I found a solution thanks to this questionthis question
 Basically I am testing two files a.txt and b.txt with this script:
#!/bin/bash
first_cmp=$(diff --unchanged-line-format= --old-line-format= --new-line-format='%L' "$1" "$2" | wc -l)
second_cmp=$(diff --unchanged-line-format= --old-line-format= --new-line-format='%L' "$2" "$1" | wc -l)
if [ "$first_cmp" -eq "0" -o "$second_cmp" -eq "0" ]
then
    echo "Subset"
    exit 0
else
    echo "Not subset"
    exit 1
fi
 If one is subset of the other the script return 0 for True otherwise 1.
 
                