# EVAL

> Execute a Lua script.

Use `EVAL` to run a Lua script on the server.

`<numkeys>` says how many of the arguments that follow are key names. The script receives those in the `KEYS` table and every remaining argument in `ARGV`. Passing key names as keys rather than hardcoding them in the script body matters, because Redis uses that list for routing and access checks. Inside the script, `redis.call` runs Redis commands and its return value is converted to a Lua value.

The script runs as a single atomic step, which makes it the standard way to do read, decide, and write logic, such as a rate limiter or a compare-and-set update, in one round trip and without a transaction. Keep scripts short, since a script that holds the database blocks everything else, and keep them deterministic by deriving values from `KEYS`, `ARGV`, or data read inside the script rather than from clock or random sources.

Upstash isolates a script with a lock. By default that is the global lock, because the engine cannot know in advance which keys the script will touch, so no other command runs while the script does. Adding the `allow-key-locking` flag to the script's shebang line makes it lock only the keys passed in `KEYS` instead, so calls that work on disjoint keys run in parallel:

```lua
#!lua flags=allow-key-locking

redis.call('INCR', KEYS[1])
return 1
```

With the flag set, every key the script touches must appear in `KEYS`, and commands that need database-wide access, such as `FLUSHDB`, are rejected. See [Key-Based Locking](/redis/features/key-locking) for the full rules.

<Warning>
  Pass every key the script touches through `KEYS`, even when the script runs
  under the global lock. Upstash keeps idle entries
  [on disk](/redis/features/durability): declared keys are loaded before the
  script starts and the lock is released during that read, but a key that the
  script builds while it runs is read from disk with the lock held, stalling
  every command waiting on it. See
  [Dynamic Keys and Latency](/redis/features/key-locking#dynamic-keys-and-latency).
</Warning>

Sending a script also caches it under its SHA1 digest, so later calls can use [`EVALSHA`](/redis/commands/scripting/evalsha) and avoid resending the body. Use [`EVAL_RO`](/redis/commands/scripting/eval-ro) for scripts that only read.

## Syntax

```redis
EVAL <script> <numkeys> [<key> [<key> ...]] [<arg> [<arg> ...]]
```

## Arguments

| Argument | Required | Repeatable | Description |
| --- | --- | --- | --- |
| `<script>` | Yes | No | Lua script source. |
| `<numkeys>` | Yes | No | Number of key arguments that follow. |
| `<key>` | No | Yes | Redis key targeted by the command. |
| `<arg>` | No | Yes | Additional argument, available to the script as `ARGV`. |

## Important points

- `numkeys` must equal the number of key arguments that immediately follow it; remaining arguments are available to the script or function as ordinary arguments.
- The script takes the global lock unless its shebang sets the `allow-key-locking` flag, in which case it locks only the keys passed in `KEYS`. See [Key-Based Locking](/redis/features/key-locking).
- A script queued inside a `MULTI`/`EXEC` transaction always runs under the global lock, even when it sets `allow-key-locking`. Call it directly if you want per-key locking.
- Pass every key the script touches through `KEYS` whether or not `allow-key-locking` is set. A key built inside the script is read from disk under the lock when it is not in memory, and it is rejected outright when the flag is set. See [Dynamic Keys and Latency](/redis/features/key-locking#dynamic-keys-and-latency).

## Response

The reply reports the result of the operation. Error replies have the same shape in RESP2 and RESP3 and are surfaced as exceptions by the SDKs below.

| Protocol | Reply |
| --- | --- |
| RESP2 | Reply produced by the evaluated script |
| RESP3 | Reply produced by the evaluated script |

<Note>
  Client libraries often decode bulk strings, maps, sets, and numeric strings into language-native values. The table describes the Redis wire reply.
</Note>

## Examples

TCP examples use the TLS `REDIS_URL` from the Upstash console. REST examples use `UPSTASH_REDIS_REST_URL` and `UPSTASH_REDIS_REST_TOKEN`.

<AccordionGroup>

<Accordion title="Redis CLI" icon="terminal">

```bash
EVAL "return ARGV[1]" 0 hello
```

</Accordion>

<Accordion title="@upstash/redis" icon="node-js" iconType="brands">

```ts
import { Redis } from "@upstash/redis";

const redis = Redis.fromEnv();

const script = `
    return ARGV[1]
`
const result = await redis.eval(script, [], ["hello"]);
console.log(result) // "hello"
```

</Accordion>

<Accordion title="upstash_redis" icon="python" iconType="brands">

```python
from upstash_redis import Redis

redis = Redis.from_env()
result = redis.eval("return ARGV[1]", args=["value"])
print(result)
```

</Accordion>

<Accordion title="ioredis" icon="node-js" iconType="brands">

```ts
import Redis from "ioredis";

const redis = new Redis(process.env.REDIS_URL!);
const result = await redis.eval("return ARGV[1]", "0", "value");
console.log(result);
```

</Accordion>

<Accordion title="node-redis" icon="node-js" iconType="brands">

```ts
import { createClient } from "redis";

const client = await createClient({ url: process.env.REDIS_URL })
  .on("error", console.error)
  .connect();
const result = await client.eval("return ARGV[1]", { arguments: ["value"] });
console.log(result);
```

</Accordion>

<Accordion title="redis-py" icon="python" iconType="brands">

```python
import os
import redis

client = redis.from_url(os.environ["REDIS_URL"])
result = client.eval("return ARGV[1]", 0, "value")
print(result)
```

</Accordion>

<Accordion title="go-redis" icon="golang" iconType="brands">

```go
package main

import (
    "context"
    "fmt"
    "os"

    "github.com/redis/go-redis/v9"
)

func main() {
    opts, err := redis.ParseURL(os.Getenv("REDIS_URL"))
    if err != nil {
        panic(err)
    }
    client := redis.NewClient(opts)
    result, err := client.Eval(context.Background(), "return ARGV[1]", nil, "value").Result()
    if err != nil {
        panic(err)
    }
    fmt.Println(result)
}
```

</Accordion>

<Accordion title="jedis" icon="java" iconType="brands">

```java
import java.net.URI;

import redis.clients.jedis.Jedis;

try (Jedis jedis = new Jedis(new URI(System.getenv("REDIS_URL")))) {
  Object result = jedis.eval("return ARGV[1]", java.util.List.of(), java.util.List.of("value"));
  System.out.println(result);
}
```

</Accordion>

<Accordion title="redis-rs" icon="rust" iconType="brands">

```rust
fn main() -> redis::RedisResult<()> {
    let url = std::env::var("REDIS_URL").expect("REDIS_URL is not set");
    let client = redis::Client::open(url)?;
    let mut connection = client.get_connection()?;

    let mut command = redis::cmd("EVAL");
    command.arg("return ARGV[1]");
    command.arg("1");
    let result: redis::Value = command.query(&mut connection)?;
    println!("{result:?}");
    Ok(())
}
```

</Accordion>

</AccordionGroup>