The "dmesg" Command

The dmesg command in ChromeOS provides access to the kernel message buffer, allowing users to view and analyze system messages. This section explains how to use this powerful diagnostic tool effectively.

Command Overview

The basic syntax for the command is: bash dmesg [options]

Command Options

Time Display Options

• -d, --show-delta: Display the time delta between messages
• -T, --ctime: Show human-readable timestamps
• -t, --notime: Don't show any timestamps

Output Format Options

• -H, --human: Enable human-readable output
• -L, --color: Colorize the output
• -p, --force-prefix: Add facility/level/timestamp prefix to each line
• -r, --raw: Print the raw message buffer
• -x, --decode: Decode facility and level numbers to readable prefixes

Message Filter Options

• -k, --kernel: Display kernel messages
• -u, --userspace: Display userspace messages

Understanding dmesg Output

Message Types

The command displays various system messages including: - Kernel boot messages - Hardware detection - Driver initialization - System errors - Device status changes

Time Formats

1. Delta Time
2. Shows time between messages
3. Useful for troubleshooting
4. Helps track event sequence

5. Identifies delays

6. Human-Readable Time

7. Standard datetime format
8. Easy to understand
9. Shows actual event times
10. Aids in log correlation

Using the Command

Basic Usage

1. View all messages: bash dmesg

Formatted Output

1. Human-readable format with color: bash dmesg -H -L

2. With timestamps: bash dmesg -T

Filtered Output

1. Kernel messages only: bash dmesg -k

2. Userspace messages: bash dmesg -u

Best Practices

Log Analysis

1. Initial Review
2. Use human-readable format
3. Enable timestamps
4. Check message types

5. Note error patterns

6. Detailed Analysis

7. Use delta timing
8. Enable message decoding
9. Filter relevant messages
10. Document findings

Troubleshooting

1. System Issues
2. Check recent messages
3. Look for errors
4. Track timing

5. Note sequences

6. Documentation

7. Save relevant output
8. Note timestamps
9. Record error patterns
10. Document resolutions

Common Use Cases

System Diagnostics

1. Boot Problems
2. Check boot messages
3. Identify failures
4. Track initialization

5. Debug startup issues

6. Hardware Issues

7. Monitor device messages
8. Check driver loading
9. Track hardware status
10. Debug device problems

Safety Considerations

System Impact

1. Resource Usage
2. Minimal impact
3. Read-only operation
4. Low system load

5. Safe to use frequently

6. Security

7. System messages visible
8. May contain sensitive info
9. Consider output sharing
10. Handle logs carefully

Troubleshooting

Common Issues

1. Output Problems
2. Message formatting
3. Timestamp accuracy
4. Buffer overflow

5. Display issues

6. Analysis Challenges

7. Message volume
8. Error identification
9. Time correlation
10. Pattern recognition