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Cheat Sheet #day30 - dmesg

Cheat Sheet #day30 - dmesg

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dmesg Cheatsheet

Basic Usage

Display the kernel ring buffer messages:
```
  dmesg
```

Filtering Output

Filter messages using grep:
```
  dmesg | grep <pattern>
```
Show messages with timestamps:
```
  dmesg -T
```

Controlling Output

Clear the ring buffer (requires root):
```
  sudo dmesg -C
```
Limit the number of lines displayed:
```
  dmesg | tail -n <number_of_lines>
```
Show messages in real-time:
```
  dmesg -w
```

Specific Message Types

Show only kernel messages:
```
  dmesg -k
```
Show only userspace messages:
```
  dmesg -u
```
Show messages by facility (e.g., auth, daemon, kern, etc.):
```
  dmesg -f <facility>
```
Show messages by level (e.g., emerg, alert, crit, err, etc.):
```
  dmesg -l <level>
```

Time and Date Formatting

Show human-readable timestamps:
```
  dmesg -H
```
Show messages with nanosecond timestamps:
```
  dmesg -L
```

Kernel Log Buffer Size

Display the size of the kernel log buffer:
```
  dmesg -s <buffer_size>
```

Advanced Usage

Decode devicetree boot messages:
```
  dmesg -D
```
Display raw message buffer data:
```
  dmesg -r
```

Redirecting Output

Save output to a file:
```
  dmesg > /path/to/file
```
Append output to a file:
```
  dmesg >> /path/to/file
```

Example Commands

Filter for USB-related messages:
```
  dmesg | grep -i usb
```
Show only error messages:
```
  dmesg -l err
```
Follow new kernel messages in real-time:
```
  dmesg -wH
```
Clear the ring buffer and display messages:
```
  sudo dmesg -C && dmesg
```

This cheatsheet provides a quick reference to the most commonly used dmesg commands and options. dmesg is a powerful tool for system administrators and developers to troubleshoot and monitor system activity by accessing kernel messages.

dmesg Linux cheatsheet cheat sheet