Not matching files

Shell Programming and Scripting
1

Hi
I need to check list of files aganinst a.txt and return those files which are not available in a.txt


ls -lrt 

file1.txt
file2.txt
file3.txt


a.txt
=====
file1.txt
file2.txt
   
Expecting o/p

file3.txt

Thanks,
Akil

2 
var="$(ls -lrt folder | awk 'NF>3{print $NF}')";echo "$var" >>a.txt;cat a.txt | sort | uniq -u

result:

file3.txt
3

you can use grep

ls -1|grep -v -f a.txt
4 
comm -3 <(ls -1) <(sort a.txt)