How can I start a WSL-based Linux executable from within PHP on Windows and capture its stdout?

I have a program that only works on Linux, and I have my development environment on Windows.

• When I run a WSL command from cmd in Windows it works as expected.

  • When I run C:wsl echo "foo" in cmd.exe I get foo back via stdout.

• When I run the same command from within php via exec I don’t get any stdout output and get an exit code -1073740791 which I assume is an error.

---

Cmd.exe:

C:wsl echo "foo"
foo

PHP:

<?php
    $cmd = 'wsl echo "foo"';
    exec($cmd, $out, $code);
    dd($out, $code);
?>
// $out is []
// $code is -1073740791

Answer

As discussed in the comments, the underlying issue was that your systemwide Apache web-server service (wampapache64) was configured to run as NT AuthoritySYSTEM (aka Local System or LocalSystem).

…and running anything like a web-server as LocalSystem is a very, very bad idea for other reasons, especially when it’s running PHP code – a simple mistake in uploaded-file handling or a failure to correctly sanitize user input could potentially hose your entire computer, especially when it’s a publicly-exposed web-server that a malicious attacker can connect to.

---

Anyway:

Apparently when a process is running as SYSTEM it cannot use Win32’s CreateProcess to invoke wsl to then indirectly invoke a Linux program.

• PHP’s exec function is implemented like so:

  • The exec function wraps PHP’s internal VCWD_POPEN.
  • VCWD_POPEN is a macro defined as #define VCWD_POPEN(command, type) virtual_popen(command, type)
  • virtual_popen varies based on OS:
    • On Win32, virtual_popen simply invokes popen_ex(command, type, CWDG(cwd).cwd, NULL).
      • And popen_ex calls CreateProcessAsUserW or CreateProcessW to actually start the new process.
      • I note that popen_ex uses OpenThreadToken to ensure the new process is created with the same user-account and permissions as the parent process – in this case NT AuthoritySYSTEM.

• So PHP’s exec() is not just a simple wrapper over C’s system().

• Whereas if exec() was a wrapper over system() then programs would be invoked via cmd.exe which will behave differently (though that too would likely fail as well)

• And WSL does not support running under Local System.

So by changing your web-server to run as a real user account that meant that the CreateProcess... call invoked wsl within a supported environment.