Discuss how Golang implements the tail function

Golang is a modern programming language that is increasingly popular and supported by the public. With its efficient and stable features, it can not only meet developers' needs for high performance and low resource consumption, but also be able to implement a variety of functions.

In this article, we will explore how Golang implements the tail function, which monitors changes in a file and outputs the last few lines of information in the file.

Implementation Ideas

Before we start to implement the tail function, we need to first understand the basic ideas for its implementation. In general, we need to implement the following functions:

1. Open the file at the given path and read the entire file content.
2. Get the size of the file and save the file pointer position.
3. Use goroutine of Go language to monitor file changes. If the file changes, output and update the last few lines of information.
4. Check file size changes in turn until the file is closed or the thread is stopped.

Implement parsing

First, we need to introduce the os package to read and close the file. We also need to define a structure type to represent the tail functionality.

package main

import (
    "fmt"
    "os"
)

type Tail struct {
    File   *os.File // 文件指针
    Size   int64    // 文件大小
    Cursor int64    // 文件指针所在位置
    Rows   int      // 输出行数
}

We have defined a Tail structure type, in which the File field is the file pointer, the Size field is the file size, the Cursor field represents the current position of the file pointer, and the Rows field represents the number of output lines.

Next, we need to implement the file opening function. In Golang, reading file contents can be implemented through the os package. After we open the file, we can obtain the file information through the os.Stat() function, including file size, file pointer position, etc. We also need to remember to position the file pointer to the end of the file.

func (t *Tail) OpenFile(path string, rows int) error {
    var err error

    // 打开文件
    t.File, err = os.Open(path)
    if err != nil {
        fmt.Printf("open file %s err: %s\n", path, err.Error())
        return err
    }

    // 获取文件信息
    fi, err := t.File.Stat()
    if err != nil {
        fmt.Printf("get file info err:%s", err.Error())
        return err
    }

    // 获取文件大小
    t.Size = fi.Size()

    // 将文件指针定位到文件末尾
    _, err = t.File.Seek(0, os.SEEK_END)
    if err != nil {
        fmt.Printf("move file pointer failed. err:%s\n", err.Error())
        return err
    }

    // 设置输出行数
    t.Rows = rows
    return nil
}

In the code, we first open the file through the os.Open() function and obtain the file information through the os.Stat() function. Next, we use the os.Seek() function to point the file pointer to the end of the file to ensure that the program reads the latest file information.

According to the number of output lines input by the user, we record the line number information. It should be noted here that we need to divide the number of lines by 2, because one line of some files may consist of multiple lines, and the specific implementation is left to the reader to think about.

Next, we implement monitoring of output changes and implement it in goroutine.

func (t *Tail) Follow() {
    defer t.File.Close()

    // 开始监视文件变化
    for {
        fi, err := t.File.Stat()
        if err != nil {
            fmt.Printf("get file info error: %s\n", err.Error())
            return
        }

        // 如果指针超过文件大小，将指针移到文件末尾
        if t.Cursor > fi.Size() {
            _, err = t.File.Seek(0, os.SEEK_END)
            if err != nil {
                fmt.Printf("move file pointer failed. err:%s\n", err.Error())
                return
            }
            t.Cursor = fi.Size()
        }

        // 读取差异部分的内容，并输出
        if fi.Size() > t.Cursor {
            data := make([]byte, fi.Size()-t.Cursor)
            _, err = t.File.ReadAt(data, t.Cursor)
            if err != nil {
                fmt.Printf("read file failed. err:%s\n", err.Error())
                return
            }

            lines := strings.Split(string(data), "\n")
            for i := len(lines) - t.Rows/2; i < len(lines); i++ {
                fmt.Println(lines[i])
            }
            t.Cursor += int64(len(data))

            fmt.Printf("Cursor:%d\n", t.Cursor)
        }

        time.Sleep(1 * time.Second)
    }
}

In goroutine, we obtain file information through the os.File.Stat() function and check whether the file size has changed. If the file size changes, we read the contents of the difference part and output it, and update the file pointer position.

We read the file content in lines and output only the last few lines. This is to avoid the problem of new lines exceeding the output range.

In fact, starting a goroutine can monitor file changes, and a change in file size means that the file content has changed. At this time, we can use the os.File.ReadAt() function to read the difference. content and then output it.

Finally, we need to implement the printing of error logs.

func main() {
    // 构造 Tail 结构体
    t := &Tail{}

    // 打开文件
    err := t.OpenFile("test.log", 20)
    if err != nil {
        return
    }

    // 监听文件变化
    fmt.Println("start following...")
    t.Follow()

    fmt.Println("tail finish.")
}

In the main() function, we first open the file through the OpenFile() function, and then monitor the file changes through the Follow() function to implement the tail function. Here we monitor changes in the file and continuously output the last few lines of the file until the file is closed or the program stops.

Conclusion

The above is how Golang implements the tail function. It is simple to implement and practical at the same time. In actual development, corresponding optimization can be carried out according to specific needs, such as reading more lines, outputting to a specified log file, etc. This implementation can help developers better monitor file changes and better meet development needs.

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