AWK Syntax Essentials

Syntax is based on The AWK Programming Language, 2nd Edition by Alfred V. Aho, Brian W. Kernighan, and Peter J. Weinberger.

Pattern

A pattern determines when an action is executed. When a pattern matches an input line, its associated action is executed.

If no action is specified, the default is print $0.

Syntax: pattern { action }

Examples

1. BEGIN — runs once before any input is read

BEGIN { FS=":" }

2. END — runs once after all input is processed

END { print "total:", total }

3. Expression — executes on every input line where the condition is true

   Skip the header line

NR > 1 { print $0 }

4. Regex — executes on every line matching the pattern

/error/ { print $0 }

5. Range — matches all lines from pattern1 through pattern2, inclusive

/start/,/end/ { print $0 }

Conditionals

AWK supports standard conditionals for branching logic.

Examples

1. if

   Skip empty lines

if (NF > 0) print $0

2. if-else

if ($1 > 0) print "positive"; else print "negative"

3. if-else if

if ($1 > 0) print "positive"
else if ($1 < 0) print "negative"
else print "zero"

Ternary Expression

The ternary operator (?:) is a compact, C-style alternative to the if-else statement. By using this operator, a concise one-line ternary expression can be constructed, which, unlike a statement, returns a value that can be used directly within calculations or commands.

Syntax: condition ? value_if_true : value_if_false

Examples

1. Absolute Value

AWK lacks a built-in abs() function. The ternary operator handles this math logic efficiently:

$1 = ($1 < 0) ? -$1 : $1

2. Truthiness & Success Labels

AWK treats 0 and "" as false, and everything else as true. Use this to label status codes:

print $1, ($1 ? "SUCCESS" : "FAILURE")

or

printf("%s\t%s\n", $1, ($1 ? "SUCCESS" : "FAILURE"))

Associative Arrays

AWK arrays are associative — keys can be strings or numbers, making them ideal for counting, grouping, and lookups without any pre-declaration.

Syntax: array[key] = value

Examples

1. Populate an array with lines

x[NR] = $0

2. Deduplicate lines

!x[$0]++

3. Populate a 2D matrix with all fields

for (i=1; i<=NF; i++) x[NR, i] = $i

AWK simulates 2D arrays by concatenating keys with a built-in separator (SUBSEP), so x[row, col] is stored internally as x[row SUBSEP col].

Loops

AWK supports standard C-style loops as well as a dedicated form for traversing associative arrays.

Examples

1. for loop

for (i=1; i<=NF; i++) print $i

2. while loop

while (i <= NF) { print $i; i++ }

3. do-while loop

do { print $i; i++ } while (i <= NF)

4. Iterate over an associative array

for (k in array) print k, array[k]

Pipes

AWK can send output to, or receive input from, external shell commands using pipes, enabling seamless integration with standard Unix tools.

Syntax: command | getline [var] / print | "command"

Examples

1. Send output to a shell command

print $1 | "sort"

2. Read a shell command's output into a variable

"date" | getline today

3. Pipe to multiple commands (close between uses)

print $1 | "sort | uniq -c"

Comparison Operators

Logical Operators

Syntax: condition1 operator condition2

Built-in variables

Built-in Arithmetic Functions

Built-in String Functions

Expression Operators

printf

printf format-control characters

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