How PHP operates extended attributes of files

In the files of the operating system, there is also a file attribute that we can define ourselves. These properties are not stored in the file content, nor are they directly visible. They can permanently associate a key-value pair information with a file. Generally, current Linux systems support such file extended attributes.

In the operating system, we can operate them through the commands setfattr, getfattr, and attr. Of course, PHP also provides us with an extension that can be used to operate on the extended attributes of files.

Add extended attributes

$file = __FILE__;

var_dump(xattr_set($file, 'Author', 'ZyBlog')); // bool(true)
var_dump(xattr_set($file, 'Num.', 121 )); // bool(true)
var_dump(xattr_set($file, 'Description', 'shuo ming', XATTR_ROOT)); // bool(true)

First we define the file to be operated on. Here we directly use the \_\_FILE\_\_ magic constant to operate the php file we are currently testing. Then use xattr_set() to set the extended attributes of the file. The extended attributes of files have the concept of namespace, and PHP also provides us with two forms: ordinary (user) namespace and XATTR_ROOT (root command space).

The attributes in the root namespace can be set by the super user and are not visible to other users, while the user namespace is defined based on the permissions of the file, that is, the user who can currently operate the file can read it. Get these extended attributes set in the user namespace of this file.

View extended attribute list

var_dump(xattr_list($file, XATTR_ROOT));
// array(1) {
//     [0]=>
//     string(11) "Description"
//   }

var_dump(xattr_list($file));
// array(2) {
//     [0]=>
//     string(4) "Num."
//     [1]=>
//     string(6) "Author"
//   }

xattr_list() function can get the keys of all namespaces defined in the file. It also distinguishes between user and root namespaces.

Get extended attribute content

var_dump(xattr_get($file, 'Author')); // string(6) "ZyBlog"
var_dump(xattr_get($file, 'Description')); // bool(false)
var_dump(xattr_get($file, 'Description', XATTR_ROOT)); // string(9) "shuo ming"

xattr_get() function is used to obtain the extended attribute content of the specified key. With the xattr_list() function above, you can obtain all extended attribute information of a file. If we do not add the XATTR_ROOT parameter, we cannot read the contents of the root namespace.

Delete extended attributes

var_dump(xattr_remove($file, 'Num.')); // bool(true)
var_dump(xattr_list($file));
// array(1) {
//     [0]=>
//     string(6) "Author"
//   }

xattr_remove() is used to delete the extended attributes of the file. We directly deleted the Num. attribute in the user namespace of the test file. Looking at its xattr_list() again only Author is left. Similarly, this function also supports a third parameter to specify whether it is a root namespace operation.

Verify whether the system supports extended attribute operations

var_dump(xattr_supported($file)); // bool(true)

Finally, there is a xattr_supported() function, which is used to verify whether the file system of the current operating system supports xattr-related operations.

Summary

Today’s content is very simple and clear. The function of the extended attribute of this file. To be honest, I saw this function extension in PHP before going back to check the relevant documents in the Linux system. Therefore, learning is all related. When we are learning PHP, we are also learning Linux. At the same time, we are often exposed to relevant knowledge of MySQL, Nginx and other applications. The best way to learn is to focus on one area while expanding your knowledge in other areas.

Test code:

https://github.com/zhangyue0503/dev-blog/blob/master/php/202010/source/9.在PHP中操作文件的扩展属性.php

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