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APPEND

Appends a value to the end of the string stored at a specified key. If the key does not exist, it is created and set as an empty string before appending the value.

Syntax​

APPEND key value

Parameters​

  • key: The name of the key where the string is stored.
  • value: The value to append to the existing string at the key.

Details​

  • Available since: 2.0.0
  • Time complexity: O(1) for the average case (when appending to an existing string), but can be O(N) where N is the length of the resulting string.
  • ACL categories: @write, @string, @fast

The APPEND command adds the specified value to the end of the string stored at key. If the key does not already hold a string, a new string is created with the specified value as its content.

Examples​

Basic Usage​

Assume we have a key called mykey with the value "Hello":

SET mykey "Hello"

To append the string " World" to mykey:

APPEND mykey " World"

This will return:

(integer) 11

After running this command, the value of mykey will be "Hello World".

Creating a New String​

If the key does not exist, APPEND will create a new string:

APPEND newkey "FirstValue"

This will return:

(integer) 10

The value of newkey will now be "FirstValue".

Edge Cases​

  • If the key does not exist, APPEND creates a new string with the provided value.
  • If the key holds a non-string value, an error is returned.
  • If the resulting string exceeds the maximum allowed length for strings (512 MB), an error is returned.

RESP2/RESP3 Reply​

  • Integer reply: the length of the string after the append operation.