How To Manage Lists in Redis
Redis is an open-source, in-memory key-value data store. In Redis, a list is a collection of strings sorted by insertion order, similar to linked lists. This tutorial covers how to create and work with elements in Redis lists.
How To Use This Guide
This guide is written as a cheat sheet with self-contained examples. We encourage you to jump to any section that is relevant to the task you’re trying to complete.
The commands shown in this guide were tested on an Ubuntu 18.04 server running Redis version 4.0.9. To set up a similar environment, you can follow Step 1 of our guide on How To Install and Secure Redis on Ubuntu 18.04. We will demonstrate how these commands behave by running them with
redis-cli, the Redis command line interface. Note that if you’re using a different Redis interface — Redli, for example — the exact output of certain commands may differ.
Alternatively, you could provision a managed Redis database instance to test these commands, but note that depending on the level of control allowed by your database provider, some commands in this guide may not work as described. To provision a DigitalOcean Managed Database, follow our Managed Databases product documentation. Then, you must either install Redli or set up a TLS tunnel in order to connect to the Managed Database over TLS.
A key can only hold one list, although any list can hold over four billion elements. Redis reads lists from left to right, and you can add new list elements to the head of a list (the “left” end) with the
lpush command or the tail (the “right” end) with
rpush. You can also use
rpush to create a new list:
- lpush key value
Both commands will output an integer showing how many elements are in the list. To illustrate, run the following commands to create a list containing the dictum “I think therefore I am”:
- lpush key_philosophy1 "therefore"
- lpush key_philosophy1 "think"
- rpush key_philosophy1 "I"
- lpush key_philosophy1 "I"
- rpush key_philosophy1 "am"
The output from the last command will read:
Note that you can add multiple list elements with a single
- rpush key_philosophy1 "-" "Rene" "Decartes"
rpushx commands are also used to add elements to lists, but will only work if the given list already exists. If either command fails, it will return
- rpushx key_philosophy2 "Happiness" "is" "the" "highest" "good" "–" "Aristotle"
To change an existing element in a list, run the
lset command followed by the key name, the index of the element you want to change, and the new value:
- lset key_philosophy1 5 "sayeth"
If you try adding a list element to an existing key that does not contain a list, it will lead to a clash in data types and return an error. For example, the following
set command creates a key holding a string, so the following attempt to add a list element to it with
lpush will fail:
- set key_philosophy3 "What is love?"
- lpush key_philosophy3 "Baby don't hurt me"
Output(error) WRONGTYPE Operation against a key holding the wrong kind of value
It isn’t possible to convert Redis keys from one data type to another, so to turn
key_philosophy3 into a list you would need to delete the key and start over with an
Retrieving Elements from a List
To retrieve a range of items in a list, use the
lrange command followed by a start offset and a stop offset. Each offset is a zero-based index, meaning that
0 represents the first element in the list,
1 represents the next, and so on.
The following command will return all the elements from the example list created in the previous section:
- lrange key_philosophy1 0 7
Output1) "I" 2) "think" 3) "therefore" 4) "I" 5) "am" 6) "sayeth" 7) "Rene" 8) "Decartes"
The offsets passed to
lrange can also be negative numbers. When used in this case,
-1 represents the final element in the list,
-2 represents the second-to-last element in the list, and so on. The following example returns the last three elements of the list held in
- lrange key_philosophy1 -3 -1
Output1) "I" 2) "am" 3) "sayeth"
To retrieve a single element from a list, you can use the
lindex command. However, this command requires you to supply the element’s index as an argument. As with
lrange, the index is zero-based, meaning that the first element is at index
0, the second is at index
1, and so on:
- lindex key_philosophy1 4
To find out how many elements are in a given list, use the
llen command, which is short for “list length”:
- llen key_philosophy1
If the value stored at the given key does not exist,
llen will return an error.
Removing Elements from a List
lrem command removes the first of a defined number of occurrences that match a given value. To experiment with this, create the following list:
- rpush key_Bond "Never" "Say" "Never" "Again" "You" "Only" "Live" "Twice" "Live" "and" "Let" "Die" "Tomorrow" "Never" "Dies"
lrem example will remove the first occurence of the value
- lrem key_Bond 1 "Live"
This command will output the number of elements removed from the list:
The number passed to an
lrem command can also be negative. The following example will remove the last two occurences of the value
- lrem key_Bond -2 "Never"
lpop command removes and returns the first, or “leftmost” element from a list:
- lpop key_Bond
Likewise, to remove and return the last or “rightmost” element from a list, use
- rpop key_Bond
Redis also includes the
rpoplpush command, which removes the last element from a list and pushes it to the beginning of another list:
- rpoplpush key_Bond key_AfterToday
If the source and destination keys passed to
rpoplpush command are the same, it will essentially rotate the elements in the list.
This guide details a number of commands that you can use to create and manage lists in Redis. If there are other related commands, arguments, or procedures you’d like to see outlined in this guide, please ask or make suggestions in the comments below.
For more information on Redis commands, see our tutorial series on How to Manage a Redis Database.