第五章：操作文件和目录

At this point, we are ready for some real work! This chapter will introduce the following commands:

此时此刻，我们已经准备好了做些真正的工作！这一章节将会介绍以下命令：

These five commands are among the most frequently used Linux commands. They are used for manipulating both files and directories.

这五个命令属于最常使用的 Linux 命令之列。它们用来操作文件和目录。

Now, to be frank, some of the tasks performed by these commands are more easily done with a graphical file manager. With a file manager, we can drag and drop a file from one directory to another, cut and paste files, delete files, etc. So why use these old command line programs?

现在，坦诚地说，用图形文件管理器来完成一些由这些命令执行的任务会更容易些。使用文件管理器， 我们可以把文件从一个目录拖放到另一个目录、剪贴和粘贴文件、删除文件等等。那么， 为什么还使用早期的命令行程序呢？

The answer is power and flexibility. While it is easy to perform simple file manipulations with a graphical file manager, complicated tasks can be easier with the command line programs. For example, how could we copy all the HTML files from one directory to another, but only copy files that do not exist in the destination directory or are newer than the versions in the destination directory? Pretty hard with a file manager. Pretty easy with the command line:

答案是命令行程序，功能强大灵活。虽然图形文件管理器能轻松地实现简单的文件操作，但是对于 复杂的文件操作任务，则使用命令行程序比较容易完成。例如，怎样拷贝一个目录下所有的HTML文件 ——这些文件在目标目录不存在或者版本比目标目录的文件更新——到目标目录呢？ 要完成这个任务，使用文件管理器相当难，使用命令行相当容易：

cp -u *.html destination

Before we begin using our commands, we need to talk about a shell feature that makes these commands so powerful. Since the shell uses filenames so much, it provides special characters to help you rapidly specify groups of filenames. These special characters are called wildcards. Using wildcards (which is also known as globbing) allow you to select filenames based on patterns of characters. The table below lists the wildcards and what they select:

在开始使用命令之前，我们需要介绍一个使命令行如此强大的 shell 特性。因为 shell 频繁地使用 文件名，shell 提供了特殊字符来帮助你快速指定一组文件名。这些特殊字符叫做通配符。 使用通配符（也以文件名代换著称）允许你依据字符的组合模式来选择文件名。下表列出这些通配符 以及它们所选择的对象：

Table 5-1: Wildcards

表5-1: 通配符

Table 5-2 lists the most commonly used character classes:

表5-2列出了最常使用的字符类：

Table 5-2: Commonly Used Character Classes

表5-2: 普遍使用的字符类

Using wildcards makes it possible to construct very sophisticated selection criteria for filenames. Here are some examples of patterns and what they match:

借助通配符，为文件名构建非常复杂的选择标准成为可能。下面是一些类型匹配的范例:

Table 5-3: Wildcard Examples

表5-3: 通配符范例

Wildcards can be used with any command that accepts filenames as arguments, but we’ll talk more about that in Chapter 8.

接受文件名作为参数的任何命令，都可以使用通配符，我们会在第八章更深入地谈到这个知识点。

The mkdir command is used to create directories. It works like this:

mkdir 命令是用来创建目录的。它这样工作：

mkdir directory...

A note on notation: When three periods follow an argument in the description of a command (as above), it means that the argument can be repeated, thus:

注意表示法: 在描述一个命令时（如上所示），当有三个圆点跟在一个命令的参数后面， 这意味着那个参数可以重复，就像这样：

mkdir dir1

would create a single directory named “dir1”, while

会创建一个名为”dir1”的目录，而

mkdir dir1 dir2 dir3

would create three directokries named “dir1”, “dir2”, “dir3”.

会创建三个目录，名为 dir1, dir2, dir3。

The cp command copies files or directories. It can be used two different ways:

cp 命令，复制文件或者目录。它有两种使用方法：

cp item1 item2

to copy the single file or directory “item1” to file or directory “item2” and:

复制单个文件或目录”item1”到文件或目录”item2”，和：

cp item... directory

to copy multiple items (either files or directories) into a directory.

复制多个项目（文件或目录）到一个目录下。

Here are some of the commonly used options (the short option and the equivalent long option) for cp:

这里列举了 cp 命令一些有用的选项（短选项和等效的长选项）：

Table 5-4: cp Options

表5-4: cp 选项

Table 5-5: cp Examples

表5-5: cp 实例

The mv command performs both file moving and file renaming, depending on how it is used. In either case, the original filename no longer exists after the operation. mv is used in much the same way as cp:

mv 命令可以执行文件移动和文件命名任务，这依赖于你怎样使用它。任何一种 情况下，完成操作之后，原来的文件名不再存在。mv 使用方法与 cp 很相像：

mv item1 item2

to move or rename file or directory “item1” to “item2” or:

把文件或目录 “item1” 移动或重命名为 “item2”, 或者：

mv item... directory

to move one or more items from one directory to another.

把一个或多个条目从一个目录移动到另一个目录中。

mv shares many of the same options as cp:

mv 与 cp 共享了很多一样的选项：

Table 5-6: mv options

表5-6: mv 选项

Table 5-7: mv Examples

表5-7: mv 实例

The rm command is used to remove(delete)files and directories:

rm 命令用来移除（删除）文件和目录：

rm item...

where “item” is one or more files or directories.

“item”代表一个或多个文件或目录。

Here are some of the common options for rm:

下表是一些普遍使用的 rm 选项：

Table 5-8: rm Options

表5-8: rm 选项

Table 5-9: rm Examples

表5-9: rm 实例

The ln command is used to create either hard or symbolic links. It is used in one of two ways:

ln 命令既可创建硬链接，也可以创建符号链接。可以用其中一种方法来使用它：

ln file link

to create a hard link, and:

创建硬链接，和：

ln -s item link

to create a symbolic link where “item” is either a file or a directory.

创建符号链接，”item” 可以是一个文件或是一个目录。

Hard links are the original Unix way of creating links; symbolic links are more modern. By default, every file has a single hard link that gives the file its name. When we create a hard link, we create an additional directory entry for a file. Hard links have two important limitations:

与更加现代的符号链接相比，硬链接是最初 Unix 创建链接的方式。每个文件默认会有一个硬链接， 这个硬链接给予文件名字。我们每创建一个硬链接，就为一个文件创建了一个额外的目录项。 硬链接有两个重要局限性：

A hard link is indistinguishable from the file itself. Unlike a symbolic link, when you list a directory containing a hard link you will see no special indication of the link. When a hard link is deleted, the link is removed but the contents of the file itself continue to exist (that is, its space is not deallocated) until all links to the file are deleted. It is important to be aware of hard links because you might encounter them from time to time, but modern practice prefers symbolic links, which we will cover next.

一个硬链接和文件本身没有什么区别。不像符号链接，当你列出一个包含硬链接的目录 内容时，你会看到没有特殊的链接指示说明。当一个硬链接被删除时，这个链接 被删除，但是文件本身的内容仍然存在（这是说，它所占的磁盘空间不会被重新分配）， 直到所有关联这个文件的链接都删除掉。知道硬链接很重要，因为你可能有时 会遇到它们，但现在实际中更喜欢使用符号链接，下一步我们会讨论符号链接。

Symbolic links were created to overcome the limitations of hard links. Symbolic links work by creating a special type of file that contains a text pointer to the referenced file or directory. In this regard, they operate in much the same way as a Windows shortcut though of course, they predate the Windows feature by many years ;-)

创建符号链接是为了克服硬链接的局限性。符号链接生效，是通过创建一个 特殊类型的文件，这个文件包含一个关联文件或目录的文本指针。在这一方面， 它们和 Windows 的快捷方式差不多，当然，符号链接早于 Windows 的快捷方式 很多年;-)

A file pointed to by a symbolic link, and the symbolic link itself are largely indistinguishable from one another. For example, if you write some something to the symbolic link, the referenced file is also written to. However when you delete a symbolic link, only the link is deleted, not the file itself. If the file is deleted before the symbolic link, the link will continue to exist, but will point to nothing. In this case, the link is said to be broken. In many implementations, the ls command will display broken links in a distinguishing color, such as red, to reveal their presence.

一个符号链接指向一个文件，而且这个符号链接本身与其它的符号链接几乎没有区别。 例如，如果你往一个符号链接里面写入东西，那么相关联的文件也被写入。然而， 当你删除一个符号链接时，只有这个链接被删除，而不是文件自身。如果先于符号链接 删除文件，这个链接仍然存在，但是不指向任何东西。在这种情况下，这个链接被称为 坏链接。在许多实现中，ls 命令会以不同的颜色展示坏链接，比如说红色，来显示它们 的存在。

The concept of links can seem very confusing, but hang in there. We’re going to try all this stuff and it will, hopefully, become clear.

关于链接的概念，看起来很迷惑，但不要胆怯。我们将要努力地练习 这些命令而且尽可能得使它变得清晰。

Since we are going to do some real file manipulation, let’s build a safe place to “play” with our file manipulation commands. First we need a directory to work in. We’ll create one in our home directory and call it “playground.”

下面我们将要做些真正的文件操作，让我们先建立一个安全地带， 来玩一下文件操作命令。首先，我们需要一个工作目录。在我们的 家目录下创建一个叫做“playground”的目录。

The mkdir command is used to create a directory. To create our playground directory we will first make sure we are in our home directory and will then create the new directory:

mkdir 命令被用来创建目录。首先确定我们在我们的家目录下，然后创建 playground 目录：

[me@linuxbox ~]$ cd
[me@linuxbox ~]$ mkdir playground

To make our playground a little more interesting, let’s create a couple of directories inside it called “dir1” and “dir2”. To do this, we will change our current working directory to playground and execute another mkdir:

为了让我们的游戏场更加有趣，在 playground 目录下创建一对目录 ，分别叫做 “dir1” 和 “dir2”。更改我们的当前工作目录到 playground，然后 执行 mkdir 命令：

[me@linuxbox ~]$ cd playground
[me@linuxbox playground]$ mkdir dir1 dir2

Notice that the mkdir command will accept multiple arguments allowing us to create both directories with a single command.

注意到 mkdir 命令可以接受多个参数，它允许我们用一个命令来创建这两个目录。

###　复制文件

Next, let’s get some data into our playground. We’ll do this by copying a file. Using the cp command, we’ll copy the passwd file from the /etc directory to the current working directory:

下一步，让我们输入一些数据到我们的游戏场中。我们可以通过复制一个文件来实现目的。 我们使用 cp 命令从 /etc 目录复制 passwd 文件到当前工作目录下：

[me@linuxbox playground]$ cp /etc/passwd .

Notice how we used the shorthand for the current working directory, the single trailing period. So now if we perform an ls, we will see our file:

请注意，我们使用命令末尾的一个圆点来简化当前工作目录的写法。如果我们执行 ls 命令， 可以看到我们的文件：

[me@linuxbox playground]$ ls -l
total 12
drwxrwxr-x 2  me  me   4096 2008-01-10 16:40 dir1
drwxrwxr-x 2  me  me   4096 2008-01-10 16:40 dir2
-rw-r--r-- 1  me  me   1650 2008-01-10 16:07 passwd

Now, just for fun, let’s repeat the copy using the “-v” option (verbose) to see what it does:

现在，仅仅是为了高兴，重复操作复制命令，使用”-v”选项（详细），看看它做了些什么：

[me@linuxbox playground]$ cp -v /etc/passwd .
`/etc/passwd' -> `./passwd'

The cp command performed the copy again, but this time displayed a concise message indicating what operation it was performing. Notice that cp overwrote the first copy without any warning. Again this is a case of cp assuming that you know what you’re are doing. To get a warning, we’ll include the “-i” (interactive) option:

cp 命令再一次执行了复制操作，但是这次显示了一条简洁的信息，指明它 进行了什么操作。注意，cp 没有警告，就重写了第一次复制的文件。这是一个案例， cp 会假设你知道自己在做什么。如果希望得到警告的话，需要加入“-i”（互动）选项：

[me@linuxbox playground]$ cp -i /etc/passwd .
cp: overwrite `./passwd'?

Responding to the prompt by entering a “y” will cause the file to be overwritten, any other character (for example, “n”) will cause cp to leave the file alone.

在提示信息后输入”y”，文件就会被覆盖，输入其它的字符（例如，”n”） cp 命令会保留原文件。

Now, the name “passwd” doesn’t seem very playful and this is a playground, so let’s change it to something else:

现在，”passwd” 这个名字，看起来不怎么有趣，这是个游戏场，所以我们给它改个名字：

[me@linuxbox playground]$ mv passwd fun

Let’s pass the fun around a little by moving our renamed file to each of the directories and back again:

让我们来传送 fun 文件，通过移动重命名的文件到各个子目录， 然后再把它移回到当前目录：

[me@linuxbox playground]$ mv fun dir1

to move it first to directory dir1, then:

首先，把 fun 文件移动目录 dir1 中，然后：

[me@linuxbox playground]$ mv dir1/fun dir2

to move it from dir1 to dir2, then:

再把 fun 文件从 dir1 移到目录 dir2, 然后：

[me@linuxbox playground]$ mv dir2/fun .

to finally bringing it back to the current working directory. Next, let’s see the effect of mv on directories. First we will move our data file into dir1 again:

最后，再把 fun 文件带回到当前工作目录。接下来，来看看移动目录的效果。 首先，我们先移动我们的数据文件到 dir1 目录：

[me@linuxbox playground]$ mv fun dir1

then move dir1 into dir2 and confirm it with ls:

然后移动 dir1到 dir2目录，用 ls 来确认执行结果:

[me@linuxbox playground]$ mv dir1 dir2
[me@linuxbox playground]$ ls -l dir2
total 4
drwxrwxr-x 2 me me 4096 2008-01-11 06:06 dir1
[me@linuxbox playground]$ ls -l dir2/dir1
total 4
-rw-r--r-- 1 me me 1650 2008-01-10 16:33 fun

Note that since dir2 already existed, mv moved dir1 into dir2. If dir2 had not existed, mv would have renamed dir1 to dir2. Lastly, let’s put everything back:

注意：因为目录 dir2 已经存在，mv 命令会把 dir1 移动到 dir2 目录中。如果 dir2 不存在， mv 会把dir1重命名为 dir2。最后，让我们把所有的东西放回原处：

[me@linuxbox playground]$ mv dir2/dir1 .
[me@linuxbox playground]$ mv dir1/fun .

Now we’ll try some links. First the hard links. We’ll create some links to our data file like so:

现在，我们试着创建链接。首先是硬链接。我们创建一些关联我们 数据文件的链接：

[me@linuxbox playground]$ ln fun fun-hard
[me@linuxbox playground]$ ln fun dir1/fun-hard
[me@linuxbox playground]$ ln fun dir2/fun-hard

So now we have four instances of the file “fun”. Let’s take a look our playground directory:

所以现在，我们有四个文件”fun”的实例。看一下目录 playground 中的内容：

[me@linuxbox playground]$ ls -l
total 16
drwxrwxr-x 2 me  me 4096 2008-01-14 16:17 dir1
drwxrwxr-x 2 me  me 4096 2008-01-14 16:17 dir2
-rw-r--r-- 4 me  me 1650 2008-01-10 16:33 fun
-rw-r--r-- 4 me  me 1650 2008-01-10 16:33 fun-hard

One thing you notice is that the second field in the listing for fun and fun-hard both contain a “4” which is the number of hard links that now exist for the file. You’ll remember that a file will always have at least one because the file’s name is created by a link. So, how do we know that fun and fun-hard are, in fact, the same file? In this case, ls is not very helpful. While we can see that fun and fun-hard are both the same size (field 5), our listing provides no way to be sure. To solve this problem, we’re going to have to dig a little deeper.

注意到一件事，列表中，文件 fun 和 fun-hard 的第二个字段是”4”，这个数字 是文件”fun”的硬链接数目。你要记得一个文件至少有一个硬链接，因为文件 名就是由链接创建的。那么，我们怎样知道实际上 fun 和 fun-hard 是同一个文件呢？ 在这个例子里，ls 不是很有用。虽然我们能够看到 fun 和 fun-hard 文件大小一样 （第五字段），但我们的列表没有提供可靠的信息来确定（这两个文件一样）。 为了解决这个问题，我们更深入的研究一下。

When thinking about hard links, it is helpful to imagine that files are made up of two parts: the data part containing the file’s contents and the name part which holds the file’s name. When we create hard links, we are actually creating additional name parts that all refer to the same data part. The system assigns a chain of disk blocks to what is called an inode, which is then associated with the name part. Each hard link therefore refers to a specific inode containing the file’s contents.

当考虑到硬链接的时候，我们可以假设文件由两部分组成：包含文件内容的数据部分和持有文件名的名字部分 ，这将有助于我们理解这个概念。当我们创建文件硬链接的时候，实际上是为文件创建了额外的名字部分， 并且这些名字都关联到相同的数据部分。这时系统会分配一连串的磁盘块给所谓的索引节点，然后索引节点与文 件名字部分相关联。因此每一个硬链接都关系到一个具体的包含文件内容的索引节点。

The ls command has a way to reveal this information. It is invoked with the “-i” option:

ls 命令有一种方法，来展示（文件索引节点）的信息。在命令中加上”-i”选项：

[me@linuxbox playground]$ ls -li
total 16
12353539 drwxrwxr-x 2 me  me 4096  2008-01-14  16:17  dir1
12353540 drwxrwxr-x 2 me  me 4096  2008-01-14  16:17  dir2
12353538 -rw-r--r-- 4 me  me 1650  2008-01-10  16:33  fun
12353538 -rw-r--r-- 4 me  me 1650  2008-01-10  16:33  fun-hard

In this version of the listing, the first field is the inode number and, as we can see, both fun and fun-hard share the same inode number, which confirms they are the same file.

在这个版本的列表中，第一字段表示文件索引节点号，正如我们所见到的， fun 和 fun-hard 共享一样的索引节点号，这就证实这两个文件是同一个文件。

Symbolic links were created to overcome the two disadvantages of hard links: hard links cannot span physical devices and hard links cannot reference directories, only files. Symbolic links are a special type of file that contains a text pointer to the target file or directory.

建立符号链接的目的是为了克服硬链接的两个缺点：硬链接不能跨越物理设备， 硬链接不能关联目录，只能是文件。符号链接是文件的特殊类型，它包含一个指向 目标文件或目录的文本指针。

Creating symbolic links is similar to creating hard links:

符号链接的建立过程相似于创建硬链接：

[me@linuxbox playground]$ ln -s fun fun-sym
[me@linuxbox playground]$ ln -s ../fun dir1/fun-sym
[me@linuxbox playground]$ ln -s ../fun dir2/fun-sym

The first example is pretty straightforward, we simply add the “-s” option to create a symbolic link rather than a hard link. But what about the next two? Remember, when we create a symbolic link, we are creating a text description of where the target file is relative to the symbolic link. It’s easier to see if we look at the ls output:

第一个例子相当直接，在 ln 命令中，简单地加上”-s”选项就可以创建一个符号链接， 而不是一个硬链接。下面两个例子又是怎样呢？ 记住，当我们创建一个符号链接 的时候，会建立一个目标文件在哪里和符号链接有关联的文本描述。如果我们看看 ls 命令的输出结果，比较容易理解。

[me@linuxbox playground]$ ls -l dir1
total 4
-rw-r--r-- 4 me  me 1650 2008-01-10 16:33 fun-hard
lrwxrwxrwx 1 me  me    6 2008-01-15 15:17 fun-sym -> ../fun

The listing for fun-sym in dir1 shows that is it a symbolic link by the leading “l” in the first field and that it points to “../fun”, which is correct. Relative to the location of fun-sym, fun is in the directory above it. Notice too, that the length of the symbolic link file is 6, the number of characters in the string “../fun” rather than the length of the file to which it is pointing.

目录 dir1 中，fun-sym 的列表说明了它是一个符号链接，通过在第一字段中的首字符”l” 可知，并且它还指向”../fun”，也是正确的。相对于 fun-sym 的存储位置，fun 在它的 上一个目录。同时注意，符号链接文件的长度是6，这是字符串”../fun”所包含的字符数， 而不是符号链接所指向的文件长度。

When creating symbolic links, you can either use absolute pathnames:

当建立符号链接时，你既可以使用绝对路径名：

ln -s /home/me/playground/fun dir1/fun-sym

or relative pathnames, as we did in our earlier example. Using relative pathnames is more desirable because it allows a directory containing symbolic links to be renamed and/or moved without breaking the links.

也可用相对路径名，正如前面例题所展示的。使用相对路径名更令人满意， 因为它允许一个包含符号链接的目录重命名或移动，而不会破坏链接。

In addition to regular files, symbolic links can also reference directories:

除了普通文件，符号链接也能关联目录：

[me@linuxbox playground]$ ln -s dir1 dir1-sym
[me@linuxbox playground]$ ls -l
total 16
...省略

As we covered earlier, the rm command is used to delete files and directories. We are going to use it to clean up our playground a little bit. First, let’s delete one of our hard links:

正如我们之前讨论的，rm 命令被用来删除文件和目录。我们将要使用它 来清理一下我们的游戏场。首先，删除一个硬链接：

[me@linuxbox playground]$ rm fun-hard
[me@linuxbox playground]$ ls -l
total 12
...省略

That worked as expected. The file fun-hard is gone and the link count shown for fun is reduced from four to three, as indicated in the second field of the directory listing. Next, we’ll delete the file fun, and just for enjoyment, we’ll include the “-i” option to show what that does:

结果不出所料。文件 fun-hard 消失了，文件 fun 的链接数从4减到3，正如 目录列表第二字段所示。下一步，我们会删除文件 fun，仅为了娱乐，我们会加入”-i” 选项，看一看它的作用：

[me@linuxbox playground]$ rm -i fun
rm: remove regular file `fun'?

Enter “y” at the prompt and the file is deleted. But let’s look at the output of ls now. Noticed what happened to fun-sym? Since it’s a symbolic link pointing to a now- nonexistent file, the link is broken:

在提示符下输入”y”，删除文件。让我们看一下 ls 的输出结果。注意，fun-sym 发生了 什么事? 因为它是一个符号链接，指向已经不存在的文件，链接已经坏了：

[me@linuxbox playground]$ ls -l
total 8
drwxrwxr-x 2 me  me     4096 2008-01-15 15:17 dir1
lrwxrwxrwx 1 me  me        4 2008-01-16 14:45 dir1-sym -> dir1
drwxrwxr-x 2 me  me     4096 2008-01-15 15:17 dir2
lrwxrwxrwx 1 me  me        3 2008-01-15 15:15 fun-sym -> fun

Most Linux distributions configure ls to display broken links. On a Fedora box, broken links are displayed in blinking red text! The presence of a broken link is not, in and of itself dangerous but it is rather messy. If we try to use a broken link we will see this:

大多数 Linux 的发行版本配置 ls 显示损坏的链接。在 Fedora 系统中，坏的链接以闪烁的 红色文本显示！损坏链接的出现，并不危险，但是相当混乱。如果我们试着使用 损坏的链接，会看到以下情况：

[me@linuxbox playground]$ less fun-sym
fun-sym: No such file or directory

Let’s clean up a little. We’ll delete the symbolic links:

稍微清理一下现场。删除符号链接：

[me@linuxbox playground]$ rm fun-sym dir1-sym
[me@linuxbox playground]$ ls -l
total 8
drwxrwxr-x 2 me  me    4096 2008-01-15 15:17 dir1
drwxrwxr-x 2 me  me    4096 2008-01-15 15:17 dir2

One thing to remember about symbolic links is that most file operations are carried out on the link’s target, not the link itself. rm is an exception. When you delete a link, it is the link that is deleted, not the target.

对于符号链接，有一点值得记住，执行的大多数文件操作是针对链接的对象，而不是链接本身。 而 rm 命令是个特例。当你删除链接的时候，删除链接本身，而不是链接的对象。

Finally, we will remove our playground. To do this, we will return to our home directory and use rm with the recursive option (-r) to delete playground and all of its contents, including its subdirectories:

最后，我们将删除我们的游戏场。为了完成这个工作，我们将返回到 我们的家目录，然后用 rm 命令加上选项(-r)，来删除目录 playground， 和目录下的所有内容，包括子目录：

[me@linuxbox playground]$ cd
[me@linuxbox ~]$ rm -r playground

We’ve covered a lot of ground here and it will take a while to fully sink in. Perform the playground exercise over and over until it makes sense. It is important to get a good understanding of basic file manipulation commands and wildcards. Feel free to expand on the playground exercise by adding more files and directories, using wildcards to specify files for various operations. The concept of links is a little confusing at first, but take the time to learn how they work. They can be a real lifesaver.

在这一章中，我们已经研究了许多基础知识。我们得花费一些时间来全面地理解。 反复练习 playground 例题，直到你觉得它有意义。能够良好地理解基本文件操作 命令和通配符，非常重要。随意通过添加文件和目录来拓展 playground 练习， 使用通配符来为各种各样的操作命令指定文件。关于链接的概念，在刚开始接触 时会觉得有点迷惑，花些时间来学习它们是怎样工作的。它们能成为真正的救星。