Redis CLI (redis-cli) basics - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis CLI command?

Assume the Redis database is empty. You run the following commands in redis-cli:

SET user:1 "Alice"
GET user:1

What will be the output of the GET command?

Redis

SET user:1 "Alice"
GET user:1

A "Alice"

B user:1

C nil

D ERROR: key not found

💡 Hint

GET returns the value stored at the key if it exists.

The SET command stores the string "Alice" at key user:1. The GET command retrieves that value, so it returns "Alice".

📝 Syntax

intermediate

Which Redis CLI command syntax is correct to add a member to a set?

You want to add the member "blue" to the set stored at key "colors". Which command is correct?

A PUSH colors blue

B SADD colors blue

C SET colors blue

D ADD colors blue

💡 Hint

The command to add members to a set starts with SADD.

SADD is the Redis command to add one or more members to a set. ADD, SET, and PUSH are either invalid or used for other data types.

❓ query_result

advanced

What is the output of this Redis CLI command sequence?

Assuming the database is empty, you run:

LPUSH fruits apple banana cherry
LRANGE fruits 0 1

What is the output of the LRANGE command?

Redis

LPUSH fruits apple banana cherry
LRANGE fruits 0 1

A

1) "cherry"
2) "banana"

B

1) "apple"
2) "banana"

C

1) "banana"
2) "apple"

D

1) "apple"
2) "cherry"

💡 Hint

LPUSH adds elements to the head of the list in the order given.

LPUSH adds elements from left to right to the head, so the list becomes [cherry, banana, apple]. LRANGE 0 1 returns the first two elements: cherry and banana.

🔧 Debug

advanced

Why does this Redis CLI command return an error?

You run the command:

INCR mykey

But Redis returns an error: "ERR value is not an integer or out of range". What is the most likely cause?

Redis

INCR mykey

A The key 'mykey' does not exist in the database.

B The INCR command is misspelled.

C The key 'mykey' holds a string value that is not an integer.

D Redis does not support the INCR command.

💡 Hint

INCR only works on keys holding integer values or keys that do not exist.

If 'mykey' exists but holds a non-integer string, INCR returns an error. If the key does not exist, INCR creates it with value 1.

🧠 Conceptual

expert

Which Redis CLI command sequence correctly sets a key with expiration and verifies it?

You want to set key "session" to "active" with a 10-second expiration, then check the remaining time to live. Which sequence is correct?

A

SETEX session 10 active
TTL session

B

SET session active PX 10000
TTL session

C

SET session active
EXPIRE session 10
TTL session

D

SET session active EX 10
TTL session

💡 Hint

SETEX sets a key with expiration in seconds in one command.

SETEX key seconds value sets the key with expiration in seconds. TTL returns the remaining time to live in seconds. Option D uses SET with EX correctly.

Practice

1. What does the Redis CLI command SET key value do?

easy

A. Retrieves the value of the key

B. Deletes the key from Redis

C. Stores the value under the given key in Redis

D. Lists all keys in Redis

Solution

1. Step 1: Understand the SET command

   The SET command in Redis stores a value under a specified key.

2. Step 2: Match command to action

   Since SET assigns a value to a key, it stores data rather than deleting or retrieving it.

3. Final Answer:

   Stores the value under the given key in Redis -> Option C

4. Quick Check:

   SET command = store value [OK]

Hint: SET means store a value for a key [OK]

Common Mistakes:

• Confusing SET with GET
• Thinking SET deletes keys
• Assuming SET lists keys

2. Which of the following is the correct syntax to get the value of a key named user1 in Redis CLI?

easy

A. KEYS user1

B. GET user1

C. DEL user1

D. SET user1

Solution

1. Step 1: Identify the command to retrieve a value

   The GET command is used to fetch the value stored at a key in Redis.

2. Step 2: Check syntax correctness

   GET followed by the key name is the correct syntax to get the value of 'user1'.

3. Final Answer:

   GET user1 -> Option B

4. Quick Check:

   GET command syntax = GET key [OK]

Hint: GET retrieves value, SET stores, DEL deletes [OK]

Common Mistakes:

• Using SET instead of GET
• Using DEL to get value
• Using KEYS to get value

3. What will be the output of the following Redis CLI commands?

SET count 10
INCR count
GET count

medium

A. "11"

B. "10"

C. Error: wrong type operation

D. nil

Solution

1. Step 1: Analyze the commands step-by-step

   SET count 10 stores the string "10" under key 'count'. INCR count increments the integer value by 1.

2. Step 2: Understand INCR effect and GET result

   INCR converts the string "10" to integer 10, adds 1 to make 11. GET count returns "11" as a string.

3. Final Answer:

   "11" -> Option A

4. Quick Check:

   INCR on "10" = "11" [OK]

Hint: INCR adds 1 to numeric string values [OK]

Common Mistakes:

• Expecting original value "10"
• Thinking INCR causes error
• Expecting nil for existing key

4. You run the command GET mykey but get an error: WRONGTYPE Operation against a key holding the wrong kind of value. What is the likely cause?

medium

A. The key holds a non-string value like a list or set

B. The key does not exist

C. The Redis server is down

D. The GET command is misspelled

Solution

1. Step 1: Understand the error message

   The error means GET was used on a key that stores a data type other than string, like a list or set.

2. Step 2: Identify the cause from options

   Since the key exists but is not a string, GET cannot retrieve it, causing the error.

3. Final Answer:

   The key holds a non-string value like a list or set -> Option A

4. Quick Check:

   WRONGTYPE error = wrong data type for GET [OK]

Hint: GET works only on string keys, not lists or sets [OK]

Common Mistakes:

• Thinking key does not exist
• Assuming server is down
• Misspelling GET causes this error

5. You want to store a user's name and age in Redis using redis-cli. Which sequence of commands correctly stores the name "Alice" and age 30, then retrieves both values?

hard

A. SET user:name Alice
SET user:age 30
HGET user:name
HGET user:age

B. LPUSH user Alice 30
LRANGE user 1 1

C. SET user Alice 30
GET user

D. HSET user name Alice age 30
HGET user name
HGET user age

Solution

1. Step 1: Identify the best data structure for multiple fields

   Using a hash (HSET) allows storing multiple fields like name and age under one key.

2. Step 2: Check commands for storing and retrieving fields

   HSET user name Alice age 30 stores both fields. HGET user name and HGET user age retrieve them separately.

3. Final Answer:

   HSET user name Alice age 30 HGET user name HGET user age -> Option D

4. Quick Check:

   Use hashes for multiple related fields [OK]

Hint: Use HSET/HGET for multiple fields in one key [OK]

Common Mistakes:

• Using SET with multiple values
• Using list commands for key-value pairs
• Trying to GET multiple values at once