When attempting to start MySQL, the following error may occur:

Job for mysqld.service failed because the control process exited with error code. See "systemctl status mysqld.service" and "journalctl -xe" for details.

To investigate, run journalctl -xe and examine the logs:

[linga@localhost ~]$ journalctl -xe
-- Subject: Unit mysqld.service has begun start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit mysqld.service has begun starting up.
Aug 30 04:32:12 localhost.localdomain mysqld[2312]: Initialization of mysqld failed: 0
Aug 30 04:32:12 localhost.localdomain systemd[1]: mysqld.service: control process exited, code=exited status=1
Aug 30 04:32:14 localhost.localdomain systemd[1]: Failed to start MySQL Server.
-- Subject: Unit mysqld.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit mysqld.service has failed.
-- 
-- The result is failed.
Aug 30 04:32:14 localhost.localdomain systemd[1]: Unit mysqld.service entered failed state.
Aug 30 04:32:14 localhost.localdomain systemd[1]: mysqld.service failed.
Aug 30 04:32:14 localhost.localdomain systemd[1]: mysqld.service holdoff time over, scheduling restart.
Aug 30 04:32:14 localhost.localdomain systemd[1]: Starting MySQL Server...
-- Subject: Unit mysqld.service has begun start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit mysqld.service has begun starting up.
Aug 30 04:32:14 localhost.localdomain mysqld[2359]: Initialization of mysqld failed: 0
Aug 30 04:32:14 localhost.localdomain systemd[1]: mysqld.service: control process exited, code=exited status=1
Aug 30 04:32:16 localhost.localdomain systemd[1]: Failed to start MySQL Server.
-- Subject: Unit mysqld.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit mysqld.service has failed.
-- 
-- The result is failed.
Aug 30 04:32:16 localhost.localdomain systemd[1]: Unit mysqld.service entered failed state.
Aug 30 04:32:16 localhost.localdomain systemd[1]: mysqld.service failed.

In this case, the error message indicaets "Initialiaztion of mysqld failed: 0". Attempting to kill the process using its PID (3306) fails:

[linga@localhost ~]$ kill 3306
-bash: kill: (3306) - No such process

Review the MySQL configuration file /etc/my.cnf:

[linga@localhost ~]$ cat /etc/my.cnf
# For advice on how to change settings please see
# http://dev.mysql.com/doc/refman/5.7/en/server-configuration-defaults.html

[mysqld]
#
# Remove leading # and set to the amount of RAM for the most important data
# cache in MySQL. Start at 70% of total RAM for dedicated server, else 10%.
# innodb_buffer_pool_size = 128M
#
# Remove leading # to turn on a very important data integrity option: logging
# changes to the binary log between backups.
# log_bin
#
# Remove leading # to set options mainly useful for reporting servers.
# The server defaults are faster for transactions and fast SELECTs.
# Adjust sizes as needed, experiment to find the optimal values.
# join_buffer_size = 128M
# sort_buffer_size = 2M
# read_rnd_buffer_size = 2M
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock

# Disabling symbolic-links is recommended to prevent assorted security risks
symbolic-links=0

log-error=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid


# Add UTF-8
character_set=utf8
init_connect='SET NAMES utf8'

The problematic line is character_set=utf8. This directive is invalid for the [mysqld] section. The correct option should be character_set_server=utf8 or use the proper utf8mb4 charset. After correcting the configuration file by removing or fixing the invalid setting, restart the service:

[linga@localhost ~]$ sudo systemctl start mysqld
[linga@localhost ~]$ sudo systemctl status mysqld
● mysqld.service - MySQL Server
   Loaded: loaded (/usr/lib/systemd/system/mysqld.service; enabled; vendor preset: disabled)
   Active: active (running) since Thu 2018-08-30 04:41:33 EDT; 12s ago
     Docs: man:mysqld(8)
           http://dev.mysql.com/doc/refman/en/using-systemd.html
  Process: 14018 ExecStart=/usr/sbin/mysqld --daemonize --pid-file=/var/run/mysqld/mysqld.pid $MYSQLD_OPTS (code=exited, status=0/SUCCESS)
  Process: 14001 ExecStartPre=/usr/bin/mysqld_pre_systemd (code=exited, status=0/SUCCESS)
 Main PID: 14021 (mysqld)
   CGroup: /system.slice/mysqld.service
           └─14021 /usr/sbin/mysqld --daemonize --pid-file=/var/run/mysqld/mysqld.pid

Aug 30 04:41:32 localhost.localdomain systemd[1]: mysqld.service holdoff time over, scheduling restart.
Aug 30 04:41:32 localhost.localdomain systemd[1]: Starting MySQL Server...
Aug 30 04:41:33 localhost.localdomain systemd[1]: Started MySQL Server.

Startup failures frequently stem from misconfigured settings in the configuration file. Always create a backup of the original configuration before making changes.