Useful AWK one-liners

While reading Masterminds of Programming I came across the interview with Alfred Aho and his thoughts about the design of the AWK programming language. I’ve always known about the existence of awk, but have never really felt the urge to dive in. Then I came across this sentence:

If I had to choose a word to describe our centering forces in language design, I’d say Kernighan emphasized ease of learning; Weiberger, soundness of implementation; and I, utility. I think AWK has all three of these properties.

This got me curious enough to put down the book and start looking through a few AWK tutorials. After mucking around with it for a day or two, I am really glad that I did so. AWK is a simple yet powerful language to do some quick data processing for which languages like Python or Ruby may be an overkill.

While learning, I’ve collected a few one-liners together, as a cheatsheat for myself.

Remove duplicate lines from a file

awk '!a[$0]++' data.txt

Print account name and role from /etc/passwd

awk -F : '/^[^(#|_)]/ {print $1, "=>", $5}' /etc/passwd

Prefixing line with increasing number

awk '{print FNR ". " $0}' data1.txt

Find sum of columns for each row

awk '{s=0; for (i=1; i<=NF; i++) s=s+$i; print s}' data1.txt

Find the line containing largest value in the first column

awk '$1 > max {max=$1; maxline=$0}; END{ print maxline}' data1.txt

Find and replace in file

# Replace only first instance in line
awk '{sub(/HTML/, "html")}; 1' data1.txt > data2.txt

# Replace all instances in line
awk '{gsub(/HTML/, "html")}; 1' data1.txt > data2.txt

Swap first two columns of every line

awk '{temp=$1; $1=$2; $2=temp} 1' data2.txt

Delete second column from every line

awk '{ $2 = ""; print }' data2.txt

Perform search similar to SQL ‘where’

awk '$2 == "hello"' data2.txt
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