Redis and Php-Redis with drupal 9 on Linux

Install Redis

sudo apt-get update && sudo apt-get upgrade
sudo apt install redis-server
sudo apt install php-redis

Check

sudo systemctl status redis-server
redis-cli ping

Drupal 9 configuration

Add following to the settings.php

// REDIS default configurations.
$settings['redis.connection']['interface'] = 'PhpRedis';
$settings['redis.connection']['host']      = '127.0.0.1';  // Your Redis instance hostname.
$settings['cache']['default'] = 'cache.backend.redis';
$conf['redis_cache_socket'] = '/tmp/redis.sock';