What is a file?
Files in a computer are a collection of data stored on external media (usually disk), which are divided into text files and binary files.

Open and close files

The os.Open() function can open a file and return a *File and an error.Calling the close() method on the resulting file instance closes the file.

func main(){
    file,err := os.Open("./main.go")
    if err != nil{
        fmt.Println("open file failed!,err",err)
        return 
    }
    fmt.Println("cool!")
    file.Close()
}

To prevent the file from forgetting to close, we usually use defer to register the file close statement.

read file

file.Read()

Basic Use

The method of Read is defined as follows:
func(f *File) Read(b []byte)(n int, err error)
It receives a byte slice, returns the number of bytes read and possible specific errors, and returns 0 and io.EOF at the end of the file.

package main
import (
    "fmt"
    "os"
    "io"
)
func main(){
    file,err := os.Open("./main.go")
    if err != nil{
        fmt.Println("open file failed!,err", err)
        return 
    }
    defer file.Close()
    var tmp = make([]byte,128)
    n, err := file.Read(tmp)
    if err == io.EOF {
        fmt.Println("File Read Out")
        return
    }
    if err != nil{
        fmt.Println("read file failed,err",err)
        return 
    }
    fmt.Printf("Read%d Byte data of\n",n)
    fmt.Println(string(tmp[:n]))
}

Loop Read

Use a for loop to read all the data in the file.

package main
import (
    "fmt"
    "os"
    "io"
)
func main(){
    file, err := os.Open("./main.go")
    if err != nil {
        fmt.Println("open file failed!, err", err)
        return 
    }
    defer file.Close()
    var content []byte
    var tmp = make([]byte,128)
    for {
        n ,err := file.Read(tmp)
        if err == io.EOF{
            fmt.Println("File Read Out")
            break
        }
        if err != nil{
            fmt.Println("read file failed,err",err)
            return
        }
        content = append(content,tmp[:n]...)

    }
    fmt.Println(string(content))
}

bufio Read File

bufio encapsulates an API based on file to support more functions.

package main
import (
    "fmt"
    "os"
    "io"
    "bufio"
)
func main(){
    file , err := os.Open("./main.go")
    if err != nil{
        fmt.Println("open file failed,err",err)
        return
    }
    defer file.Close()
    reader := bufio.NewReader(file)
    for {
        line, err := reader.ReadString('\n')
        if err == io.EOF{
            fmt.Println("File Read Out")
            break
        }
        if err != nil{
            fmt.Println("read file failed!,err",err)
            return
        }
        fmt.Println(line)
    }
}

Use ioutil to read the entire file

The ReadFile method of the io/ioutil package reads the entire file, simply passing in the file name as a parameter.

package main
import (
    "fmt"
    "io/ioutil"
)
func main(){
    content, err := ioutil.ReadFile("./main.go")
    if err != nil{
        fmt.Println("read file failed,err:",err)
        return 

    }
    fmt.Println(string(content))
}

File Write Operation

The os.OpenFile() function can open a file in a specified mode, thereby enabling file writing.

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

Where:
Name is the name of the file to be opened, flag is the mode in which the file is opened. There are several modes:

perm file permissions, an octal number.R (read) 04, w (write) 02, x (execute) 01.

Write and WriteString

func main() {
    file, err := os.OpenFile("xx.txt", os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0666)
    if err != nil {
        fmt.Println("open file failed, err:", err)
        return
    }
    defer file.Close()
    str := "hello Shahe River"
    file.Write([]byte(str))       //Write Byte Slice Data
    file.WriteString("hello Princeling") //Write string data directly
}

bufio.NewWriter

import (
    "io"
        "fmt"
)
func main() {
    file, err := os.OpenFile("xx.txt", os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0666)
    if err != nil {
        fmt.Println("open file failed, err:", err)
        return
    }
    defer file.Close()
    writer := bufio.NewWriter(file)
    for i := 0; i < 10; i++ {
        writer.WriteString("hello Shahe River\n") //Write data to cache first
    }
    writer.Flush() //Write the contents of the cache to a file
}

ioutil.WriteFile

func main() {
    str := "hello Shahe River"
    err := ioutil.WriteFile("./xx.txt", []byte(str), 0666)
    if err != nil {
        fmt.Println("write file failed, err:", err)
        return
    }
}

Practice

copyFile

func CopyFile(dstName, srcName string)(written int64,err error){
    src,err := os.Open(srcName)
    if err != nil{
        fmt.Printf("open %s failed,err: %v.\n", srcName,err)
        return
    }
    defer src.Close()
    dst ,err := os.OpenFile(dstName,os.O_CREATE|os.O_WRONLY,0644)
    if err != nil{
        fmt.Printf("open %s failed,err:%v.\n",dstName,err)
        return 
    }
    defer dst.Close()
    return io.Copy(dst,src)
}
func main(){
    _,err := CopyFile("dst.txt","src.txt")
    if err != nil{
        fmt.Println("copy file failed,err",err)
        return 
    }
    fmt.Println("Copy done!")
}