Shell Programming and Scripting

#!/usr/bin/env python
d={}
for line in open("file"):
    line = line.strip().split()
    d.setdefault(line[0],[])
    d[line[0]].append(line[-1])
for i,j in d.iteritems():
    print i,','.join(j)

# ./test.py
a1 test1,test2
a2 test1,test2
b1 test1,test2
b2 test1,test2

awk 'END { print f, r[f] }
f && $1 != f { print f, r[f] } 
{ r[f = $1] = r[$1] ? r[$1] ", " $2 : $2 }
' infile

awk '{r[$1]=r[$1] ", " $2} END {for (f in r) print f, substr(r[f], 3)}' infile

awk 'END { print r } 
f && $1 != f { print r; r = x } 
{ r = r ? r ", " $2 : (f = $1) FS $2 }
' infile