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+## Use `-n` instead of `! -z`.
+
+(or "Use `-z` instead of `! -n`)
+
+### Problematic code:
+
+```sh
+if [ ! -n "$JAVA_HOME" ]; then echo "JAVA_HOME not specified"; fi
+if [ ! -z "$STY" ];       then echo "You are already running screen"; fi
+```
+
+### Correct code:
+
+```sh
+if [ -z "$JAVA_HOME" ]; then echo "JAVA_HOME not specified"; fi
+if [ -n "$STY" ];       then echo "You are already running screen"; fi
+```
+
+### Rationale:
+
+You have negated `test -z` or `test -n`, resulting in a needless double-negative. You can just use the other operator instead:
+
+    # Identical tests to verify that a value is assigned
+    [ ! -z foo ] # Not has no value
+    [ -n foo ]   # Has value
+
+    # Identical tests to verify that a value is empty
+    [ ! -n foo ] # Not is non-empty
+    [ -z foo ]   # Is empty
+
+### Exceptions:
+
+This is a stylistic issue that does not affect correctness. If you prefer the original expression, you can't not [[Ignore]] it with a directive or flag.
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!
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