APPEND for string concatenation in Redis - Step-by-Step Execution

Concept Flow - APPEND for string concatenation

Start with key and initial string

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Call APPEND command with key and new string

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Redis checks if key exists

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Return new length of string stored at key

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End

APPEND adds a new string to the existing string stored at a key or creates the key if it doesn't exist, then returns the new string length.

Execution Sample

Redis

SET greeting "Hello"
APPEND greeting " World"
GET greeting

Starts with 'Hello', appends ' World', then retrieves the full string.

Execution Table

Step	Command	Key Exists?	Action	String Value at Key	Return Value
1	SET greeting "Hello"	No	Create key 'greeting' with 'Hello'	Hello	OK
2	APPEND greeting " World"	Yes	Concatenate ' World' to 'Hello'	Hello World	11
3	GET greeting	Yes	Retrieve value	Hello World	Hello World
4	APPEND newkey "Hi"	No	Create key 'newkey' with 'Hi'	Hi	2
5	GET newkey	Yes	Retrieve value	Hi	Hi

💡 Commands complete; APPEND returns new string length, GET returns full string.

Variable Tracker

Key	Start	After Step 1	After Step 2	After Step 4
greeting	null	Hello	Hello World	Hello World
newkey	null	null	null	Hi

Key Moments - 3 Insights

What happens if the key does not exist when APPEND is called?

Redis creates the key with the new string as its value, as shown in step 4 of the execution_table.

What does APPEND return after adding the new string?

It returns the length of the new string stored at the key, for example 11 after step 2.

Does APPEND overwrite the existing string or add to it?

APPEND adds to the existing string, it does not overwrite it, as seen in step 2 where ' World' is added to 'Hello'.

Visual Quiz - 3 Questions

Look at the execution_table, what is the string value at key 'greeting' after step 2?

A " World"

B "Hello"

C "Hello World"

D null

💡 Hint

Check the 'String Value at Key' column for step 2.

At which step does APPEND create a new key because it does not exist?

A Step 4

B Step 3

C Step 2

D Step 1

💡 Hint

Look for 'Key Exists?' = No and APPEND command in the execution_table.

If you APPEND "!!!" to 'greeting' after step 2, what would the return value be?

A 11

B 14

C 8

D null

💡 Hint

Add the length of '!!!' (3) to the current string length (11) from step 2.

Concept Snapshot

APPEND key value
- Adds 'value' to string at 'key'.
- Creates key if missing.
- Returns new string length.
- Does not overwrite existing string.
- Useful for building strings step-by-step.

Full Transcript

The APPEND command in Redis adds a new string to the existing string stored at a key. If the key does not exist, Redis creates it with the new string as its value. After appending, the command returns the length of the new string stored at the key. For example, starting with key 'greeting' set to 'Hello', APPEND ' World' results in 'Hello World' with length 11. This command is useful for concatenating strings incrementally without overwriting the original content.