For variables i think you could do the following:

Code (Skript):

1. Something:
2.     set {slot} to 0
3.     loop 54 times:
4.         set {Item} to items in {slot} of player
5.         add {Item} to {Inv.%player%::*}
6.         add 1 to {slot}

and ... im not sure it will work

 

Try:

Code (Skript):

1. command /inv [<string>]:
2.     trigger:
3.         if arg 1 is "backup":
4.             set {_slot} to 0
5.             loop 36 times:
6.                 set {_Item.%{_slot}%.%player%} to items in slot {_slot} of player's inventory
7.                 add 1 to {_slot}
8.         if arg 1 is "restore":
9.             set {_slot} to 0
10.             loop 36 times:
11.                 set slot {_slot} of player's inventory to {_Item.%{_slot}%.%player%}
12.                 add 1 to {_slot}
13.