php执行python脚本并在会话中保存输出

i have this php code :

$command = escapeshellcmd('python acn.py -c'.$username);
$output = shell_exec($command);

i need output the result of acn.py .. on windows get output, on linux not give output. python script work, php not print output ! Where is my wrong ? chmod +x * allready executed !