Created SC2070 (markdown)

SC2070.md

@@ -0,0 +1,46 @@
+## Always true because you failed to quote. Use [[ ]] instead.
+
+### Problematic code:
+
+    if [ -n $var ]
+    then
+      echo "var has a value"
+    else
+      echo "var is empty"
+    fi
+
+### Correct code:
+
+In bash/ksh:
+
+    if [[ -n $var ]]
+    then
+      echo "var has a value"
+    else
+      echo "var is empty"
+    fi
+
+In POSIX:
+
+    if [ -n "$var" ]
+    then
+      echo "var has a value"
+    else
+      echo "var is empty"
+    fi
+
+
+### Rationale:
+
+When `$var` is unquoted, a blank value will cause it to wordsplit and disappear. If `$var` is empty, these two statements are identical:
+
+    [ -n $var ]
+    [ -n ]
+
+`[ string ]` is shorthand for testing if a string is empty. This is still true if `string` happens to be `-n`. `[ -n ]` is therefore true, and by extension so is `[ -n $var ]`.
+
+To fix this, either use `[[ -n $var ]]` which has fewer caveats than `[`, or quote the variable.
+
+### Exceptions:
+
+None