cmp

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.

s

Silent

Return status 0 if equal, 1 otherwise.

Prints nothing.

cmp -s "$F" "$G"
if [ $? -eq 1 ]; then
  echo neq
else
  echo eq
fi

More than two files

http://unix.stackexchange.com/questions/33638/diff-several-files-true-if-all-not-equal