http://pubs.opengroup.org/onlinepubs/9699919799/utilities/cmp.html, Ubuntu GNU Diffutils package.
Compares F and G byte by byte, until first difference is found.
cmp "$F" "$G"
If equal, print nothing.
Else, print location of first difference.
Silent
Return status 0 if equal, 1 otherwise.
Prints nothing.
cmp -s "$F" "$G"
if [ $? -eq 1 ]; then
echo neq
else
echo eq
fi
http://unix.stackexchange.com/questions/33638/diff-several-files-true-if-all-not-equal