Shell Programming and Scripting

Group1:*:gid1:user,user
Group2:*:gid2:user,user
Group3:*:gid3:user,user,user,user,user,user,user
Group4:*:gid4:user,user

 
cat group |tr "," "\n" | while read X
do
   echo $X | grep \: > /dev/null
   if [ $? -eq 0 ]
   then
      echo $X | tr ":" " " | while read GRP GARBAGE GID FIRSTNAME
      do
       export GRP GID FIRSTNAME
       echo "$GRP,$GID,$FIRSTNAME"
      done
   else
      echo "$GRP,$GID,${X}"
   fi
done

Group1,gid1,user
Group1,gid1,user
Group2,gid2,user
Group2,gid2,user
Group2,gid2,user
Group2,gid2,user
Group3,gid3,user

Group1,gid1,user
,,user
Group2,gid2,user
,,user
,,user
,,user
Group3,gid3,user

awk -F '[:,]' '{for(i=3;++i<=NF;) print $1,$3,$i}' OFS=\, infile