Awk and Sed

awk and sed are text manipulation programs. You can use them, for example, to replace strings:

echo image.jpg | sed 's/\.jpg/.png/'
image.png

or change the order of strings:

echo "hello world" | awk '{print $2, $1}'
world hello

awk and sed are harder to learn than some other basic Linux CLI tools because they contain their own small programming language.

Awk Basics

awk transforms lines of text (stdin) into any other text, using a set of instructions (called the awk program):

awk program input-files

An awk program can also contain one or more actions, for example, calculating values or printing text. These actions run only when an input matches a pattern:

pattern {action}

Typical patterns include:

• BEGIN runs once before awk processes any input
• END runs once after awk has processed all the input
• A regex surrounded by forward slashes, example: /^[A-Z]/
• Other awk-specific expressions, example: FNR>5 tells awk to skip the first five lines of input

A pattern with no action runs the default action {print}.

awk can also perform calculations, such as:

seq 1 100 | awk '{s+=$1} END {print s}'
5050

(This sums numbers from 1 to 100.)

Sed Basics

sed, like awk, can be used to transform text from stdin to any other text using instructions called sed scripts:

sed script input-files

The most common use case for sed is text replacement, as in this example:

echo "Windows eats Linux for breakfast" | sed s/Windows/Linux/g | sed s/Linux/Windows/2

This article was last updated on April 5, 2024.