Created SC2123 (markdown)

SC2123.md

@@ -0,0 +1,30 @@
+## PATH is the shell search path. Use another name.
+
+### Problematic code:
+
+    PATH=/my/dir
+    cat "$PATH/myfile"
+
+### Correct code:
+
+Good practice: always use lowercase for unexported variables.
+
+    path=/my/dir
+    cat "$path/myfile"
+
+Bad practice: use another uppercase name.
+
+    MYPATH=/my/dir
+    cat "$MYPATH/myfile"
+
+### Rationale:
+
+`PATH` is where the shell looks for the commands it executes. By inadvertently overwriting it, the shell will be unable to find commands (like `cat` in this case).
+
+You get this warning when ShellCheck suspects that you didn't meant to overwrite it (because it contains a non-`/bin` path but no path separators (`:`)). 
+
+Best shell scripting practice is to always use lowercase variable names to avoid accidentally overwriting exported and internal variables.
+
+### Contraindications
+
+If you're aware of the above and really do want to set your shell search path to `/my/dir`, you can ignore this warning.