SC2210

if x > 5; then echo "true"; fi

or

foo > /dev/null 2>1

if (( x > 5 )); then echo "true"; fi

or

foo > /dev/null 2>&1

You are redirecting to or from a filename that is an integer. For example, ls > file where file happens to be 3.

This is not likely to be intentional. The most common causes are:

1. Trying to compare two numbers, as in x > 5. This should instead be [ "$x" -gt 5 ] or (( x > 5 )).
2. Trying similarly to compare command output, as in grep -c foo file > 100 instead of [ "$(grep -c foo file)" -gt 100 ]
3. Malformed FD operations, such as writing 1>2 instead of 1>&2.

If you do want to create a file named 4, you can quote it to silence shellcheck and make it more clear to humans that it's not supposed to be taken numerically.

If you use the &> form of redirection, as in foo > /dev/null 2&>1, it will trigger this warning. You can safely ignore this warning if that is what triggered it, or change your redirection operator to the semantically preferable >&.