Go1.8从文件结尾读取文件

package mainimport (    "bufio"    "bytes"    "flag"    "io"    "log"    "os"    "strconv"    "sync")const readbuf = 1 << 10 //1024type flags struct {    line   int    lines  int    size   string    offset string    output string    file   string}var args flagsfunc init() {    flag.StringVar(&args.output, "o", "", "-o 指定输出的路径,不指定则输出到标准输出")    flag.IntVar(&args.line, "l", 0, "-l 指定从倒数第几行开始读取")    flag.IntVar(&args.lines, "n", 0, "-n 指定读取的行数")    flag.StringVar(&args.offset, "i", "", "-i 指定开始读取的位置,单位:b,kb,mb,默认单位:b")    flag.StringVar(&args.size, "s", "", "-s 指定读取的大小,单位:b,kb,mb,默认单位:b")    flag.StringVar(&args.file, "f", "", "-f 指定要查看的文件路径")}var Tail = &Command{    UsageLine: `tail -f main.go -l 10 -n 5 -o tmp.txt`,    Run:       tail,    Short:     "从文件结尾或指定位置读取内容",    Long: `从文件结尾或指定位置读取内容,可以按行读取,也可以按大小读取,-i 和 -l同时使用的话-i生效,-s 与 -n 同时使用的话-s生效`,}func tail(cmd *Command, arg []string) bool {    File, err := os.OpenFile(args.file, os.O_RDONLY, 0644)    if err != nil {        log.Printf("打开文件失败:%s\n", err.Error())        return true    }    var w io.Writer    if args.output != "" {        w, err = os.Create(args.output)        if err != nil {            log.Printf("创建输出文件失败:%s\n", err.Error())            return true        }    } else {        w = os.Stdout    }    f := NewTail(File)    defer f.Close()    if args.offset != "" {        offset := parseCompany(args.offset)        if offset == 0 {            return false        }        size := parseCompany(args.size)        if err = f.Read(offset, size, args.lines, w); err != nil {            log.Printf("读取内容错误:%s\n", err.Error())            return true        }    }    if args.line > 0 {        if err = f.TailLine(args.line, args.lines, w); err != nil {            log.Printf("读取内容错误:%s\n", err.Error())            return true        }    }    return false}func parseCompany(c string) int64 {    if len(c) < 1 {        return 0    }    if c[len(c)-1] != 'b' {        c += "b"    }    index := len(c) - 2    var company int64 = 1    switch c[index] {    case 'k':        company = 1 << 10 //1024    case 'm':        company = 1 << 20 //1024*1024    default:        index++    }    size, err := strconv.ParseInt(c[:index], 10, 0)    if err != nil {        return -1    }    return size * company}func NewTail(File *os.File) *TailFile {    offset, _ := File.Seek(0, 2)    return &TailFile{file: File, size: offset, offset: offset}}type TailFile struct {    mu     sync.Mutex    file   *os.File    size   int64    offset int64}func (f *TailFile) read(p []byte) (n int, err error) {    if f.offset == 0 {        return 0, io.EOF    }    var (        offset int        length = int64(len(p))    )    if f.offset >= length {        f.offset -= length    } else {        offset = int(f.offset)        f.offset = 0    }    _, err = f.file.Seek(f.offset, 0)    if err == nil {        n, err = f.file.Read(p)        if offset != 0 && offset < n {            n = offset        }    }    return}func (f *TailFile) Read(offset, size, lines int64, w io.Writer) error {    f.mu.Lock()    defer f.mu.Unlock()    if f.size < offset {        return io.EOF    }    f.file.Seek(offset, 0)    switch {    case size > 0:        if _, err := io.CopyN(w, f.file, size); err != io.EOF {            return err        }    case lines > 0:        buf := bufio.NewReader(f.file)        for i := 0; i < int(lines); i++ {            line, err := buf.ReadBytes('\n')            if err != nil {                return err            }            if _, err = w.Write(line); err != nil {                return err            }        }    }    return nil}//暂时用不到func (f *TailFile) ReadLine() ([]byte, error) {    var list []*[]byte    f.mu.Lock()    defer f.mu.Unlock()    if f.offset == 0 {        return nil, io.EOF    }    for {        var buf = make([]byte, 256)        n, err := f.read(buf[:])        if err != nil {            if err == io.EOF {                break            }            return nil, err        }        index := bytes.LastIndexByte(buf[:n], '\n')        if index >= 0 {            f.offset += int64(index)            f.file.Seek(f.offset, 0)            buf = buf[index:n]            list = append(list, &buf)            break        } else {            buf = buf[:n]            list = append(list, &buf)        }    }    var (        last int        line = make([]byte, len(list)*256)    )    for i := len(list) - 1; i >= 0; i-- {        copy(line[last:], *list[i])        last += len(*list[i])    }    return line[:last], nil}func (f *TailFile) TailLine(line, lines int, w io.Writer) (err error) {    buf := make([]byte, readbuf)    f.mu.Lock()    defer f.mu.Unlock()    var (        n, l int        sep  = []byte("\n")    )    for {        n, err = f.read(buf)        if err != nil {            if err == io.EOF {                f.file.Seek(0, 0)                break            }        }        l += bytes.Count(buf[:n], sep)        if l >= line {            var seek, i int            buf = buf[:n]            for l > line {                i = bytes.Index(buf, sep) + 1                buf = buf[i:]                seek += i                l--            }            f.file.Seek(f.offset+int64(seek), 0)            break        }    }    if lines == 0 {        _, err = io.Copy(w, f.file)    } else {        buf := bufio.NewReader(f.file)        var line []byte        for i := 0; i < lines; i++ {            line, err = buf.ReadBytes('\n')            if err != nil {                return            }            if _, err = w.Write(line); err != nil {                return            }        }    }    return}func (f *TailFile) Close() error {    return f.file.Close()}