Insert characters Into a string in bash

echo "2013abcd07" | sed 's/^\(.\{8\}\)/\1-/'

output:  2013abcd-07
it will insert character '-' after 8 characters

time=125959  ## expected : 12:59:59
echo ${time:0:2}:${time:2:2}:${time:4:2}

Example:
input: "abcd" 

output: ".abcd
a.bcd
ab.cd
abc.d
abcd." 


input=abcd
for ((i=0; i<${#input}+1; i++))
do
    echo ${input::$i}.${input:$i}
done