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AWK Cheatsheet

A concise cheat sheet covering essential AWK syntax, patterns, actions, and built-in functions, designed to help you quickly write and understand AWK scripts.

AWK Basics

Syntax

awk 'pattern { action }' file

AWK scripts consist of patterns and actions. For each line in the input file, AWK checks if the pattern matches. If it does, the action is executed. If no pattern is given, the action is performed for every input line. If no action is given, the matching line is printed.

awk '{ print $1 }' file

Prints the first field of each line in file. Fields are separated by whitespace by default.

awk -F',' '{ print $1, $2 }' file

Uses , as the field separator and prints the first and second fields of each line.

awk 'BEGIN { print "Start" } { print $0 } END { print "End" }' file

BEGIN block is executed before processing any input. END block is executed after processing all input. The { print $0 } action prints each line of the input file.

Patterns

BEGIN

Executed before any input is read.

END

Executed after all input is read.

expression

A boolean expression that determines whether the action is executed.
Example: $1 > 10

pattern1, pattern2

A range pattern that matches all lines from a line matching pattern1 to a line matching pattern2.

!pattern

Negates the pattern. The action is executed if the line does not match the pattern.

Actions

print: Prints the current line or specified fields.
Example: print $1, $3

printf: Formatted printing, similar to C’s printf.
Example: printf "%s: %d\n", $1, $2

next: Skips the current line and proceeds to the next input line.

exit: Terminates the AWK script.

delete array[index]: Deletes an element from an array.

Variables and Operators

Built-in Variables

$0

The entire current line.

$1, $2, ...

The first, second, … field of the current line.

NF

The number of fields in the current line.

NR

The number of the current line.

FILENAME

The name of the current input file.

FS

The field separator (default is whitespace). Can be changed with -F option or by assigning a value to FS.

RS

The record separator (default is newline).

OFS

The output field separator (default is whitespace).

ORS

The output record separator (default is newline).

Operators

=

Assignment operator.

==, !=

Equality and inequality operators.

>, <, >=, `<=``

Comparison operators.

~, !~

Regular expression match and non-match operators.

&&, ||, !

Logical AND, OR, and NOT operators.

+, -, *, /, ^, %

Arithmetic operators: addition, subtraction, multiplication, division, exponentiation, modulus.

++, --

Increment and decrement operators.

+=, -=, *=, /=, %=, ^=

Compound assignment operators.

User-defined Variables

Variables can be defined and used within AWK scripts.

Example:

BEGIN { count = 0 }
{ count++ }
END { print "Total lines:", count }

Variables are initialized to zero or the empty string if not explicitly initialized.

Functions

Built-in Functions

length(string)

Returns the length of the string.

substr(string, start, length)

Returns a substring of the string starting at start with the given length.

index(string, substring)

Returns the starting position of substring in string, or 0 if not found.

split(string, array, separator)

Splits the string into elements of the array using separator as the delimiter. Returns the number of elements.

match(string, regex)

Returns the starting position of the regular expression regex in string, or 0 if not found. Sets RSTART and RLENGTH.

gsub(regex, replacement, string)

Globally substitutes all matches of the regular expression regex in string with replacement. Returns the number of substitutions made.

tolower(string)

Converts the string to lowercase.

toupper(string)

Converts the string to uppercase.

sprintf(format, expr1, expr2, ...)

Formats expressions expr1, expr2, … according to the format string format (similar to C’s sprintf).

User-Defined Functions

You can define your own functions in AWK.

Syntax:

function function_name(parameter1, parameter2, ...) {
  # Function body
  return value
}

Example:

function max(x, y) {
  return (x > y ? x : y)
}

{ print max($1, $2) }

Examples

Simple Examples

Print lines longer than 80 characters:
awk 'length($0) > 80 { print }' file

Print the total number of fields in the input:
awk '{ total += NF } END { print "Total fields:", total }' file

Print lines containing the word ‘error’:
awk '/error/ { print }' file

Print the last field of each line:
awk '{ print $NF }' file

Advanced Examples

Calculate the average of the values in the first field:
awk '{ sum += $1; count++ } END { if (count > 0) print "Average:", sum / count }' file

Print unique lines in a file:
awk '!seen[$0]++' file

Sum values in a specific column based on a condition:
awk '$2 == "active" { sum += $1 } END { print "Sum of active values:", sum }' file
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