In-place-style transform with awk

Global substitution and write back to same file

awk '{gsub(/foo/, "bar"); print}' input.txt > tmp && mv tmp input.txt

More awk commands

Process CSV with quotes (gawk)

gawk -v FPAT='([^,]+)|("[^"]+")' '{print $2}' data.csv

Parse quoted CSV fields correctly with FPAT in gawk
BEGIN/END blocks

awk 'BEGIN{print "Start"} {lines++} END{print lines" lines processed"}' file.txt

Run code before and after processing all lines
Two-file join on key

awk 'NR==FNR{a[$1]=$2; next} $1 in a {print $0, a[$1]}' lookup.txt data.txt

Left-join data.txt with lookup.txt on first field
Print specific column

awk '{print $3}' file.txt

Print the 3rd whitespace-delimited field
Custom field separator

awk -F: '{print $1, $3}' /etc/passwd

Use colon as field separator, print user and UID