$ Linux Command

Text

awk

Process text files by fields and patterns.

textfieldscolumnsreportsfilter

Additional Notes

awk is a text-processing language built around patterns and actions. It reads input line by line, splits each line into fields, and lets you print, filter, calculate, and format results.

It is especially useful for column-oriented text such as logs, CSV-like data, command output, and /etc/passwd.

Syntax

awk [options] 'pattern { action }' [file...]

Parameters

  • options: Flags that change how awk behaves.
  • file: Text file to read or process.
  • 'pattern { action }': Awk program with a pattern to match lines and an action to execute.

Important Variables

  • $0: The whole current line.
  • $1, $2, $3: First, second, third field, and so on.
  • NF: Number of fields in the current line.
  • NR: Current input record number, usually the line number.
  • FS: Input field separator.
  • OFS: Output field separator.

Common Options

  • -F SEP: Set input field separator.
  • -v NAME=VALUE: Pass a variable into awk.
  • -f FILE: Read awk program from a file.

Examples

awk '{print $1}' access.log

Print the first field from each line.

awk -F: '{print $1}' /etc/passwd

Print usernames from /etc/passwd.

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

Print line numbers with each line.

awk '$9 >= 500 {print $0}' access.log

Print web log lines where field 9 is 500 or greater.

awk '{sum += $1} END {print sum}' numbers.txt

Add numbers from the first column.

df -h | awk 'NR > 1 {print $1, $5, $6}'

Print selected columns from df output.

awk -F, 'NR > 1 {print $1, $3}' data.csv

Read comma-separated data and skip the header line.

Pattern Examples

  • NR == 1: Match first line.
  • NR > 1: Skip first line.
  • $3 == "active": Match lines where field 3 equals active.
  • $5 > 100: Match lines where field 5 is greater than 100.
  • /error/: Match lines containing error.
  • END { ... }: Run after all input is processed.

Practical Notes

  • Use awk when data has columns or fields.
  • Use sed for stream editing and substitutions.
  • Use grep for searching lines.
  • The default field separator is whitespace.
  • For real CSV with quotes and embedded commas, use a proper CSV parser.