(PHP 4, PHP 5)
exec — Execute an external program
exec() executes the given command .
The command that will be executed.
If the output argument is present, then the specified array will be filled with every line of output from the command. Trailing whitespace, such as \n, is not included in this array. Note that if the array already contains some elements, exec() will append to the end of the array. If you do not want the function to append elements, call unset() on the array before passing it to exec().
If the return_var argument is present along with the output argument, then the return status of the executed command will be written to this variable.
The last line from the result of the command. If you need to execute a command and have all the data from the command passed directly back without any interference, use the passthru() function.
To get the output of the executed command, be sure to set and use the output parameter.
Example#1 An exec() example
<?php
// outputs the username that owns the running php/httpd process
// (on a system with the "whoami" executable in the path)
echo exec('whoami');
?>
如果想允许用户输入的数据被传入本函数，则应使用 escapeshellarg() 或 escapeshellcmd() 函数来确保用户不能欺骗系统从而执行任意命令。
Note: 如果用本函数启动一个程序并希望保持在后台运行，必须确保该程序的输出被重定向到一个文件或者其它输出流去，否则 PHP 会在程序执行结束前挂起。
Note: 在打开了安全模式时，只能执行在 safe_mode_exec_dir 之内的程序。为实用起见目前不能在指向程序的路径中包含 .. 成分。
在打开了安全模式时，命令字符串会被 escapeshellcmd() 转义。从而 echo y | echo x 就成了 echo "y | echo x"。