LTRIM key start stop
Trim an existing list so that it will contain only the specified range of elements specified. Both
stop are zero-based indexes, where
0 is the first element of the list (the head),
1 the next element and so on.
LTRIM foobar 0 2 will modify the list stored at
foobar so that only the first three elements of the list will remain.
end can also be negative numbers indicating offsets from the end of the list, where
-1 is the last element of the list,
-2 the penultimate element and so on.
Out of range indexes will not produce an error: if
start is larger than the end of the list, or
start > end, the result will be an empty list (which causes
key to be removed). If
end is larger than the end of the list, Redis will treat it like the last element of the list.
A common use of LTRIM is together with LPUSH / RPUSH. For example:
LPUSH mylist someelement LTRIM mylist 0 99
This pair of commands will push a new element on the list, while making sure that the list will not grow larger than 100 elements. This is very useful when using Redis to store logs for example. It is important to note that when used in this way LTRIM is an O(1) operation because in the average case just one element is removed from the tail of the list.
(integer) 1redis> RPUSH mylist "two"
(integer) 2redis> RPUSH mylist "three"
(integer) 3redis> LTRIM mylist 1 -1
"OK"redis> LRANGE mylist 0 -1
1) "two" 2) "three"
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