uniq
-information on repeated lines in a file
-generally works better if the lines are initially sorted
-can count number of repeated lines
-can show lines that are repeated

more dfruit
apple
pear
peach
peach
apple
blueberry
blueberry
blueberry
apple
grape
blueberry

uniq dfruit
apple
pear
peach
apple
blueberry
apple
grape
blueberry

1. notice that repeated lines have been collapsed into one

sort dfruit |uniq
apple
blueberry
grape
peach
pear

how many copies of a line?
sort dfruit |uniq -c

    3 apple
    4 blueberry
    1 grape
    2 peach
    1 pear

which lines are repeated?
sort dfruit |uniq -d
apple
blueberry
peach

which lines are not repeated?
sort dfruit |uniq -u
grape
pear