Add notes for dynamic variables

README.md

@@ -679,9 +679,33 @@ done
 
 ## Name and access a variable based on another variable
 
+### Set the dynamic part of the variable name
+
 ```shell
 $ var="world"
+```
+
+#### Option [1]: Set the variable using export
+
+**Warning:** Actually exports the variable, it will be inherited by called programs. Remember to unset.
+
+```shell
+$ export "hello_$var=value"
+```
+
+#### Option [2]: Set the variable using eval
+
+**Warning:** Does not export the variables, but is more dangerous than using export, as eval can execute arbitrary commands (try with `var="=;echo pwned;_"`). Remember to sanitize any data to only have alphanumerals and underscores.
+
+```shell
 $ eval "hello_$var=value"
+```
+
+### Access the variable in a command
+
+**NOTE:** The entire sequence is interpreted an extra time by the shell when calling eval. Note the double-escaping in the printf format.
+
+```shell
 $ eval printf '%s\\n' "\$hello_$var"
 value
 ```