在 Linux 文件系统中使用 attr 添加扩展属性
attr
![](/data/attachment/album/202309/15/081240syst7uisttadnky6.jpg)



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> 我使用开源的 XFS 文件系统是为了其扩展属性带来的小小便利。扩展属性是一种为我的数据添加上下文的独特方式。
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“文件系统” 是一个描述你的计算机怎样跟踪你创建的所有文件的完美词语。你的计算机存储有大量的数据，无论是文档、配置文件还是数以千计的照片。这需要一种对人和机器都友好的方式。诸如 Ext4、XFS、JFS、BtrFS 的文件系统是你的计算机用来跟踪文件的“语言”。


你的桌面或终端可以帮助你快速查找文件。例如，你的文件管理器可能有过滤功能，以便让你可以快速查看家目录内的图片，它也可能带有搜索功能以让你使用文件名定位文件。这些特性被称作*文件属性*，正如其名，它们是由文件头或者文件系统代码定义的文件对象的属性。大多数的文件系统记录了标准的文件属性，例如文件名、文件大小、文件类型、创建时间、上次访问时间等。


我在我的计算机上使用开源的 XFS 文件系统，不是为了其稳定性和高性能，而是为了其扩展属性带来的小小便利。


### 常见的文件属性


当你保存一个文件，文件相关的数据也会随同保存。常见的属性会告诉操作系统是否更新访问时间，什么时间同步数据到硬盘等逻辑细节。哪些属性被保存取决于底层文件系统的功能和特性。


在标准文件属性（如果有标准属性的话）之外，XFS、Ext4 和 BtrFS 文件系统都可以使用扩展属性。


### 扩展属性


XFS、Ext4 和 BtrFs 允许你创建自己的任意文件属性。由于是自己创建的，操作系统不会有内建支持，不过我以“标签”的方式使用它们，就像图片的 EXIF 数据一样。开发者们也许会选择使用扩展属性去开发应用内的自定义的功能。


XFS 中的属性有两个“命名空间”：用户（`user`）和根（`root`）。当创建属性时，你必须将其加入其中一个命名空间。要将属性添加到根命名空间，必须使用 `sudo` 命令或以 `root` 身份登录。


### 增加一个属性


你可以使用 `attr` 和 `setfattr` 命令在 XFS 文件系统中为文件增加属性。


`attr` 命令默认在 `user` 命名空间下进行，所以你只需要为属性设置一个名字（`-s`）和值（`-V`）。



```
$ attr -s flavor -V vanilla example.txt
Attribute "flavor" set to a 7 byte value for example.txt:
vanilla

```

`setfattr` 命令需要你指定目标命名空间：



```
$ setfattr --name user.flavor --value chocolate example.txt

```

### 列出扩展文件属性


使用 `attr` 或者 `getfattr` 命令可以看已添加到文件上的扩展属性。`attr` 命令默认使用 `user` 空间，使用 `-g` 以获取扩展属性：



```
$ attr -g flavor example.txt
Attribute "flavor" had a 9 byte value for example.txt:
chocolate

```

`getfattr` 命令需要属性的命名空间和名字：



```
$ getfattr --name user.flavor example.txt 
# file: example.txt
user.flavor="chocolate"

```

### 列出所有扩展属性


要看一个文件的所有扩展属性，你可以使用 `attr -l`：



```
$ attr -l example.txt
Attribute "md5sum" has a 32 byte value for example.txt
Attribute "flavor" has a 9 byte value for example.txt

```

或者，你也可以使用 `getfattr -d`：



```
$ getfattr -d example.txt
# file: example.txt
user.flavor="chocolate"
user.md5sum="969181e76237567018e14fe1448dfd11"

```

扩展文件属性可以使用 `attr` 和 `setfattr` 更新，就像你创建这些属性一样：



```
$ setfattr --name user.flavor --value strawberry example.txt

$ getfattr -d example.txt
# file: example.txt
user.flavor="strawberry"
user.md5sum="969181e76237567018e14fe1448dfd11"

```

### 其他文件系统上的属性


使用扩展属性最大的风险是忘记这些属性是特定于某个文件系统的。这意味着当你从一个磁盘或分区复制文件到另外一个磁盘或分区时，这些属性都会丢失，**即使**目标位置的文件系统也支持扩展属性。


为了避免丢失扩展属性，你需要使用支持保留这些属性的工具，例如 `rsync` 命令。



```
$ rsync --archive --xattrs ~/example.txt /tmp/

```

但无论你使用什么工具，如果你传输文件到一个不知道如何处理扩展属性的文件系统，这些属性都会被丢弃。


### 搜索属性


与扩展属性交互的机制并不多，所以使用这些添加的文件属性的方法也有限。我使用扩展属性作为标签机制，这让我可以将没有明显关系的文件联系起来。例如，假设我需要在一个项目中用“知识共享”的图形。假如我预见性地将 `license` 属性添加到了我的图形库中，我就可以使用 `find` 和 `getfattr` 在图形文件夹中寻找：



```
find ~/Graphics/ -type f \
    -exec getfattr \
    --name user.license \
    -m cc-by-sa {} \; 2>/dev/null

# file: /home/tux/Graphics/Linux/kde-eco-award.png
user.license="cc-by-sa"
user.md5sum="969181e76237567018e14fe1448dfd11"

```

### 文件系统的秘密


文件系统一般不会引起你的注意。它们实际是定义文件的系统。这不是计算机做的最让人兴奋的任务，也不是用户应该关心的东西。但是有些文件系统可以给你有趣、安全的特殊能力，扩展文件属性就是一个好例子。它的用途可能有限，但是扩展属性是你为数据增加上下文的独特方法。


*（题图：MJ/06c0c478-7af7-49e4-836b-a9923db9dc0c）*




---


via: <https://opensource.com/article/22/6/linux-attr-command>


作者：[Seth Kenlon](https://opensource.com/users/seth) 选题：[lkxed](https://github.com/lkxed) 译者：[wznmickey](https://github.com/wznmickey) 校对：[wxy](https://github.com/wxy)


本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出


OK
The term *filesystem* is a fancy word to describe how your computer keeps track of all the files you create. Whether it's an office document, a configuration file, or thousands of digital photos, your computer has to store a lot of data in a way that's useful for both you and it. Filesystems like Ext4, XFS, JFS, BtrFS, and so on are the "languages" your computer uses to keep track of data.

Your desktop or terminal can do a lot to help you find your data quickly. Your file manager might have, for instance, a filter function so you can quickly see just the image files in your home directory, or it might have a search function that can locate a file by its filename, and so on. These qualities are known as *file attributes* because they are exactly that: Attributes of the data object, defined by code in file headers and within the filesystem itself. Most filesystems record standard file attributes such as filename, file size, file type, time stamps for when it was created, and time stamps for when it was last visited.

I use the open source XFS filesystem on my computers not for its reliability and high performance but for the subtle convenience of extended attributes.

## Common file attributes

When you save a file, data about it are saved along with it. Common attributes tell your operating system whether to update the access time, when to synchronize the data in the file back to disk, and other logistical details. Which attributes get saved depends on the capabilities and features of the underlying filesystem.

In addition to standard file attributes (insofar as there are standard attributes), the XFS, Ext4, and BtrFS filesystems can all use extending filesystems.

## Extended attributes

XFS, Ext4, and BtrFS allow you to create your own arbitrary file attributes. Because you're making up attributes, there's nothing built into your operating system to utilize them, but I use them as "tags" for files in much the same way I use EXIF data on photos. Developers might choose to use extended attributes to develop custom capabilities in applications.

There are two "namespaces" for attributes in XFS: **user** and **root**. When creating an attribute, you must add your attribute to one of these namespaces. To add an attribute to the **root** namespace, you must use the `sudo`

command or be logged in as root.

## Add an attribute

You can add an attribute to a file on an XFS filesystem with the `attr`

or `setfattr`

commands.

The `attr`

command assumes the `user`

namespace, so you only have to set (`-s`

) a name for your attribute followed by a value (`-V`

):

```
``````
$ attr -s flavor -V vanilla example.txt
Attribute "flavor" set to a 7 byte value for example.txt:
vanilla
```


The `setfattr`

command requires that you specify the target namespace:

```
````$ setfattr --name user.flavor --value chocolate example.txt`


## List extended file attributes

Use the `attr`

or `getfattr`

commands to see extended attributes you've added to a file. The `attr`

command defaults to the **user** namespace and uses the `-g`

option to *get* extended attributes:

```
``````
$ attr -g flavor example.txt
Attribute "flavor" had a 9 byte value for example.txt:
chocolate
```


The `getfattr`

command requires the namespace and name of the attribute:

```
``````
$ getfattr --name user.flavor example.txt
# file: example.txt
user.flavor="chocolate"
```


## List all extended attributes

To see all extended attributes on a file, you can use `attr -l`

:

```
``````
$ attr -l example.txt
Attribute "md5sum" has a 32 byte value for example.txt
Attribute "flavor" has a 9 byte value for example.txt
```


Alternately, you can use `getfattr -d`

:

```
``````
$ getfattr -d example.txt
# file: example.txt
user.flavor="chocolate"
user.md5sum="969181e76237567018e14fe1448dfd11"
```


Any extended file attribute can be updated with `attr`

or `setfattr`

, just as if you were creating the attribute:

```
``````
$ setfattr --name user.flavor --value strawberry example.txt
$ getfattr -d example.txt
# file: example.txt
user.flavor="strawberry"
user.md5sum="969181e76237567018e14fe1448dfd11"
```


## Attributes on other filesystems

The greatest risk when using extended attributes is forgetting that these attributes are specific to the filesystem they're on. That means when you copy a file from one drive or partition to another, the attributes are lost *even if the target filesystem supports extended attributes*.

To avoid losing extended attributes, you must use a tool that supports retaining them, such as the `rsync`

command.

```
````$ rsync --archive --xattrs ~/example.txt /tmp/`


No matter what tool you use, if you transfer a file to a filesystem that doesn't know what to do with extended attributes, those attributes are dropped.


## Search for attributes

There aren't many mechanisms to interact with extended attributes, so the options for using the file attributes you've added are limited. I use extended attributes as a tagging mechanism, which allows me to associate files that have no obvious relation to one another. For instance, suppose I need a Creative Commons graphic for a project I'm working on. Assume I've had the foresight to add the extended attribute **license** to my collection of graphics. I could search my graphic folder with `find`

and `getfattr`

together:

```
``````
find ~/Graphics/ -type f \
-exec getfattr \
--name user.license \
-m cc-by-sa {} \; 2>/dev/null
# file: /home/tux/Graphics/Linux/kde-eco-award.png
user.license="cc-by-sa"
user.md5sum="969181e76237567018e14fe1448dfd11"
```


## Secrets of your filesystem

Filesystems aren't generally something you're meant to notice. They're literally systems for defining a file. It's not the most exciting task a computer performs, and it's not something users are supposed to have to be concerned with. But some filesystems give you some fun, and safe, special abilities, and extended file attributes are a good example. Its use may be limited, but extended attributes are a unique way to add context to your data.

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