These come up constantly in scripting and interview questions:
grep "pattern" file.txt # find lines matching pattern
grep -r "pattern" /path # recursive search
grep -v "pattern" # invert (lines NOT matching)
grep -i "pattern" # case-insensitive
grep -E "regex" # extended regex
grep -c "pattern" # count matches
awk '{print $2}' # print second field
awk -F: '{print $1}' /etc/passwd # custom field separator
awk '$3 > 100 {print $1}' # conditional print
sed 's/old/new/g' file # substitute
sed -i 's/old/new/g' file # in-place edit (Linux)
sort file | uniq -c | sort -rn # count and rank unique lines
cut -d',' -f1,3 file.csv # cut columns from CSV
wc -l file # line count
head -n 20 file # first 20 lines
tail -n 20 file # last 20 lines
tail -f file # follow (live updates)