See for example [this stackoverflow question](https://stackoverflow.com/questions/8515411/what-is-indirect-expansion-what-does-var-mean).

```bash
MYVAR=Hello
A=MYVAR
echo ${!A}
```
will print `Hello`. The exceptions are
```bash
echo ${!PA*}
```
will print a (IFS-delimited) list of variables beginning with PA. And
```bash
echo ${!name[@]}
```
will list the array keys for the array variable `$name`.