Best way to simulate “group by” from bash?

Question

Suppose you have a file that contains IP addresses, one address in each line:

10.0.10.1
10.0.10.1
10.0.10.3
10.0.10.2
10.0.10.1

You need a

Answer 1

I feel awk associative array is also handy in this case

$ awk '{count[$1]++}END{for(j in count) print j,count[j]}' ips.txt

A group by post here