In Bash, I'd do it this way:
```bash
[ -v IFS ] && oldIFS="$IFS" || unset oldIFS

IFS=something
some commands

[ -v oldIFS ] && IFS="$oldIFS"
```

or this way:

```bash
[ "${IFS+set}" ] && oldIFS="$IFS" || unset oldIFS

IFS=something
some commands

[ "${oldIFS+set}" ] && IFS="$oldIFS"
```