歸檔和備份

One of the primary tasks of a computer system’s administrator is keeping the system’s data secure. One way this is done is by performing timely backups of the system’s files. Even if you’re not system administrators, it is often useful to make copies of things and to move large collections of files from place to place and from device to device. In this chapter, we will look at several common programs that are used to manage collections of files. There are the file compression programs:

計算機系統管理員的一個主要任務就是保護系統的資料安全，其中一種方法是透過時時備份系統檔案，來保護 資料。即使你不是一名系統管理員，像做做拷貝或者在各個位置和裝置之間移動大量的檔案，通常也是很有幫助的。 在這一章中，我們將會看看幾個經常用來管理檔案集合的程式。它們就是檔案壓縮程式：

• gzip – Compress or expand files

• gzip – 壓縮或者展開檔案

• bzip2 – A block sorting file compressor

• bzip2 – 塊排序檔案壓縮器

The archiving programs:

歸檔程式：

• tar – Tape archiving utility

• tar – 磁帶打包工具

• zip – Package and compress files

• zip – 打包和壓縮檔案

And the file synchronization program:

還有檔案同步程式：

• rsync – Remote file and directory synchronization

• rsync – 同步遠端檔案和目錄

壓縮檔案

Throughout the history of computing, there has been a struggle to get the most data into the smallest available space, whether that space be memory, storage devices or network bandwidth. Many of the data services that we take for granted today, such as portable music players, high definition television, or broadband Internet, owe their existence to effective data compression techniques.

縱觀計算領域的發展歷史，人們努力想把最多的資料存放到到最小的可用空間中，不管是記憶體，儲存裝置 還是網路頻寬。今天我們把許多資料服務都看作是理所當然的事情，但是諸如行動式音樂播放器， 高清電視，或寬頻網路之類別的存在都應歸功於高效的資料壓縮技術。

Data compression is the process of removing redundancy from data. Let's consider an imaginary example. Say we had an entirely black picture file with the dimensions of one hundred pixels by one hundred pixels. In terms of data storage (assuming twenty-four bits, or three bytes per pixel), the image will occupy thirty thousand bytes of storage:

資料壓縮就是一個刪除冗餘資料的過程。讓我們考慮一個假想的例子，比方說我們有一張100*100畫素的 純黑的圖片檔案。根據資料儲存方案（假定每個畫素佔24位，或者3個位元組），那麼這張影象將會佔用 30,000個位元組的儲存空間：

100 * 100 * 3 = 30,000

An image that is all one color contains entirely redundant data. If we were clever, we could encode the data in such a way that we simply describe the fact that we have a block of thirty thousand black pixels. So, instead of storing a block of data containing thirty thousand zeros (black is usually represented in image files as zero), we could compress the data into the number 30,000, followed by a zero to represent our data. Such a data compression scheme is called run-length encoding and is one of the most rudimentary compression techniques. Today’s techniques are much more advanced and complex but the basic goal remains the same—get rid of redundant data.

一張單色影象包含的資料全是多餘的。我們要是聰明的話，可以用這種方法來編碼這些資料， 我們只要簡單地描述這個事實，我們有3萬個黑色的畫素資料塊。所以，我們不儲存包含3萬個0 （通常在影象檔案中，黑色由0來表示）的資料塊，取而代之，我們把這些資料壓縮為數字30,000， 後跟一個0，來表示我們的資料。這種資料壓縮方案被稱為遊程編碼，是一種最基本的壓縮技術。今天的技術更加先進和複雜，但是基本目標依然不變——避免多餘資料。

Compression algorithms (the mathematical techniques used to carry out the compression) fall into two general categories, lossless and lossy. Lossless compression preserves all the data contained in the original. This means that when a file is restored from a compressed version, the restored file is exactly the same as the original, uncompressed version. Lossy compression, on the other hand, removes data as the compression is performed, to allow more compression to be applied. When a lossy file is restored, it does not match the original version; rather, it is a close approximation. Examples of lossy compression are JPEG (for images) and MP3 (for music.) In our discussion, we will look exclusively at lossless compression, since most data on computers cannot tolerate any data loss.

壓縮演算法（數學技巧被用來執行壓縮任務）分為兩大類別，無失真壓縮和有失真壓縮。無失真壓縮保留了 原始檔案的所有資料。這意味著，當還原一個壓縮檔案的時候，還原的檔案與原檔案一模一樣。 而另一方面，有失真壓縮，執行壓縮操作時會刪除資料，允許更大的壓縮。當一個有損檔案被還原的時候， 它與原檔案不相匹配; 相反，它是一個近似值。有失真壓縮的例子有 JPEG（影象）檔案和 MP3（音訊）檔案。 在我們的討論中，我們將看看完全無失真壓縮，因為計算機中的大多數資料是不能容忍丟失任何資料的。

gzip

The gzip program is used to compress one or more files. When executed, it replaces the original file with a compressed version of the original. The corresponding gunzip program is used to restore compressed files to their original, uncompressed form. Here is an example:

這個 gzip 程式被用來壓縮一個或多個檔案。當執行 gzip 命令時，則原始檔案的壓縮版會替代原始檔案。 相對應的 gunzip 程式被用來把壓縮檔案復原為沒有被壓縮的版本。這裡有個例子：

[me@linuxbox ~]$ ls -l /etc > foo.txt
[me@linuxbox ~]$ ls -l foo.*
-rw-r--r-- 1 me     me 15738 2008-10-14 07:15 foo.txt
[me@linuxbox ~]$ gzip foo.txt
[me@linuxbox ~]$ ls -l foo.*
-rw-r--r-- 1 me     me 3230 2008-10-14 07:15 foo.txt.gz
[me@linuxbox ~]$ gunzip foo.txt.gz
[me@linuxbox ~]$ ls -l foo.*
-rw-r--r-- 1 me     me 15738 2008-10-14 07:15 foo.txt

In this example, we create a text file named foo.txt from a directory listing. Next, we run gzip, which replaces the original file with a compressed version named foo.txt.gz. In the directory listing of foo.*, we see that the original file has been replaced with the compressed version, and that the compressed version about one-fifth the size of the original. We can also see that the compressed file has the same permissions and time stamp as the original.

在這個例子裡，我們建立了一個名為 foo.txt 的文字檔案，其內容包含一個目錄的列表清單。 接下來，我們執行 gzip 命令，它會把原始檔案替換為一個叫做 foo.txt.gz 的壓縮檔案。在 foo.* 檔案列表中，我們看到原始檔案已經被壓縮檔案替代了，並將這個壓縮檔案大約是原始 檔案的五分之一。我們也能看到壓縮檔案與原始檔案有著相同的許可權和時間戳。

Next, we run the gunzip program to uncompress the file. Afterward, we can see that the compressed version of the file has been replaced with the original, again with the permissions and time stamp preserved.

接下來，我們執行 gunzip 程式來解壓縮檔案。隨後，我們能見到壓縮檔案已經被原始檔案替代了， 同樣地保留了相同的許可權和時間戳。

gzip has many options. Here are a few:

gzip 命令有許多選項。這裡列出了一些：

Going back to our earlier example:

返回到我們之前的例子中：

[me@linuxbox ~]$ gzip foo.txt
[me@linuxbox ~]$ gzip -tv foo.txt.gz
foo.txt.gz: OK
[me@linuxbox ~]$ gzip -d foo.txt.gz

Here, we replaced the file foo.txt with a compressed version, named foo.txt.gz. Next, we tested the integrity of the compressed version, using the -t and -v options. Finally, we decompressed the file back to its original form. gzip can also be used in interesting ways via standard input and output:

這裡，我們用壓縮檔案來替代檔案 foo.txt，壓縮檔名為 foo.txt.gz。下一步，我們測試了壓縮檔案 的完整性，使用了-t 和-v 選項。

[me@linuxbox ~]$ ls -l /etc | gzip > foo.txt.gz

This command creates a compressed version of a directory listing.

這個命令建立了一個目錄列表的壓縮檔案。

The gunzip program, which uncompresses gzip files, assumes that filenames end in the extension .gz, so it’s not necessary to specify it, as long as the specified name is not in conflict with an existing uncompressed file:

這個 gunzip 程式，會解壓縮 gzip 檔案，假定那些檔名的副檔名是.gz，所以沒有必要指定它， 只要指定的名字與現有的未壓縮檔案不衝突就可以：

[me@linuxbox ~]$ gunzip foo.txt.gz

If our goal were only to view the contents of a compressed text file, we can do this:

如果我們的目標只是為了瀏覽一下壓縮文字檔案的內容，我們可以這樣做：

[me@linuxbox ~]$ gunzip -c foo.txt.gz | less

Alternately, there is a program supplied with gzip, called zcat, that is equivalent to gunzip with the -c option. It can be used like the cat command on gzip compressed files:

另外，對應於 gzip 還有一個程式，叫做 zcat，它等同於帶有-c 選項的 gunzip 命令。 它可以被用來如 cat 命令作用於 gzip 壓縮檔案：

[me@linuxbox ~]$ zcat foo.txt.gz | less

Tip: There is a zless program, too. It performs the same function as the pipeline above.

小貼士: 還有一個 zless 程式。它與上面的管道線有相同的功能。

bzip2

The bzip2 program, by Julian Seward, is similar to gzip, but uses a different compression algorithm that achieves higher levels of compression at the cost of compression speed. In most regards, it works in the same fashion as gzip. A file compressed with bzip2 is denoted with the extension .bz2:

這個 bzip2 程式，由 Julian Seward 開發，與 gzip 程式相似，但是使用了不同的壓縮演算法， 捨棄了壓縮速度，而實現了更高的壓縮級別。在大多數情況下，它的工作模式等同於 gzip。 由 bzip2 壓縮的檔案，用副檔名 .bz2 來表示：

[me@linuxbox ~]$ ls -l /etc > foo.txt
[me@linuxbox ~]$ ls -l foo.txt
-rw-r--r-- 1 me     me      15738 2008-10-17 13:51 foo.txt
[me@linuxbox ~]$ bzip2 foo.txt
[me@linuxbox ~]$ ls -l foo.txt.bz2
-rw-r--r-- 1 me     me      2792 2008-10-17 13:51 foo.txt.bz2
[me@linuxbox ~]$ bunzip2 foo.txt.bz2

As we can see, bzip2 can be used the same way as gzip. All the options (except for -r) that we discussed for gzip are also supported in bzip2. Note, however, that the compression level option (-number) has a somewhat different meaning to bzip2. bzip2 comes with bunzip2 and bzcat for decompressing files. bzip2 also comes with the bzip2recover program, which will try to recover damaged .bz2 files.

正如我們所看到的，bzip2 程式使用起來和 gzip 程式一樣。我們之前討論的 gzip 程式的所有選項（除了-r） ，bzip2 程式同樣也支援。注意，然而，壓縮級別選項（-number）對於 bzip2 程式來說，有少許不同的含義。 伴隨著 bzip2 程式，有 bunzip2 和 bzcat 程式來解壓縮檔案。bzip2 檔案也帶有 bzip2recover 程式，其會 試圖恢復受損的 .bz2 檔案。

Don’t Be Compressive Compulsive

不要強迫性壓縮

I occasionally see people attempting to compress a file, which has been already compressed with an effective compression algorithm, by doing something like this:

我偶然見到人們試圖用高效的壓縮演算法，來壓縮一個已經被壓縮過的檔案，透過這樣做：

$ gzip picture.jpg

Don’t do it. You’re probably just wasting time and space! If you apply compression to a file that is already compressed, you will actually end up a larger file. This is because all compression techniques involve some overhead that is added to the file to describe the compression. If you try to compress a file that already contains no redundant information, the compression will not result in any savings to offset the additional overhead.

不要這樣。你可能只是在浪費時間和空間！如果你再次壓縮已經壓縮過的檔案，實際上你 會得到一個更大的檔案。這是因為所有的壓縮技術都會涉及一些開銷，檔案中會被新增描述 此次壓縮過程的資訊。如果你試圖壓縮一個已經不包含多餘資訊的檔案，那麼再次壓縮不會節省 空間，以抵消額外的花費。

歸檔檔案

A common file management task used in conjunction with compression is archiving. Archiving is the process of gathering up many files and bundling them together into a single large file. Archiving is often done as a part of system backups. It is also used when old data is moved from a system to some type of long-term storage.

一個常見的，與檔案壓縮結合一塊使用的檔案管理任務是歸檔。歸檔就是收集許多檔案，並把它們 捆綁成一個大檔案的過程。歸檔經常作為系統備份的一部分來使用。當把舊資料從一個系統移到某 種類型的長期儲存裝置中時，也會用到歸檔程式。

tar

In the Unix-like world of software, the tar program is the classic tool for archiving files. Its name, short for tape archive, reveals its roots as a tool for making backup tapes. While it is still used for that traditional task, it is equally adept on other storage devices as well. We often see filenames that end with the extension .tar or .tgz which indicate a “plain” tar archive and a gzipped archive, respectively. A tar archive can consist of a group of separate files, one or more directory hierarchies, or a mixture of both. The command syntax works like this:

在類別 Unix 的軟體世界中，這個 tar 程式是用來歸檔檔案的經典工具。它的名字，是 tape archive 的簡稱，揭示了它的根源，它是一款製作磁帶備份的工具。而它仍然被用來完成傳統任務， 它也同樣適用於其它的儲存裝置。我們經常看到副檔名為 .tar 或者 .tgz 的檔案，它們各自表示“普通” 的 tar 包和被 gzip 程式壓縮過的 tar 包。一個 tar 包可以由一組獨立的檔案，一個或者多個目錄，或者 兩者混合體組成。命令語法如下：

tar mode[options] pathname...

where mode is one of the following operating modes (only a partial list is shown here; see the tar man page for a complete list):

這裡的 mode 是指以下操作模式（這裡只展示了一部分，檢視 tar 的手冊來得到完整列表）之一：

tar uses a slightly odd way of expressing options, so we’ll need some examples to show how it works. First, Let's re-create our playground from the previous chapter:

tar 命令使用了稍微有點奇怪的方式來表達它的選項，所以我們需要一些例子來展示它是 怎樣工作的。首先，讓我們重新建立之前我們用過的操練場:

[me@linuxbox ~]$ mkdir -p playground/dir-{00{1..9},0{10..99},100}
[me@linuxbox ~]$ touch playground/dir-{00{1..9},0{10..99},100}/file-{A..Z}

Next, Let's create a tar archive of the entire playground:

下一步，讓我們建立整個操練場的 tar 包：

[me@linuxbox ~]$ tar cf playground.tar playground

This command creates a tar archive named playground.tar that contains the entire playground directory hierarchy. We can see that the mode and the f option, which is used to specify the name of the tar archive, may be joined together, and do not require a leading dash. Note, however, that the mode must always be specified first, before any other option.

這個命令建立了一個名為 playground.tar 的 tar 包，其包含整個 playground 目錄層次結果。我們 可以看到模式 c 和選項 f，其被用來指定這個 tar 包的名字，模式和選項可以寫在一起，而且不 需要開頭的短橫線。注意，然而，必須首先指定模式，然後才是其它的選項。

To list the contents of the archive, we can do this:

要想列出歸檔檔案的內容，我們可以這樣做：

[me@linuxbox ~]$ tar tf playground.tar

For a more detailed listing, we can add the v (verbose) option:

為了得到更詳細的列表資訊，我們可以新增選項 v：

[me@linuxbox ~]$ tar tvf playground.tar

Now, Let's extract the playground in a new location. We will do this by creating a new directory named foo, and changing the directory and extracting the tar archive:

現在，抽取 tar 包 playground 到一個新位置。我們先建立一個名為 foo 的新目錄，更改目錄， 然後抽取 tar 包中的檔案：

[me@linuxbox ~]$ mkdir foo
[me@linuxbox ~]$ cd foo
[me@linuxbox ~]$ tar xf ../playground.tar
[me@linuxbox ~]$ ls
playground

If we examine the contents of ~/foo/playground, we see that the archive was successfully installed, creating a precise reproduction of the original files. There is one caveat, however: unless you are operating as the superuser, files and directories extracted from archives take on the ownership of the user performing the restoration, rather than the original owner.

如果我們檢查 ~/foo/playground 目錄中的內容，會看到這個歸檔檔案已經被成功地安裝了，也即建立了 一個精確的原始檔案的副本。然而，這裡有一個警告：除非你是超級使用者，要不然從歸檔檔案中抽取的檔案 和目錄的所有權由執行此復原操作的使用者所擁有，而不屬於原始所有者。

Another interesting behavior of tar is the way it handles pathnames in archives. The default for pathnames is relative, rather than absolute. tar does this by simply removing any leading slash from the pathname when creating the archive. To demonstrate, we will recreate our archive, this time specifying an absolute pathname:

tar 命令另一個有趣的行為是它處理歸檔檔案路徑名的方式。預設情況下，路徑名是相對的，而不是絕對 路徑。當以相對路徑建立歸檔檔案的時候，tar 命令會簡單地刪除路徑名開頭的斜槓。為了說明問題，我們將會 重新建立我們的歸檔檔案，但是這次指定用絕對路徑建立：

[me@linuxbox foo]$ cd
[me@linuxbox ~]$ tar cf playground2.tar ~/playground

Remember, ~/playground will expand into /home/me/playground when we press the enter key, so we will get an absolute pathname for our demonstration. Next, we will extract the archive as before and watch what happens:

記住，當按下回車鍵後，~/playground 會展開成 /home/me/playground，所以我們將會得到一個 絕對路徑名。接下來，和之前一樣我們會抽取歸檔檔案，觀察發生什麼事情：

[me@linuxbox ~]$ cd foo
[me@linuxbox foo]$ tar xf ../playground2.tar
[me@linuxbox foo]$ ls
home     playground
[me@linuxbox foo]$ ls home
me
[me@linuxbox foo]$ ls home/me
playground

Here we can see that when we extracted our second archive, it recreated the directory home/me/playground relative to our current working directory, ~/foo, not relative to the root directory, as would have been the case with an absolute pathname. This may seem like an odd way for it to work, but it’s actually more useful this way, as it allows us to extract archives to any location rather than being forced to extract them to their original locations. Repeating the exercise with the inclusion of the verbose option (v) will give a clearer picture of what’s going on.

這裡我們看到當我們抽取第二個歸檔檔案時，它重新建立了 home/me/playground 目錄， 相對於我們當前的工作目錄，~/foo，而不是相對於 root 目錄，作為帶有絕對路徑名的案例。 這看起來似乎是一種奇怪的工作方式，但事實上這種方式很有用，因為這樣就允許我們抽取檔案 到任意位置，而不是強制地把抽取的檔案放置到原始目錄下。加上 verbose（v）選項，重做 這個練習，將會展現更加詳細的資訊。

Let's consider a hypothetical, yet practical example, of tar in action. Imagine we want to copy the home directory and its contents from one system to another and we have a large USB hard drive that we can use for the transfer. On our modern Linux system, the drive is “automagically” mounted in the /media directory. Let's also imagine that the disk has a volume name of BigDisk when we attach it. To make the tar archive, we can do the following:

讓我們考慮一個假設，tar 命令的實際應用。假定我們想要複製家目錄及其內容到另一個系統中， 並且有一個大容量的 USB 硬碟，可以把它作為傳輸工具。在現代 Linux 系統中， 這個硬碟會被“自動地”掛載到 /media 目錄下。我們也假定硬碟中有一個名為 BigDisk 的邏輯卷。 為了製作 tar 包，我們可以這樣做：

[me@linuxbox ~]$ sudo tar cf /media/BigDisk/home.tar /home

After the tar file is written, we unmount the drive and attach it to the second computer. Again, it is mounted at /media/BigDisk. To extract the archive, we do this:

tar 包製作完成之後，我們解除安裝硬碟，然後把它連線到第二個計算機上。再一次，此硬碟被 掛載到 /media/BigDisk 目錄下。為了抽取歸檔檔案，我們這樣做：

[me@linuxbox2 ~]$ cd /
[me@linuxbox2 /]$ sudo tar xf /media/BigDisk/home.tar

What’s important to see here is that we must first change directory to /, so that the extraction is relative to the root directory, since all pathnames within the archive are relative.

值得注意的一點是，因為歸檔檔案中的所有路徑名都是相對的，所以首先我們必須更改目錄到根目錄下， 這樣抽取的檔案路徑就相對於根目錄了。

When extracting an archive, it’s possible to limit what is extracted from the archive. For example, if we wanted to extract a single file from an archive, it could be done like this:

當抽取一個歸檔檔案時，有可能限制從歸檔檔案中抽取什麼內容。例如，如果我們想要抽取單個檔案， 可以這樣實現：

tar xf archive.tar pathname

By adding the trailing pathname to the command, tar will only restore the specified file. Multiple pathnames may be specified. Note that the pathname must be the full, exact relative pathname as stored in the archive. When specifying pathnames, wildcards are not normally supported; however, the GNU version of tar (which is the version most often found in Linux distributions) supports them with the –wildcards option. Here is an example using our previous playground.tar file:

透過給命令新增末尾的路徑名，tar 命令就只會恢復指定的檔案。可以指定多個路徑名。注意 路徑名必須是完全的，精準的相對路徑名，就如儲存在歸檔檔案中的一樣。當指定路徑名的時候， 通常不支援萬用字元；然而，GNU 版本的 tar 命令（在 Linux 發行版中最常出現）透過 --wildcards 選項來 支援萬用字元。這個例子使用了之前 playground.tar 檔案：

[me@linuxbox ~]$ cd foo
[me@linuxbox foo]$ tar xf ../playground2.tar --wildcards 'home/me/playground/dir-\*/file-A'

This command will extract only files matching the specified pathname including the wildcard dir-*.

這個命令將只會抽取匹配特定路徑名的檔案，路徑名中包含了萬用字元 dir-*。

tar is often used in conjunction with find to produce archives. In this example, we will use find to produce a set of files to include in an archive:

tar 命令經常結合 find 命令一起來製作歸檔檔案。在這個例子裡，我們將會使用 find 命令來 產生一個檔案集合，然後這些檔案被包含到歸檔檔案中。

[me@linuxbox ~]$ find playground -name 'file-A' -exec tar rf playground.tar '{}' '+'

Here we use find to match all the files in playground named file-A and then, using the -exec action, we invoke tar in the append mode (r) to add the matching files to the archive playground.tar.

這裡我們使用 find 命令來匹配 playground 目錄中所有名為 file-A 的檔案，然後使用-exec 行為，來 喚醒帶有追加模式（r）的 tar 命令，把匹配的檔案新增到歸檔檔案 playground.tar 裡面。

Using tar with find is a good way of creating incremental backups of a directory tree or an entire system. By using find to match files newer than a timestamp file, we could create an archive that only contains files newer than the last archive, assuming that the timestamp file is updated right after each archive is created.

使用 tar 和 find 命令，來建立逐漸增加的目錄樹或者整個系統的備份，是個不錯的方法。透過 find 命令匹配新於某個時間戳的檔案，我們就能夠建立一個歸檔檔案，其只包含新於上一個 tar 包的檔案， 假定這個時間戳檔案恰好在每個歸檔檔案建立之後被更新了。

tar can also make use of both standard input and output. Here is a comprehensive example:

tar 命令也可以利用標準輸出和輸入。這裡是一個完整的例子:

[me@linuxbox foo]$ cd
[me@linuxbox ~]$ find playground -name 'file-A' | tar cf - --files-from=-
   | gzip > playground.tgz

In this example, we used the find program to produce a list of matching files and piped them into tar. If the filename “-” is specified, it is taken to mean standard input or output, as needed (by the way, this convention of using “-” to represent standard input/output is used by a number of other programs, too.) The –files-from option (which may be also be specified as -T) causes tar to read its list of pathnames from a file rather than the command line. Lastly, the archive produced by tar is piped into gzip to create the compressed archive playground.tgz. The .tgz extension is the conventional extension given to gzip-compressed tar files. The extension .tar.gz is also used sometimes.

在這個例子裡面，我們使用 find 程式產生了一個匹配檔案列表，然後把它們管道到 tar 命令中。 如果指定了檔名“-”，則其被看作是標準輸入或輸出，正是所需（順便說一下，使用“-”來表示 標準輸入／輸出的慣例，也被大量的其它程式使用）。這個 --file-from 選項（也可以用 -T 來指定） 導致 tar 命令從一個檔案而不是命令列來讀入它的路徑名列表。最後，這個由 tar 命令產生的歸檔 檔案被管道到 gzip 命令中，然後建立了壓縮歸檔檔案 playground.tgz。此 .tgz 副檔名是命名 由 gzip 壓縮的 tar 檔案的常規副檔名。有時候也會使用 .tar.gz 這個副檔名。

While we used the gzip program externally to produced our compressed archive, modern versions of GNU tar support both gzip and bzip2 compression directly, with the use of the z and j options, respectively. Using our previous example as a base, we can simplify it this way:

雖然我們使用 gzip 程式來製作我們的壓縮歸檔檔案，但是現在的 GUN 版本的 tar 命令 ，gzip 和 bzip2 壓縮兩者都直接支援，各自使用 z 和 j 選項。以我們之前的例子為基礎， 我們可以這樣簡化它：

[me@linuxbox ~]$ find playground -name 'file-A' | tar czf playground.tgz -T -

If we had wanted to create a bzip2 compressed archive instead, we could have done this:

如果我們本要建立一個由 bzip2 壓縮的歸檔檔案，我們可以這樣做：

[me@linuxbox ~]$ find playground -name 'file-A' | tar cjf playground.tbz -T -

By simply changing the compression option from z to j (and changing the output file’s extension to .tbz to indicate a bzip2 compressed file) we enabled bzip2 compression. Another interesting use of standard input and output with the tar command involves transferring files between systems over a network. Imagine that we had two machines running a Unix-like system equipped with tar and ssh. In such a scenario, we could transfer a directory from a remote system (named remote-sys for this example) to our local system:

透過簡單地修改壓縮選項，把 z 改為 j（並且把輸出檔案的副檔名改為 .tbz，來指示一個 bzip2 壓縮檔案）， 就使 bzip2 命令壓縮生效了。另一個 tar 命令與標準輸入和輸出的有趣使用，涉及到在系統之間經過 網路傳輸檔案。假定我們有兩臺機器，每臺都執行著類別 Unix，且裝備著 tar 和 ssh 工具的作業系統。 在這種情景下，我們可以把一個目錄從遠端系統（名為 remote-sys）傳輸到我們的本地系統中：

[me@linuxbox ~]$ mkdir remote-stuff
[me@linuxbox ~]$ cd remote-stuff
[me@linuxbox remote-stuff]$ ssh remote-sys 'tar cf - Documents' | tar xf -
me@remote-sys’s password:
[me@linuxbox remote-stuff]$ ls
Documents

Here we were able to copy a directory named Documents from the remote system remote-sys to a directory within the directory named remote-stuff on the local system. How did we do this? First, we launched the tar program on the remote system using ssh. You will recall that ssh allows us to execute a program remotely on a networked computer and “see” the results on the local system—the standard output produced on the remote system is sent to the local system for viewing. We can take advantage of this by having tar create an archive (the c mode) and send it to standard output, rather than a file (the f option with the dash argument), thereby transporting the archive over the encrypted tunnel provided by ssh to the local system. On the local system, we execute tar and have it expand an archive (the x mode) supplied from standard input (again, the f option with the dash argument).

這裡我們能夠從遠端系統 remote-sys 中複製目錄 Documents 到本地系統名為 remote-stuff 目錄中。 我們怎樣做的呢？首先，透過使用 ssh 命令在遠端系統中啟動 tar 程式。你可記得 ssh 允許我們 在遠端聯網的計算機上執行程式，並且在本地系統中看到執行結果——遠端系統中產生的輸出結果 被髮送到本地系統中檢視。我們可以利用。在本地系統中，我們執行 tar 命令，

zip

The zip program is both a compression tool and an archiver. The file format used by the program is familiar to Windows users, as it reads and writes .zip files. In Linux, however, gzip is the predominant compression program with bzip2 being a close second.

這個 zip 程式既是壓縮工具，也是一個打包工具。這程式使用的檔案格式，Windows 使用者比較熟悉， 因為它讀取和寫入.zip 檔案。然而，在 Linux 中 gzip 是主要的壓縮程式，而 bzip2則位居第二。

In its most basic usage, zip is invoked like this:

在 zip 命令最基本的使用中，可以這樣喚醒 zip 命令：

zip options zipfile file...

For example, to make a zip archive of our playground, we would do this:

例如，製作一個 playground 的 zip 版本的檔案包，這樣做：

[me@linuxbox ~]$ zip -r playground.zip playground

Unless we include the -r option for recursion, only the playground directory (but none of its contents) is stored. Although the addition of the extension .zip is automatic a, we will include the file extension for clarity.

除非我們包含-r 選項，要不然只有 playground 目錄（沒有任何它的內容）被儲存。雖然會自動新增 .zip 副檔名，但為了清晰起見，我們還是包含副檔名。

During the creation of the zip archive, zip will normally display a series of messages like this:

在建立 zip 版本的檔案包時，zip 命令通常會顯示一系列的資訊：

adding: playground/dir-020/file-Z (stored 0%)
adding: playground/dir-020/file-Y (stored 0%)
adding: playground/dir-020/file-X (stored 0%)
adding: playground/dir-087/ (stored 0%)
adding: playground/dir-087/file-S (stored 0%)

These messages show the status of each file added to the archive. zip will add files to the archive using one of two storage methods: either it will “store” a file without compression, as shown here, or it will “deflate” the file which performs compression. The numeric value displayed after the storage method indicates the amount of compression achieved. Since our playground only contains empty files, no compression is performed on its contents.

這些資訊顯示了新增到檔案包中每個檔案的狀態。zip 命令會使用兩種儲存方法之一，來新增 檔案到檔案包中：要不它會“store”沒有壓縮的檔案，正如這裡所示，或者它會“deflate”檔案， 執行壓縮操作。在儲存方法之後顯示的數值表明了壓縮量。因為我們的 playground 目錄 只是包含空檔案，沒有對它的內容執行壓縮操作。

Extracting the contents of a zip file is straightforward when using the unzip program:

使用 unzip 程式，來直接抽取一個 zip 檔案的內容。

[me@linuxbox ~]$ cd foo
[me@linuxbox foo]$ unzip ../playground.zip

One thing to note about zip (as opposed to tar) is that if an existing archive is specified, it is updated rather than replaced. This means that the existing archive is preserved, but new files are added and matching files are replaced. Files may be listed and extracted selectively from a zip archive by specifying them to unzip:

對於 zip 命令（與 tar 命令相反）要注意一點，就是如果指定了一個已經存在的檔案包，其被更新 而不是被替代。這意味著會保留此檔案包，但是會新增新檔案，同時替換匹配的檔案。可以列出 檔案或者有選擇地從一個 zip 檔案包中抽取檔案，只要給 unzip 命令指定檔名：

[me@linuxbox ~]$ unzip -l playground.zip playground/dir-87/file-Z
Archive: ../playground.zip
    Length      Date    Time    Name

         0    10-05-08  09:25   playground/dir-87/file-Z

         0                      1 file
[me@linuxbox ~]$ cd foo
[me@linuxbox foo]$ unzip ./playground.zip playground/dir-87/file-Z
Archive: ../playground.zip
replace playground/dir-87/file-Z? [y]es, [n]o, [A]ll, [N]one,
[r]ename: y
extracting: playground/dir-87/file-Z

Using the -l option causes unzip to merely list the contents of the archive without extracting the file. If no file(s) are specified, unzip will list all files in the archive. The -v option can be added to increase the verbosity of the listing. Note that when the archive extraction conflicts with an existing file, the user is prompted before the file is replaced.

使用-l 選項，導致 unzip 命令只是列出檔案包中的內容而沒有抽取檔案。如果沒有指定檔案， unzip 程式將會列出檔案包中的所有檔案。新增這個-v 選項會增加列表的冗餘資訊。注意當抽取的 檔案與已經存在的檔案衝突時，會在替代此檔案之前提醒使用者。

Like tar, zip can make use of standard input and output, though its implementation is somewhat less useful. It is possible to pipe a list of filenames to zip via the -@ option:

像 tar 命令一樣，zip 命令能夠利用標準輸入和輸出，雖然它的實施不大有用。透過-@選項，有可能把一系列的 檔名管道到 zip 命令。

[me@linuxbox foo]$ cd
[me@linuxbox ~]$ find playground -name "file-A" | zip -@ file-A.zip

Here we use find to generate a list of files matching the test -name “file-A”, and pipe the list into zip, which creates the archive file-A.zip containing the selected files.

這裡我們使用 find 命令產生一系列與“file-A”相匹配的檔案列表，並且把此列表管道到 zip 命令， 然後建立包含所選檔案的檔案包 file-A.zip。

zip also supports writing its output to standard output, but its use is limited because very few programs can make use of the output. Unfortunately, the unzip program, does not accept standard input. This prevents zip and unzip from being used together to perform network file copying like tar.

zip 命令也支援把它的輸出寫入到標準輸出，但是它的使用是有限的，因為很少的程式能利用輸出。 不幸地是，這個 unzip 程式，不接受標準輸入。這就阻止了 zip 和 unzip 一塊使用，像 tar 命令那樣， 來複制網路上的檔案。

zip can, however, accept standard input, so it can be used to compress the output of other programs:

然而，zip 命令可以接受標準輸入，所以它可以被用來壓縮其它程式的輸出：

[me@linuxbox ~]$ ls -l /etc/ | zip ls-etc.zip -
adding: - (deflated 80%)

In this example we pipe the output of ls into zip. Like tar, zip interprets the trailing dash as “use standard input for the input file.”

在這個例子裡，我們把 ls 命令的輸出管道到 zip 命令。像 tar 命令，zip 命令把末尾的橫槓解釋為 “使用標準輸入作為輸入檔案。”

The unzip program allows its output to be sent to standard output when the -p (for pipe) option is specified:

這個 unzip 程式允許它的輸出傳送到標準輸出，當指定了-p 選項之後：

[me@linuxbox ~]$ unzip -p ls-etc.zip | less

We touched on some of the basic things that zip/unzip can do. They both have a lot of options that add to their flexibility, though some are platform specific to other systems. The man pages for both zip and unzip are pretty good and contain useful examples. However, the main use of these programs is for exchanging files with Windows systems, rather than performing compression and archiving on Linux, where tar and gzip are greatly preferred.

我們討論了一些 zip/unzip 可以完成的基本操作。它們兩個都有許多選項，其增加了 命令的靈活性，雖然一些選項只針對於特定的平臺。zip 和 unzip 命令的說明手冊都相當不錯， 並且包含了有用的範例。然而，這些程式的主要用途是為了和 Windows 系統交換檔案， 而不是在 Linux 系統中執行壓縮和打包操作，tar 和 gzip 程式在 Linux 系統中更受歡迎。

同步檔案和目錄

A common strategy for maintaining a backup copy of a system involves keeping one or more directories synchronized with another directory (or directories) located on either the local system (usually a removable storage device of some kind) or with a remote system. We might, for example, have a local copy of a web site under development and synchronize it from time to time with the “live” copy on a remote web server. In the Unix-like world, the preferred tool for this task is rsync. This program can synchronize both local and remote directories by using the rsync remote-update protocol, which allows rsync to quickly detect the differences between two directories and perform the minimum amount of copying required to bring them into sync. This makes rsync very fast and economical to use, compared to other kinds of copy programs.

維護系統備份的常見策略是保持一個或多個目錄與另一個本地系統（通常是某種可移動的儲存裝置） 或者遠端系統中的目錄（或多個目錄）同步。我們可能，例如有一個正在開發的網站的本地備份， 需要時不時的與遠端網路伺服器中的檔案備份保持同步。在類別 Unix 系統的世界裡，能完成此任務且 備受人們喜愛的工具是 rsync。這個程式能同步本地與遠端的目錄，透過使用 rsync 遠端更新協議，此協議 允許 rsync 快速地檢測兩個目錄的差異，執行最小量的複製來達到目錄間的同步。比起其它種類的複製程式， 這就使 rsync 命令非常快速和高效。

rsync is invoked like this:

rsync 被這樣喚醒：

rsync options source destination

where source and destination are one of the following:

這裡 source 和 destination 是下列選項之一：

• A local file or directory

• A remote file or directory in the form of [user@]host:path

• A remote rsync server specified with a URI of rsync://[user@]host[:port]/path

• 一個本地檔案或目錄

• 一個遠端檔案或目錄，以[user@]host:path 的形式存在

• 一個遠端 rsync 伺服器，由 rsync://[user@]host[:port]/path 指定

Note that either the source or destination must be a local file. Remote to remote copying is not supported.

注意 source 和 destination 兩者之一必須是本地檔案。rsync 不支援遠端到遠端的複製

Let's try rsync out on some local files. First, Let's clean out our foo directory:

讓我們試著對一些本地檔案使用 rsync 命令。首先，清空我們的 foo 目錄：

[me@linuxbox ~]$ rm -rf foo/*

Next, we’ll synchronize the playground directory with a corresponding copy in foo:

下一步，我們將同步 playground 目錄和它在 foo 目錄中相對應的副本

[me@linuxbox ~]$ rsync -av playground foo

We’ve included both the -a option (for archiving—causes recursion and preservation of file attributes) and the -v option (verbose output) to make a mirror of the playground directory within foo. While the command runs, we will see a list of the files and directories being copied. At the end, we will see a summary message like this:

我們包括了-a 選項（遞迴和保護檔案屬性）和-v 選項（冗餘輸出）， 來在 foo 目錄中製作一個 playground 目錄的映象。當這個命令執行的時候， 我們將會看到一系列的檔案和目錄被複制。在最後，我們將看到一條像這樣的總結資訊：

sent 135759 bytes received 57870 bytes 387258.00 bytes/sec
total size is 3230 speedup is 0.02

indicating the amount of copying performed. If we run the command again, we will see a different result:

說明覆制的數量。如果我們再次執行這個命令，我們將會看到不同的結果：

[me@linuxbox ~]$ rsync -av playgound foo
building file list ... done
sent 22635 bytes received 20 bytes
total size is 3230 speedup is 0.14
45310.00 bytes/sec

Notice that there was no listing of files. This is because rsync detected that there were no differences between ~/playground and ~/foo/playground, and therefore it didn’t need to copy anything. If we modify a file in playground and run rsync again:

注意到沒有檔案列表。這是因為 rsync 程式檢測到在目錄~/playground 和 ~/foo/playground 之間 不存在差異，因此它不需要複製任何資料。如果我們在 playground 目錄中修改一個檔案，然後 再次執行 rsync 命令：

[me@linuxbox ~]$ touch playground/dir-099/file-Z
[me@linuxbox ~]$ rsync -av playground foo
building file list ... done
playground/dir-099/file-Z
sent 22685 bytes received 42 bytes 45454.00 bytes/sec
total size is 3230 speedup is 0.14

we see that rsync detected the change and copied only the updated file. As a practical example, Let's consider the imaginary external hard drive that we used earlier with tar. If we attach the drive to our system and, once again, it is mounted at / media/BigDisk, we can perform a useful system backup by first creating a directory, named /backup on the external drive and then using rsync to copy the most important stuff from our system to the external drive:

我們看到 rsync 命令檢測到更改，並且只是複製了更新的檔案。作為一個實際的例子， 讓我們考慮一個假想的外部硬碟，之前我們在 tar 命令中用到過的。如果我們再次把此 硬碟連線到我們的系統中，它被掛載到/media/BigDisk 目錄下，我們可以執行一個有 用的系統備份了，首先在外部硬碟上建立一個目錄，名為/backup，然後使用 rsync 程式 從我們的系統中複製最重要的資料到此外部硬碟上：

[me@linuxbox ~]$ mkdir /media/BigDisk/backup
[me@linuxbox ~]$ sudo rsync -av --delete /etc /home /usr/local /media/BigDisk/backup

In this example, we copied the /etc, /home, and /usr/local directories from our system to our imaginary storage device. We included the –delete option to remove files that may have existed on the backup device that no longer existed on the source device (this is irrelevant the first time we make a backup, but will be useful on subsequent copies.) Repeating the procedure of attaching the external drive and running this rsync command would be a useful (though not ideal) way of keeping a small system backed up. Of course, an alias would be helpful here, too. We could create an alias and add it to our .bashrc file to provide this feature:

在這個例子裡，我們把/etc，/home，和/usr/local 目錄從我們的系統中複製到假想的儲存裝置中。 我們包含了–delete 這個選項，來刪除可能在備份裝置中已經存在但卻不再存在於源裝置中的檔案， （這與我們第一次建立備份無關，但是會在隨後的複製操作中有用途）。掛載外部驅動器，執行 rsync 命令，不斷重複這個過程，是一個不錯的（雖然不理想）方式來儲存少量的系統備份檔案。 當然，別名會對這個操作更有幫助些。我們將會建立一個別名，並把它新增到.bashrc 檔案中， 來提供這個特性：

alias backup='sudo rsync -av --delete /etc /home /usr/local /media/BigDisk/backup'

Now all we have to do is attach our external drive and run the backup command to do the job.

現在我們所做的事情就是連線外部驅動器，然後執行 backup 命令來完成工作。

在網路間使用 rsync 命令

One of the real beauties of rsync is that it can be used to copy files over a network. After all, the “r” in rsync stands for “remote.” Remote copying can be done in one of two ways. The first way is with another system that has rsync installed, along with a remote shell program such as ssh. Let's say we had another system on our local network with a lot of available hard drive space and we wanted to perform our backup operation using the remote system instead of an external drive. Assuming that it already had a directory named /backup where we could deliver our files, we could do this:

rsync 程式的真正好處之一，是它可以被用來在網路間複製檔案。畢竟，rsync 中的“r”象徵著“remote”。 遠端複製可以透過兩種方法完成。第一個方法要求另一個系統已經安裝了 rsync 程式，還安裝了 遠端 shell 程式，比如 ssh。比方說我們本地網路中的一個系統有大量可用的硬碟空間，我們想要 用遠端系統來代替一個外部驅動器，來執行檔案備份操作。假定遠端系統中有一個名為/backup 的目錄， 其用來存放我們傳送的檔案，我們這樣做：

[me@linuxbox ~]$ sudo rsync -av --delete --rsh=ssh /etc /home /usr/local remote-sys:/backup

We made two changes to our command to facilitate the network copy. First, we added the –rsh=ssh option, which instructs rsync to use the ssh program as its remote shell. In this way, we were able to use an ssh encrypted tunnel to securely transfer the data from the local system to the remote host. Second, we specified the remote host by prefixing its name (in this case the remote host is named remote-sys) to the destination path name.

我們對命令做了兩處修改，來方便網路間檔案複製。首先，我們添加了--rsh=ssh 選項，其指示 rsync 使用 ssh 程式作為它的遠端 shell。以這種方式，我們就能夠使用一個 ssh 加密通道，把資料 安全地傳送到遠端主機中。其次，透過在目標路徑名前加上遠端主機的名字（在這種情況下， 遠端主機名為 remote-sys），來指定遠端主機。

The second way that rsync can be used to synchronize files over a network is by using an rysnc server. rsync can be configured to run as a daemon and listen to incoming requests for synchronization. This is often done to allow mirroring of a remote system. For example, Red Hat Software maintains a large repository of software packages under development for its Fedora distribution. It is useful for software testers to mirror this collection during the testing phase of the distribution release cycle. Since files in the repository change frequently (often more than once a day), it is desirable to maintain a local mirror by periodic synchronization, rather than by bulk copying of the repository. One of these repositories is kept at Georgia Tech; we could mirror it using our local copy of rsync and their rsync server like this:

rsync 可以被用來在網路間同步檔案的第二種方式是透過使用 rsync 伺服器。rsync 可以被配置為一個 守護程序，監聽即將到來的同步請求。這樣做經常是為了進行一個遠端系統的映象操作。例如，Red Hat 軟體中心為它的 Fedora 發行版，維護著一個巨大的正在開發中的軟體包的儲存庫。對於軟體測試人員， 在發行週期的測試階段，定期映象這些軟體集合是非常有幫助的。因為儲存庫中的這些檔案會頻繁地 （通常每天不止一次）改動，定期同步本地映象而不是大量地拷貝軟體儲存庫，這是更為明智的。 這些軟體函式庫之一被維護在喬治亞理工大學；我們可以使用本地 rsync 程式和它們的 rsync 伺服器來映象它。

[me@linuxbox ~]$ mkdir fedora-devel
[me@linuxbox ~]$ rsync -av -delete rsync://rsync.gtlib.gatech.edu/fedora-linux-
 core/development/i386/os fedora-devel

In this example, we use the URI of the remote rsync server, which consists of a protocol (rsync://), followed by the remote host name (rsync.gtlib.gatech.edu), followed by the pathname of the repository.

在這個例子裡，我們使用了遠端 rsync 伺服器的 URI，其由協議（rsync://），遠端主機名 （rsync.gtlib.gatech.edu），和軟體儲存庫的路徑名組成。

拓展閱讀

• The man pages for all of the commands discussed here are pretty clear and contain useful examples. In addition, the GNU Project has a good online manual for its version of tar. It can be found here:

• 在這裡討論的所有命令的手冊文件都相當清楚明白，並且包含了有用的例子。另外， GNU 版本的 tar 命令有一個不錯的線上文件。可以在下面連結處找到：

  http://www.gnu.org/software/tar/manual/index.html