Created SC2230 (markdown)

SC2230.md

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+## which is non-standard. Use builtin 'command -v' instead.
+
+### Problematic code:
+
+```sh
+which grep
+```
+
+### Correct code:
+
+```sh
+command -v grep
+```
+
+### Rationale:
+
+`which` is a non-standard, external tool that locates an executable in PATH. `command -v` is a POSIX standard builtin, which uses the same lookup mechanism that the shell itself would.
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* [Check if a program exists from a Bash script](https://stackoverflow.com/a/677212/1899640) on StackOverflow.