php - 如何在 PHP 中只读取文本文件的最后 5 行？

Question

我有一个名为 file.txt 的文件，它通过添加行来更新。

我正在通过这段代码阅读它:

$fp = fopen("file.txt", "r");
$data = "";
while(!feof($fp))
{
$data .= fgets($fp, 4096);
}
echo $data;

然后出现大量行。我只想回显文件的最后 5 行

我该怎么做？

---

file.txt是这样的:

11111111111111
22222222222

33333333333333
44444444444

55555555555555
66666666666

Answer 1

对于一个大文件，使用 file() 将所有行读入一个数组有点浪费。以下是读取文件并维护最后 5 行缓冲区的方法:

$lines=array();
$fp = fopen("file.txt", "r");
while(!feof($fp))
{
   $line = fgets($fp, 4096);
   array_push($lines, $line);
   if (count($lines)>5)
       array_shift($lines);
}
fclose($fp);

您可以通过一些关于可能行长度的启发式方法来进一步优化这一点，方法是寻找一个位置，例如，距离末端大约 10 行，如果这不会产生 5 行，则再往前走。这是一个简单的实现，它证明了这一点:

//how many lines?
$linecount=5;

//what's a typical line length?
$length=40;

//which file?
$file="test.txt";

//we double the offset factor on each iteration
//if our first guess at the file offset doesn't
//yield $linecount lines
$offset_factor=1;


$bytes=filesize($file);

$fp = fopen($file, "r") or die("Can't open $file");


$complete=false;
while (!$complete)
{
    //seek to a position close to end of file
    $offset = $linecount * $length * $offset_factor;
    fseek($fp, -$offset, SEEK_END);


    //we might seek mid-line, so read partial line
    //if our offset means we're reading the whole file, 
    //we don't skip...
    if ($offset<$bytes)
        fgets($fp);

    //read all following lines, store last x
    $lines=array();
    while(!feof($fp))
    {
        $line = fgets($fp);
        array_push($lines, $line);
        if (count($lines)>$linecount)
        {
            array_shift($lines);
            $complete=true;
        }
    }

    //if we read the whole file, we're done, even if we
    //don't have enough lines
    if ($offset>=$bytes)
        $complete=true;
    else
        $offset_factor*=2; //otherwise let's seek even further back

}
fclose($fp);

var_dump($lines);