Shell Programming and Scripting

#!/bin/ksh
# or bash, bsh, ...
# join.sh
prev=""
saveid=""
sort -t , -k 3,3 | while IFS="," read  id num email xstr
do
       [ "$prev" = "" ] && prev="$email"  #1st line
       [ "$prev" = "$email" ] && saveid="$saveid$id," && continue
       # buf out
       echo "$saveid$prev"
       prev="$email"
done
# last buf
echo "$saveid$prev"

chmod a+rx join.sh
# using ex.
cat file | ./join.sh | ...
./join.sh < file  > file2

awk -F"," 'NR>1{arr[$3]=arr[$3]","$1}END {for (i in arr)print arr[i]","i}' yourfile

awk -F"," 'NR>1{arr[$3]=arr[$3]","$1}END {for (i in arr)print arr[i]","i}' yourfile

awk -F"," 'NR>1{arr[$3]=arr[$3]","$1}END {for (i in arr)print arr[i]","i}' yourfile | sed 's/^\,//'

#  nawk -F"," '{OFS=",";t[$3]?t[$3]=t[$3]","$1:t[$3]=$1}END{for (i in t){print t[i],i}}' infile
20,payal@gmail.com
10,30,40,ravi@yahoo.com

./join.sh < testfile.txt

20,payal@gmail.com
20,30,40,ravi@yahoo.com

./join.sh < testfile.txt > result.txt