mirror of
https://github.com/LCTT/TranslateProject.git
synced 2026-08-23 04:03:29 +08:00
Xingyu.Wang
GitHub
@@ -1,252 +0,0 @@
|
||||
[#]: collector: (lujun9972)
|
||||
[#]: translator: (liwenwensnow)
|
||||
[#]: reviewer: ( )
|
||||
[#]: publisher: ( )
|
||||
[#]: url: ( )
|
||||
[#]: subject: (Fields, records, and variables in awk)
|
||||
[#]: via: (https://opensource.com/article/19/11/fields-records-variables-awk)
|
||||
[#]: author: (Seth Kenlon https://opensource.com/users/seth)
|
||||
|
||||
Fields, records, and variables in awk
|
||||
======
|
||||
In the second article in this intro to awk series, learn about fields,
|
||||
records, and some powerful awk variables.
|
||||
![Man at laptop on a mountain][1]
|
||||
|
||||
Awk comes in several varieties: There is the original **awk**, written in 1977 at AT&T Bell Laboratories, and several reimplementations, such as **mawk**, **nawk**, and the one that ships with most Linux distributions, GNU awk, or **gawk**. On most Linux distributions, awk and gawk are synonyms referring to GNU awk, and typing either invokes the same awk command. See the [GNU awk user's guide][2] for the full history of awk and gawk.
|
||||
|
||||
The [first article][3] in this series showed that awk is invoked on the command line with this syntax:
|
||||
|
||||
|
||||
```
|
||||
`$ awk [options] 'pattern {action}' inputfile`
|
||||
```
|
||||
|
||||
Awk is the command, and it can take options (such as **-F** to define the field separator). The action you want awk to perform is contained in single quotes, at least when it's issued in a terminal. To further emphasize which part of the awk command is the action you want it to take, you can precede your program with the **-e** option (but it's not required):
|
||||
|
||||
|
||||
```
|
||||
$ awk -F, -e '{print $2;}' colours.txt
|
||||
yellow
|
||||
blue
|
||||
green
|
||||
[...]
|
||||
```
|
||||
|
||||
### Records and fields
|
||||
|
||||
Awk views its input data as a series of _records_, which are usually newline-delimited lines. In other words, awk generally sees each line in a text file as a new record. Each record contains a series of _fields_. A field is a component of a record delimited by a _field separator_.
|
||||
|
||||
By default, awk sees whitespace, such as spaces, tabs, and newlines, as indicators of a new field. Specifically, awk treats multiple _space_ separators as one, so this line contains two fields:
|
||||
|
||||
|
||||
```
|
||||
`raspberry red`
|
||||
```
|
||||
|
||||
As does this one:
|
||||
|
||||
|
||||
```
|
||||
`tuxedo black`
|
||||
```
|
||||
|
||||
Other separators are not treated this way. Assuming that the field separator is a comma, the following example record contains three fields, with one probably being zero characters long (assuming a non-printable character isn't hiding in that field):
|
||||
|
||||
|
||||
```
|
||||
`a,,b`
|
||||
```
|
||||
|
||||
### The awk program
|
||||
|
||||
The _program_ part of an awk command consists of a series of rules. Normally, each rule begins on a new line in the program (although this is not mandatory). Each rule consists of a pattern and one or more actions:
|
||||
|
||||
|
||||
```
|
||||
`pattern { action }`
|
||||
```
|
||||
|
||||
In a rule, you can define a pattern as a condition to control whether the action will run on a record. Patterns can be simple comparisons, regular expressions, combinations of the two, and more.
|
||||
|
||||
For instance, this will print a record _only_ if it contains the word "raspberry":
|
||||
|
||||
|
||||
```
|
||||
$ awk '/raspberry/ { print $0 }' colours.txt
|
||||
raspberry red 99
|
||||
```
|
||||
|
||||
If there is no qualifying pattern, the action is applied to every record.
|
||||
|
||||
Also, a rule can consist of only a pattern, in which case the entire record is written as if the action was **{ print }**.
|
||||
|
||||
Awk programs are essentially _data-driven_ in that actions depend on the data, so they are quite a bit different from programs in many other programming languages.
|
||||
|
||||
### The NF variable
|
||||
|
||||
Each field has a variable as a designation, but there are special variables for fields and records, too. The variable **NF** stores the number of fields awk finds in the current record. This can be printed or used in tests. Here is an example using the [text file][3] from the previous article:
|
||||
|
||||
|
||||
```
|
||||
$ awk '{ print $0 " (" NF ")" }' colours.txt
|
||||
name color amount (3)
|
||||
apple red 4 (3)
|
||||
banana yellow 6 (3)
|
||||
[...]
|
||||
```
|
||||
|
||||
Awk's **print** function takes a series of arguments (which may be variables or strings) and concatenates them together. This is why, at the end of each line in this example, awk prints the number of fields as an integer enclosed by parentheses.
|
||||
|
||||
### The NR variable
|
||||
|
||||
In addition to counting the fields in each record, awk also counts input records. The record number is held in the variable **NR**, and it can be used in the same way as any other variable. For example, to print the record number before each line:
|
||||
|
||||
|
||||
```
|
||||
$ awk '{ print NR ": " $0 }' colours.txt
|
||||
1: name color amount
|
||||
2: apple red 4
|
||||
3: banana yellow 6
|
||||
4: raspberry red 3
|
||||
5: grape purple 10
|
||||
[...]
|
||||
```
|
||||
|
||||
Note that it's acceptable to write this command with no spaces other than the one after **print**, although it's more difficult for a human to parse:
|
||||
|
||||
|
||||
```
|
||||
`$ awk '{print NR": "$0}' colours.txt`
|
||||
```
|
||||
|
||||
### The printf() function
|
||||
|
||||
For greater flexibility in how the output is formatted, you can use the awk **printf()** function. This is similar to **printf** in C, Lua, Bash, and other languages. It takes a _format_ argument followed by a comma-separated list of items. The argument list may be enclosed in parentheses.
|
||||
|
||||
|
||||
```
|
||||
`$ printf format, item1, item2, ...`
|
||||
```
|
||||
|
||||
The format argument (or _format string_) defines how each of the other arguments will be output. It uses _format specifiers_ to do this, including **%s** to output a string and **%d** to output a decimal number. The following **printf** statement outputs the record followed by the number of fields in parentheses:
|
||||
|
||||
|
||||
```
|
||||
$ awk 'printf "%s (%d)\n",$0,NF}' colours.txt
|
||||
name color amount (3)
|
||||
raspberry red 4 (3)
|
||||
banana yellow 6 (3)
|
||||
[...]
|
||||
```
|
||||
|
||||
In this example, **%s (%d)** provides the structure for each line, while **$0,NF** defines the data to be inserted into the **%s** and **%d** positions. Note that, unlike with the **print** function, no newline is generated without explicit instructions. The escape sequence **\n** does this.
|
||||
|
||||
### Awk scripting
|
||||
|
||||
All of the awk code in this article has been written and executed in an interactive Bash prompt. For more complex programs, it's often easier to place your commands into a file or _script_. The option **-f FILE** (not to be confused with **-F**, which denotes the field separator) may be used to invoke a file containing a program.
|
||||
|
||||
For example, here is a simple awk script. Create a file called **example1.awk** with this content:
|
||||
|
||||
|
||||
```
|
||||
/^a/ {print "A: " $0}
|
||||
/^b/ {print "B: " $0}
|
||||
```
|
||||
|
||||
It's conventional to give such files the extension **.awk** to make it clear that they hold an awk program. This naming is not mandatory, but it gives file managers and editors (and you) a useful clue about what the file is.
|
||||
|
||||
Run the script:
|
||||
|
||||
|
||||
```
|
||||
$ awk -f example1.awk colours.txt
|
||||
A: raspberry red 4
|
||||
B: banana yellow 6
|
||||
A: apple green 8
|
||||
```
|
||||
|
||||
A file containing awk instructions can be made into a script by adding a **#!** line at the top and making it executable. Create a file called **example2.awk** with these contents:
|
||||
|
||||
|
||||
```
|
||||
#!/usr/bin/awk -f
|
||||
#
|
||||
# Print all but line 1 with the line number on the front
|
||||
#
|
||||
|
||||
NR > 1 {
|
||||
printf "%d: %s\n",NR,$0
|
||||
}
|
||||
```
|
||||
|
||||
Arguably, there's no advantage to having just one line in a script, but sometimes it's easier to execute a script than to remember and type even a single line. A script file also provides a good opportunity to document what a command does. Lines starting with the **#** symbol are comments, which awk ignores.
|
||||
|
||||
Grant the file executable permission:
|
||||
|
||||
|
||||
```
|
||||
`$ chmod u+x example2.awk`
|
||||
```
|
||||
|
||||
Run the script:
|
||||
|
||||
|
||||
```
|
||||
$ ./example2.awk colours.txt
|
||||
2: apple red 4
|
||||
2: banana yellow 6
|
||||
4: raspberry red 3
|
||||
5: grape purple 10
|
||||
[...]
|
||||
```
|
||||
|
||||
An advantage of placing your awk instructions in a script file is that it's easier to format and edit. While you can write awk on a single line in your terminal, it can get overwhelming when it spans several lines.
|
||||
|
||||
### Try it
|
||||
|
||||
You now know enough about how awk processes your instructions to be able to write a complex awk program. Try writing an awk script with more than one rule and at least one conditional pattern. If you want to try more functions than just **print** and **printf**, refer to [the gawk manual][4] online.
|
||||
|
||||
Here's an idea to get you started:
|
||||
|
||||
|
||||
```
|
||||
#!/usr/bin/awk -f
|
||||
#
|
||||
# Print each record EXCEPT
|
||||
# IF the first record contains "raspberry",
|
||||
# THEN replace "red" with "pi"
|
||||
|
||||
$1 == "raspberry" {
|
||||
gsub(/red/,"pi")
|
||||
}
|
||||
|
||||
{ print }
|
||||
```
|
||||
|
||||
Try this script to see what it does, and then try to write your own.
|
||||
|
||||
The next article in this series will introduce more functions for even more complex (and useful!) scripts.
|
||||
|
||||
* * *
|
||||
|
||||
_This article is adapted from an episode of [Hacker Public Radio][5], a community technology podcast._
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
via: https://opensource.com/article/19/11/fields-records-variables-awk
|
||||
|
||||
作者：[Seth Kenlon][a]
|
||||
选题：[lujun9972][b]
|
||||
译者：[译者ID](https://github.com/译者ID)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]: https://opensource.com/users/seth
|
||||
[b]: https://github.com/lujun9972
|
||||
[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/computer_laptop_code_programming_mountain_view.jpg?itok=yx5buqkr (Man at laptop on a mountain)
|
||||
[2]: https://www.gnu.org/software/gawk/manual/html_node/History.html#History
|
||||
[3]: https://opensource.com/article/19/10/intro-awk
|
||||
[4]: https://www.gnu.org/software/gawk/manual/
|
||||
[5]: http://hackerpublicradio.org/eps.php?id=2129
|
||||
@@ -0,0 +1,249 @@
|
||||
[#]: collector: (lujun9972)
|
||||
[#]: translator: (liwenwensnow)
|
||||
[#]: reviewer: ( )
|
||||
[#]: publisher: ( )
|
||||
[#]: url: ( )
|
||||
[#]: subject: (Fields, records, and variables in awk)
|
||||
[#]: via: (https://opensource.com/article/19/11/fields-records-variables-awk)
|
||||
[#]: author: (Seth Kenlon https://opensource.com/users/seth)
|
||||
|
||||
awk中的字段，记录和变量
|
||||
======
|
||||
这个系列的第二篇，我们会学习 字段，记录和一些非常有用的awk变量。
|
||||
![Man at laptop on a mountain][1]
|
||||
|
||||
Awk 有好几个变种： 最早的 **awk**, 是1977 年 AT&T Bell 实验室所创。它还有一些重构版本，例如 **mawk**, **nawk**。在大多数Linux 发行版中能见到的，是 GNU awk，也叫**gawk**。 在大多数 Linux 发行版中，awk 和 gawk 都是指向 GNU awk 的链接。 输入awk命令，也是同样的效果。 [GNU awk 用户手册][2]中，能看到 awk 和 gawk 的全部历史。
|
||||
|
||||
这一系列的[第一篇文章][3] 介绍了awk 命令的基本格式：
|
||||
|
||||
```
|
||||
`$ awk [options] 'pattern {action}' inputfile`
|
||||
```
|
||||
|
||||
Awk 是一个命令，后面要接选项 （比如用 **-F** 来定义字段分隔符）。 想让awk 执行的部分需要写在 两个单引号之间，至少在终端中需要这么做。 在awk 命令中，为了进一步强调你想要执行的部分，可以用 **-e** 选项来突出显示 (但这不是必须的):
|
||||
|
||||
|
||||
```
|
||||
$ awk -F, -e '{print $2;}' colours.txt
|
||||
yellow
|
||||
blue
|
||||
green
|
||||
[...]
|
||||
```
|
||||
|
||||
### 记录和字段
|
||||
|
||||
Awk 将输入数据视为 一系列 _记录_ ， 通常是按行分割的。 换句话说，awk 将文本中的每一行视作一个记录。每一记录包含多个 _字段_. 一个字段由 _字段分隔符_ 分隔开来，字段是记录的一部分.
|
||||
|
||||
默认情况下，awk 将各种空白符，如空格，tab,换行符等视为分隔符。 值得注意的是，在awk 中，多个 _空格_ 将被视为一个分隔符。所以下面这行文本有两个字段：
|
||||
|
||||
|
||||
```
|
||||
`raspberry red`
|
||||
```
|
||||
|
||||
这行也是：
|
||||
|
||||
|
||||
```
|
||||
`tuxedo black`
|
||||
```
|
||||
|
||||
其他分隔符，在程序中不是这么处理的。假设字段分隔符是逗号，如下所示的记录，就有三个字段。其中一个字段可能会是0个字节（假设这一字段中不包含隐藏字符）
|
||||
|
||||
```
|
||||
`a,,b`
|
||||
```
|
||||
|
||||
### awk 程序
|
||||
|
||||
awk 命令的 _程序部分_ 是由一系列规则组成的。通常来说，程序中每个规则占一行(尽管这不是必须的)。 每个规则由一个模式，或一个/多个动作组成：
|
||||
|
||||
```
|
||||
`pattern { action }`
|
||||
```
|
||||
|
||||
在一个规则中，你可以通过定义模式，来确定行动是否会在记录中执行。 模式可以是简单的比较条件，正则表达式，甚至两者结合等等。
|
||||
|
||||
这个例子中，程序 _只会_ 显示包含 单词 “raspberry” 的记录：
|
||||
|
||||
|
||||
```
|
||||
$ awk '/raspberry/ { print $0 }' colours.txt
|
||||
raspberry red 99
|
||||
```
|
||||
|
||||
如果没有文本符合模式，最终结果会对应所有记录。
|
||||
|
||||
并且，在一条规则只包含一个模式时，相当于对整个记录执行 **{ print }** 。
|
||||
|
||||
Awk 程序本质上是 _数据驱动_ 的，命令执行结果取决于数据。所以，与其他编程语言中的程序相比，它还是有些区别的。
|
||||
|
||||
### NF 变量
|
||||
|
||||
每个字段都有指定变量，但针对字段和记录，也存在一些特殊变量。 **NF** 变量，能存储awk在当前记录中找到的数字字段。其内容可在屏幕上显示，也可用于测试。 下面例子中的数据，来自上篇文章[文本][3]：
|
||||
|
||||
|
||||
```
|
||||
$ awk '{ print $0 " (" NF ")" }' colours.txt
|
||||
name color amount (3)
|
||||
apple red 4 (3)
|
||||
banana yellow 6 (3)
|
||||
[...]
|
||||
```
|
||||
|
||||
Awk 的 **print** 函数会接受一系列参数（可以是变量或者字符），并将它们拼接起来。这就是为什么在这个例子里，每行结尾处，awk 会显示一个被括号括起来的整数。
|
||||
|
||||
### NR 变量
|
||||
|
||||
另外，为了计算每个记录中的字段数，awk 也计算输入记录。 记录数目被存储在变量 **NR** 中，它的使用方法和其他变量没有任何区别。例如，为了在每一行开头显示行号：
|
||||
|
||||
```
|
||||
$ awk '{ print NR ": " $0 }' colours.txt
|
||||
1: name color amount
|
||||
2: apple red 4
|
||||
3: banana yellow 6
|
||||
4: raspberry red 3
|
||||
5: grape purple 10
|
||||
[...]
|
||||
```
|
||||
|
||||
注意，在这个命令下输入数据时，可以不遵循在 **print** 后的规则，参数间可以不写空格，尽管这样会降低可读性：
|
||||
|
||||
|
||||
```
|
||||
`$ awk '{print NR": "$0}' colours.txt`
|
||||
```
|
||||
|
||||
### printf() 函数
|
||||
|
||||
为了输出结果时格式更灵活，你可以使用 awk 的 **printf()** 函数。 它与C，Lua,Bash和其他语言中的 **printf** 相类似。 它也接受 _格式_ ，加逗号分隔的参数。参数列表需要写在括号里。
|
||||
|
||||
|
||||
```
|
||||
`$ printf format, item1, item2, ...`
|
||||
```
|
||||
|
||||
格式这一参数（也叫 _格式符_ ） 定义了其他参数如何显示。 这一功能是用 _格式修饰符_ 来实现的。 **%s** 显示字符， **%d** 显示数字。 下面的**printf** 语句，会在括号内显示字段数量：
|
||||
|
||||
```
|
||||
$ awk 'printf "%s (%d)\n",$0,NF}' colours.txt
|
||||
name color amount (3)
|
||||
raspberry red 4 (3)
|
||||
banana yellow 6 (3)
|
||||
[...]
|
||||
```
|
||||
|
||||
|
||||
在这个例子里， **%s (%d)** 确定了每一行的输出格式，**$0,NF** 定义了插入 **%s** 和 **%d** 位置的数据。注意，和**print** 函数不同，在没有明确指令时，输出不会转到下一行。出现 转义字符 **\n** 时才会换行。
|
||||
|
||||
### Awk 脚本编程
|
||||
|
||||
这篇文章中出现的所有awk代码，都在Bash终端中执行过。 面对更复杂的程序，将命令放在文件（ _脚本_ ）中会更容易。 **-f FILE** 选项（不要和 **-F** 弄混了，那个选项用于字段分隔符），可用于指明包含可执行程序的文件。
|
||||
|
||||
举个例子，下面是一个简单的awk 脚本。 创建一个名为 **example1.awk** 的文件，包含以下内容：
|
||||
|
||||
|
||||
```
|
||||
/^a/ {print "A: " $0}
|
||||
/^b/ {print "B: " $0}
|
||||
```
|
||||
|
||||
如果一个文件包含 awk 程序，那么在给文件命名时，最好写上 **.awk** 的扩展名。 这样命名不是强制的，但这么做，会给文件管理器，编辑者（和你）一个关于文件内容的，很有用的提示。
|
||||
|
||||
执行这一脚本：
|
||||
|
||||
|
||||
```
|
||||
$ awk -f example1.awk colours.txt
|
||||
A: raspberry red 4
|
||||
B: banana yellow 6
|
||||
A: apple green 8
|
||||
```
|
||||
|
||||
一个包含 awk 命令的文件，在最开头一行加上 **#!** ，就能变成可执行脚本。 创建一个名为 **example2.awk** 的文件，包含以下内容：
|
||||
|
||||
```
|
||||
#!/usr/bin/awk -f
|
||||
#
|
||||
# 除了第一行，在其他行前显示行号
|
||||
#
|
||||
|
||||
NR > 1 {
|
||||
printf "%d: %s\n",NR,$0
|
||||
}
|
||||
```
|
||||
|
||||
可以说，脚本中只有一行，大多数情况下没什么用。 但在某些情况下，执行一个脚本，比记住，然后打一条命令要容易的多。 一个脚本文件，也提供了一个记录命令具体作用的好机会。 以 **#** 号开头的行是注释，awk 会忽略它们。
|
||||
|
||||
给文件可执行权限：
|
||||
|
||||
|
||||
```
|
||||
`$ chmod u+x example2.awk`
|
||||
```
|
||||
|
||||
执行脚本：
|
||||
|
||||
|
||||
```
|
||||
$ ./example2.awk colours.txt
|
||||
2: apple red 4
|
||||
2: banana yellow 6
|
||||
4: raspberry red 3
|
||||
5: grape purple 10
|
||||
[...]
|
||||
```
|
||||
|
||||
|
||||
将awk 命令放在脚本文件中，有一个好处就是，修改和格式化输出会更容易。在终端中，如果能用一行执行多条awk命令，那么输入多行，才能达到同样效果，就显得有些多余了。
|
||||
|
||||
### 试一试
|
||||
|
||||
你现在已经足够了解， awk 是如何执行指令的了。现在你应该能编写复杂的awk 程序了。 试着编写一个awk 脚本，它需要: 至少包括一个条件模式，以及多个规则。如果你想使用除 **print** 和 **printf** 以外的函数，可以参考在线[ gawk 手册][4] .
|
||||
|
||||
|
||||
下面这个例子是个很好的切入点：
|
||||
|
||||
|
||||
```
|
||||
#!/usr/bin/awk -f
|
||||
#
|
||||
# 显示所有记录 除了出现以下情况
|
||||
# 如果第一个记录 包含 “raspberry”
|
||||
# 将 “red” 替换成 “pi”
|
||||
|
||||
$1 == "raspberry" {
|
||||
gsub(/red/,"pi")
|
||||
}
|
||||
|
||||
{ print }
|
||||
```
|
||||
|
||||
试着执行这个脚本，看看输出是什么。接下来就看你自己的了。
|
||||
|
||||
|
||||
这一系列的下一篇文章，将会介绍更多，能在更复杂(更有用!) 脚本中使用的函数。
|
||||
|
||||
* * *
|
||||
|
||||
_这篇文章改编自 [Hacker Public Radio][5] 系列,一个技术社区博客_
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
via: https://opensource.com/article/19/11/fields-records-variables-awk
|
||||
|
||||
作者：[Seth Kenlon][a]
|
||||
选题：[lujun9972][b]
|
||||
译者：[译者ID](https://github.com/译者ID)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]: https://opensource.com/users/seth
|
||||
[b]: https://github.com/lujun9972
|
||||
[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/computer_laptop_code_programming_mountain_view.jpg?itok=yx5buqkr (Man at laptop on a mountain)
|
||||
[2]: https://www.gnu.org/software/gawk/manual/html_node/History.html#History
|
||||
[3]: https://opensource.com/article/19/10/intro-awk
|
||||
[4]: https://www.gnu.org/software/gawk/manual/
|
||||
[5]: http://hackerpublicradio.org/eps.php?id=2129
|
||||
Reference in New Issue
Block a user