(PHP 4, PHP 5)
system — Execute an external program and display the output
system() is just like the C version of the function in that it executes the given command and outputs the result.
The system() call also tries to automatically flush the web server's output buffer after each line of output if PHP is running as a server module.
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.
The command that will be executed.
If the return_var argument is present, then the return status of the executed command will be written to this variable.
Returns the last line of the command output on success, and FALSE on failure.
Example #1 system() example
<?php
echo '<pre>';
// Outputs all the result of shellcommand "ls", and returns
// the last output line into $last_line. Stores the return value
// of the shell command in $retval.
$last_line = system('ls', $retval);
// Printing additional info
echo '
</pre>
<hr />Last line of the output: ' . $last_line . '
<hr />Return value: ' . $retval;
?>
当用户提供的数据传入此函数，使用 escapeshellarg() 或 escapeshellcmd() 来确保用户欺骗系统从而执行任意命令。
Note:
如何程序使用此函数启动，为了能保持在后台运行，此程序必须将输出重定向到文件或其它输出流。 否则会导致 PHP 挂起，直至程序执行结束。
WarningNote: 安全模式 启用时，可仅可用 safe_mode_exec_dir 执行文件。实际上，现在不允许在到可执行的路径中存在 .. 组件。
安全模式 启用时，命令字符串会被 escapeshellcmd() 转换。因此， echo y | echo x 会变成 echo y | echo x。