SC2148

Add a shebang line to the top of your script:

#!/bin/bash

Or, for scripts that will not be executed (e.g., ~/.bashrc), use a directive:

# shellcheck shell=bash

If neither of those options are possible or desirable, you can invoke ShellCheck with the --shell switch:

shellcheck --shell=sh without-shebang.sh

See shellcheck --help for a list of supported flavors (sh, bash, dash, ksh).