How to Use Comparison Operators with Awk in Linux – Part 4

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.

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.

What are Comparison operators in Awk?

Comparison operators in Awk are used to compare the value of numbers or strings and they include the following:

  1. > – greater than
  2. < – less than
  3. >= – greater than or equal to
  4. <= – less than or equal to
  5. == – equal to
  6. != – not equal to
  7. some_value ~ / pattern/ – true if some_value matches pattern
  8. some_value !~ / pattern/ – true if some_value does not match pattern

Now that we have looked at the various comparison operators in Awk, let us understand them better using an example.

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.

File – food_list.txt
No      Item_Name               Quantity        Price
1       Mangoes                    45           $3.45
2       Apples                     25           $2.45
3       Pineapples                 5            $4.45
4       Tomatoes                   25           $3.45
5       Onions                     15           $1.45
6       Bananas                    30           $3.45

The general syntax for using comparison operators in Awk is:

# expression { actions; }

To achieve the above goal, I will have to run the command below:

# awk '$3 <= 30 { printf "%s\t%s\n", $0,"**" ; } $3 > 30 { print $0 ;}' food_list.txt

No	Item_Name`		Quantity	Price
1	Mangoes	      		   45		$3.45
2	Apples			   25		$2.45	**
3	Pineapples		   5		$4.45	**
4	Tomatoes		   25		$3.45	**
5	Onions			   15           $1.45	**
6	Bananas			   30           $3.45	**

In the above example, there are two important things that happen:

  1. 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.
  2. The second expression { action ; } combination, $3 > 30 { print $0 ;} prints out lines unchanged since their quantity is greater then 30.

One more example:

# awk '$3 <= 20 { printf "%s\t%s\n", $0,"TRUE" ; } $3 > 20  { print $0 ;} ' food_list.txt 

No	Item_Name		Quantity	Price
1	Mangoes			   45		$3.45
2	Apples			   25		$2.45
3	Pineapples		   5		$4.45	TRUE
4	Tomatoes		   25		$3.45
5	Onions			   15           $1.45	TRUE
6       Bananas	                   30           $3.45

In this example, we want to indicate lines with quantity less or equal to 20 with the word (TRUE) at the end.

Summary

This is an introductory tutorial to comparison operators in Awk, therefore you need to try out many other options and discover more.

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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Aaron Kili

Aaron Kili is a Linux and F.O.S.S enthusiast, an upcoming Linux SysAdmin, web developer, and currently a content creator for TecMint who loves working with computers and strongly believes in sharing knowledge.

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9 Responses

  1. Dan says:

    Apparently the script is not smart enough to validate if $3 is a number of character.

    $ awk '$3 <= 30 { print $0," 30 { print $0, "<-- quantity greater than 30" ;}' food_list.txt
    
    No      Item_Name               Quantity        Price <-- quantity greater than 30
    1       Mangoes                    45           $3.45 <-- quantity greater than 30
    2       Apples                     25           $2.45 <-- quantity is less than or equal to 30
    3       Pineapples                 5            $4.45 <-- quantity is less than or equal to 30
    4       Tomatoes                   25           $3.45 <-- quantity is less than or equal to 30
    5       Onions                     15           $1.45 <-- quantity is less than or equal to 30
    6       Bananas                    30           $3.45 <-- quantity is less than or equal to 30
    
    • Aaron Kili says:

      @Dan

      Sure, you can modify it for that purpose and share it with us.

      • Dan says:

        My try script, while the output is good however I do feel it’s a bit redundant particularly checking ($3 ~ /^[0-9]/) twice. Please simply :)

        $ awk ‘($3 ~ “^[a-zA-Z]”) { print $0} (($3 ~ /^[0-9]/) && ($3 <= 30)) { print $0," 30)) { print $0, “<– quantity is greater than 30" ;}' food_list.txt

        No      Item_Name               Quantity        Price
        1       Mangoes                    45           $3.45 <-- quantity is greater than 30
        2       Apples                     25           $2.45 <-- quantity is less than or equal to 30
        3       Pineapples                 5            $4.45 <-- quantity is less than or equal to 30
        4       Tomatoes                   25           $3.45 <-- quantity is less than or equal to 30
        5       Onions                     15           $1.45 <-- quantity is less than or equal to 30
        6       Bananas                    30           $3.45 <-- quantity is less than or equal to 30
        
  2. upkar says:

    What is the difference between using these expression directly and using this expressions with if statement. How it makes difference.

  3. jumen says:

    awk ‘NR==1 {print }; NR>1 && $3 > 30 { printf “%s\t%s\n”, $0,”**” ; };NR>1 && $3 <= 30 { print $0 ;}' food_list.txt
    will be better

  4. jumen says:

    if I want filter the quantity greater than 30; there will be something wrong
    awk ‘$3 > 30 { printf “%s\t%s\n”, $0,”**” ; } $3 1 && $3 > 30 { printf “%s\t%s\n”, $0,”**” ; };NR>1 && $3 <= 30 { print $0 ;}' food_list.txt

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