很多 Linux® 系统管理员都需要做一些整理纯文本配置文件的乏味工作。幸运的是，Linux 有很多源自于 UNIX® 的数据提取工具，包括 head、tail、grep、egrep、fgrep、cut、paste、join、awk 等。本文给出了几个真实的例子，它们可以展示如何使用这些简单的命令行工具更好地服务于系统管理工作。本文将逐一介绍这些数据提取工具及其操作，将它们应用到日常工作所使用的典型文件中，并介绍一下为什么这些工具对于从这些文件中提取数据来说非常重要。

　　Linux 操作系统中有很多文件：配置文件、文本文件、文档文件、日志文件、用户文件，这个清单还在不断增长。通常，这些文件都包含了要查找重要数据所需要访问的一些信息。尽管我们可以简单地使用诸如 cat、more 之类的标准工具将大部分文件的内容输出到屏幕上，但是系统中有更加合适的工具可以对文本进行过滤和处理，这样就可以只关心我们想要的内容。

　　在阅读本文的过程中，您可以打开 shell 并体验一下每个工具的例子。

　　正则表达式

　　在开始之前，我们需要首先理解什么是正则表达式，以及如何使用正则表达式。

　　在最简单的形式中，正则表达式（regular expression）是用来在文件中定位文本的一些搜索标准。例如，要查找所有包含单词 “admin” 的行，我们就可以对 “admin” 进行搜索。因此，“admin” 就构成了一个正则表达式。如果我们不但希望查找 “admin”，而且还想将其替换成 “root”，那么我们就可以在一个工具中使用适当的命令将 “admin” 替换成 “root”。它们都构成了正则表达式。

　　正则表达式所采用的一些基本规则如下：

• 任何单个字符或一串字符都可以匹配字符本身，例如上面的 “admin” 的例子。
  
• ^ 符号（^）表示一行的开始；$ 符号（$）表示一行的结束。
  
• 要搜索特殊字符（例如 $ 符号），需要在这些字符前面加上反斜线（\）。例如， \$ 就表示查找 $，而不是一行的末尾。
  
• 点（.）代表任何单个字符。例如，ad..n 代表 5 个字符项，前两个字符是 “ad”，最后一个字符是 “n”。中间两个字符可以是任何字符，但是只能是由两个字符组成。
  
• 任何时候如果正则表达式包含在斜线中（例如 /re/），搜索就是通过文件顺序进行的。如果正则表达式包含在问号中（例如，?re?），搜索就是通过文件逆序进行的。
  
• 方括号（[]）表示多个值，减号（-）表示值的范围。例如，[0-9] 与 [0123456789] 相同，[a-z] 就等效于搜索任何小写字符。如果一个列表的首字符是 ^ 符号，它就匹配不在这个清单中的任何字符。

　　表 1 给出了这些规则是如何真正进行匹配的。

　　了解了这些信息，下面让我们开始看一下相关工具。

　　grep

　　grep 工具的工作方式是对文件的每一行搜索给定字符串的首次出现。如果找到了这个字符串，就打印该行的内容；否则就不对该行进行打印。下面这个文件我称之为 “memo”，阐述了 grep 的用法和结果。

To: All Employees

From: Human Resources

In order to better serve the needs of our mass market customers, ABC Publishing is integrating the groups selling to this channel for ABC General Reference and ABC Computer Publishing. This change will allow us to better coordinate our selling and marketing efforts, as well as simplify ABC's relationships with these customers in the areas of customer service, co-op management, and credit and collection. Two national account managers, Ricky Ponting and Greeme Smith, have joined the sales team as a result of these changes.

To achieve this goal, we have also organized the new mass sales group into three distinct teams reporting to our current sales directors, Stephen Fleming and Boris Baker. I have outlined below the national account managers and their respective accounts in each of the teams. We have also hired two new national account managers and a new sales administrator to complete our account coverage. They include:

Sachin Tendulkar, who joins us from XYZ Consumer Electronics as a national account manager covering traditional mass merchants.

Brian Lara, who comes to us via PQR Company and will be responsible for managing our West Coast territory.

Shane Warne, who will become an account administrator for our warehouse clubs business and joins us from DEF division.

Effectively, we have seven new faces on board:

1. RICKY PONTING
2. GREEME SMITH
3. STEPHEN FLEMING
4. BORIS BAKER
5. SACHIN TENDULKAR
6. BRIAN LARA
7. SHANE WARNE

Please join me in welcoming each of our new team members.

　　举一个简单的例子来说，要查找包含 “welcoming” 单词的行，最好的方法是使用下面的命令行：

# grep welcoming memo 
Please join me in welcoming each of our new team members. 

　　如果您想查找单词 “market”，结果会有很大的不同，如下所示：

# grep market memo 
In order to better serve the needs of our mass 
market customers, ABC Publishing is 
integrating the groups selling to this channel 
for ABC General Reference and ABC Computer 
Publishing. This change will allow us to 
better coordinate our selling and marketing 
efforts, as well as simplify ABC's 
relationships with these customers in the 
areas of customer service, co-op management, 
and credit and collection. Two national 
account managers, Ricky Ponting and Greeme 
Smith, have joined the sales team as a result 
of these changes. 

　　注意我们找到了两个匹配项：我们希望找的 “market” 和 “marketing”。如果在这个文件中还存在 “marketable” 或 “marketed” 之类的单词，那么上面的命令也会显示包含这些单词的行的内容。

　　在 grep 中可以使用通配符和元字符，我强烈建议将它们放到引号中，这样 shell 就不会将它们解释成命令了。

　　要查找所有包含数字的行，请使用下面的命令：

# grep  "[0-9]" memo 
1. RICKY PONTING 
2. GREEME SMITH 
3. STEPHEN FLEMING 
4. BORIS BAKER 
5. SACHIN TENDULKAR 
6. BRIAN LARA 
7. SHANE WARNE 

　　要查找所有包含 “the” 的行，请使用下面的命令：

# grep the memo 
In order to better serve the needs of our mass 
market customers, ABC Publishing is 
integrating the groups selling to this channel 
for ABC General Reference and ABC Computer 
Publishing. This change will allow us to 
better coordinate our selling and marketing 
efforts, as well as simplify ABC's 
relationships with these customers in the 
areas of customer service, co-op management, 
and credit and collection. Two national 
account managers, Ricky Ponting and Greeme 
Smith, have joined the sales team as a result 
of these changes. 

To achieve this goal, we have also organized 
the new mass sales group into three distinct 
teams reporting to our current sales 
directors, Stephen Flemming and Boris Baker. I 
have outlined below the national account 
managers and their respective accounts in each 
of the teams. We have also hired two new 
national account managers and a new sales 
administrator to complete our account 
coverage. They include: