"mysql -u a -p -S /var/run/mysqld/mysqld.sock" works but "mysql -u a -p" doesn't

When I use the following command:

mysql -u a -p -S /var/run/mysqld/mysqld.sock

It works but if I don't specify -S /var/run/mysqld/mysqld.sock it gives me the following error:

ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/tmp/mysql.sock' (2)

I also remember it was working the last time I used this computer(maybe a month ago) but today I tried it and it doesn't work!

What is the problem and how can I fix it?

EDIT: This is /etc/mysql/my.cnf:

!includedir /etc/mysql/conf.d/!includedir /etc/mysql/mysql.conf.d/

This is /etc/mysql/mysql.conf.d/mysql.cnf:

[mysql]

This is /etc/mysql/mysql.conf.d/mysqld.cnf:

[mysqld]## * Basic Settings#user            = mysqlpid-file        = /var/run/mysqld/mysqld.pidsocket  = /var/run/mysqld/mysqld.sockport            = 3306datadir = /var/lib/mysql# If MySQL is running as a replication slave, this should be# changed. Ref https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_tmpdir# tmpdir                = /tmp## Instead of skip-networking the default is now to listen only on# localhost which is more compatible and is not less secure.bind-address            = 127.0.0.1mysqlx-bind-address     = 127.0.0.1## * Fine Tuning#key_buffer_size         = 16M# max_allowed_packet    = 64M# thread_stack          = 256K# thread_cache_size       = -1# This replaces the startup script and checks MyISAM tables if needed# the first time they are touchedmyisam-recover-options  = BACKUP# max_connections        = 151# table_open_cache       = 4000## * Logging and Replication## Both location gets rotated by the cronjob.## Log all queries# Be aware that this log type is a performance killer.# general_log_file        = /var/log/mysql/query.log# general_log             = 1## Error log - should be very few entries.#log_error = /var/log/mysql/error.log## Here you can see queries with especially long duration# slow_query_log                = 1# slow_query_log_file   = /var/log/mysql/mysql-slow.log# long_query_time = 2# log-queries-not-using-indexes## The following can be used as easy to replay backup logs or for replication.# note: if you are setting up a replication slave, see README.Debian about#       other settings you may need to change.# server-id             = 1# log_bin                       = /var/log/mysql/mysql-bin.log# binlog_expire_logs_seconds    = 2592000max_binlog_size   = 100M# binlog_do_db          = include_database_name# binlog_ignore_db      = include_database_name