Update 20170622 A users guide to links in the Linux filesystem.md

sources/tech/20170622 A users guide to links in the Linux filesystem.md

@@ -23,11 +23,11 @@ linux 文件系统中有两种链接：硬链接和软链接。虽然二者差
 
 *   [linux 高级命令速查表][4]
 
-*   [Our latest Linux articles][5]
+*   [我们最新的 linux 文章][5]
 
-I have found, for instance, that some programs required a particular version of a library. When a library upgrade replaced the old version, the program would crash with an error specifying the name of the old, now-missing library. Usually, the only change in the library name was the version number. Acting on a hunch, I simply added a link to the new library but named the link after the old library name. I tried the program again and it worked perfectly. And, okay, the program was a game, and everyone knows the lengths that gamers will go to in order to keep their games running.
+举个例子，我曾发现一些程序要求特定的版本库方可运行。 当用升级后的库替代旧库后，程序会崩溃，提示就版本库缺失。 同城库中唯一变化是版本号。出于该直觉，我仅仅给程序添加了一个新的库链接，并以旧库名称命名。我试着再次启动程序，运行良好。 程序就是一个游戏，人人都明白，每个玩家都会尽力使游戏进行下去。
 
-In fact, almost all applications are linked to libraries using a generic name with only a major version number in the link name, while the link points to the actual library file that also has a minor version number. In other instances, required files have been moved from one directory to another to comply with the Linux file specification, and there are links in the old directories for backwards compatibility with those programs that have not yet caught up with the new locations. If you do a long listing of the **/lib64** directory, you can find many examples of both.
+事实上，几乎所有的应用程序链接库都使用通用的命名规则，链接名称中包含了住版本号，链接所指文件的文件名中同样包含了最小版本号。再比如，程序的一些必需文件为了迎合 linux 文件系统的规范从一个目录移动到另一个目录中，系统为了向后兼容那些不能获取这些文件新位置的程序在旧的目录中存放了这些文件的链接。如果你对 /lib64 目录做一个长清单列表，你会发现很多这样的例子。 
 
 ```
 lrwxrwxrwx.  1 root root       36 Dec  8  2016 cracklib_dict.hwm -> ../../usr/share/cracklib/pw_dict.hwm 
@@ -45,56 +45,56 @@ lrwxrwxrwx.  1 root root       30 Jan 16 16:39 libakonadi-calendar.so.4 -> libak
 lrwxrwxrwx.  1 root root       29 Jan 16 16:39 libakonadi-contact.so.4 -> libakonadi-contact.so.4.14.26 
 ```
 
-A few of the links in the **/lib64** directory
+**/lib64** 目录下的一些链接
 
-The long listing of the **/lib64** directory above shows that the first character in the filemode is the letter "l," which means that each is a soft or symbolic link.
+T在上面展示的 **/lib64** 目录清单列表中，文件模式第一个字母 I 表示这是一个符号链接或软链接。
 
-### Hard links
+### 硬链接
 
-In [An introduction to Linux's EXT4 filesystem][15], I discussed the fact that each file has one inode that contains information about that file, including the location of the data belonging to that file. [Figure 2][16] in that article shows a single directory entry that points to the inode. Every file must have at least one directory entry that points to the inode that describes the file. The directory entry is a hard link, thus every file has at least one hard link.
+在 [An introduction to Linux's EXT4 filesystem][15]一文中，我曾探讨过这样一个事实，每个文件都有一个包含该文件信息的节点，包含了该文件的位置信息。上述文章中的[图2][16]展示了一个指向文件节点的单一目录项。每个文件都至少有一个目录项指向描述该文件信息的文件节点，目录项是一个硬链接，因此每个文件至少都有一个硬链接。
 
-In Figure 1 below, multiple directory entries point to a single inode. These are all hard links. I have abbreviated the locations of three of the directory entries using the tilde (**~**) convention for the home directory, so that **~** is equivalent to **/home/user** in this example. Note that the fourth directory entry is in a completely different directory, **/home/shared**, which might be a location for sharing files between users of the computer.
+如下图1所示，多个目录项指向了同一文件节点。这些目录项都是硬链接。我曾使用波浪线 (**~**) 表示三级目录项的缩写，这是用户目录的惯例表示，因此在该例中波浪线等同于 **/home/user** 。值得注意的是，四级目录项是一个完全不同的目录，**/home/shared** 可能是该计算机上用户的共享文件目录。
 
 ![fig1directory_entries.png](https://opensource.com/sites/default/files/images/life/fig1directory_entries.png)
 Figure 1
 
-Hard links are limited to files contained within a single filesystem. "Filesystem" is used here in the sense of a partition or logical volume (LV) that is mounted on a specified mount point, in this case **/home**. This is because inode numbers are unique only within each filesystem, and a different filesystem, for example, **/var**or **/opt**, will have inodes with the same number as the inode for our file.
+单一文件系统中的文件硬链接数是有限制的。”文件系统“ 是就挂载在特定挂载点上的分区或逻辑卷而言的，此例中是 /home。这是因为文件系统中的节点号都是唯一的。在不同的文件系统中，如 **/var** 或 **/opt**，会有和 **/home** 中相同的节点号。
 
-Because all the hard links point to the single inode that contains the metadata about the file, all of these attributes are part of the file, such as ownerships, permissions, and the total number of hard links to the inode, and cannot be different for each hard link. It is one file with one set of attributes. The only attribute that can be different is the file name, which is not contained in the inode. Hard links to a single **file/inode** located in the same directory must have different names, due to the fact that there can be no duplicate file names within a single directory.
+因为所有的硬链接都指向了包含文件元信息的节点，这些特性都是文件的一部分，像所属关系，权限，节点硬链接数目，这些特性不能区分不同的硬链接。这是一个文件所具有的一组属性。唯一能区分这些文件的是包含在节点信息中的文件名。对单靠 **file/inode** 来定位文件的同一目录中的硬链接必须拥有不同的文件名，基于上述事实，同一目录下不能存在重复的文件名。
 
-The number of hard links for a file is displayed with the **ls -l** command. If you want to display the actual inode numbers, the command **ls -li** does that.
+文件的硬链接数目可通过 **ls -l** 来查看，如果你想查看实际节点号，可使用 **ls -li** 命令。
 
-### Symbolic (soft) links
+### 符号（软）链接
 
-The difference between a hard link and a soft link, also known as a symbolic link (or symlink), is that, while hard links point directly to the inode belonging to the file, soft links point to a directory entry, i.e., one of the hard links. Because soft links point to a hard link for the file and not the inode, they are not dependent upon the inode number and can work across filesystems, spanning partitions and LVs.
+软链接（符号链接）和硬链接的区别在于，硬链接直接指向文件中的节点而软链接直接指向一个目录项，即一个硬链接。因为软链接指向一个文件的硬链接而非该文件的节点信息，所以它们并不依赖于文件节点，这使得它们能在不同的文件系统中起作用，跨越不同的分区和逻辑卷。
 
-The downside to this is: If the hard link to which the symlink points is deleted or renamed, the symlink is broken. The symlink is still there, but it points to a hard link that no longer exists. Fortunately, the **ls** command highlights broken links with flashing white text on a red background in a long listing.
+软链接的缺点是，一旦它所指向的硬链接被删除或重命名后，该软链接就失效了。软链接虽然还在，但所指向的硬链接已不存在。所幸的是，**ls** 命令能以红底白字的方式在其列表中高亮显示失效的软链接。
 
-### Lab project: experimenting with links
+### 实验项目: 链接实验
 
-I think the easiest way to understand the use of and differences between hard and soft links is with a lab project that you can do. This project should be done in an empty directory as a  _non-root user_ . I created the **~/temp** directory for this project, and you should, too. It creates a safe place to do the project and provides a new, empty directory to work in so that only files associated with this project will be located there.
+我认为最容易理解链接用法及其差异的方法即使动手搭建一个项目。这个项目应以非超级用户的身份在一个空目录下进行。我创建了 **~/tmp** 目录做这个实验，你也可以这么做。这么做可为项目创建一个安全的环境且提供一个新的空目录让程序运作，如此以来这儿仅存放和程序有关的文件。
 
-### **Initial setup**
+### **初始工作**
 
-First, create the temporary directory in which you will perform the tasks needed for this project. Ensure that the present working directory (PWD) is your home directory, then enter the following command.
+首先，在你要进行实验的目录下为该项目中的任务创建一个临时目录，确保当前工作目录（PWD）是你的主目录，然后键入下列命令。
 
 ```
 mkdir temp
 ```
 
-Change into **~/temp** to make it the PWD with this command.
+使用这个命名将当前工作目录切换到 *~/temp** 
 
 ```
 cd temp
 ```
 
-To get started, we need to create a file we can link to. The following command does that and provides some content as well.
+实验开始，我们创建一个能够链接的文件，下列命令可完成该工作并向其填充内容。
 
 ```
 du -h > main.file.txt
 ```
 
-Use the **ls -l** long list to verify that the file was created correctly. It should look similar to my results. Note that the file size is only 7 bytes, but yours may vary by a byte or two.
+使用 *ls -l** 长列表命名确认文件被正确地创建。运行结果应类似于我的。注意文件大小只有 7 字节，但你的可能会有 1～2 字节的变动。
 
 ```
 [dboth@david temp]$ ls -l 
@@ -102,11 +102,11 @@ total 4
 -rw-rw-r-- 1 dboth dboth 7 Jun 13 07:34 main.file.txt
 ```
 
-Notice the number "1" following the file mode in the listing. That number represents the number of hard links that exist for the file. For now, it should be 1 because we have not created any additional links to our test file.
+在列表中，文件模式串后的数字 1 代表存在于该文件上的硬链接数。现在应该是 1 ，因为我们还没有为这个测试文件建立任何硬链接。
 
-### **Experimenting with hard links**
+### **对硬链接进行实验**
 
-Hard links create a new directory entry pointing to the same inode, so when hard links are added to a file, you will see the number of links increase. Ensure that the PWD is still **~/temp**. Create a hard link to the file **main.file.txt**, then do another long list of the directory.
+硬链接创建一个指向同一文件节点的目录项，当为文件添加一个硬链接时，你会看到链接数目的增加。确保当前工作目录仍为 **~/temp**。创建一个指向 **main.file.txt** 的硬链接，然后查看该目录下文件列表。
 
 ```
 [dboth@david temp]$ ln main.file.txt link1.file.txt 
@@ -116,7 +116,7 @@ total 8
 -rw-rw-r-- 2 dboth dboth 7 Jun 13 07:34 main.file.txt
 ```
 
-Notice that both files have two links and are exactly the same size. The date stamp is also the same. This is really one file with one inode and two links, i.e., directory entries to it. Create a second hard link to this file and list the directory contents. You can create the link to either of the existing ones: **link1.file.txt** or **main.file.txt**.
+目录中两个文件都有两个链接且大小相同，时间戳也一样。这是同一文件节点的两个不同的硬链接，即该文件的目录项。再建立一个该文件的硬链接，并列出目录清单内容，你可以建立 **link1.file.txt** 或 **main.file.txt** 的硬链接。
 
 ```
 [dboth@david temp]$ ln link1.file.txt link2.file.txt ; ls -l
@@ -126,22 +126,21 @@ total 16
 -rw-rw-r-- 3 dboth dboth 7 Jun 13 07:34 main.file.txt
 ```
 
-Notice that each new hard link in this directory must have a different name because two files—really directory entries—cannot have the same name within the same directory. Try to create another link with a target name the same as one of the existing ones.
+注意，该目录下的每个硬链接必须使用不同的名称，因为同一目录下的两个文件不能拥有相同的文件名。试着创建一个和现存链接名称相同的硬链接。
 
 ```
 [dboth@david temp]$ ln main.file.txt link2.file.txt 
 ln: failed to create hard link 'link2.file.txt': File exists
 ```
 
-Clearly that does not work, because **link2.file.txt** already exists. So far, we have created only hard links in the same directory. So, create a link in your home directory, the parent of the temp directory in which we have been working so far.
+显然不行，因为 **link2.file.txt** 已经存在。目前为止我们只在同一目录下创建硬链接，接着在临时目录的父目录，你的主目录中创建一个链接。
 
 ```
 [dboth@david temp]$ ln main.file.txt ../main.file.txt ; ls -l ../main*
 -rw-rw-r--    4 dboth dboth     7 Jun 13 07:34 main.file.txt
 ```
 
-The **ls** command in the above listing shows that the **main.file.txt** file does exist in the home directory with the same name as the file in the temp directory. Of course, these are not different files; they are the same file with multiple links—directory entries—to the same inode. To help illustrate the next point, add a file that is not a link.
-
+ 上面的 **ls** 命令显示 **main.file.txt** 文件确实存在于主目录中，且与该文件在 temp 目录中的名称一致。当然它们是没有区别的两个文件，它们是同一文件的两个链接，指向了同一文件的目录项。为了帮助说明下一点，在 temp 目录中添加一个非链接文件。
 ```
 [dboth@david temp]$ touch unlinked.file ; ls -l
 total 12
@@ -151,7 +150,7 @@ total 12
 -rw-rw-r-- 1 dboth dboth 0 Jun 14 08:18 unlinked.file
 ```
 
-Look at the inode number of the hard links and that of the new file using the **-i**option to the **ls** command.
+使用 **ls** 命令的 **i** 选项查看文件节点的硬链接号和新创建文件的硬链接号。
 
 ```
 [dboth@david temp]$ ls -li
@@ -162,9 +161,9 @@ total 12
 657863 -rw-rw-r-- 1 dboth dboth 0 Jun 14 08:18 unlinked.file
 ```
 
-Notice the number **657024** to the left of the file mode in the example above. That is the inode number, and all three file links point to the same inode. You can use the **-i** option to view the inode number for the link we created in the home directory as well, and that will also show the same value. The inode number of the file that has only one link is different from the others. Note that the inode numbers will be different on your system.
+注意上面文件模式左边的数字 **657024** ，这是三个硬链接文件所指的同一文件的节点号，你也可以使用 **i** 选项查看主目录中所创建的链接节点号，和该值相同。只有一个链接的文件节点号和其他的不同，在你的系统上看到的不同于本文中的。
 
-Let's change the size of one of the hard-linked files.
+接着改变其中一个硬链接文件的大小。
 
 ```
 [dboth@david temp]$ df -h > link2.file.txt ; ls -li
@@ -175,11 +174,11 @@ total 12
 657863 -rw-rw-r-- 1 dboth dboth    0 Jun 14 08:18 unlinked.file
 ```
 
-The file size of all the hard-linked files is now larger than before. That is because there is really only one file that is linked to by multiple directory entries.
+现在的硬链接文件大小比原来大，因为多个目录项链接着同一文件。
 
-I know this next experiment will work on my computer because my **/tmp**directory is on a separate LV. If you have a separate LV or a filesystem on a different partition (if you're not using LVs), determine whether or not you have access to that LV or partition. If you don't, you can try to insert a USB memory stick and mount it. If one of those options works for you, you can do this experiment.
+我知道下个实验在我的电脑上会成功，因为我的 **/tmp** 目录是一个独立的逻辑卷，如果你有单独的逻辑卷或文件系统在不同的分区上（如果未使用逻辑卷），确定你是否能访问那个分区或逻辑卷，如果不能，你可以在电脑上挂载一个 U盘，如果上述选项适合你，你可以进行这个实验。
 
-Try to create a link to one of the files in your **~/temp** directory in **/tmp** (or wherever your different filesystem directory is located).
+试着在 **/tmp** 目录中建立一个 **~/temp** 目录下文件的链接（或你的文件系统所在的位置）
 
 ```
 [dboth@david temp]$ ln link2.file.txt /tmp/link3.file.txt
@@ -187,7 +186,7 @@ ln: failed to create hard link '/tmp/link3.file.txt' => 'link2.file.txt':
 Invalid cross-device link
 ```
 
-Why does this error occur? The reason is each separate mountable filesystem has its own set of inode numbers. Simply referring to a file by an inode number across the entire Linux directory structure can result in confusion because the same inode number can exist in each mounted filesystem.
+为什么会出现这个错误呢？ 原因是每一个单独的挂载文件系统都有一套自己的节点号。 Simply referring to a file by an inode number across the entire Linux directory structure can result in confusion because the same inode number can exist in each mounted filesystem.
 
 There may be a time when you will want to locate all the hard links that belong to a single inode. You can find the inode number using the **ls -li** command. Then you can use the **find** command to locate all links with that inode number.