AWK Cheat Sheet: Commands, Variables, Functions & One-Liners

This is a reference to keep open while you work: the AWK syntax you use most, grouped by task, with the result of each example. Every entry was run on gawk 5.2.1 and mawk 1.3.4. Where the two disagree, the entry says so, and a portability table near the end collects the differences in one place. Print this page from your browser if you want a paper copy.

If you are new to AWK, read the beginner's guide first. This sheet assumes you know what a field and a record are.

Sample files used below

printf 'start\nx\ny\nend\nz\n' > rng.txt
printf 'one two\nthree four five\n\nsix\n' > t4.txt
printf 'p1 l1\np1 l2\n\np2 l1\n' > para.txt

Run AWK

Wrap the program in single quotes. Double quotes let the shell replace $1 before AWK sees it.

Program structure

BEGIN { ... }          # once, before any input
pattern { action }     # for every line that matches the pattern
{ action }             # no pattern: every line
pattern                # no action: print the matching lines
END { ... }            # once, after the last line

Fields and built-in variables

awk '{print FILENAME, NR, FNR}' t4.txt rng.txt | sed -n '3,6p'
# t4.txt 3 3
# t4.txt 4 4
# rng.txt 5 1
# rng.txt 6 2

awk 'BEGIN{RS=""} {print NR ": " $1, NF}' para.txt
# 1: p1 4
# 2: p2 2

awk 'BEGIN{ORS="|"} {print $1} END{printf "\n"}' t4.txt
# one|three||six|

echo 'a b c' | awk '{NF=2; print}'        # a b    (assigning NF cuts the line)
echo 'a b'   | awk '{$5="e"; print NF}'    # 5      (assigning past the end adds empty fields)

Patterns

Operators

The ** power operator works in gawk only; mawk gives a syntax error. Use ^, which works in both.

String functions

awk 'BEGIN{s="a-b-c"; gsub(/-/,"&&",s); print s}'                 # a--b--c   (& means "the matched text")
awk 'BEGIN{n=split("a1b22c",p,/[0-9]+/); print n, p[1], p[2], p[3]}'  # 3 a b c   (separator can be a regex)
awk 'BEGIN{s="  trim me  "; gsub(/^ +| +$/,"",s); print "[" s "]"}'   # [trim me]

Two things to remember. First, a string used as a pattern is a regular expression, so gsub(".", "-", s) replaces every character, not every dot. On a.b.c it gave -----, while gsub(/\./, "-", t) gave a-b-c. Second, substr() with a start below 1 is not portable: substr("hello", -1, 3) gave hel in gawk and hell in mawk. Keep the start at 1 or more.

Numbers and printf

Plain print shows at most six significant digits, so use printf when exact output matters. A single line that uses most formats:

awk 'BEGIN{printf "%d %5.2f %-6s| %05d %x %o %e %c %%\n", 42.9, 3.14159, "ab", 42, 255, 8, 12345.678, 65}'
# 42  3.14 ab    | 00042 ff 10 1.234568e+04 A %

printf does not add a newline. End the format with \n.

Control flow

awk 'BEGIN{for(i=1;i<=3;i++) printf "%d ", i; print ""}'                       # 1 2 3
awk 'BEGIN{i=1; while(i<=3){printf "%d ", i; i++} print ""}'                  # 1 2 3
awk 'BEGIN{i=5; do {printf "%d ", i; i++} while(i<3); print ""}'              # 5   (do-while runs once first)
awk 'BEGIN{for(i=1;i<=10;i++){if(i%2) continue; if(i>6) break; printf "%d ", i} print ""}'   # 2 4 6
awk 'BEGIN{x=7; if (x<5) print "low"; else if (x<10) print "mid"; else print "high"}'          # mid

Arrays

Input and output

Close a file with close("out.txt") before you read it back in the same program. I wrote $1 of every line of t4.txt to a file, closed it, and read it back with getline, and got one, three, an empty line and six.

User-defined functions

function max(a, b) { return a > b ? a : b }
function fact(n)   { return n <= 1 ? 1 : n * fact(n - 1) }
function f(x,  tmp) { tmp = x * 2; return tmp }   # extra parameters are local variables

BEGIN { print max(3, 9), max("b", "a") }          # 9 b
BEGIN { print fact(5) }                           # 120
BEGIN { tmp = "keep"; print f(4), tmp }           # 8 keep

AWK has no local keyword. Extra parameters you never pass behave as local variables, which is why tmp kept its value in the last line. By habit, put two spaces before them to show they are locals.

One-liners

What works in gawk but not in mawk

Most of this sheet runs on every awk. These features do not. The worst ones fail with no message at all, so your script runs and gives wrong output.

These do work in both: length(array), nextfile, delete array, systime(), strftime(), /dev/stderr, -v and -f.

Common mistakes

• Double quotes around the program. awk "{print $1}" prints whole lines, because the shell empties $1.
• Header row in a numeric test. $4 > 5 also matches a header like qty. Add NR>1 &&.
• Array loop order. for (k in a) has no guaranteed order. Pipe to sort.
• Setting FS inside a rule. It only affects the next line. Set it in BEGIN or use -F.
• $(NF-1) on a blank line. It stops AWK with an error. Guard with NF>1.
• A string where you meant a regex. gsub(".", ...) matches any character.