UNIX for Dummies Questions & Answers

#!/bin/sh

COL=4

# Break file into livable chunks
split -C 100K < megadata.txt

for FILE in x??
do
        sort -k $COL < "${FILE}" > "${FILE}.tmp"
        rm -f "${FILE}"
        mv "${FILE}.tmp" "${FILE}"
        shift
done

sort -k $COL -m ??? | awk -v COL=${COL} '{
        if(! $0) { }
        else if($COL == LAST)
        {
                if(orig)
                {       print orig;     orig="";        }

                print;
        }
        else
        {
                LAST=$COL;
                orig=$0;
        }

                }' > output.txt
rm -f x??