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Atomically sets `key` to `value` and returns the old value stored at `key`.
Returns an error when `key` exists but does not hold a string value.
## Design pattern
`GETSET` can be used together with `INCR` for counting with atomic reset. For
example: a process may call `INCR` against the key `mycounter` every time some
event occurs, but from time to time we need to get the value of the counter and
reset it to zero atomically. This can be done using `GETSET mycounter "0"`:
```cli
INCR mycounter
GETSET mycounter "0"
GET mycounter
```
@return
@bulk-string-reply: the old value stored at `key`, or `nil` when `key` did not
exist.
@examples
```cli
SET mykey "Hello"
GETSET mykey "World"
GET mykey
```
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