When MySQL fails to start and reports a missing socket file (/var/run/mysqld/mysqld.sock), it typically indicates one of several fundamental issues:

# Common error you'll see in logs
$ tail -n 20 /var/log/mysql/error.log
[ERROR] Can't start server: Bind on Unix socket: No such file or directory
[ERROR] Do you already have another mysqld server running on socket: /var/run/mysqld/mysqld.sock?

Before attempting fixes, verify these critical points:

# Check if MySQL process is running
$ ps aux | grep mysqld

# Verify socket directory exists
$ ls -la /var/run/mysqld/

# Check permissions (mysql user should own the directory)
$ ls -ld /var/run/mysqld

1. Create Missing Directory Structure

Modern systems using systemd may require manual directory creation:

# Create directory with proper permissions
$ sudo mkdir -p /var/run/mysqld
$ sudo chown mysql:mysql /var/run/mysqld
$ sudo chmod 755 /var/run/mysqld

2. Configure Alternative Socket Path

If filesystem restrictions exist, modify my.cnf:

[mysqld]
socket=/tmp/mysql.sock

[client]
socket=/tmp/mysql.sock

3. Systemd-Specific Fixes

For systems using systemd, create proper runtime directory:

# Create systemd service override
$ sudo mkdir -p /etc/systemd/system/mysql.service.d
$ sudo nano /etc/systemd/system/mysql.service.d/socket.conf

[Service]
RuntimeDirectory=mysqld
RuntimeDirectoryMode=755

Checking File Descriptors and Process Limits

# Check system limits
$ ulimit -n

# Verify in-use sockets
$ sudo lsof -U | grep mysql

Debugging with MySQL Safe Mode

$ mysqld_safe --skip-grant-tables --skip-networking &
$ mysql -u root

Add these to your server maintenance checklist:

• Configure log rotation for MySQL error logs
• Implement monitoring for socket file existence
• Create startup pre-checks in init scripts

#!/bin/bash
# Sample pre-start check
if [ ! -d "/var/run/mysqld" ]; then
    mkdir -p /var/run/mysqld
    chown mysql:mysql /var/run/mysqld
fi

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When your MySQL server refuses to start after a system reboot and complains about a missing socket file, you're facing a common yet frustrating issue. The error message clearly indicates the system cannot locate /var/run/mysqld/mysqld.sock, which is crucial for local connections.

Before diving into solutions, verify the current state with these commands:

sudo systemctl status mysql
ls -la /var/run/mysqld/
ps aux | grep mysql

• Permissions issues in /var/run/mysqld/
• Incorrect my.cnf configuration
• MySQL not having write access to the directory
• AppArmor/SELinux restrictions
• Filesystem corruption or full disk

First, ensure the directory exists with proper permissions:

sudo mkdir -p /var/run/mysqld
sudo chown mysql:mysql /var/run/mysqld

Then check your MySQL configuration (typically in /etc/mysql/my.cnf or /etc/my.cnf):

[mysqld]
socket=/var/run/mysqld/mysqld.sock

[client]
socket=/var/run/mysqld/mysqld.sock

If the issue persists, try forcing a clean start:

sudo systemctl stop mysql
sudo rm -f /var/lib/mysql/ib_logfile*
sudo mysqld --initialize
sudo systemctl start mysql

For systems with AppArmor/SELinux:

# For AppArmor
sudo aa-status | grep mysql
sudo aa-complain /usr/sbin/mysqld

# For SELinux
sudo restorecon -v /var/run/mysqld/mysqld.sock

While fixing the socket issue, you can temporarily connect via TCP:

mysql -h 127.0.0.1 -u root -p

Update your applications to use TCP connection if necessary:

$db = new mysqli('127.0.0.1', 'user', 'password', 'database', 3306);

Add these to your server maintenance checklist:

• Regularly verify MySQL's systemd unit file
• Monitor disk space in /var
• Implement proper log rotation
• Test service recovery after reboots