Read and write files in Go

Reading file line by line

如果要一行一行地讀
建議用 bufio.Scanner
但是 Scanner 有個缺點
就是一行太長(超過 64K)的時候會出現 bufio.Scanner: token too long 的錯誤
這時候還是得用 bufio.Reader

fin, err := os.Open(path)
if err != nil {
    fmt.Println(err)
}
defer fin.Close()

scanner := bufio.NewScanner(fin)
for scanner.Scan() {
    line := scanner.Text()
    fmt.Fprintln(os.Stdin, line)
}

if err := scanner.Err(); err != nil {
    fmt.Fprintln(os.Stderr, err)
}

If you know the maximum length of the tokens you will be reading, copy the bufio.Scanner code into your project and change the const MaxScanTokenSize value.

ref:
http://stackoverflow.com/questions/6141604/go-readline-string
http://stackoverflow.com/questions/1821811/how-to-read-write-from-to-file
http://stackoverflow.com/questions/8757389/reading-file-line-by-line-in-go
http://stackoverflow.com/questions/5884154/golang-read-text-file-into-string-array-and-write
https://github.com/polaris1119/The-Golang-Standard-Library-by-Example/blob/master/chapter01/01.4.md

bufio
https://golang.org/pkg/bufio/
https://golang.org/pkg/bufio/#Scanner

Reading and writing file line by line

fmt.Fprintln(writer, line)bw.WriteString(line) 還要快

func FileSpacing(filename string, w io.Writer) (err error) {
    fr, err := os.Open(filename)
    if err != nil {
        return err
    }
    defer fr.Close()

    br := bufio.NewReader(fr)
    bw := bufio.NewWriter(w)

    for {
        line, err := br.ReadString('\n')
        if err == nil {
            fmt.Fprint(bw, TextSpacing(line))
        } else {
            if err == io.EOF {
                fmt.Fprint(bw, TextSpacing(line))
                break
            }
            return err
        }
    }
    defer bw.Flush()

    return nil
}

Copy a file

fin, _ := os.Open("source.txt")
fout, _ := os.Create("destination.txt")

io.Copy(fout, fin)

defer fout.Close()
defer fin.Close()

ref:
http://stackoverflow.com/questions/23272663/transfer-a-big-file-in-golang
http://golang.org/pkg/io/#Copy

Compute MD5 of a file

func md5Of(filename string) string {
    var result []byte

    file, err := os.Open(filename)
    checkError(err)
    defer file.Close()

    hash := md5.New()
    _, err = io.Copy(hash, file)
    checkError(err)

    checksum := hex.EncodeToString(hash.Sum(result))

    return checksum
}

ref:
http://stackoverflow.com/questions/29505089/how-can-i-compare-two-files-in-golang