os.File bufio ioutil
文件较小 可使用ioutil

bufio
const (
    defaultBufSize = 4096
)

func NewReader(rd io.Reader) *Reader {
    return NewReaderSize(rd, defaultBufSize)
}

func NewReaderSize(rd io.Reader, size int) *Reader



//使用File自带的Read
func read1(filename string) int {
    fi, err := os.Open(filename)
    if err != nil {
        panic(err)
    }
    defer fi.Close()
    buf := make([]byte, 4096)
    var nbytes int
    for {
        n, err := fi.Read(buf)
        if err != nil && err != io.EOF {
            panic(err)
        }
        if n == 0 {
            break
        }
        nbytes += n

    }
    return nbytes
}


//使用bufio
func read2(filename string) int {
    fi, err := os.Open(filename)
    if err != nil {
        panic(err)
    }
    defer fi.Close()
    buf := make([]byte, 4096)
    var nbytes int
    rd := bufio.NewReader(fi)
    for {
        n, err := rd.Read(buf)
        if err != nil && err != io.EOF {
            panic(err)
        }
        if n == 0 {
            break
        }
        nbytes += n
    }
    return nbytes
}

//使用ioutil
func read3(filename string) int {
    fi, err := os.Open(filename)
    if err != nil {
        panic(err)
    }
    defer fi.Close()
    fd, err := ioutil.ReadAll(fi)
    nbytes := len(fd)
    return nbytes
}

func testfile1(filename string) {
    fmt.Printf("============test1 %s ===========\n", filename)
    start := time.Now()
    size1 := read1(filename)
    t1 := time.Now()
    fmt.Printf("Read 1 cost: %v, size: %d\n", t1.Sub(start), size1)
    size2 := read2(filename)
    t2 := time.Now()
    fmt.Printf("Read 2 cost: %v, size: %d\n", t2.Sub(t1), size2)
    size3 := read3(filename)
    t3 := time.Now()
    fmt.Printf("Read 3 cost: %v, size: %d\n", t3.Sub(t2), size3)
}





bufio 读取一行数据
ReadBytes
ReadString
ReadSlice
ReadLine

处理大文件时，ReadLine和ReadSlice效率相近，明显快于ReadString和ReadBytes
ReadSlice 切片式读取，根据分隔符切片
ReadLine 在切片读取的基础上，对换行符\n和\r\n做了一些处理


func readline1(filename string) {
    fi, err := os.Open(filename)
    if err != nil {
        panic(err)
    }
    defer fi.Close()
    rd := bufio.NewReader(fi)
    for {
        _, err := rd.ReadBytes('\n')
        if err != nil || err == io.EOF {
            break
        }
    }
}

func readline2(filename string) {
    fi, err := os.Open(filename)
    if err != nil {
        panic(err)
    }
    defer fi.Close()
    rd := bufio.NewReader(fi)
    for {
        _, err := rd.ReadString('\n')
        if err != nil || err == io.EOF {
            break
        }
    }
}
func readline3(filename string) {
    fi, err := os.Open(filename)
    if err != nil {
        panic(err)
    }
    defer fi.Close()
    rd := bufio.NewReader(fi)
    for {
        _, err := rd.ReadSlice('\n')
        if err != nil || err == io.EOF {
            break
        }
    }
}
func readline4(filename string) {
    fi, err := os.Open(filename)
    if err != nil {
        panic(err)
    }
    defer fi.Close()
    rd := bufio.NewReader(fi)
    for {
        _, _, err := rd.ReadLine()
        if err != nil || err == io.EOF {
            break
        }
    }
}

func testfile2(filename string) {
    fmt.Printf("============test2 %s ===========\n", filename)
    start := time.Now()
    readline1(filename)
    t1 := time.Now()
    fmt.Printf("Readline 1 cost: %v\n", t1.Sub(start))
    readline2(filename)
    t2 := time.Now()
    fmt.Printf("Readline 2 cost: %v\n", t2.Sub(t1))
    readline3(filename)
    t3 := time.Now()
    fmt.Printf("Readline 3 cost: %v\n", t3.Sub(t2))
    readline4(filename)
    t4 := time.Now()
    fmt.Printf("Readline 4 cost: %v\n", t4.Sub(t3))
}




完整测试代码
https://gitee.com/dyyx/hellocode/blob/master/web/tech/go/demo/fileread.go

go run fileread.go -filename fileread.go 
filename= fileread.go
============test1 fileread.go ===========
Read 1 cost: 79.591µs, size: 3616
Read 2 cost: 172.882µs, size: 3616
Read 3 cost: 113.971µs, size: 3616
============test2 fileread.go ===========
Readline 1 cost: 73µs
Readline 2 cost: 73.806µs
Readline 3 cost: 44.578µs
Readline 4 cost: 35.342µs


go run fileread.go -filename=fileread.go 
stat -filename=fileread.go: no such file or directory

为何 -filename=fileread.go  不行 ？


 GO flag 命令行参数解析