在exec（）结束后运行后台程序之前，AJAX脚本不会响应

Question

I run a background PHP program with exec() like this :

exec('/usr/bin/php bgScript.php "arg1" "arg2" > /dev/null 2>&1 &');

It works and the program does run in background.

Problem

I have Output Buffering Enabled and would like to keep it that way.

My whole script is this :

exec('/usr/bin/php bgScript.php "arg1" "arg2" > /dev/null 2>&1 &');
echo json_encode(array(
  "status" => "started"
));

When an AJAX request is made to the file above, the process is started and is in background. I assume this because, further requests to the server returns data and doesn't wait for the previous AJAX script to finish.

But, the problem is that the JSON data is not outputted until the background process is completed.

Since the program is made to run in the background, shouldn't the JSON Data be outputted without waiting for the exec() to end ? I don't know how to say this techinically (Forgive me) : Why does the Output Buffer continue until exec() ends ?

How can I make the script output the JSON Data right after the program is started in the background and close the connection between the AJAX script and browser ?

Answer 1

The command does not run in the background if it's stdio is not redirected. From the official documentation

Note: If a program is started with this function, in order for it to continue running in the background, the output of the program must be redirected to a file or another output stream. Failing to do so will cause PHP to hang until the execution of the program ends.

There are several methods to alter this behavior in Unix like systems you can run the command with command >/dev/null & on windows you can use start command.