Learn How to Use Awk Built-in Variables – Part 10

As we uncover the section of Awk features, in this part of the series, we shall walk through the concept of built-in variables in Awk. There are two types of variables you can use in Awk, these are; user-defined variables, which we covered in Part 8 and built-in variables.

Awk Built in Variables Examples
Awk Built in Variables Examples

Built-in variables have values already defined in Awk, but we can also carefully alter those values, the built-in variables include:

  1. FILENAME : current input file name( do not change variable name)
  2. FR : number of the current input line (that is input line 1, 2, 3… so on, do not change variable name)
  3. NF : number of fields in current input line (do not change variable name)
  4. OFS : output field separator
  5. FS : input field separator
  6. ORS : output record separator
  7. RS : input record separator

Let us proceed to illustrate the use of some of the Awk built-in variables above:

To read the filename of the current input file, you can use the FILENAME built-in variable as follows:

$ awk ' { print FILENAME } ' ~/domains.txt 
Awk FILENAME Variable
Awk FILENAME Variable

You will realize that, the filename is printed out for each input line, that is the default behavior of Awk when you use FILENAME built-in variable.

Using NR to count the number of lines (records) in an input file, remember that, it also counts the empty lines, as we shall see in the example below.

When we view the file domains.txt using cat command, it contains 14 lines with text and empty 2 lines:

$ cat ~/domains.txt
Print Contents of File
Print Contents of File
$ awk ' END { print "Number of records in file is: ", NR } ' ~/domains.txt 
Awk Count Number of Lines
Awk Count Number of Lines

To count the number of fields in a record or line, we use the NR built-in variable as follows:

$ cat ~/names.txt
List File Contents
List File Contents
$ awk '{ print "Record:",NR,"has",NF,"fields" ; }' ~/names.txt
Awk Count Number of Fields in File
Awk Count Number of Fields in File

Next, you can also specify an input field separator using the FS built-in variable, it defines how Awk divides input lines into fields.

The default value for FS is space and tab, but we can change the value of FS to any character that will instruct Awk to divide input lines accordingly.

There are two methods to do this:

  1. one method is to use the FS built-in variable
  2. and the second is to invoke the -F Awk option

Consider the file /etc/passwd on a Linux system, the fields in this file are divided using the : character, so we can specify it as the new input field separator when we want to filter out certain fields as in the following examples:

We can use the -F option as follows:

$ awk -F':' '{ print $1, $4 ;}' /etc/passwd
Awk Filter Fields in Password File
Awk Filter Fields in Password File

Optionally, we can also take advantage of the FS built-in variable as below:

$ awk ' BEGIN {  FS=“:” ; }  { print $1, $4  ; } ' /etc/passwd
Filter Fields in File Using Awk
Filter Fields in File Using Awk

To specify an output field separator, use the OFS built-in variable, it defines how the output fields will be separated using the character we use as in the example below:

$ awk -F':' ' BEGIN { OFS="==>" ;} { print $1, $4 ;}' /etc/passwd
Add Separator to Field in File
Add Separator to Field in File

In this Part 10, we have explored the idea of using Awk built-in variables which come with predefined values. But we can also change these values, though, it is not recommended to do so unless you know what you are doing, with adequate understanding.

After this, we shall progress to cover how we can use shell variables in Awk command operations, therefore, stay connected to Tecmint.

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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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