🚀 Master Linux Debugging with dmesg & journalctl – Essential Commands for Troubleshooting!

🚀 Master Linux Debugging with dmesg & journalctl – Essential Commands for Troubleshooting!
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1️. dmesg - Kernel Logs for Hardware & System Issues
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✅ What It Does?
dmesg (display message) shows kernel logs, including hardware issues, driver failures, disk errors, and boot messages.

✅ Use Cases:
✔️ Diagnosing boot issues
✔️ Identifying disk I/O errors
✔️ Checking kernel panics
✔️ Debugging network and device issues

🛠 dmesg Commands for Troubleshooting:
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1. 🔹 View the latest kernel messages:
dmesg | tail -50

2. 🔹 Filter messages by time (useful for debugging a past issue):
dmesg --since "10 minutes ago"

3. 🔹 Check for hardware errors (disks, RAM, etc.):
dmesg | grep -i "error\|fail\|warn"

4. 🔹 Check for disk failures:
dmesg | grep -i "ext4\|xfs\|sda\|nvme"

5. 🔹Check if a network interface (e.g., eth0) is failing:
dmesg | grep -i eth0

6. 🔹Check CPU or memory errors:
dmesg | grep -i "CPU\|memory\|OOM"

7. 🔹Monitor logs in real-time (like tail -f):
dmesg -w
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2️. journalctl - Systemd Logs for Services & Authentication
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✅ What It Does?
journalctl provides detailed logs of system events, user logins, authentication failures, and systemd service failures.

✅ Use Cases:
✔️ Checking failed services
✔️ Investigating login failures
✔️ Debugging system crashes
✔️ Viewing specific service logs

🛠 journalctl Commands for Troubleshooting:
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1. 🔹 View system logs (all logs from boot time)
journalctl -n 50

2. 🔹 Show logs in real-time (like tail -f)
journalctl -f

3. 🔹 Check logs from the last system boot
journalctl -b

4. 🔹 Check failed systemd services
systemctl --failed
journalctl -p 3 -xb

5. 🔹 View logs for a specific service (e.g., Nginx)
journalctl -u nginx --since "1 hour ago"

6. 🔹 Find user login attempts & authentication failures
journalctl _UID=1000 | grep "authentication failure"

7. 🔹 View logs from a specific time
journalctl --since "2024-02-03 12:00:00" --until "2024-02-03 15:00:00"

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Example: Debugging a Slow Server
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🔍 Step 1: Check CPU Usage
top -o %CPU

🔍 Step 2: Check Disk I/O Issues
iostat -x 1

🔍 Step 3: Check for Disk Errors
dmesg | grep -i "I/O error\|ext4\|xfs"

🔍 Step 4: Check if a Process is Being Killed
journalctl -k | grep -i "killed process"
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