In Bash, I'd do it this way:
[ -v IFS ] && oldIFS="$IFS" || unset oldIFS IFS=something some commands [ -v oldIFS ] && IFS="$oldIFS" or this way:
[ "${IFS+set}" ] && oldIFS="$IFS" || unset oldIFS IFS=something some commands [ "${oldIFS+set}" ] && IFS="$oldIFS"