File I/O

Syntax#

file, err := os.Open(name) // Opens a file in read-only mode. A non-nil error is returned if the file could not be opened.
file, err := os.Create(name) // Creates or opens a file if it already exists in write-only mode. The file is overwritten to if it already exists. A non-nil error is returned if the file could not be opened.
file, err := os.OpenFile(name, flags, perm) // Opens a file in the mode specified by the flags. A non-nil error is returned if the file could not be opened.
data, err := ioutil.ReadFile(name) // Reads the entire file and returns it. A non-nil error is returned if the entire file could not be read.
err := ioutil.WriteFile(name, data, perm) // Creates or overwrites a file with the provided data and UNIX permission bits. A non-nil error is returned if the file failed to be written to.
err := os.Remove(name) // Deletes a file. A non-nil error is returned if the file could not be deleted.
err := os.RemoveAll(name) // Deletes a file or whole directory hierarchy. A non-nil error is returned if the file or directory could not be deleted.
err := os.Rename(oldName, newName) // Renames or moves a file (can be across directories). A non-nil error is returned if the file could not be moved.

Parameters#

Parameter	Details
name	A filename or path of type string. For example: `"hello.txt"`.
err	An `error`. If not `nil`, it represents an error that occurred when the function was called.
file	A file handler of type `*os.File` returned by the `os` package file related functions. It implements an `io.ReadWriter`, meaning you can call `Read(data)` and `Write(data)` on it. Note that these functions may not be able to be called depending on the open flags of the file.
data	A slice of bytes (`[]byte`) representing the raw data of a file.
perm	The UNIX permission bits used to open a file with of type `os.FileMode`. Several constants are available to help with the use of permission bits.
flag	File open flags that determine the methods that can be called on the file handler of type `int`. Several constants are available to help with the use of flags. They are: `os.O_RDONLY` , `os.O_WRONLY` , `os.O_RDWR` , `os.O_APPEND`, `os.O_CREATE`, `os.O_EXCL`, `os.O_SYNC`, and `os.O_TRUNC`.
## Reading and writing to a file using ioutil
A simple program that writes “Hello, world!” to `test.txt`, reads back the data, and prints it out. Demonstrates simple file I/O operations.

package main

import (
    "fmt"
    "io/ioutil"
)

func main() {
    hello := []byte("Hello, world!")

    // Write `Hello, world!` to test.txt that can read/written by user and read by others 
    err := ioutil.WriteFile("test.txt", hello, 0644)
    if err != nil {
        panic(err)
    }

    // Read test.txt
    data, err := ioutil.ReadFile("test.txt")
    if err != nil {
        panic(err)
    }

    // Should output: `The file contains: Hello, world!`
    fmt.Println("The file contains: " + string(data))
}

Listing all the files and folders in the current directory

package main

import (
    "fmt"
    "io/ioutil"
)

func main() {
    files, err := ioutil.ReadDir(".")
    if err != nil {
        panic(err)
    }

    fmt.Println("Files and folders in the current directory:")

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

Listing all folders in the current directory

package main

import (
    "fmt"
    "io/ioutil"
)

func main() {
    files, err := ioutil.ReadDir(".")
    if err != nil {
        panic(err)
    }

    fmt.Println("Folders in the current directory:")

    for _, fileInfo := range files {
        if fileInfo.IsDir() {
            fmt.Println(fileInfo.Name())
        }
    }
}