NAME
uniq - report or omit repeated lines SYNOPSIS
uniq [OPTION]... [INPUT [OUTPUT]] DESCRIPTION
Filter adjacent matching lines from INPUT (or standard input), writing to OUTPUT (or standard output). With no options, matching lines are merged to the first occurrence. Mandatory arguments to long options are mandatory for short options too. -c, --count
prefix lines by the number of occurrences -d, --repeated
only print duplicate lines -D, --all-repeated[=delimit-method]
print all duplicate lines delimit-method={none(default),prepend,separate} Delimiting is done with blank lines -f, --skip-fields=N
avoid comparing the first N fields -i, --ignore-case
ignore differences in case when comparing -s, --skip-chars=N
avoid comparing the first N characters -u, --unique
only print unique lines -z, --zero-terminated
end lines with byte, not newline -w, --check-chars=N
compare no more than N characters in lines --help display this help and exit --version
output version information and exit A field is a run of blanks (usually spaces and/or TABs), then non-blank characters. Fields are skipped before chars. Note: 'uniq' does not detect repeated lines unless they are adjacent. You may want to sort the input first, or use 'sort -u' without 'uniq'. Also, comparisons honor the rules specified by 'LC_COL‐
LATE'.
以上是man输出。
从最后的note中可以知道当使用uniq进行去重，要求输入重复项是相邻的。这个比较好理解，要求重复项时连续的话可以省去一个hashmap的空间来做统计。为了获得这样的一个输入，可以先对数据进行一个排序操作，这样重复项必然是连续相邻的。
有如下文本文件：
the
day
is
sunny
the
the
sunny
day
is
today
is
sunny
day
UASE CASE 1.
首先对单词内容做一个去重处理（先排序，再去重）
$ sort words.txt | uniq
day
is
sunny
the
today
USE CASE 2.
重复统计：
$ sort words.txt | uniq -c
day
is
sunny
the
today
USE CASE 3.
只输出重复项或者只输出唯一项：
$ sort words.txt | uniq -d
day
is
sunny
the
$ sort words.txt | uniq -u
today