Redis Exists Command

Redis is a key-value database that is lightweight and easy to use. To fetch a value in Redis, you have to reference the associated key. Although this is incredibly easy, it does provide one challenge, what happens if the key does not exist?

To create a simple error handling mechanism in our applications, we can use the Redis EXISTS command to verify the key before executing a query.

Redis SET Key

Let us assume you have a key-value pair as shown below:

127.0.0.1:6379> SET mykey myvalue

OK

We create a new Redis key and value using the SET command in the previous command.

To get the value associated with a specific key, we can use the GET command as shown below:

127.0.0.1:6379> GET mykey

"myvalue"

This should return the value stored by the key “mykey”.

Redis EXISTS Command

The EXISTS command in Redis allows us to determine if a specified key exists in the database.

For example, to check if the key “mykey” exists in the selected database, we execute the following command:

127.0.0.1:6379> EXISTS mykey

(integer) 1

The command returns (integer) 1 if the key is found and (integer) 0 if not. The following example is provided:

127.0.0.1:6379> EXISTS nokey

(integer) 0

Note that you can pass multiple keys as the parameters. An example is provided below:

127.0.0.1:6379> EXISTS key1 mykey nokey key2

(integer) 1

The command will take the provided keys and check how many exist within the selected database.

For example, the previous command returns (integer) 1 since only one key exists in the database.

What happens if you specify the same key more than once?

Take the following example commands below:

127.0.0.1:6379> set key1 one

127.0.0.1:6379> set key2 two

127.0.0.1:6379> set key3 three

We can check the same key multiple times using the following command shown:

127.0.0.1:6379> EXISTS key1 key1 key 2

(integer) 2

Redis ignores the repeated keys and counts the unique values in this case.

NOTE: The existence of the key is only checked in the currently selected database. For the following example, let us switch to database 2:

127.0.0.1:6379> select 2

OK

127.0.0.1:6379[2]>

Note: the prompt changes to reflect the currently selected database.

In this database, let us create a new key and value pair as shown below:

127.0.0.1:6379[2]> SET user1 "mr_fantastic"

OK

Switch back to the database at index 0.

127.0.0.1:6379[2]> SELECT 0

OK

127.0.0.1:6379>

If you check the existence of the key “user1”, Redis will return 0.

127.0.0.1:6379> EXISTS user1

(integer) 0

The EXISTS command only checks the key in the currently selected database.

Conclusion

This article details how to check if a key exists within the currently selected database within a Redis instance. We hope you found this article helpful. Check the other Linux Hint articles for more tips and information.

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