diff --git a/SC2154.md b/SC2154.md
index 19bff74..4492210 100644
--- a/SC2154.md
+++ b/SC2154.md
@@ -1,6 +1,63 @@
-RUN \
-  export http_proxy=${http_proxy}; \
-  export https_proxy=${https_proxy}; \
-  dnf -y upgrade; \
-  dnf -y install libXext libXrender libXtst xorg-x11-server-Xvfb fontconfig unzip sudo telnet; \
-  dnf -y clean all
\ No newline at end of file
+## var is referenced but not assigned.
+
+### Problematic code:
+
+```sh
+var=name
+n=42
+echo "$var_$n.jpg"   # overextended
+```
+
+or
+
+```sh
+target="world"
+echo "hello $tagret"  # misspelled
+```
+
+or
+
+```sh
+echo "Result: ${mycmd -a myfile}"  # trying to execute commands
+```
+
+### Correct code:
+
+```sh
+var=name
+n=42
+echo "${var}_${n}.jpg"
+```
+
+or
+
+```sh
+target="world"
+echo "hello $target"
+```
+
+or
+
+```sh
+echo "Result: $(mycmd -a myfile)"
+```
+
+### Rationale:
+
+ShellCheck has noticed that you reference a variable that is not assigned. Double check that the variable is indeed assigned, and that the name is not misspelled.
+
+Note: This message only triggers for variables with lowercase characters in their name (`foo` and `kFOO` but not `FOO`) due to the standard convention of using lowercase variable names for unexported, local variables.
+
+### Exceptions:
+
+ShellCheck intentionally does not attempt to figure out runtime or dynamic assignments like with `source "$(date +%F).sh"` or `eval var=value`. See [[SC2034]] for an extended discussion of why this is the case.
+
+If you know for a fact that the variable is set, you can use `${var:?}` to fail if the variable is unset (or empty), initialize it to a default value if uninitialized with `: "${var:=}"`, or explicitly initialize/declare it with `var=""` or `declare var`. You can also disable the message with a [[directive]].
+
+---
+
+## Read more:
+
+POSIX - Parameter expansion:
+* https://stackoverflow.com/a/16753536/2309247
+* http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02
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