Shell Programming and Scripting

awk -F'_' 'length($(NF-1))!=8{$(NF-1)="20"$(NF-1)}1' OFS='_'

#!/bin/bash
for i in *.txt
do a=`ls $i | awk -F"_" 'length($(NF-1))!=8{$(NF-1)="20"$(NF-1)}{print}' OFS='_'`
mv $i $a
done

for i in *.txt
do
   printf "%s" "$i" | awk ..
done

for file in *.txt
do
        dt=${file%_*}
        dt=${dt##*_}
        if [ ${#dt} -ne 8 ]
        then
                n_dt="20${dt}"
                n_file="${file/$dt/$n_dt}"
                echo mv "$file" "$n_file"
        fi
done