

func WriteFile 

func WriteFile(filename string, data []byte, perm fs.FileMode) error

WriteFile writes data to a file named by filename. If the file does not exist, WriteFile creates it with permissions perm (before umask); otherwise WriteFile truncates it before writing, without changing permissions.

As of Go 1.16, this function simply calls os.WriteFile.

Example

Code:

message := []byte("Hello, Gophers!")
err := ioutil.WriteFile("hello", message, 0644)
if err != nil {
		log.Fatal(err)
}