Basic Syntax

nawk reads input line by line and applies rules of the form pattern { action }. If the pattern matches the current line, the action is executed. Either part is optional.

nawk 'pattern { action }' file
nawk -f script.awk file       # read program from a file
nawk -F: '{ print $1 }' file  # set field separator to :

Each input line is automatically split into fields. $0 is the entire line; $1, $2, … are individual fields. The default field separator is whitespace.

Built-in Variables

Patterns

Patterns control which lines trigger an action. The four main types:

BEGIN { print "Start" }
/error/ { print "Found error on line", NR }
NF > 5  { print "Long line:", $0 }
END   { print "Done. Total lines:", NR }

print and printf

print outputs values separated by OFS and terminated by ORS. printf gives C-style formatted output with no automatic newline.

{ print $1, $3 }                      # print fields 1 and 3
{ print $1 > "output.txt" }           # redirect to file
{ print $1 | "sort" }                 # pipe to command
{ printf "%-10s %5d\n", $1, $2 }      # formatted output

Common printf format specifiers:

String Functions

Control Flow

# if / else
if ($3 > 100) { print "high" } else { print "low" }

# while loop
{ i = 1; while (i <= NF) { print $i; i++ } }

# for loop
{ for (i = 1; i <= NF; i++) print $i }

# next  -- skip to next line
/^#/ { next }          # skip comment lines

# exit  -- stop processing
NR > 100 { exit }      # stop after 100 lines

Arithmetic operators: + - * / % ^. Comparison: == != < > <= >=. String match: ~ !~.

Arrays

nawk arrays are associative (keyed by string). No declaration is needed.

# count occurrences of each value in field 1
{ count[$1]++ }
END { for (k in count) print k, count[k] }

# delete an element
delete count["key"]

# check if a key exists
if ("key" in count) print "found"

Multi-dimensional arrays use comma-separated keys: arr[i,j].

Common One-liners