os - Go

Overview

Package os provides a platform-independent interface to operating system functionality. The design is Unix-like, although the error handling is Go-like; failing calls return values of type error rather than error numbers.

File Operations

Open

Opens the named file for reading.

func Open(name string) (*File, error)

name

string

required

Path to the file to open

file

*File

Opened file with O_RDONLY mode

err

error

Error of type *PathError if operation fails

Example:

file, err := os.Open("example.txt")
if err != nil {
    log.Fatal(err)
}
defer file.Close()

data := make([]byte, 100)
n, err := file.Read(data)

Create

Creates or truncates the named file.

func Create(name string) (*File, error)

name

string

required

Path to the file to create

file

*File

Created file with mode 0666 and O_RDWR mode

err

error

Error if creation fails

Example:

file, err := os.Create("output.txt")
if err != nil {
    log.Fatal(err)
}
defer file.Close()

file.WriteString("Hello, World!")

OpenFile

Generalized open call with specified flags and permissions.

func OpenFile(name string, flag int, perm FileMode) (*File, error)

name

string

required

Path to the file

flag

int

required

Flags: O_RDONLY, O_WRONLY, O_RDWR, O_APPEND, O_CREATE, O_TRUNC, etc.

perm

FileMode

required

Unix permission bits (e.g., 0644)

Example:

file, err := os.OpenFile("log.txt", os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
if err != nil {
    log.Fatal(err)
}
defer file.Close()

File.Read

Reads up to len(b) bytes from the File.

func (f *File) Read(b []byte) (n int, err error)

[]byte

required

Buffer to read data into

int

Number of bytes read

err

error

io.EOF at end of file, or other error

File.Write

Writes len(b) bytes to the File.

func (f *File) Write(b []byte) (n int, err error)

[]byte

required

Data to write

int

Number of bytes written

err

error

Error if n != len(b)

Example:

data := []byte("Hello, World!")
n, err := file.Write(data)

File.WriteString

Writes the contents of string s to the file.

func (f *File) WriteString(s string) (n int, err error)

Example:

n, err := file.WriteString("Hello, World!\n")

File.Close

Closes the File.

func (f *File) Close() error

Example:

defer file.Close()

File.Seek

Sets the offset for the next Read or Write.

func (f *File) Seek(offset int64, whence int) (ret int64, err error)

offset

int64

required

Offset to seek to

whence

int

required

0 = relative to origin, 1 = relative to current, 2 = relative to end

Example:

// Seek to beginning
file.Seek(0, 0)

// Seek to end
file.Seek(0, 2)

File System Operations

ReadFile

Reads the named file and returns the contents.

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

name

string

required

Path to the file to read

data

[]byte

File contents

err

error

Error if read fails (EOF is not an error)

Example:

data, err := os.ReadFile("config.json")
if err != nil {
    log.Fatal(err)
}
fmt.Printf("File size: %d bytes\n", len(data))

WriteFile

Writes data to the named file, creating it if necessary.

func WriteFile(name string, data []byte, perm FileMode) error

name

string

required

Path to the file

data

[]byte

required

Data to write

perm

FileMode

required

Unix permission bits (e.g., 0644)

Example:

data := []byte("Hello, World!\n")
err := os.WriteFile("output.txt", data, 0644)
if err != nil {
    log.Fatal(err)
}

Remove

Removes the named file or (empty) directory.

func Remove(name string) error

Example:

err := os.Remove("temp.txt")
if err != nil {
    log.Fatal(err)
}

RemoveAll

Removes path and any children it contains.

func RemoveAll(path string) error

Example:

err := os.RemoveAll("temp_dir")

Rename

Renames (moves) oldpath to newpath.

func Rename(oldpath, newpath string) error

Example:

err := os.Rename("old.txt", "new.txt")

Mkdir

Creates a new directory with the specified name and permission bits.

func Mkdir(name string, perm FileMode) error

name

string

required

Directory name

perm

FileMode

required

Unix permission bits (e.g., 0755)

Example:

err := os.Mkdir("mydir", 0755)
if err != nil {
    log.Fatal(err)
}

MkdirAll

Creates a directory named path, along with any necessary parents.

func MkdirAll(path string, perm FileMode) error

Example:

err := os.MkdirAll("path/to/dir", 0755)

File Info

Stat

Returns a FileInfo describing the named file.

func Stat(name string) (FileInfo, error)

name

string

required

File path

info

FileInfo

File information

Example:

info, err := os.Stat("example.txt")
if err != nil {
    log.Fatal(err)
}
fmt.Printf("Size: %d bytes\n", info.Size())
fmt.Printf("Modified: %v\n", info.ModTime())
fmt.Printf("Is directory: %v\n", info.IsDir())

FileInfo Interface

type FileInfo interface {
    Name() string       // base name of the file
    Size() int64        // length in bytes
    Mode() FileMode     // file mode bits
    ModTime() time.Time // modification time
    IsDir() bool        // abbreviation for Mode().IsDir()
    Sys() any          // underlying data source
}

Directory Operations

ReadDir

Reads the named directory, returning all its directory entries sorted by filename.

func ReadDir(name string) ([]DirEntry, error)

name

string

required

Directory path

entries

[]DirEntry

Directory entries sorted by filename

Example:

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

for _, entry := range entries {
    fmt.Println(entry.Name())
}

Getwd

Returns a rooted path name corresponding to the current directory.

func Getwd() (dir string, err error)

Example:

dir, err := os.Getwd()
if err != nil {
    log.Fatal(err)
}
fmt.Printf("Current directory: %s\n", dir)

Chdir

Changes the current working directory to the named directory.

func Chdir(dir string) error

Example:

err := os.Chdir("/tmp")

Environment Variables

Getenv

Retrieves the value of the environment variable named by the key.

func Getenv(key string) string

Example:

path := os.Getenv("PATH")
home := os.Getenv("HOME")

Setenv

Sets the value of the environment variable named by the key.

func Setenv(key, value string) error

Example:

err := os.Setenv("MY_VAR", "value")

Environ

Returns a copy of strings representing the environment.

func Environ() []string

Example:

for _, env := range os.Environ() {
    fmt.Println(env)
}

Process Operations

Exit

Causes the current program to exit with the given status code.

func Exit(code int)

Example:

if err != nil {
    fmt.Fprintf(os.Stderr, "Error: %v\n", err)
    os.Exit(1)
}
os.Exit(0)

Getpid

Returns the process id of the caller.

func Getpid() int

Example:

pid := os.Getpid()
fmt.Printf("Process ID: %d\n", pid)

Standard File Descriptors

var (
    Stdin  = NewFile(uintptr(syscall.Stdin), "/dev/stdin")
    Stdout = NewFile(uintptr(syscall.Stdout), "/dev/stdout")
    Stderr = NewFile(uintptr(syscall.Stderr), "/dev/stderr")
)

Example:

fmt.Fprintln(os.Stdout, "Standard output")
fmt.Fprintln(os.Stderr, "Error message")

File Mode Flags

const (
    O_RDONLY int = syscall.O_RDONLY // open read-only
    O_WRONLY int = syscall.O_WRONLY // open write-only
    O_RDWR   int = syscall.O_RDWR   // open read-write
    O_APPEND int = syscall.O_APPEND // append data to file
    O_CREATE int = syscall.O_CREAT  // create file if none exists
    O_EXCL   int = syscall.O_EXCL   // used with O_CREATE, file must not exist
    O_SYNC   int = syscall.O_SYNC   // open for synchronous I/O
    O_TRUNC  int = syscall.O_TRUNC  // truncate file when opened
)

Complete Example

package main

import (
    "fmt"
    "log"
    "os"
)

func main() {
    // Write to file
    err := os.WriteFile("example.txt", []byte("Hello, World!\n"), 0644)
    if err != nil {
        log.Fatal(err)
    }
    
    // Read from file
    data, err := os.ReadFile("example.txt")
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("File contents: %s", data)
    
    // Get file info
    info, err := os.Stat("example.txt")
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("Size: %d bytes\n", info.Size())
    
    // Create directory
    err = os.Mkdir("testdir", 0755)
    if err != nil {
        log.Fatal(err)
    }
    
    // List directory
    entries, err := os.ReadDir(".")
    if err != nil {
        log.Fatal(err)
    }
    
    fmt.Println("Directory contents:")
    for _, entry := range entries {
        fmt.Printf("  %s (dir: %v)\n", entry.Name(), entry.IsDir())
    }
    
    // Clean up
    os.Remove("example.txt")
    os.Remove("testdir")
}

Language Constructs

I/O & System

Data Structures

Networking

Encoding

Cryptography

Concurrency

Testing & Debugging

Reflection & Parsing

​Overview

​File Operations

​Open

​Create

​OpenFile

​File.Read

​File.Write

​File.WriteString

​File.Close

​File.Seek

​File System Operations

​ReadFile

​WriteFile

​Remove

​RemoveAll

​Rename

​Mkdir

​MkdirAll

​File Info

​Stat

​FileInfo Interface

​Directory Operations

​ReadDir

​Getwd

​Chdir

​Environment Variables

​Getenv

​Setenv

​Environ

​Process Operations

​Exit

​Getpid

​Standard File Descriptors

​File Mode Flags

​Complete Example

Build docs developers (and LLMs) love

Overview

File Operations

Open

Create

OpenFile

File.Read

File.Write

File.WriteString

File.Close

File.Seek

File System Operations

ReadFile

WriteFile

Remove

RemoveAll

Rename

Mkdir

MkdirAll

File Info

Stat

FileInfo Interface

Directory Operations

ReadDir

Getwd

Chdir

Environment Variables

Getenv

Setenv

Environ

Process Operations

Exit

Getpid

Standard File Descriptors

File Mode Flags

Complete Example