Created SC2210 (markdown)

SC2210.md

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+## This is a file redirection. Was it supposed to be a comparison or fd operation?
+
+### Problematic code:
+
+```sh
+if x > 5; then echo "true"; fi
+```
+or
+```sh
+foo > /dev/null 2>1
+```
+
+### Correct code:
+
+```sh
+if (( x > 5 )); then echo "true"; fi
+```
+or
+```sh
+foo > /dev/null 2>&1
+```
+### Rationale:
+
+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 `[ "$x" -gt 5 ]` or `(( x > 5 ))`.
+1. Trying similarly to compare command output, as in `grep -c foo file > 100` instead of `[ "$(grep -c foo file)" -gt 100 ]`
+1. Malformed FD operations, such as writing `1>2` instead of `1>&2`.
+
+### Exceptions:
+
+None. 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.