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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"

In Bash, I'd do it this way:

[ -v IFS ] && oldIFS="$IFS" || unset oldIFS

IFS=something
some commands

[ -v oldIFS ] && IFS="$oldIFS" || unset IFS

or this way:

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

IFS=something
some commands

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

In Bash, I'd do it this way:

[ -v IFS ] && oldIFS="$IFS" || unset oldIFS

IFS=something
some commands

[ -v oldIFS ] && IFS="$oldIFS" || unset IFS

or this way:

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

IFS=something
some commands

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

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"

In Bash, I'd do it this way:

[ -v IFS ] && oldIFS="$IFS" || unset oldIFS

IFS=something
some commands

[ -v oldIFS ] && IFS="$oldIFS" || unset IFS

this may be better:

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

IFS=something
some commands

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

In Bash, I'd do it this way:

[ -v IFS ] && oldIFS="$IFS" || unset oldIFS

IFS=something
some commands

[ -v oldIFS ] && IFS="$oldIFS" || unset IFS

or this way:

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

IFS=something
some commands

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