See,

$ find . -name "*.sh" | xargs shellcheck -i SC2034

In ./roll-tarballs.sh line 34:
NULL=""
^--^ SC2034: NULL appears unused. Verify use (or export if used externally).

For more information:
  https://www.shellcheck.net/wiki/SC2034 -- NULL appears unused. Verify use (...
Signed-off-by: Mario Trangoni <mjtrangoni@gmail.com>