Up to this point, we have seen a series of mysterious commands, each with its own mysterious options and arguments. In this chapter, we will attempt to remove some of that mystery and even create some of our own commands. The commands introduced in this chapter are:
在這之前，我們已經知道了一系列神祕的命令，每個命令都有自己奇妙的 選項和引數。在這一章中，我們將試圖去掉一些神祕性，甚至建立我們自己 的命令。這一章將介紹以下命令：
type – Indicate how a command name is interpreted
type – 說明怎樣解釋一個命令名
which – Display which executable program will be executed
which – 顯示會執行哪個可執行程式
man – Display a command’s manual page
man – 顯示命令手冊頁
apropos – Display a list of appropriate commands
apropos – 顯示一系列適合的命令
info – Display a command’s info entry
info – 顯示命令 info
whatis – Display a very brief description of a command
whatis – 顯示一個命令的簡潔描述
alias – Create an alias for a command
alias – 建立命令別名
A command can be one of four different things:
命令可以是下面四種形式之一：
An executable program like all those files we saw in /usr/bin. Within this category, programs can be compiled binaries such as programs written in C and C++, or programs written in scripting languages such as the shell, perl, python, ruby, etc.
A command built into the shell itself. bash supports a number of commands internally called shell builtins. The cd command, for example, is a shell builtin.
A shell function. These are miniature shell scripts incorporated into the environment. We will cover configuring the environment and writing shell functions in later chapters, but for now, just be aware that they exist.
An alias. Commands that we can define ourselves, built from other commands.
是一個可執行程式，就像我們所看到的位於目錄/usr/bin 中的檔案一樣。 這一類別程式可以是用諸如 C 和 C++語言寫成的程式編譯的二進位制檔案, 也可以是由諸如shell，perl，python，ruby等等指令碼語言寫成的程式 。
是一個內建於 shell 自身的命令。bash 支援若干命令，內部叫做 shell 內部命令 (builtins)。例如，cd 命令，就是一個 shell 內部命令。
是一個 shell 函式。這些是小規模的 shell 指令碼，它們混合到環境變數中。 在後續的章節裡，我們將討論配置環境變數以及書寫 shell 函式。但是現在， 僅僅意識到它們的存在就可以了。
是一個命令別名。我們可以定義自己的命令，建立在其它命令之上。
It is often useful to know exactly which of the four kinds of commands is being used and Linux provides a couple of ways to find out.
這經常很有用，能確切地知道正在使用四類別命令中的哪一類別。Linux 提供了一對方法來 弄明白命令型別。
The type command is a shell builtin that displays the kind of command the shell will execute, given a particular command name. It works like this:
type 命令是 shell 內部命令，它會顯示命令的類別，給出一個特定的命令名（做為引數）。 它像這樣工作：
type command
Where “command” is the name of the command you want to examine. Here are some examples:
“command”是你要檢測的命令名。這裡有些例子：
[me@linuxbox ~]$ type type
type is a shell builtins
[me@linuxbox ~]$ type ls
ls is aliased to `ls --color=tty`
[me@linuxbox ~]$ type cp
cp is /bin/cp
Here we see the results for three different commands. Notice that the one for ls (taken from a Fedora system) and how the ls command is actually an alias for the ls command with the “--color=tty” option added. Now we know why the output from ls is displayed in color!
我們看到這三個不同命令的檢測結果。注意，ls 命令（在 Fedora 系統中）的檢查結果，ls 命令實際上 是 ls 命令加上選項”--color=tty”的別名。現在我們知道為什麼 ls 的輸出結果是有顏色的！
Sometimes there is more than one version of an executable program installed on a system. While this is not very common on desktop systems, it’s not unusual on large servers. To determine the exact location of a given executable, the which command is used:
有時候在一個作業系統中，不只安裝了可執行程式的一個版本。雖然在桌面系統中這並不普遍， 但在大型伺服器中卻很平常。為了確定所給定的執行程式的準確位置，使用 which 命令：
[me@linuxbox ~]$ which ls
/bin/ls
which only works for executable programs, not builtins nor aliases that are substitutes for actual executable programs. When we try to use which on a shell builtin, for example, cd, we either get no response or an error message:
這個命令只對可執行程式有效，不包括內建命令和命令別名，別名是真正的可執行程式的替代物。 當我們試著使用 shell 內建命令時，例如，cd 命令，我們或者得不到迴應，或者是個錯誤資訊：
[me@linuxbox ~]$ which cd
/usr/bin/which: no cd in
(/opt/jre1.6.0_03/bin:/usr/lib/qt-3.3/bin:/usr/kerberos/bin:/opt/jre1
.6.0_03/bin:/usr/lib/ccache:/usr/local/bin:/usr/bin:/bin:/home/me/bin)
which is a fancy way of saying “command not found.”
說“命令沒有找到”，真是很奇特。
With this knowledge of what a command is, we can now search for the documentation available for each kind of command.
知道了什麼是命令，現在我們來尋找每一類別命令的可得到的文件。
bash has a built-in help facility available for each of the shell builtins. To use it, type “help” followed by the name of the shell builtin. For example:
bash 有一個內建的幫助工具，可供每一個 shell 內建命令使用。輸入“help”，接著是 shell 內部命令名。例如：
[me@linuxbox ~]$ help cd
cd: cd [-L|-P] [dir]
Change ...
A note on notation: When square brackets appear in the description of a command’s syntax, they indicate optional items. A vertical bar character indicates mutually exclusive items. In the case of the cd command above:
注意表示法：出現在命令語法說明中的方括號，表示可選的專案。一個豎槓字元 表示互斥選項。在上面 cd 命令的例子中：
cd [-L|-P] [dir]
This notation says that the command cd may be followed optionally by either a “-L” or a “-P” and further, optionally followed by the argument “dir”.
這種表示法說明，cd 命令可能有一個“-L”選項或者“-P”選項，進一步，可能有引數“dir”。
While the output of help for the cd commands is concise and accurate, it is by no means tutorial and as we can see, it also seems to mention a lot of things we haven’t talked about yet! Don’t worry. We’ll get there.
雖然 cd 命令的幫助文件很簡潔準確，但它決不是課程。正如我們所看到的，它似乎提到了許多 我們還沒有談論到的東西！不要擔心，我們會學到的。
Many executable programs support a “--help” option that displays a description of the command’s supported syntax and options. For example:
許多可執行程式支援一個 --help 選項，這個選項是顯示命令所支援的語法和選項說明。例如：
[me@linuxbox ~]$ mkdir --help
Usage: mkdir [OPTION] DIRECTORY...
Create ...
Some programs don’t support the “--help” option, but try it anyway. Often it results in an error message that will reveal the same usage information.
一些程式不支援 --help 選項，但不管怎樣試一下。這經常會導致輸出錯誤資訊，但同時能 揭示一樣的命令用法資訊。
Most executable programs intended for command line use provide a formal piece of documentation called a manual or man page. A special paging program called man is used to view them. It is used like this:
許多希望被命令列使用的可執行程式，提供了一個正式的文件，叫做手冊或手冊頁(man page)。一個特殊的叫做 man 的分頁程式，可用來瀏覽他們。它是這樣使用的：
man program
where “program” is the name of the command to view.
“program”是要瀏覽的命令名。
Man pages vary somewhat in format but generally contain a title, a synopsis of the command’s syntax, a description of the command’s purpose, and a listing and description of each of the command’s options. Man pages, however, do not usually include examples, and are intended as a reference, not a tutorial. As an example, Let's try viewing the man page for the ls command:
手冊文件的格式有點不同，一般地包含一個標題、命令語法的綱要、命令用途的說明、 以及每個命令選項的列表和說明。然而，手冊文件通常並不包含範例，它打算 作為一本參考手冊，而不是課程。作為一個例子，瀏覽一下 ls 命令的手冊文件：
[me@linuxbox ~]$ man ls
On most Linux systems, man uses less to display the manual page, so all of the familiar less commands work while displaying the page.
在大多數 Linux 系統中，man 使用 less 工具來顯示參考手冊，所以當瀏覽文件時，你所熟悉的 less 命令都能有效。
The “manual” that man displays is broken into sections and not only covers user commands but also system administration commands, programming interfaces, file formats and more. The table below describes the layout of the manual:
man 所顯示的參考手冊，被分成幾個章節，它們不僅僅包括使用者命令，也包括系統管理員 命令、程式介面、檔案格式等等。下表描繪了手冊的佈局：
| Section | Contents |
|---|---|
| 1 | User commands |
| 2 | Programming interfaces kernel system calls |
| 3 | Programming interfaces to the C library |
| 4 | Special files such as device nodes and drivers |
| 5 | File formats |
| 6 | Games and amusements such as screen savers |
| 7 | Miscellaneous |
| 8 | System administration commands |
| 章節 | 內容 |
|---|---|
| 1 | 使用者命令 |
| 2 | 程式介面核心系統呼叫 |
| 3 | C 函式庫函式程式介面 |
| 4 | 特殊檔案，比如說裝置結點和驅動程式 |
| 5 | 檔案格式 |
| 6 | 遊戲娛樂，如螢幕保護程式 |
| 7 | 其他方面 |
| 8 | 系統管理員命令 |
Sometimes we need to look in a specific section of the manual to find what we are looking for. This is particularly true if we are looking for a file format that is also the name of a command. Without specifying a section number, we will always get the first instance of a match, probably in section 1. To specify a section number, we use man like this:
有時候，我們需要檢視參考手冊的特定章節，從而找到我們需要的資訊。 如果我們要查詢一種檔案格式，而同時它也是一個命令名時,這種情況尤其正確。 沒有指定章節號，我們總是得到第一個匹配項，可能在第一章節。我們這樣使用 man 命令， 來指定章節號：
man section search_term
For example:
例如：
[me@linuxbox ~]$ man 5 passwd
This will display the man page describing the file format of the /etc/passwd file.
命令執行結果會顯示檔案 /etc/passwd 的檔案格式說明手冊。
It is also possible to search the list of man pages for possible matches based on a search term. It’s very crude but sometimes helpful. Here is an example of a search for man pages using the search term “floppy”:
也有可能搜尋參考手冊列表，基於某個關鍵字的匹配項。雖然很粗糙但有時很有用。 下面是一個以”floppy”為關鍵詞來搜尋參考手冊的例子：
[me@linuxbox ~]$ apropos floppy
create_floppy_devices (8) - udev callout to create all possible
...
The first field in each line of output is the name of the man page, the second field shows the section. Note that the man command with the “-k” option performs the exact same function as apropos.
輸出結果每行的第一個欄位是手冊頁的名字，第二個欄位展示章節。注意，man 命令加上”-k”選項， 和 apropos 完成一樣的功能。
The whatis program displays the name and a one line description of a man page matching a specified keyword:
whatis 程式顯示匹配特定關鍵字的手冊頁的名字和一行命令說明：
The Most Brutal Man Page Of Them All
最晦澀難懂的手冊頁
As we have seen, the manual pages supplied with Linux and other Unix-like systems are intended as reference documentation and not as tutorials. Many man pages are hard to read, but I think that the grand prize for difficulty has got to go to the man page for bash. As I was doing my research for this book, I gave it careful review to ensure that I was covering most of its topics. When printed, it’s over eighty pages long and extremely dense, and its structure makes absolutely no sense to a new user.
正如我們所看到的，Linux 和類別 Unix 的系統提供的手冊頁，只是打算作為參考手冊使用， 而不是課程。許多手冊頁都很難閱讀，但是我認為由於閱讀難度而能拿到特等獎的手冊頁應該是 bash 手冊頁。因為我正在為這本書做我的研究，所以我很仔細地瀏覽了整個 bash 手冊，為的是確保我講述了 大部分的 bash 主題。當把 bash 參考手冊整個打印出來，其篇幅有八十多頁且內容極其緊密， 但對於初學者來說，其結構安排毫無意義。
On the other hand, it is very accurate and concise, as well as being extremely complete. So check it out if you dare and look forward to the day when you can read it and it all makes sense.
另一方面，bash 參考手冊的內容非常簡明精確，同時也非常完善。所以，如果你有膽量就檢視一下， 並且期望有一天你能讀懂它。
The GNU Project provides an alternative to man pages for their programs, called “info.” Info pages are displayed with a reader program named, appropriately enough, info. Info pages are hyperlinked much like web pages. Here is a sample:
GNU 專案提供了一個命令程式手冊頁的替代物，稱為”info”。info 內容可透過 info 閱讀器 程式讀取。info 頁是超級連結形式的，和網頁很相似。這有個例子：
File: coreutils.info, Node: ls invocation, Next: dir invocation,
Up: Directory listing
10.1 `ls': List directory contents
==================================
...
The info program reads info files, which are tree structured into individual nodes, each containing a single topic. Info files contain hyperlinks that can move you from node to node. A hyperlink can be identified by its leading asterisk, and is activated by placing the cursor upon it and pressing the enter key.
info 程式讀取 info 檔案，info 檔案是樹型結構，分化為各個結點，每一個包含一個題目。 info 檔案包含超級連結，它可以讓你從一個結點跳到另一個結點。一個超級連結可透過 它開頭的星號來辨別出來，把游標放在它上面並按下 enter 鍵，就可以啟用它。
To invoke info, type “info” followed optionally by the name of a program. Below is a table of commands used to control the reader while displaying an info page:
輸入”info”，接著輸入程式名稱，啟動 info。下表中的命令，當顯示一個 info 頁面時， 用來控制閱讀器。
| Command | Action |
|---|---|
| ? | Display command help |
| PgUp or Backspace | Display privious page |
| PgDn or Space | Display next page |
| n | Next - Display the next node |
| p | Previous - Display the previous node |
| u | Up - Display the parent node of the currently displayed node, usually a menu. |
| Enter | Follow the hyperlink at the cursor location |
| q | Quit |
| 命令 | 行為 |
|---|---|
| ? | 顯示命令幫助 |
| PgUp or Backspace | 顯示上一頁 |
| PgDn or Space | 顯示下一頁 |
| n | 下一個 - 顯示下一個結點 |
| p | 上一個 - 顯示上一個結點 |
| u | Up - 顯示當前所顯示結點的父結點，通常是個選單 |
| Enter | 啟用游標位置下的超級連結 |
| q | 退出 |
Most of the command line programs we have discussed so far are part of the GNU Project’s “coreutils” package, so typing:
到目前為止，我們所討論的大多數命令列程式，屬於 GNU 專案”coreutils”包，所以輸入：
[me@linuxbox ~]$ info coreutils
will display a menu page with hyperlinks to each program contained in the coreutils package.
將會顯示一個包含超級連結的手冊頁，這些超級連結指向包含在 coreutils 包中的各個程式。
Many software packages installed on your system have documentation files residing in the /usr/share/doc directory. Most of these are stored in plain text format and can be viewed with less. Some of the files are in HTML format and can be viewed with a web browser. We may encounter some files ending with a “.gz” extension. This indicates that they have been compressed with the gzip compression program. The gzip package includes a special version of less called zless that will display the contents of gzip-compressed text files.
許多安裝在你係統中的軟體，都有自己的文件檔案，這些檔案位於/usr/share/doc 目錄下。 這些檔案大多數是以文字檔案的形式儲存的，可用 less 閱讀器來瀏覽。一些檔案是 HTML 格式， 可用網頁瀏覽器來閱讀。我們可能遇到許多以”.gz”結尾的檔案。這表示 gzip 壓縮程式 已經壓縮了這些檔案。gzip 軟體包包括一個特殊的 less 版本，叫做 zless，zless 可以顯示由 gzip 壓縮的文字檔案的內容。
Now for our very first experience with programming! We will create a command of our own using the alias command. But before we start, we need to reveal a small command line trick. It’s possible to put more than one command on a line by separating each command with a semicolon character. It works like this:
現在是時候，感受第一次程式設計經歷了！我們將用 alias 命令建立我們自己的命令。但在 開始之前，我們需要展示一個命令列小技巧。可以把多個命令放在同一行上，命令之間 用”;”分開。它像這樣工作：
command1; command2; command3...
Here’s the example we will use:
我們會用到下面的例子：
[me@linuxbox ~]$ cd /usr; ls; cd -
bin games kerberos lib64 local share tmp
...
[me@linuxbox ~]$
As we can see, we have combined three commands on one line. First we change directory to /usr then list the directory and finally return to the original directory (by using ‘cd -‘) so we end up where we started. Now Let's turn this sequence into a new command using alias. The first thing we have to do is dream up a name for our new command. Let's try “test”. Before we do that, it would be a good idea to find out if the name “test” is already being used. To find out, we can use the type command again:
正如我們看到的，我們在一行上聯合了三個命令。首先更改目錄到/usr，然後列出目錄 內容，最後回到原始目錄（用命令”cd -“）,結束在開始的地方。現在，透過 alias 命令 把這一串命令轉變為一個命令。我們要做的第一件事就是為我們的新命令構想一個名字。 比方說”test”。在使用”test”之前，查明是否”test”命令名已經存在系統中，是個很不錯 的主意。為了查清此事，可以使用 type 命令：
[me@linuxbox ~]$ type test
test is a shell builtin
Oops! The name “test” is already taken. Let's try “foo”:
哦！”test”名字已經被使用了。試一下”foo”:
[me@linuxbox ~]$ type foo
bash: type: foo: not found
Great! “foo” is not taken. So Let's create our alias:
太棒了！”foo”還沒被佔用。建立命令別名：
[me@linuxbox ~]$ alias foo='cd /usr; ls; cd -'
Notice the structure of this command:
注意命令結構：
alias name='string'
After the command “alias” we give alias a name followed immediately (no whitespace allowed) by an equals sign, followed immediately by a quoted string containing the meaning to be assigned to the name. After we define our alias, it can be used anywhere the shell would expect a command. Let's try it:
在命令”alias”之後，輸入“name”，緊接著（沒有空格）是一個等號，等號之後是 一串用引號引起的字串，字串的內容要賦值給 name。我們定義了別名之後， 這個命令別名可以使用在任何地方。試一下：
[me@linuxbox ~]$ foo
bin games kerberos lib64 local share tmp
...
[me@linuxbox ~]$
We can also use the type command again to see our alias:
我們也可以使用 type 命令來檢視我們的別名：
[me@linuxbox ~]$ type foo
foo is aliased to `cd /usr; ls ; cd -'
To remove an alias, the unalias command is used, like so:
刪除別名，使用 unalias 命令，像這樣：
[me@linuxbox ~]$ unalias foo
[me@linuxbox ~]$ type foo
bash: type: foo: not found
While we purposefully avoided naming our alias with an existing command name, it is not uncommon to do so. This is often done to apply a commonly desired option to each invocation of a common command. For instance, we saw earlier how the ls command is often aliased to add color support:
雖然我們有意避免使用已經存在的命令名來命名我們的別名，但這是常做的事情。通常， 會把一個普遍用到的選項加到一個經常使用的命令後面。例如，之前見到的 ls 命令，會 帶有色彩支援：
[me@linuxbox ~]$ type ls
ls is aliased to 'ls --color=tty'
To see all the aliases defined in the environment, use the alias command without arguments. Here are some of the aliases defined by default on a Fedora system. Try and figure out what they all do:
要檢視所有定義在系統環境中的別名，使用不帶引數的 alias 命令。下面在 Fedora 系統中 預設定義的別名。試著弄明白，它們是做什麼的：
[me@linuxbox ~]$ alias
alias l.='ls -d .* --color=tty'
...
There is one tiny problem with defining aliases on the command line. They vanish when your shell session ends. In a later chapter, we will see how to add our own aliases to the files that establish the environment each time we log on, but for now, enjoy the fact that we have taken our first, albeit tiny, step into the world of shell programming!
在命令列中定義別名有點小問題。當你的 shell 會話結束時，它們會消失。隨後的章節裡， 我們會了解怎樣把自己的別名新增到檔案中去，每次我們登入系統，這些檔案會建立系統環境。 現在，好好享受我們剛經歷過的，步入 shell 程式設計世界的第一步吧，雖然微小。
Now that we have learned how to find the documentation for commands, go and look up the documentation for all the commands we have encountered so far. Study what additional options are available and try them out!
既然我們已經學習了怎樣找到命令的幫助文件，那就試著查閱，到目前為止，我們學到的所有 命令的文件。學習命令其它可用的選項，練習一下！
There are many online sources of documentation for Linux and the command line. Here are some of the best:
在網上，有許多關於 Linux 和命令列的文件。以下是一些最好的文件：
The Bash Reference Manual is a reference guide to the bash shell. It’s still a reference work but contains examples and is easier to read than the bash man page.
Bash 參考手冊是一本 bash shell 的參考指南。它仍然是一本參考書，但是包含了很多 範例，而且它比 bash 手冊頁容易閱讀。
The Bash FAQ contains answers to frequently asked questions regarding bash. This list is aimed at intermediate to advanced users, but contains a lot of good information.
Bash FAQ 包含關於 bash，而經常提到的問題的答案。這個列表面向 bash 的中高階使用者， 但它包含了許多有幫助的資訊。
The GNU Project provides extensive documentation for its programs, which form the core of the Linux command line experience. You can see a complete list here:
GUN 專案為它的程式提供了大量的文件，這些文件組成了 Linux 命令列實驗的核心。 這裡你可以看到一個完整的列表：
Wikipedia has an interesting article on man pages:
Wikipedia 有一篇關於手冊頁的有趣文章：