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AWK example scripts

In this article some small AWK scripts will be presented. Even if they're really simple, they can be useful to understand the logic behind this program, which is a bit different from the one of the traditional programming languages. For further informations you can read about AWK on Wikipedia.
AWK is an interpreted language for processing text files. For the interpreter, the text file is a big table. Each text row is a table row and is called record. Each word surrounded by spaces is a table column, and is called field. When a text file is inputed to an AWK script, these things happens:
  1. The AWK interpreter opens automatically the file.
  2. The AWK interpreter scans all the records (rows) of the text file, and applies the rules coming from the AWK commands. Note that the file is scanned automatically, the programmer doesn't have to implement any cycle for, while, foreach.
  3. The interpreter saves the file and close it.
Due to its nature, AWK is perfect for processing a big amount of data with simple rules, saving the time necessary to other operations like opening and closing the file or reading and writing it.
Another great simplification is the absence of typed variables: each variable can be indifferently a text or a number.

The structure of an AWK command to process a record is:

(condition) {action}

  • Condition represents a condition which selects a record or a part of it; on this selection an action will be performed.
  • Action represents the action to perform on the selected record.
The following examples will always refer to a text file containing a list of nations with surface, population and continent:

USSR            86250   262     Asia
USA             3615    219     Nord America
China           3692    866     Asia
Canada          3852    24      Nord America
Brazil          3286    116     South America
Australia       2968    14      Oceania
India           1269    637     Asia
Argentina       1072    26      South America
Sudan           968     19      Africa
Italy           920     60      Europe
Angola          1246    12      Africa
Austria         83      8       Europe
Spain           504     45      Europe
 

Select a field
This example shows how the fields of each record are selected, that is by numbers beginning with $ (the same as the parameters passed by command line in the Bash).

#!/usr/bin/awk -f
# This script prints only nations and continents

{print $1, $4}
 

Conditions and variables
These examples show the use of numerical or complex conditions. Note that there's no typing in AWK: any selected string can be used without any conversion as a number.

#!/usr/bin/awk -f
# This script prints all the information relative only to
# Nations with more than 50 millions people

( $3 > 50 ) { print $0 }



#!/usr/bin/awk -f
# This script prints all the information relative only to
# nations whose name is longer than 5 characters

( length($1)>=5 ) { print $0 }
 

Regular expressions
These examples shows the implementation of regular expressions in AWK. Note the slashes / to delimit the regular expression and the ˜ to refer it only to a field instead of to the whole record

#!/usr/bin/awk -f
# This script prints all the information relative only to
# Asian nations

(/.Asia/) {print $0}
 



#!/usr/bin/awk -f
# This script prints all the information relative only to
# European nations

( /Europe/) { print $0 }
 



#!/usr/bin/awk -f
# This script prints all the information relative only to
# nations beginning with A

( $1~/A./ ) { print $0 }
 

Compound conditions
The following examples shows the combination of different conditions by boolean operators, which are very similar to the ones implemented in C programming language.

#!/usr/bin/awk -f
# This script prints all the information relative only to
# Nations with more than 50 millions people and a surface
# of more than 2 millions of square km

( $3 > 50 && $2 > 2000) { print $0 }
 



#!/usr/bin/awk -f
# This script prints all the information relative only to
# African nations beginning with A

( $0~/Africa/ && $1~/^A/ ) { print $0 }
 



#!/usr/bin/awk -f
# This script prints all the information relative only to
# European nations beginning with A or S

( $0~/Europe/ && ($1~/^A/ || $1~/^S/) ) { print $0 }
 







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