Basics

Running AWK

awk '{ print }' file.txt          # print every line
awk '{ print $1 }' file.txt       # print first field
awk -F: '{ print $1 }' /etc/passwd  # custom delimiter
awk -f script.awk file.txt        # run from file
cmd | awk '{ print $2 }'          # pipe input

Program Structure

awk 'pattern { action }'	Basic form — action runs when pattern matches
BEGIN { ... }	Runs once before processing input
END { ... }	Runs once after all input is processed
No pattern	Action runs for every line
No action	Default action is `{ print }`

Patterns & Actions

Pattern Types

awk '/error/' file.txt            # regex match
awk '$3 > 100' file.txt           # comparison
awk 'NR >= 5 && NR <= 10' file.txt  # line range
awk '/start/,/end/' file.txt      # range pattern

Pattern Reference

/regex/	Match line against regex
$1 ~ /pat/	Field matches regex
$1 !~ /pat/	Field does not match regex
expr1, expr2	Range: from first match to second
expr1 && expr2	Logical AND
expr1 \|\| expr2	Logical OR
!expr	Logical NOT

Variables

Built-in Variables

NR	Current record (line) number
NF	Number of fields in current record
FS	Input field separator (default: whitespace)
OFS	Output field separator (default: space)
RS	Input record separator (default: newline)
ORS	Output record separator (default: newline)
FILENAME	Current input filename
FNR	Record number in current file

User Variables

awk '{ total += $1 } END { print total }' file.txt
awk -v threshold=50 '$1 > threshold' file.txt
awk 'BEGIN { count = 0 } /pat/ { count++ }
     END { print count }' file.txt

Fields

Field Access

$0	Entire current line
$1, $2, ...	First, second, ... field
$NF	Last field
$(NF-1)	Second-to-last field

Field Separators

awk -F, '{ print $2 }' data.csv     # comma
awk -F'\t' '{ print $1 }' data.tsv  # tab
awk 'BEGIN { FS = "[,:]" } { print $1 }' f  # multi-char
awk 'BEGIN { OFS = "," } { print $1, $3 }' f  # output sep

Control Flow

Conditionals & Loops

awk '{ if ($1 > 50) print "high"; else print "low" }' f
awk '{ for (i = 1; i <= NF; i++) print $i }' f
awk '{ i = 1; while (i <= NF) { print $i; i++ } }' f
awk '/skip/ { next } { print }' f  # skip matching lines

Control Statements

if (cond) { ... } else { ... }	Conditional
for (i = 0; i < n; i++) { ... }	C-style for loop
for (key in array) { ... }	Iterate array keys
while (cond) { ... }	While loop
do { ... } while (cond)	Do-while loop
next	Skip to next input record
exit	Stop processing, run END block

Functions

User-Defined Functions

awk 'function max(a, b) {
    return (a > b) ? a : b
}
{ print max($1, $2) }' file.txt

Numeric Functions

int(x)	Truncate to integer
sqrt(x)	Square root
sin(x), cos(x)	Trigonometric functions
log(x), exp(x)	Natural log and exponent
rand()	Random float between 0 and 1
srand(seed)	Seed the random number generator

Arrays

Associative Arrays

awk '{ count[$1]++ }
     END { for (k in count) print k, count[k] }' f
awk '{ arr[NR] = $0 }
     END { for (i = NR; i >= 1; i--) print arr[i] }' f

Array Operations

arr[key] = val	Set element
arr[key]	Get element (auto-creates on access)
key in arr	Test if key exists
delete arr[key]	Delete single element
delete arr	Delete entire array
for (k in arr)	Iterate over keys (unordered)
length(arr)	Number of elements (gawk)

String Functions

String Reference

length(s)	String length
substr(s, start, len)	Substring (1-indexed)
index(s, target)	Position of target in s (0 if not found)
split(s, arr, sep)	Split string into array
sub(pat, repl, s)	Replace first match
gsub(pat, repl, s)	Replace all matches
match(s, pat)	Position of regex match (sets RSTART, RLENGTH)
tolower(s) / toupper(s)	Case conversion
sprintf(fmt, ...)	Format string (like C printf)

String Examples

awk '{ gsub(/old/, "new"); print }' f    # sed-like replace
awk '{ print toupper($0) }' f             # uppercase all
awk '{ print substr($0, 1, 40) }' f       # truncate to 40

I/O

Output

awk '{ print $1, $2 }' f              # space-separated
awk '{ printf "%s,%d\n", $1, $2 }' f  # formatted output
awk '{ print $1 > "out.txt" }' f      # redirect to file
awk '{ print $1 >> "out.txt" }' f     # append to file

I/O Reference

print	Print with ORS (newline by default)
printf fmt, ...	Formatted print (no trailing newline)
print > file	Redirect output to file
print >> file	Append output to file
print \| cmd	Pipe output to a command
getline < file	Read a line from file
cmd \| getline var	Read command output into variable
close(file)	Close file or pipe

Common Patterns

One-Liners

awk '{ sum += $1 } END { print sum }' f   # sum column
awk 'END { print NR }' f                   # count lines
awk '!seen[$0]++' f                        # remove dupes
awk 'NF' f                                 # remove blanks
awk '{ print NF }' f                       # fields per line

Recipes

CSV to TSV	`awk -F, 'BEGIN{OFS="\t"} {$1=$1; print}'`
Sum column 2	`awk '{ s += $2 } END { print s }'`
Top N lines	`awk 'NR <= 10'` (like head)
Frequency count	`awk '{ c[$1]++ } END { for (k in c) print k, c[k] }'`
Between markers	`awk '/BEGIN/,/END/'`
Print nth field	`awk '{ print $N }'` (replace N)

AWK QUICK REFERENCE