SC2148

echo "$RANDOM"   # Does this work?

#!/bin/sh
echo "$RANDOM"  # Unsupported in sh. Produces warning.

or

#!/bin/bash
echo "$RANDOM"  # Supported in bash. No warnings.

Different shells support different features. To give effective advice, ShellCheck needs to know which shell your script is going to run on. You will get a different numbers of warnings about different things depending on your target shell.

If you add a shebang (e.g. #!/bin/bash as the first line), the OS will use this interpreter when the script is executed, and ShellCheck will use this shell when offering advice.

If you for any reason can't or won't add a shebang, there are multiple other ways to let shellcheck know which shell you're coding for:

• Specify the shell using the -s or --shell flag, e.g. shellcheck -s bash myfile
• Use a shellcheck directive, adding # shellcheck shell=ksh before the first command in the file.
• Give the script a .bash, .ksh or .dash extension (.sh will not assume --shell=sh since it's so generic)

None. Please either add a shebang, directive, extension or use -s to maximize ShellCheck's usefulness.