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98 lines
4.0 KiB
Markdown
98 lines
4.0 KiB
Markdown
chunyang-wen translating
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How to Use Comparison Operators with Awk in Linux
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===================================================
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When dealing with numerical or string values in a line of text, filtering text or strings using comparison operators comes in handy for Awk command users.
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In this part of the Awk series, we shall take a look at how you can filter text or strings using comparison operators. If you are a programmer then you must already be familiar with comparison operators but those who are not, let me explain in the section below.
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### What are Comparison operators in Awk?
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Comparison operators in Awk are used to compare the value of numbers or strings and they include the following:
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- `>` – greater than
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- `<` – less than
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- `>=` – greater than or equal to
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- `<=` – less than or equal to
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- `==` – equal to
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- `!=` – not equal to
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- `some_value ~ / pattern/` – true if some_value matches pattern
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- `some_value !~ / pattern/` – true if some_value does not match pattern
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Now that we have looked at the various comparison operators in Awk, let us understand them better using an example.
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In this example, we have a file named food_list.txt which is a shopping list for different food items and I would like to flag food items whose quantity is less than or equal 20 by adding `(**)` at the end of each line.
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```
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File – food_list.txt
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No Item_Name Quantity Price
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1 Mangoes 45 $3.45
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2 Apples 25 $2.45
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3 Pineapples 5 $4.45
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4 Tomatoes 25 $3.45
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5 Onions 15 $1.45
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6 Bananas 30 $3.45
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```
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The general syntax for using comparison operators in Awk is:
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```
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# expression { actions; }
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```
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To achieve the above goal, I will have to run the command below:
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```
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# awk '$3 <= 30 { printf "%s\t%s\n", $0,"**" ; } $3 > 30 { print $0 ;}' food_list.txt
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No Item_Name` Quantity Price
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1 Mangoes 45 $3.45
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2 Apples 25 $2.45 **
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3 Pineapples 5 $4.45 **
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4 Tomatoes 25 $3.45 **
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5 Onions 15 $1.45 **
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6 Bananas 30 $3.45 **
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```
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In the above example, there are two important things that happen:
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- The first expression `{ action ; }` combination, `$3 <= 30 { printf “%s\t%s\n”, $0,”**” ; }` prints out lines with quantity less than or equal to 30 and adds a `(**)` at the end of each line. The value of quantity is accessed using `$3` field variable.
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- The second expression `{ action ; }` combination, `$3 > 30 { print $0 ;}` prints out lines unchanged since their quantity is greater then `30`.
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One more example:
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```
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# awk '$3 <= 20 { printf "%s\t%s\n", $0,"TRUE" ; } $3 > 20 { print $0 ;} ' food_list.txt
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No Item_Name Quantity Price
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1 Mangoes 45 $3.45
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2 Apples 25 $2.45
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3 Pineapples 5 $4.45 TRUE
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4 Tomatoes 25 $3.45
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5 Onions 15 $1.45 TRUE
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6 Bananas 30 $3.45
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```
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In this example, we want to indicate lines with quantity less or equal to 20 with the word (TRUE) at the end.
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### Summary
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This is an introductory tutorial to comparison operators in Awk, therefore you need to try out many other options and discover more.
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In case of any problems you face or any additions that you have in mind, then drop a comment in the comment section below. Remember to read the next part of the Awk series where I will take you through compound expressions.
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--------------------------------------------------------------------------------
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via: http://www.tecmint.com/comparison-operators-in-awk/
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作者：[Aaron Kili][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: http://www.tecmint.com/author/aaronkili/
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