20160627-2 选题

sources/tech/awk/Part 2 - How to Use Awk to Print Fields and Columns in File.md

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+How to Use Awk to Print Fields and Columns in File
+======================================================
+
+In this part of our Linux Awk command series, we shall have a look at one of the most important features of Awk, which is field editing.
+
+It is good to know that Awk automatically divides input lines provided to it into fields, and a field can be defined as a set of characters that are separated from other fields by an internal field separator.
+
+![](http://www.tecmint.com/wp-content/uploads/2016/04/Awk-Print-Fields-and-Columns.png)
+>Awk Print Fields and Columns
+
+If you are familiar with the Unix/Linux or do [bash shell programming][1], then you should know what internal field separator (IFS) variable is. The default IFS in Awk are tab and space.
+
+This is how the idea of field separation works in Awk: when it encounters an input line, according to the IFS defined, the first set of characters is field one, which is accessed using $1, the second set of characters is field two, which is accessed using $2, the third set of characters is field three, which is accessed using $3 and so forth till the last set of character(s).
+
+To understand this Awk field editing better, let us take a look at the examples below:
+
+**Example 1**: I have created a text file called tecmintinfo.txt.
+
+```
+# vi tecmintinfo.txt
+# cat tecmintinfo.txt
+```
+
+![](http://www.tecmint.com/wp-content/uploads/2016/04/Create-File-in-Linux.png)
+>Create File in Linux
+
+Then from the command line, I try to print the first, second and third fields from the file tecmintinfo.txt using the command below:
+
+```
+$ awk '//{print $1 $2 $3 }' tecmintinfo.txt
+TecMint.comisthe
+```
+
+From the output above, you can see that the characters from the first three fields are printed based on the IFS defined which is space:
+
+- Field one which is “TecMint.com” is accessed using `$1`.
+- Field two which is “is” is accessed using `$2`.
+- Field three which is “the” is accessed using `$3`.
+
+If you have noticed in the printed output, the field values are not separated and this is how print behaves by default.
+
+To view the output clearly with space between the field values, you need to add (,) operator as follows:
+
+```
+$ awk '//{print $1, $2, $3; }' tecmintinfo.txt
+
+TecMint.com is the
+```
+
+One important thing to note and always remember is that the use of `($)` in Awk is different from its use in shell scripting.
+
+Under shell scripting `($)` is used to access the value of variables while in Awk `($)` it is used only when accessing the contents of a field but not for accessing the value of variables.
+
+**Example 2**: Let us take a look at one other example using a file which contains multiple lines called my_shoping.list.
+
+```
+No	  Item_Name		            Unit_Price	Quantity	Price
+1	    Mouse			              #20,000		   1		    #20,000
+2 	  Monitor			            #500,000	   1		    #500,000
+3	    RAM_Chips		            #150,000	   2		    #300,000
+4	    Ethernet_Cables	        #30,000		   4		    #120,000		
+```
+
+Say you wanted to only print `Unit_Price` of each item on the shopping list, you will need to run the command below:
+
+```
+$ awk '//{print $2, $3 }' my_shopping.txt 
+
+Item_Name Unit_Price
+Mouse #20,000
+Monitor #500,000
+RAM_Chips #150,000
+Ethernet_Cables #30,000
+```
+
+Awk also has a `printf` command that helps you to format your output is a nice way as you can see the above output is not clear enough.
+
+Using `printf` to format output of the Item_Name and Unit_Price:
+
+```
+$ awk '//{printf "%-10s %s\n",$2, $3 }' my_shopping.txt 
+
+Item_Name  Unit_Price
+Mouse      #20,000
+Monitor    #500,000
+RAM_Chips  #150,000
+Ethernet_Cables #30,000
+```
+
+### Summary
+Field editing is very important when using Awk to filter text or strings, it helps you get particular data in columns in a list. And always remember that the use of `($)` operator in Awk is different from that in shell scripting.
+
+I hope the article was helpful to you and for any additional information required or questions, you can post a comment in the comment section.
+
+--------------------------------------------------------------------------------
+
+via: http://www.tecmint.com/awk-print-fields-columns-with-space-separator/
+
+作者：[Aaron Kili][a]
+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]: http://www.tecmint.com/author/aaronkili/
+[1]: http://www.tecmint.com/category/bash-shell/