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+## In POSIX sh, unary `-v` (in place of `[ -n "${var+x}" ]`) is undefined.
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+if [ -v STY ]
+then
+  echo "STY is set, you are using screen"
+fi
+```
+
+### Correct code:
+
+Either switch to bash:
+
+```sh
+#!/bin/sh
+if [ -n "${STY+x}" ]
+then
+  echo "STY is set, you are using screen"
+fi
+```
+
+### Rationale:
+
+Your script uses a shell feature not supported by the shebang. Either rewrite the script to be portable, or change the shebang to explicitly require a shell like Bash.
+
+In this case, `[ -v variable ]` to check if a variable is set can be replaced with `[ -n "${variable+x}" ]` which uses the "alternative value if set" parameter expansion syntax to accomplish the same thing.
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!
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