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

You probably can use the file system itself as a hash table. Pseudo-code as follows:

for every entry in the ip address file; do
  let addr denote the ip address;

  if file "addr" does not exist; then
    create file "addr";
    write a number "0" in the file;
  else 
    read the number from "addr";
    increase the number by 1 and write it back;
  fi
done

In the end, all you need to do is to traverse all the files and print the file names and numbers in them. Alternatively, instead of keeping a count, you could append a space or a newline each time to the file, and in the end just look at the file size in bytes.