Programming

ls *\_*.txt | while read file;do echo $file | cut -d_ -f1 >> output.txt;done

cat output.txt
ABC126
AYA127

create a new file of say /tmp/filenames.lst
change the drawer to place where *.txt files are situated
ls *.txt > /tmp/listing
[optional]cat < /tmp/listing [as a visible check.]
read in the /tmp/listing to a var
create an array from var
use a loop derived from the number of array elements<and>
from each line of the array extract the first 6 characters
echo <each_set_of_6_characters> and append to /tmp/filenames.lst

ls|colrm 6