Created SC2209 (markdown)

SC2209.md

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+## Use var=$(command) to assign output (or quote to assign string).
+
+### Problematic code:
+
+```sh
+user=whoami         # Want to run whoami and assign output
+
+PAGER=cat git log   # Want to assign the string "cat"
+```
+
+### Correct code:
+
+```sh
+user=$(whoami)
+
+PAGER="cat" git log
+```
+
+### Rationale:
+
+Putting `var=` in front of a command will not assign its output. Use `var=$(my command here)` to execute the command and capture its output.
+
+If you do want to assign a literal string, use quotes to make this clear to shellcheck and humans alike. 
+
+### Exceptions:
+
+None.