How to Use APPEND Command in Redis for String Manipulation

Use the APPEND command in Redis to add a string to the end of an existing string value stored at a key. If the key does not exist, APPEND creates it with the given value. The command returns the new length of the string after appending.

Syntax

The APPEND command syntax is simple:

• APPEND key value

Here, key is the name of the string key you want to append to, and value is the string you want to add at the end.

If the key does not exist, Redis creates it with the value as its content.

The command returns the length of the string after the append operation.

redis

APPEND mykey "Hello"
APPEND mykey " World"

Output

5
11

Example

This example shows how to append strings to a Redis key and check the resulting length and value.

redis

127.0.0.1:6379> SET greeting "Hello"
OK
127.0.0.1:6379> APPEND greeting " World"
11
127.0.0.1:6379> GET greeting
"Hello World"

Output

OK
11
"Hello World"

Common Pitfalls

Common mistakes when using APPEND include:

• Trying to append to a key holding a non-string type, which causes an error.
• Assuming APPEND replaces the string instead of adding to it.
• Not checking the returned length, which can help verify the operation succeeded.

Example of wrong usage and correction:

redis

127.0.0.1:6379> LPUSH mylist "item"
(integer) 1
127.0.0.1:6379> APPEND mylist "more"
(error) WRONGTYPE Operation against a key holding the wrong kind of value

# Correct approach:
127.0.0.1:6379> SET mystring "start"
OK
127.0.0.1:6379> APPEND mystring " more"
10

Output

(integer) 1
(error) WRONGTYPE Operation against a key holding the wrong kind of value
OK
10

Quick Reference

Command	Description	Return Value
APPEND key value	Add value to end of string at key	New length of string
GET key	Retrieve the string value stored at key	String value or nil
SET key value	Set key to hold the string value	OK

Key Takeaways

APPEND adds data to the end of a string key or creates the key if it doesn't exist.

The command returns the new length of the string after appending.

APPEND only works on string keys; using it on other types causes errors.

Always check the returned length to confirm the append succeeded.

Use GET to verify the final string content after appending.