Created SC2154 (markdown)

SC2154.md

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+## var is referenced but apparently never assigned.
+
+### Problematic code:
+
+    var=name
+    n=42
+    echo "$var_$n.jpg"
+
+### Correct code:
+
+    var=name
+    n=42
+    echo "${var}_$n.jpg"
+
+### Rationale:
+
+ShellCheck has noticed that you reference a variable that is not assigned. Double check that the variable is indeed assigned, and that the name is not misspelled. 
+
+Note: This message only triggers for variables with lowercase characters in their name (`foo` and `kFOO` but not `FOO`) due to the standard convention of using lowercase variable names for unexported, local variables.
+
+### Exceptions:
+
+ShellCheck does not attempt to figure out runtime or dynamic assignments like with `source mycommonvars.sh` or `eval var=value`. 
+
+If you know for a fact that the variable is set, you can use `${var:?}` to fail if the variable is unset (or empty). You can also disable the message with a [[directive]].