Print each line once, counting instances of each.
Print duplicate lines once, but no unique lines.
Print only unique lines (no copy of duplicate entries is kept).
n fields of a line.
Fields are separated by spaces or by tabs.
n characters of a field.
Send one copy of each line from list to output file list.new:
uniq list list.new
Show which names appear more than once:
sort names | uniq -d
Show which lines appear exactly three times:
sort names | uniq -c | grep "3 "