Created SC2298 (markdown)

SC2298.md

@@ -0,0 +1,50 @@
+##  `${$x}` is invalid. For expansion, use ${x}. For indirection, use arrays, ${!x} or (for sh) eval.
+
+(or `${${x}}` is invalid)
+
+### Problematic code:
+
+```sh
+# Expecting $RETRIES or 3 if unset
+retries=${$RETRIES:-3}
+```
+
+or
+
+```sh
+mypath="/tmp/foo.txt"
+var=mypath
+result=${$var##*/}  # Expecting ${mypath##*/}, i.e. 'foo.txt'
+```
+
+### Correct code:
+
+```sh
+retries=${RETRIES:-3}
+```
+
+or
+
+```sh
+mypath="/tmp/foo.txt"
+var=mypath
+result=${!var}
+result=${result##*/}
+
+```
+
+### Rationale:
+
+ShellCheck found a parameter expansion `${..}` where the first element was a second parameter expansion, either `${$x..}` or `${${x}..}`. This is not valid.
+
+In the first example, the extra `$` was unintentional and should simply be deleted.
+
+In the second example, `${$var##*/}` was used in the hopes that it would expand to `${myvar##*/}` and subsequently strip the path. This is not possible, and `var` must instead be expanded indirectly in a separate step, before the path can be stripped as usual.
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!