Created SC3066 (markdown)

SC3066.md

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+## In POSIX sh, test -G is undefined.
+
+### Problematic code:
+
+```sh
+if [ -G file ]
+then
+  echo "File is owned by your effective group"
+fi
+```
+
+### Correct code:
+
+```sh
+if [ -n "$(find file -prune -group "$(id -g)")" ]
+then
+  echo "File is owned by your effective group"
+fi```
+
+### Rationale:
+
+`test -G` is a bash/ksh/dash/ash extension to check whether the file is owned by your effective group.
+
+To ensure compatibility with other shells, you can use `find -group "$(id -g)"`.
+
+### Exceptions:
+
+If you expect your shell to support `test -G`, specify this explicitly in the shebang (or with `# shellcheck shell=dash` directive).
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!