Replace repeated elements in a list with unique identifiers - bash

Question

I have a list like the below:
1 . Fred 1 6 78 8 09
1 1 Geni 1 4 68 9 34
2 . Sam 3 4 56 6 89
3 . Flit 2 4 56 8 34
3 4 Dog 2 5 67 8 78
3 . Pig 2 5 67 2 21
(except the real list is 40 million lines long).
There are repeated elements in the second column (i.e. the ".")
I want to replace these with unique identifers (e.g. ".1", ".2", ".3"...".n")
I tried to do this with a bash loop / sed combination, but it didn't work...
Failed attempt:
for i in 1..4
do
sed -i "s_//._//."$i"_"$i""
done
(Essentially, I was trying to get sed to replace each n th "." with ".n", but this didn't work).

Answer 1

Here's a way to do it with awk (assuming your file is called input:
$ awk '$2=="."{$2="."++counter}{print}' input
1 .1 Fred 1 6 78 8 09
1 1 Geni 1 4 68 9 34
2 .2 Sam 3 4 56 6 89
3 .3 Flit 2 4 56 8 34
3 4 Dog 2 5 67 8 78
3 .4 Pig 2 5 67 2 21
The awk program replaces the second column ($2) by a string formed by concatenating . and a pre-incremented counter (++counter) if the second column was exactly .. It then prints out all the columns it got (with $2 modified or not) ({print}).
Plain bash alternative:
c=1
while read -r a b line ; do
if [ "$b" == "." ] ; then
echo "$a ."$((c++))" $line"
else
echo "$a $b $line"
fi
done < input

Answer 2

Since your question is tagged sed and bash, here are a few examples for completeness.
Bash only
Use parameter expansion. The second column will be unique, but not sequential:
i=1; while read line; do echo ${line/\./.$((i++))}; done < input
1 .1 Fred 1 6 78 8 09
1 1 Geni 1 4 68 9 34
2 .3 Sam 3 4 56 6 89
3 .4 Flit 2 4 56 8 34
3 4 Dog 2 5 67 8 78
3 .6 Pig 2 5 67 2 21
Bash + sed
sed cannot increment variables, it has to be done externally.
For each line, increment $i if line contains a ., then let sed append $i after the .
i=0
while read line; do
[[ $line == *.* ]] && i=$((i+1))
sed "s#\.#.$i#" <<<"$line"
done < input
Output:
1 .1 Fred 1 6 78 8 09
1 1 Geni 1 4 68 9 34
2 .2 Sam 3 4 56 6 89
3 .3 Flit 2 4 56 8 34
3 4 Dog 2 5 67 8 78
3 .4 Pig 2 5 67 2 21

Answer 3

you can use this command:
awk '{gsub(/\./,c++);print}' filename
Output:
1 0 Fred 1 6 78 8 09
1 1 Geni 1 4 68 9 34
2 2 Sam 3 4 56 6 89
3 3 Flit 2 4 56 8 34
3 4 Dog 2 5 67 8 78
3 5 Pig 2 5 67 2 21