Created SC2300 (markdown)

SC2300.md

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+## Parameter expansion can't be applied to command substitutions. Use temporary variables.
+
+### Problematic code:
+
+```sh
+echo "Building ${$(git rev-parse --show-toplevel)##*/}"
+```
+
+### Correct code:
+
+```sh
+tmp=$(git rev-parse --show-toplevel)
+echo "Building ${tmp##*/}"
+```
+### Rationale:
+
+ShellCheck found a parameter expansion that begins with a command substitution, such as `$(..)` or `` `..` ``. This is not valid. Parameter expansion only works on variables (normal or special).
+
+In the example, the user hoped to apply the construct `${var##*/}`, stripping the path, to the current git root directory as output by `git rev-parse --show-toplevel`. Since parameter expansion only works on variable, the command substitution must be assigned to a variable first like in the correct example.
+
+If the goal was instead to dynamically generate a variable name to expand, see [[SC2082]].
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!