Remove first line from text file in Golang

Question

I'm trying to pop the first line of a file and thus reduce the file lines one by one. My implementation for removing the first line is as follows

type FS struct {
    ...
    File       *os.File
}

//File creation ok...


func (fs *Fs) pop() []byte {
    var buf []string
    scanner := bufio.NewScanner(fs.File)
    //Reading lines
    for scanner.Scan() {
        line := scanner.Text()
        buf = append(buf, line)
    }
    //Writing from second line on the same file
    for s := 1; s < len(buf); s++ {
        fs.File.WriteString(fmt.Println(buf[s]))
    }
    //Commit changes
    fs.File.Sync()
    fs.File.Close()

    return []byte(buf[0])
}

I get the returned []byte with the expected string but the file never changes. What am I missing here?

Answer 1

pop the first line of a file and thus reduce the file lines one by one.

---

For example,

package main

import (
    "bytes"
    "fmt"
    "io"
    "os"
)

func popLine(f *os.File) ([]byte, error) {
    fi, err := f.Stat()
    if err != nil {
        return nil, err
    }
    buf := bytes.NewBuffer(make([]byte, 0, fi.Size()))

    _, err = f.Seek(0, io.SeekStart)
    if err != nil {
        return nil, err
    }
    _, err = io.Copy(buf, f)
    if err != nil {
        return nil, err
    }

    line, err := buf.ReadBytes('\n')
    if err != nil && err != io.EOF {
        return nil, err
    }

    _, err = f.Seek(0, io.SeekStart)
    if err != nil {
        return nil, err
    }
    nw, err := io.Copy(f, buf)
    if err != nil {
        return nil, err
    }
    err = f.Truncate(nw)
    if err != nil {
        return nil, err
    }
    err = f.Sync()
    if err != nil {
        return nil, err
    }

    _, err = f.Seek(0, io.SeekStart)
    if err != nil {
        return nil, err
    }
    return line, nil
}

func main() {
    fname := `popline.txt`
    f, err := os.OpenFile(fname, os.O_RDWR|os.O_CREATE, 0666)
    if err != nil {
        fmt.Fprintln(os.Stderr, err)
        return
    }
    defer f.Close()
    line, err := popLine(f)
    if err != nil {
        fmt.Fprintln(os.Stderr, err)
        return
    }
    fmt.Println("pop:", string(line))
}

$ cat popline.txt
Lorem ipsum dolor sit amet, consectetur adipiscing elit, 
sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. 
Ut enim ad minim veniam, 
quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. 
Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. 
Excepteur sint occaecat cupidatat non proident, 
sunt in culpa qui officia deserunt mollit anim id est laborum.

$ go run popline.go
pop: Lorem ipsum dolor sit amet, consectetur adipiscing elit, 

$ cat popline.txt
sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. 
Ut enim ad minim veniam, 
quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. 
Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. 
Excepteur sint occaecat cupidatat non proident, 
sunt in culpa qui officia deserunt mollit anim id est laborum.

$