Lua script syntax in Redis - Step-by-Step Execution

Concept Flow - Lua script syntax in Redis

Start Script

↓

Define Variables

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Use Redis Commands

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Return Result

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End Script

A Lua script in Redis starts, defines variables, runs Redis commands, returns a result, and ends.

Execution Sample

Redis

local val = redis.call('GET', KEYS[1])
if val == false then
  return 'No value'
end
return val

This script gets a key's value; if missing, returns 'No value', else returns the value.

Execution Table

Step	Action	Evaluation	Result
1	Call redis.call('GET', KEYS[1])	KEYS[1] = 'mykey'	val = 'hello'
2	Check if val == false	val = 'hello'	Condition false
3	Return val	val = 'hello'	Output: 'hello'
4	End script		Script ends

💡 Script ends after returning the value or 'No value' if key missing

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
val	nil	'hello'	'hello'	'hello'

Key Moments - 2 Insights

Why does redis.call return false if the key does not exist?

redis.call returns false when the key is missing, so the script checks val == false to handle missing keys (see execution_table step 2).

What happens if we forget to return a value in the script?

If no return is given, Redis returns nil, which can cause confusion. Always use return to send a result back (see execution_table step 3).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'val' after Step 1?

A nil

B false

C 'hello'

D 'No value'

💡 Hint

Check the 'Result' column in Step 1 of the execution_table.

At which step does the script decide what to return?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Evaluation' columns in Step 2 of the execution_table.

If the key does not exist, what would the script return?

A 'No value'

B 'hello'

C nil

D false

💡 Hint

Refer to the script code in execution_sample and the condition in execution_table step 2.

Concept Snapshot

Lua scripts in Redis:
- Use redis.call('COMMAND', args) to run Redis commands
- KEYS and ARGV hold input keys and arguments
- Use local variables to store results
- Return a value with return statement
- redis.call returns false if key missing

Full Transcript

This visual execution shows how a Lua script runs in Redis. The script starts by calling redis.call to get a key's value. If the key does not exist, redis.call returns false. The script checks this and returns 'No value' if missing. Otherwise, it returns the actual value. Variables like 'val' store intermediate results. The script ends after returning a value. This step-by-step trace helps beginners see how Lua scripts interact with Redis commands and handle missing keys.