Strictly Conforming XSI Application that counts unique non-empty keys from standard input, and prints those appearing at least MINCNT times.

count.c

#define _XOPEN_SOURCE 700
#include <search.h>
#include <stdio.h>
#include <stdlib.h>

#define NEL (2 << 20)
#define MINCNT 5

main()
{
	static long nkeys, i;
	static struct key {
		char *key;
		long cnt;
	} keys[NEL];

	hcreate(NEL);

	while (!feof(stdin)) {
		ENTRY item, *found;
		char *key = NULL;
		ssize_t len = 0;
		struct key *uniq;

		getline(&key, &len, stdin);

		item.key = key;
		found = hsearch(item, FIND);
		if (found) {
			uniq = found->data;
			uniq->cnt++;

			free(key);
		} else {
			uniq = &keys[nkeys];
			uniq->key = key;
			uniq->cnt++;
			++nkeys;

			item.data = uniq;
			hsearch(item, ENTER);
		}
	}

	for (i = 0; i < nkeys; i++) {
		struct key *uniq = &keys[i];
		long cnt = uniq->cnt;

		if (MINCNT <= cnt)
			printf("%ld\t%s", cnt, uniq->key);
	}

	return 0;
}

cut.awk

$5 != "" && $16 != "" {
	print $5 "," $16
}

Makefile

SCRIPT = "awk -F , -f cut.awk <input.csv | ./count >output.txt"

.POSIX:

all: cut.awk count input.csv
	time sh -c $(SCRIPT)

clean:
	-rm -f count output.txt