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EVAL

Evaluates a Lua script in the context of the Redis server.

Syntax​

EVAL script numkeys key [key ...] arg [arg ...]

Parameters​

  • script: The Lua script to be executed.
  • numkeys: The number of keys that the script will access.
  • key: The keys that the script will operate on. There should be numkeys keys specified.
  • arg: Optional arguments that are passed to the script. They can be used inside the Lua script.

Details​

  • Available since: 2.6.0
  • Time complexity: O(N), where N is the length of the script and the complexity of the operations performed by the script.
  • ACL categories: @scripting, @write

The EVAL command allows the execution of Lua scripts directly on the Redis server. Lua scripts can be used to perform complex operations atomically. The script is executed in the context of the server, meaning it has access to the same data as Redis commands and can perform operations as needed.

Examples​

Basic Usage​

To evaluate a simple Lua script:

EVAL "return redis.call('SET', 'key', 'value')" 1 key

In this example, the Lua script sets the key key to value. The 1 indicates that the script operates on one key, which is key.

Script with Multiple Keys​

To work with multiple keys:

EVAL "return redis.call('MGET', KEYS[1], KEYS[2])" 2 key1 key2

This script retrieves the values of key1 and key2. The 2 indicates that there are two keys in the script.

Script with Arguments​

To pass additional arguments to the script:

EVAL "return redis.call('SET', KEYS[1], ARGV[1])" 1 key value

Here, ARGV[1] is the argument value, which will be set for key. The 1 specifies that the script operates on one key.

Complex Script Example​

To perform more complex operations:

EVAL "
local key = KEYS[1]
local value = ARGV[1]
local existing = redis.call('GET', key)
if existing then
    return redis.call('SET', key, value)
else
    return redis.call('SET', key, value)
end
" 1 key newvalue

This script checks if key exists and sets it to newvalue accordingly.

RESP2/RESP3 Reply​

  • The result of the script execution is returned as the reply. The reply format depends on the operations performed within the script.

Example Reply​

If the script sets a key:

"OK"

If the script retrieves a value:

"value"

Notes​

  • Lua scripts executed with EVAL are executed atomically, ensuring that no other Redis commands are executed in between.
  • To check the execution time of a script, use EVALSHA with the script hash.
  • Scripts can be preloaded into Redis using SCRIPT LOAD for improved performance with EVALSHA.