Execute system external commands in PHP

PHP, as a server-side scripting language, is fully capable of tasks such as writing simple or complex dynamic web pages. But this is not always the case. Sometimes in order to implement a certain function, you must resort to external programs (or commands) of the operating system, so that you can get twice the result with half the effort.

So, is it possible to call external commands in PHP scripts? If so, how to do it? What are your concerns? I believe that after reading this article, you will definitely be able to answer these questions.

Is it possible?

The answer is yes. PHP, like other programming languages, can call external commands within the program, and it is very simple: just use one or a few functions.

Prerequisites

Since PHP is basically used for WEB program development, security has become an important aspect that people consider. So PHP designers added a door to PHP: safe mode. If running in safe mode, the PHP script will be subject to the following four restrictions:

Execution of external commands

Some restrictions when opening files

Connecting to MySQL database

HTTP-based authentication

In safe mode , only external programs in a specific directory can be executed, and calls to other programs will be denied. This directory can be specified using the safe_mode_exec_dir directive in the php.ini file, or by adding the --with-exec-dir option when compiling PHP. The default is /usr/local/php/bin.

If you call an external command that should be able to output results (meaning that the PHP script has no errors), but you get a blank, then it is likely that your network administrator has run PHP in safe mode.

How to do it?

Calling external commands in PHP can be implemented in the following three methods:

1) Use the special functions provided by PHP

PHP provides a total of 3 special functions for executing external commands: system (), exec ( ), passthru().

system()

Prototype: string system (string command [, int return_var])

system() function is similar to that in other languages. It executes the given command, outputs and returns the result. The second parameter is optional and is used to get the status code after the command is executed.

Example:

<?system（"/usr/local/bin/webalizer/webalizer"）；?>
exec（）

Prototype: string exec (string command [, string array [, int return_var]])

The exec() function is similar to system(), and also executes the given command, but does not output the results, but is the last row of the returned result. Although it only returns the last line of the command result, using the second parameter array can get the complete result by appending the results line by line to the end of the array. So if the array is not empty, it is best to use unset() to clear it before calling it. Only when the second parameter is specified, the third parameter can be used to obtain the status code of command execution.

Example:

<?
exec（"/bin/ls -l"）；
exec（"/bin/ls -l", $res）；
#$res是一个数据，每个元素代表结果的一行
exec（"/bin/ls -l", $res, $rc）；
#$rc的值是命令/bin/ls -l的状态码。成功的情况下通常是0
?>

passthru()

Prototype: void passthru (string command [, int return_var])

passthru() only calls the command and does not return any results, but outputs the running results of the command directly to on the standard output device. Therefore, the passthru() function is often used to call programs like pbmplus (a tool for processing images under Unix that outputs a binary stream of original images). It can also get the status code of command execution.

Example:

<?
header（"Content-type: image/gif"）；
passthru（"./ppmtogif hunte.ppm"）；
?>

2) Use the popen() function to open the process

The above method can only simply execute the command, but cannot interact with the command. But sometimes you must enter something into the command. For example, when adding a Linux system user, you need to call su to change the current user to root, and the su command must enter the root password on the command line. In this case, it is obviously not possible to use the method mentioned above. The

popen() function opens a process pipe to execute the given command, returning a file handle. Since a file handle is returned, you can read and write to it. In PHP3, this kind of handle can only be used in a single operation mode, either writing or reading; starting from PHP4, it is possible to read and write at the same time. Unless the handle was opened in one mode (read or write), the pclose() function must be called to close it.

Example 1:

<?
$fp=popen（"/bin/ls -l", "r"）；
?>

Example 2 (this example comes from the PHP China Alliance website http://www.phpx.com/show.php?d=col&i=51):

380a2b4892b38628a493ee3034a91807）和从文件读入（<）等。函数EscapeShellArg是用来处理命令的参数的。它在给定的字符串两边加上单引号，并把字符串中的单引号转义，这样这个字符串就可以安全地作为命令的参数。

再来看看超时问题。如果要执行的命令要花费很长的时间，那么应该把这个命令放到系统的后台去运行。但在默认情况下，象system（）等函数要等到这个命令运行完才返回（实际上是要等命令的输出结果），这肯定会引起PHP脚本的超时。解决的办法是把命令的输出重定向到另外一个文件或流中，如：

<?
system（"/usr/local/bin/order_proc > /tmp/null &"）；
?>