20160627-6 选题

sources/tech/awk/Part 6 - How to Use ‘next’ Command with Awk in Linux.md

@@ -0,0 +1,76 @@
+How to Use ‘next’ Command with Awk in Linux
+=============================================
+
+![](http://www.tecmint.com/wp-content/uploads/2016/06/Use-next-Command-with-Awk-in-Linux.png)
+
+In this sixth part of Awk series, we shall look at using `next` command, which tells Awk to skip all remaining patterns and expressions that you have provided, but instead read the next input line.
+
+The `next` command helps you to prevent executing what I would refer to as time-wasting steps in a command execution.
+
+To understand how it works, let us consider a file called food_list.txt that looks like this:
+
+```
+Food List Items
+No      Item_Name               Price           Quantity
+1       Mangoes                 $3.45              5
+2       Apples                  $2.45              25
+3       Pineapples              $4.45              55
+4       Tomatoes                $3.45              25
+5       Onions                  $1.45              15
+6       Bananas                 $3.45              30
+```
+
+Consider running the following command that will flag food items whose quantity is less than or equal to 20 with a `(*)` sign at the end of each line:
+
+```
+# awk '$4 <= 20 { printf "%s\t%s\n", $0,"*" ; } $4 > 20 { print $0 ;} ' food_list.txt 
+
+No	Item_Name		Price		Quantity
+1	  Mangoes			$3.45		   5	*
+2	  Apples			$2.45      25
+3	  Pineapples	$4.45      55
+4	  Tomatoes		$3.45      25 
+5	  Onions			$1.45      15	*
+6	  Bananas	    $3.45      30
+```
+
+The command above actually works as follows:
+
+- First, it checks whether the quantity, fourth field of each input line is less than or equal to 20, if a value meets that condition, it is printed and flagged with the `(*)` sign at the end using expression one: `$4 <= 20`
+- Secondly, it checks if the fourth field of each input line is greater than 20, and if a line meets the condition it gets printed using expression two: `$4 > 20`
+
+But there is one problem here, when the first expression is executed, a line that we want to flag is printed using: `{ printf "%s\t%s\n", $0,"**" ; }` and then in the same step, the second expression is also checked which becomes a time wasting factor.
+
+So there is no need to execute the second expression, `$4 > 20` again after printing already flagged lines that have been printed using the first expression.
+
+To deal with this problem, you have to use the `next` command as follows:
+
+```
+# awk '$4 <= 20 { printf "%s\t%s\n", $0,"*" ; next; } $4 > 20 { print $0 ;} ' food_list.txt
+
+No	Item_Name		Price		           Quantity
+1	  Mangoes			$3.45		           5	*
+2	  Apples			$2.45              25
+3	  Pineapples	$4.45              55
+4	  Tomatoes		$3.45              25 
+5	  Onions			$1.45              15	*
+6	  Bananas	    $3.45              30
+```
+
+After a single input line is printed using `$4 <= 20` `{ printf "%s\t%s\n", $0,"*" ; next ; }`, the `next` command included will help skip the second expression `$4 > 20` `{ print $0 ;}`, so execution goes to the next input line without having to waste time on checking whether the quantity is greater than 20.
+
+The next command is very important is writing efficient commands and where necessary, you can always use to speed up the execution of a script. Prepare for the next part of the series where we shall look at using standard input (STDIN) as input for Awk.
+
+Hope you find this how to guide helpful and you can as always put your thoughts in writing by leaving a comment in the comment section below.
+
+--------------------------------------------------------------------------------
+
+via: http://www.tecmint.com/use-next-command-with-awk-in-linux/
+
+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/