场景

[root@localhost bin]# ./mysqld
2020-01-15 17:52:02 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2020-01-15 17:52:02 0 [Note] --secure-file-priv is set to NULL. Operations related to importing and exporting data are disabled
2020-01-15 17:52:02 0 [Note] ./mysqld (mysqld 5.6.44) starting as process 90367 ...
2020-01-15 17:52:02 90367 [Note] Plugin 'FEDERATED' is disabled.
2020-01-15 17:52:02 90367 [Note] InnoDB: Using atomics to ref count buffer pool pages
2020-01-15 17:52:02 90367 [Note] InnoDB: The InnoDB memory heap is disabled
2020-01-15 17:52:02 90367 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
2020-01-15 17:52:02 90367 [Note] InnoDB: GCC builtin __atomic_thread_fence() is used for memory barrier
2020-01-15 17:52:02 90367 [Note] InnoDB: Compressed tables use zlib 1.2.11
2020-01-15 17:52:02 90367 [Note] InnoDB: Using Linux native AIO
2020-01-15 17:52:02 90367 [Note] InnoDB: Not using CPU crc32 instructions
2020-01-15 17:52:02 90367 [Note] InnoDB: Initializing buffer pool, size = 128.0M
2020-01-15 17:52:02 90367 [Note] InnoDB: Completed initialization of buffer pool
2020-01-15 17:52:02 90367 [Note] InnoDB: Highest supported file format is Barracuda.
2020-01-15 17:52:02 90367 [Note] InnoDB: 128 rollback segment(s) are active.
2020-01-15 17:52:02 90367 [Note] InnoDB: Waiting for purge to start
2020-01-15 17:52:02 90367 [Note] InnoDB: 5.6.44 started; log sequence number 18755961
2020-01-15 17:52:02 90367 [Note] Server hostname (bind-address): '*'; port: 3306
2020-01-15 17:52:02 90367 [Note] IPv6 is available.
2020-01-15 17:52:02 90367 [Note]   - '::' resolves to '::';
2020-01-15 17:52:02 90367 [Note] Server socket created on IP: '::'.
2020-01-15 17:52:02 90367 [ERROR] Can't start server : Bind on unix socket: Address already in use
2020-01-15 17:52:02 90367 [ERROR] Do you already have another mysqld server running on socket: /tmp/mysql.sock ?
2020-01-15 17:52:02 90367 [ERROR] Aborting

排查过程

1）管道文件已经存在

2020-01-15 17:52:02 90367 [ERROR] Can't start server : Bind on unix socket: Address already in use
2020-01-15 17:52:02 90367 [ERROR] Do you already have another mysqld server running on socket: /tmp/mysql.sock ?

2）查看文件属性以及查看进程占用

[root@localhost tmp]# file mysqld.sock
mysqld.sock: socket

[root@localhost tmp]# fuser /tmp/mysql.sock

没有发现任何进程占用，说明该文件不是由mysqld进程创建的，并且没有任何的服务占用该管道文件，该管道文件一般用于本地的mysql服务器和客户端通信的通道。默认情况下，mysql主进程在启动的时候创建该文件，退出的时候，会删除该文件，通过删除该文件，解决上面的问题。

3）启动mysql服务，查看占用过程

[root@localhost tmp]# fuser /tmp/mysql.sock
/tmp/mysqld.sock:    76533
[root@localhost tmp]# lsof /tmp/mysql.sock
COMMAND   PID  USER   FD   TYPE             DEVICE SIZE/OFF    NODE NAME
mysqld  76533 mysql   22u  unix 0x00000000f901123c      0t0 8234599 /tmp/mysql.sock