Created SC2251 (markdown)

SC2251.md

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+## This ! is not on a condition and skips errexit. Use { ! ...; } to errexit, or verify usage.
+
+### Problematic code:
+
+```sh
+set -e
+! false
+```
+
+### Correct code:
+
+```sh
+set -e
+{ ! false; }
+```
+
+### Rationale:
+
+ShellCheck has found a command inverted with `!` that may have no effect. In particular, it does not appear as a condition in an `if` statement or `while` loop, or as the final command in a script or function.
+
+The most common reason for this is thinking that it'll trigger `set -e` aka `errexit` if a command succeeds, as in the example. This is not the case: `!` will inhibit errexit both on success and failure of the inverted command.
+
+### Exceptions:
+
+ShellCheck will not detect cases where `$?` is implicitly or explicitly used to check the value afterwards:
+
+```
+check_success() { [ $? -eq 0 ] || exit 1; }
+! false; check_success
+! true; check_success
+```
+
+In this case, you can [[ignore]] the warning, or optionally wrap them in `{ ! false; }`.
+
+### Related resources:
+
+* StackOverflow: [Why do I need parenthesis In bash `set -e` and negated return code](https://stackoverflow.com/questions/39581150/why-do-i-need-parenthesis-in-bash-set-e-and-negated-return-code/39582012)