Learn awk by examples

AWK is a pattern-directed scanning and manipulating language, designed expecially for text processing.
It scans each input file for lines that match any of a set of patterns specified literally in prog or in one or more files specified as -f file. With each pattern there can be an associated action that will be performed when a line of a file matches the pattern.

A program basing on AWK is made of a list of rules that specify a pattern and the relative action to execute if the pattern is satisfied:

  pattern { action }

Example usages.

Suppose you have this file.txt:

Red     Blue    1
Orange  Pink    2
Yellow  Black   3
Green   White   4
Red     Purple  5

• Print the lines from file.txt that contains the word "Blue"

$ awk '/Blue/ { print }' file.txt

Red     Blue    1 

 

• Print the whole file:

$ awk '{print $0}' file.txt

Red     Blue    1

Orange  Pink    2

Yellow  Black   3

Green   White   4

Red     Purple  5 

 

• Print the first column of the file:

$ awk '{print $1}' file.txt

Red

Orange

Yellow

Green

Red 

 

• Print the second column of the file:

$ awk '{print $2}' file.txt

Blue

Pink

Black

White

Purple

• Preprocessing: add an header to your output:

$ awk 'BEGIN {print "Second column:"} {print $2}' file.txt

Second column:

Blue

Pink

Black

White

Purple 

 

• Postprocessing: add a footer to your output:

$ awk '{print $2} END {print "End of file!"}' file.txt

Blue

Pink

Black

White

Purple

End of file! 

• Substitution: change the field separator (FS) with an output field separator (OFS) in file:

$ awk 'BEGIN{FS=" "; OFS="-"} {print $1,$2,$3}' file.txt

Red-Blue-1

Orange-Pink-2

Yellow-Black-3

Green-White-4

Red-Purple-5