ShellCheck noticed you're using
**-notation with two asterisks to obtain an exponent's value. Some examples:
#!/bin/sh echo $((2 ** 3)) # Using `**` for an exponent is undefined in POSIX sh.
Other possibilities exist:
#!/bin/sh echo $((2 * 2 * 2)) # equivalent to `bash`'s `echo $((2 ** 3))` echo '2 ^ 3' | bc # piping the formula to `bc` to parse the output https://stackoverflow.com/a/13111995 printf '2 ^ 3\n' | bc # piping using `printf` (newline `\n` is mandatory)
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.