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PHP实现linux命令tail -f

今天突然想到之前有人问过我的一个问题,如何通过PHP实现linux中的命令 tail -f ,这里就来分析实现下。

这个想一想也挺简单,通过一个循环检测文件,看文件的大小是否有变化,如果有变化,输出文件变化的部分,当然了这里面会有好多的细节,这里具体分析下。

如果初始文件太大或者改变内容太多

这个时候一下输出好多内容可能看不清,因此我这里设置了一个阈值 8192 ,当内容长度超过这个阈值的时候,只输出最后面的 8192 个字节,这样就不会出现大面积的刷新导致看不清的问题。

如何检测文件大小的变化

这个问题是这个程序的核心,能不能成功,性能的好坏就靠这部分了。我在这里的实现是下面这样:

  • 打开文件句柄 $fp ,这里要注意,这里的文件句柄全程需中只打开一次关闭一次,因此要将他放在循环的外面。
  • 初始化当前文件大小 file_size 和 file_size_new 都为0。
    • 循环里面更新 file_size_new 文件大小,这里要注意,php中获取文件大小之前一定要运行函数 clearstatcache() ,清除文件状态缓存,否则获取文件大小可能会有偏差。
    • 计算 add_size = file_size_new - file_size ,看文件大小是否有变化,如果有变化,将文件指针移动到指定位置,然后输出新加的内容,更新 file_size 值为 new_file_size 。
    • usleep(50000),睡眠1/20秒。

代码实现

#!/usr/bin/env php <?phpif(2 != count($argv)){    fwrite(        STDERR,        "调用格式错误!使用格式 ./tail filename".PHP_EOL    );      return 1;}$file_name      = $argv[1];define("MAX_SHOW", 8192);$file_size      = 0;$file_size_new  = 0;$add_size       = 0;$ignore_size    = 0;$fp = fopen($file_name, "r");while(1){    clearstatcache();    $file_size_new  = filesize($file_name);    $add_size       = $file_size_new - $file_size;    if($add_size > 0){         if($add_size > MAX_SHOW){            $ignore_size    = $add_size - MAX_SHOW;            $add_size       = MAX_SHOW;            fseek($fp, $file_size + $ignore_size);        }           fwrite(            STDOUT,            fread($fp, $add_size)        );          $file_size  = $file_size_new;    }    usleep(50000);}fclose($fp);

代码实现这里第一行的 #!/usr/bin/env php 是告诉可执行文件,可执行文件 php 在系统 PATH 中查找,这样的好处就是移植性好。

2016-02-22 11:28:51改进

查了PHP官方手册, fseek 函数这里可以改进改进,这个函数还接受第三个参数,表示偏移指针的类型,默认是 SEEK_SET ,从开始偏移,还可以设置为 SEEK_CUR ,表示从当前位置偏移,因此这里改为 fseek($fp, $ignore_size, $ignore_size);

下面是结果

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