Join lines of CSV file on column field number specified (starting at 0).

joinonfield.py

import sys
i=int(sys.argv[1])
d=dict()
for l in sys.stdin.readlines():
    l=l.strip().split(',')
    k=l.pop(i)
    d[k]=d.get(k,[])+l
for k in d:
    print ','.join([k]+d[k])