Linux Troubleshooting Commands With Examples & Cheat Sheet

Linux Troubleshooting Commands with examples

Enterprise infrastructure, development environments, and servers all frequently employ Linux systems. Even though Linux is renowned for its dependability, difficulties like disk problems, network outages, excessive CPU consumption, or service breakdowns can still happen. The process of locating, evaluating, and resolving these problems is called troubleshooting.

Numerous command-line tools that Linux offers assist administrators in rapidly identifying the cause of an issue. Users can examine system logs, keep an eye on processes, verify hardware conditions, and examine network connectivity with these commands.

This tutorial provides basic examples and explanations of frequently used Linux troubleshooting commands.

1. dmesg Command

The dmesg command displays messages from the Linux kernel ring buffer. It is useful for diagnosing hardware problems or driver issues.

Example

bash

dmesg

This shows kernel messages generated during system startup.

To view the most recent messages:

bash

dmesg | tail

Use Case: If a USB device or hard disk is not detected, dmesg it often shows related error messages.

2. journalctl Command

Modern Linux systems that use systemd store logs in a journal. The journalctl command is used to read these logs.

Example

View all system logs:

bash

journalctl

View logs from the current boot:

bash

journalctl -b

View logs for a specific service:

bash

journalctl -u ssh

Use Case: Helps identify why a service failed to start or crashed.

3. top Command

The top command provides a real-time view of system processes, CPU usage, memory usage, and load average.

Example

bash

top

Use Case: If the system becomes slow, top helps identify which process is consuming the most CPU or memory.

Also read about Linux Security Features, Tools, And Why Linux Is Secure

4. ps Command

The ps command displays information about active processes.

Example

bash

ps aux

Use Case: Useful for locating running applications or identifying suspicious processes.

To search for a specific process:

bash

ps aux | grep apache

5. df Command

The df command checks disk space usage on mounted file systems.

Example

bash

df -h

The -h option shows output in a human-readable format.

Use Case: Helps determine whether a disk is full, which can cause application or system failures.

6. du Command

The du command estimates disk usage for directories and files.

Example

bash

du -sh /var/log

Use Case: Used to identify which directory is consuming large amounts of disk space.

7. free Command

The free command displays information about system memory usage.

Example

bash

free -h

Use Case: If applications crash or the system becomes slow, checking available RAM can help diagnose the problem.

8. ping Command

The ping command tests network connectivity between two systems.

Example

bash

ping google.com

Use Case: Helps determine whether the network connection is working.

9. traceroute Command

The traceroute command shows the path that packets take to reach a destination host.

Example

bash

traceroute google.com

Use Case: Useful for diagnosing network delays or routing issues.

Also read about What Is Kernel Loading In Linux? Kernel Loading Process

10. netstat Command

The netstat command displays network connections, routing tables, and listening ports.

Example

bash

netstat -tuln

Use Case: Helps identify which services are listening on network ports.

11. systemctl Command

The systemctl command is used to manage services on systems running systemd.

Example

Check service status:

bash

systemctl status nginx

Restart a service:

bash

systemctl restart nginx

Use Case: Used to troubleshoot services that fail to start or stop unexpectedly.

12. lsblk Command

The lsblk command lists information about block devices such as disks and partitions.

Example

bash

lsblk

Use Case: Helps verify whether storage devices are detected by the system.

Linux Troubleshooting Cheat Sheet

Linux Troubleshooting Cheat Sheet, featuring the 10 most essential commands every administrator should know to diagnose and fix a system.

Command	Category	Purpose
top or htop	Performance	Shows real-time CPU, RAM usage, and running processes.
dmesg	Kernel	Lists kernel-level messages (useful for hardware/driver errors).
journalctl -xe	Logs	Shows the most recent system errors and service failures.
df -h	Storage	Checks disk space usage in a human-readable format.
free -m	Memory	Displays total, used, and available RAM in Megabytes.
ip addr	Network	Verifies if network interfaces are up and have an IP address.
ping -c 4 8.8.8.8	Network	Checks if the server can reach the outside internet.
lsblk	Storage	Lists all blocked devices (hard drives and partitions).
ps aux	Process	Gives a detailed snapshot of every running program.
netstat -tulpn	Network	Shows which services are listening on which ports.

Also read about What Are The Runlevels In Linux? & Common Systemd Targets

Linux Troubleshooting Interview Questions

System Performance & Load

• Question: A server is running slowly. What is the first command you run?
  • Answer: Use top or htop to check CPU usage, memory consumption, and the load average.
• Question: What does a high “iowait” in top indicate?
  • Answer: It means the CPU is sitting idle because it is waiting for a slow disk (I/O) to finish reading or writing data.
• Question: How do you find which process is using the most memory?
  • Answer: Run ps aux --sort=-%mem | head to list the top memory-consuming processes.

Storage & Filesystems

• Question: How do you find which directory is consuming the most disk space?
  • Answer: Use du -sh /* | sort -h to calculate and sort the sizes of all top-level folders.
• Question: What do you do if df -h it shows a full disk, but you can’t find the large files?
  • Answer: Run lsof +L1 to find “deleted” files that are still being held open by a running process.
• Question: How do you fix a “Read-only file system” error?
  • Answer: Check for hardware errors in dmesg, then try to remount it using mount -o remount,rw /.

Networking

• Question: A user cannot connect to a web server. How do you check if the port is open?
  • Answer: Use netstat -tulpn or ss -tulpn to see if the service is listening on the expected port (e.g., 80 or 443).
• Question: How do you verify if a DNS issue is causing a connection failure?
  • Answer: Use nslookup or dig followed by the domain name to see if it resolves to an IP address.
• Question: How can you tell if a firewall is blocking a specific IP address?
  • Answer: Check the active rules with iptables -L -n or ufw status numbered.

Boot & Kernel

• Question: What is a “Kernel Panic” and how do you investigate it?
  • Answer: It is a critical system crash; you investigate it by checking the /var/log/messages or serial console logs.
• Question: How do you change the boot parameters if the system fails to start?
  • Answer: Press e at the GRUB menu to edit the boot entry and add single or init=/bin/bash to enter rescue mode.
• Question: Where can you find hardware-related error messages?
  • Answer: Run the dmesg command or check the /var/log/kern.log file.

Service Management

• Question: How do you check why a service failed to start?
  • Answer: Run systemctl status <service> followed by journalctl -u <service> -e for detailed error logs.
• Question: What is the difference between SIGHUP, SIGTERM, and SIGKILL?
  • Answer: SIGHUP reloads configs, SIGTERM asks a process to stop gracefully, and SIGKILL forces it to stop immediately.
• Question: How do you see all logs generated in the last 10 minutes?
  • Answer: Run journalctl --since "10 min ago".

Practical Scenario Questions

• Scenario: “The system says ‘No space left on device’ but df -h shows 50% free. Why?”
  • Answer: The filesystem has likely run out of Inodes; check this by running df -i.
• Scenario: “You edited /etc/fstab , and now the system won’t boot. How do you fix it?”
  • Answer: Boot into a Live USB or Maintenance Mode, mount the root partition, and fix the typo in the fstab file.

Also read about Basic Linux Commands For Beginners With Easy Examples