Redis Commands

At its core, Redis is a data storage engine that allows data to be stored in key-value pairs.

The key serves as a unique identifier. The value can be one of the supported data structures, such as strings, lists, sets, hashes and more.

To allow storing, retrieving, and manipulating data, Redis offers over 200 commands.

In this series, we will learn about 8 commonly used Redis commands — PING, ECHO, SET, GET, DEL, KEYS, EXPIRE and TTL.

PING

The PING command checks the connection status of a Redis server by testing whether a connection is still alive.

It returns "PONG" if the server is running. This allows applications to ensure the availability of the Redis server before executing commands or queries.

redis> PING
“PONG”

ECHO

The ECHO command accepts an argument and simply returns it.

Similar to PING, the ECHO command can be used to test whether a connection is active and healthy. It is often used for diagnostic purposes and helps developers to identify and solve connection problems.

redis> ECHO “hello world!”
“hello world!”

SET

The SET command assigns a specific value to a key. It accepts two arguments, the key and its corresponding value.

redis> SET username Salvatore_Sanfilippo
OK

Here, username is the key and Salvatore_Sanfilippo is the corresponding value.

If the save operation is successful, Redis returns OK. In exceptional cases, e.g. if there is a lack of memory, a corresponding error message is returned.

redis> SET new_username Linus
OK

The SET command also updates the value of a specific key. The old value is overridden by this operation.

redis> SET new_username Linus_Torvalds
OK

The SET command also supports several options that modify its behavior. The use of options makes it possible to combine several operations in a single command. This is important to reduce the number of round-trips to the server.

redis> SET key value [options]

One commonly used option is EX, which sets the expiry for a key.

redis> SET name Gates EX 5
OK

The above example sets the value Gates for the key name which would expire within 5 seconds.

GET

The GET command retrieves the value associated with a specific key. The key is passed as an argument, and the corresponding value is returned.

redis> GET username
“Salvatore_Sanfilippo”

This retrieves the value stored under the key username, which is "Salvatore_Sanfilippo".

The GET command can also be used to check the existence of a key. If a non-existent key is passed as an argument, Redis will return a special value, nil.

redis> GET non_existing_key
(nil)

DEL

The DEL command deletes one or more key-value pairs from the data store, permanently. Once deleted, the key-value pairs cannot be restored later.

The keys are passed as arguments to the command.

redis> DEL username new_username
(integer) 2

It returns an integer value indicating the number of keys that have been deleted from the data store.

Redis ignores the keys that are not present in its data store.

redis> DEL username new_username name
(integer) 2

In the above example, Redis returns a response of 2. The DEL command has ignored the third key - name - which does not exist.

KEYS

The KEYS command retrieves all keys that match a certain pattern.

redis> KEYS *
1) username
2) new_username

The pattern * is specified as an argument and returns all keys that are present in the data store.

redis> KEYS h?llo
1) hello
2) hpllo
3) hxllo 

In the above example, the pattern ? matches any character that is to be used in its place.

So h?llo matches hello, hpllo and hxllo.

There are several such patterns which can be used to search for the keys in Redis.

EXPIRE

The EXPIRE command is used to set a timeout for a key in seconds.

redis> EXPIRE username 10
(integer) 1

It is important for scenarios in which keys are to be deleted automatically after a certain time. After the specified time, the key-value pair is automatically removed from the Redis data store.

It takes the key as an argument together with the time in seconds.

redis> EXPIRE username 5
(integer) 1

The above example sets the timeout for the key - username to 5 seconds. After this timeout, username is automatically deleted along with its value.

It returns the integer value 0 if the timeout has not been set. Otherwise, it returns the value 1.

TTL

The TTL command checks the remaining lifetime of a key. The key must have an expiry time attached to it.

This command only accepts the key as the argument.

redis> TTL username0 8

redis> TTL username1
-1

redis> TTL username3
-2

The TTL command returns the remaining time until the key expires.

For keys that exist but have no expiry time, -1 is returned.

For keys that do not exist or have expired, -2 is returned.

Summary

You have now mastered 8 basic Redis commands with which you can seamlessly perform the most important tasks in a key-value store.

Here's a quick overview of the commands you've learnt so far:

• PING: Checks the connection status of a Redis server.
• ECHO: Accepts an argument and returns it.
• SET: Assigns a value to a key.
• GET: Retrieves the value associated with a key.
• DEL: Permanently deletes one or more key-value pairs.
• KEYS: Retrieves all keys that match a specific pattern.
• EXPIRE: Defines a timeout for a key in seconds.
• TTL: Checks the remaining lifetime of a key.

These commands provide essential functions for storing, retrieving and managing data in Redis and facilitate efficient data operations and system maintenance.

Links for further reading:

• Commands (Official Redis documentation)