Created SC2242 (markdown)

SC2242.md

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+## Can only exit with status 0-255. Other data should be written to stdout/stderr.
+
+### Problematic code:
+
+```sh
+exit "Bad filename"
+```
+
+### Correct code:
+
+```sh
+echo "Bad filename" >&2
+exit 1
+```
+
+### Rationale:
+
+`exit` can only be used to signal success or failure (0 = success, 1-255 = failure). It can not be used to return string data, and it can not be used to print error messages.
+
+String data should be written stdout, before an `exit 0` to exit with success.
+
+Errors should instead be written to stderr, with an `exit 1` (or higher) to exit with failure:
+
+```
+if [ ! -f "$1" ]
+then
+   echo "$1 is not a regular file" >&2
+   exit 1
+fi
+```
+
+Note in particular that `exit -1` is equivalent to `exit 255`, but that `exit 1` is the more canonical way of expressing the first possible error code.
+
+### Exceptions:
+
+None