PHP implements linux command tail

PHP implements linux command tail -f

Today I suddenly thought of a question someone asked me before, how to implement the command tail -f in Linux through PHP. Here I will analyze and implement it.

This is quite simple when you think about it. It detects the file through a loop to see if the size of the file has changed. If there is a change, output the changed part of the file. Of course, there will be a lot of details in it. Here is a detailed analysis.

If the initial file is too large or changes too much

At this time, you may not be able to see clearly if you output a lot of content at once, so I set a threshold of 8192 here. When the content length exceeds this threshold, only the last 8192 bytes will be output, so that there will be no large-area refresh. The problem of not seeing clearly.

How to detect file size changes

This problem is the core of this program. Whether it can be successful or not, the performance depends on this part. My implementation here is as follows:

Open the file handle $fp. It should be noted here that the file handle here needs to be opened and closed only once in the whole process, so it must be placed outside the loop. Initialize the current file size file_size and file_size_new to both 0. The file_size_new file size is updated in the loop. It should be noted here that before obtaining the file size in PHP, the function clearstatcache() must be run to clear the file status cache, otherwise there may be deviation in obtaining the file size. Calculate add_size = file_size_new - file_size to see if the file size changes. If there is a change, move the file pointer to the specified position, then output the newly added content, and update the file_size value to new_file_size. usleep(50000), sleep for 1/20 seconds. Code

#!/usr/bin/env php

if(2 != count($argv)){
 fwrite(
 STDERR,
 "Call format error! Use format ./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);

The first line of code implementation here, #!/usr/bin/env php, tells the executable file that the executable file php is to be found in the system PATH. The advantage of this is that it has good portability.

2016-02-22 11:28:51 Improvement

I checked the PHP official manual and found that the fseek function can be improved here. This function also accepts a third parameter, indicating the type of offset pointer. The default is SEEK_SET, which means offset from the beginning. It can also be set to SEEK_CUR, which means offset from the current position. Shift, so this is changed to fseek($fp, $ignore_size, $ignore_size);

Below is the result

PHP, linux, tail