awk patterns and actions in Linux CLI - Commands & Configuration

Introduction

Sometimes you want to find and process specific parts of text in files or command output. Awk helps by letting you match patterns and then do actions on those matches, like printing or calculating.

When you want to print only lines containing a certain word from a log file.

When you need to sum numbers in a specific column of a data file.

When you want to extract and reformat parts of text from command output.

When you want to count how many lines match a pattern in a file.

When you want to replace or modify text only on lines that match a pattern.

Commands

This command searches the syslog file for lines containing the word 'error' and prints those entire lines. The pattern is '/error/' and the action is '{print $0}', which means print the whole line.

Terminal

awk '/error/ {print $0}' /var/log/syslog

Expected Output

Apr 26 10:15:32 server1 kernel: [12345.678901] error: device not found

Apr 26 10:16:45 server1 systemd: error: failed to start service

This command reads 'data.txt' and prints the first and third columns only for lines where the third column is greater than 50. The pattern is '$3 > 50' and the action is '{print $1, $3}'.

Terminal

awk '$3 > 50 {print $1, $3}' data.txt

Expected Output

John 75

Alice 82

This command sums all numbers in the second column of 'numbers.txt'. The BEGIN block initializes sum to 0, the main block adds each second column value to sum, and the END block prints the total sum.

Terminal

awk 'BEGIN {sum=0} {sum += $2} END {print sum}' numbers.txt

Expected Output

210

This command pipes three fruit names to awk, which prints only lines containing the letter 'a'. It shows how awk can process piped input with a pattern and action.

Terminal

echo -e "apple\nbanana\ncherry" | awk '/a/ {print $0}'

Expected Output

apple

banana

Key Concept

If you remember nothing else from awk patterns and actions, remember: awk looks at each line, checks if it matches the pattern, and if yes, runs the action on that line.

Common Mistakes

Using single quotes inside the awk pattern without escaping

This breaks the shell command because the shell misinterprets the quotes.

Use double quotes for the shell command or escape single quotes inside the awk script.

Forgetting to specify an action, expecting awk to print matching lines automatically

Awk prints the whole line only if no action is given; if you provide an empty action block, nothing prints.

Either omit the action block to print matching lines or explicitly use '{print $0}'.

Using incorrect field numbers like $0 for a specific column

$0 means the whole line, not a column; using it for a column causes wrong output.

Use $1, $2, $3, etc., for columns; use $0 only to refer to the entire line.

Summary

Use awk patterns to select lines based on text or conditions.

Use actions to print, calculate, or modify data on matching lines.

Combine BEGIN and END blocks to initialize and finalize processing.