Common Linux troubleshooting scenarios

 

1. System Won't Boot

Symptoms: After rebooting, the system stops at the boot screen or fails to load the operating system.

Causes:

• Corrupted bootloader (e.g., GRUB).
• Missing or corrupted kernel.
• Disk errors.

Solutions:

• Boot from a live USB: Access the system via a live USB, then troubleshoot.
• Check bootloader: Use the grub-install command to reinstall GRUB.
• Repair filesystem: Run fsck on the root filesystem to detect and fix disk errors.
• Restore kernel: Reinstall or update the kernel package if it's missing.

2. Network Connectivity Issues

Symptoms: No network connectivity, unable to ping external websites, or resolve domain names.

Causes:

• Misconfigured network interface.
• Firewall blocking network traffic.
• DNS issues.

Solutions:

• Check IP configuration: Use ip a or ifconfig to check interface settings. Modify /etc/network/interfaces or use nmcli for configuration.
• Check firewall: Use ufw status or iptables -L to verify that the firewall isn't blocking traffic.
• Check DNS: Test using ping 8.8.8.8 to see if it's a DNS issue. If so, check /etc/resolv.conf for DNS server configuration.

3. Disk Space Issues

Symptoms: Error messages about "No space left on device" or inability to write files.

Causes:

• Disk is full.
• Filesystem reaching inode limit.

Solutions:

• Identify large files: Use du -sh /* to locate large files.
• Clear logs: Check /var/log and rotate or delete large log files.
• Inode check: Run df -i to see if you've run out of inodes, and delete unnecessary small files if needed.

4. Slow Performance

Symptoms: System slowdowns, lagging applications, or high CPU usage.

Causes:

• High memory or CPU consumption.
• Disk I/O bottlenecks.
• Background processes consuming resources.

Solutions:

• Monitor resources: Use top or htop to identify resource-intensive processes.
• Kill processes: Terminate high-usage processes with kill.
• Optimize disk I/O: Check iostat for I/O performance and make adjustments as needed, such as upgrading hardware or adjusting configurations.

5. Permission Denied Errors

Symptoms: "Permission denied" error messages even as root.

Causes:

• Incorrect file permissions or ownership.
• SELinux or AppArmor restrictions.

Solutions:

• Check file permissions: Use ls -l to inspect file permissions. Change them using chmod or chown.
• Verify SELinux status: If SELinux is enforcing, use getenforce to check status and setenforce 0 to temporarily disable it.
• AppArmor profiles: For Ubuntu and derivatives, check AppArmor with sudo aa-status and adjust as needed.

6. Broken Package Dependencies

Symptoms: Errors during package installation or upgrade.

Causes:

• Corrupted or missing package files.
• Conflicting dependencies.

Solutions:

• Force reinstall: Use apt-get install -f (Debian/Ubuntu) to fix broken dependencies.
• Clear cache: Run apt-get clean or dnf clean all to clear the package cache.
• Use a package manager: Tools like dpkg --configure -a or yum-complete-transaction can often resolve these issues.

7. File Not Found in Path

Symptoms: Command not found error even if the program is installed.

Causes:

• Binary location not included in the system’s PATH.
• Application not installed correctly.

Solutions:

• Locate binary: Use which <command> to find where the binary is located. Add the location to PATH if necessary (export PATH=$PATH:/path/to/directory).
• Install application: If the program isn't installed, use the appropriate package manager to install it.

8. Kernel Panic

Symptoms: System crashes with a kernel panic error.

Causes:

• Hardware issues.
• Corrupted kernel or drivers.
• Memory errors.

Solutions:

• Check hardware: Test hardware components (e.g., RAM, disk).
• Update kernel: Try reinstalling or updating to a stable kernel version.
• View kernel logs: Review /var/log/kern.log for specific causes.

 

9. USB Drive Not Detected

Symptoms: Inserting a USB drive does not mount or display the drive in the file manager.

Causes:

• USB drive failure or hardware issues.
• Lack of necessary drivers or permissions.
• Incorrect USB port or connection.

Solutions:

• Check dmesg: Run dmesg | tail immediately after inserting the drive to see any system messages related to USB detection.
• Check mount points: Use lsblk or fdisk -l to verify if the drive is detected. If so, manually mount it using mount.
• Install drivers: Ensure you have usbutils and udisk2 packages installed for handling USB devices.

10. SSH Connection Refused

Symptoms: Unable to connect via SSH, seeing “Connection refused” errors.

Causes:

• SSH server (sshd) not running or misconfigured.
• Firewall blocking SSH port (default 22).
• Network issues between client and server.

Solutions:

• Check SSH service: Verify SSH server status with sudo systemctl status ssh or sudo service ssh status.
• Open firewall ports: Ensure port 22 is open using ufw allow 22 or iptables.
• Test network: Use ping, traceroute, or telnet <server> 22 to confirm network connectivity to the server.

11. Frozen GUI/Desktop Environment

Symptoms: The system’s graphical interface is unresponsive, mouse and keyboard inputs are not registering.

Causes:

• High memory or CPU usage.
• Graphical server crash.
• Hardware issues.

Solutions:

• Switch to terminal: Press Ctrl + Alt + F2 to switch to a different terminal, then check processes with top or htop.
• Restart display manager: Run sudo systemctl restart gdm (for GNOME), sddm (for KDE), or lightdm as needed.
• Force restart: If unable to recover, force a reboot using REISUB keys (Alt + SysRq + R E I S U B).

12. Outdated System Time

Symptoms: Incorrect system time, leading to issues with timestamps and SSL connections.

Causes:

• Incorrect time zone or NTP settings.
• CMOS battery failure (hardware clock).

Solutions:

• Synchronize with NTP: Run sudo timedatectl set-ntp true to enable NTP sync.
• Set timezone: Use sudo timedatectl set-timezone <timezone> to set the correct timezone.
• Replace CMOS battery: If system time resets after shutdown, consider replacing the CMOS battery.

13. Software Installation Fails Due to Missing Repositories

Symptoms: Package manager errors indicating missing repositories.

Causes:

• Repositories were removed or are outdated.
• Network issues preventing repository access.

Solutions:

• Update repository list: Refresh using sudo apt update or equivalent.
• Add missing repositories: Locate and add missing repositories in /etc/apt/sources.list (Debian/Ubuntu) or /etc/yum.repos.d/ (RHEL/CentOS).
• Check network: Ensure you can access the repository URLs in a browser or with ping.

14. High Load Average

Symptoms: uptime or top shows a high load average, indicating the system is overloaded.

Causes:

• Overloaded CPU or disk I/O.
• Too many processes running simultaneously.

Solutions:

• Identify high-usage processes: Use top or ps aux --sort=-%cpu to identify and kill or stop unnecessary processes.
• Schedule heavy tasks: Run CPU or disk-intensive tasks during off-peak hours or set process priority with nice and renice.
• Consider upgrading hardware: If high load persists, consider upgrading CPU, memory, or disks.

15. Failed to Load Kernel Module

Symptoms: Error messages about missing or failing to load kernel modules, often impacting hardware or software functionality.

Causes:

• Incompatible kernel version.
• Missing or corrupted kernel module.

Solutions:

• Load module manually: Use modprobe <module_name> to load the module, and check errors with dmesg.
• Reinstall module: If the module is missing, install or rebuild it using package managers or by reconfiguring the kernel (make menuconfig for custom kernels).
• Kernel update: Ensure compatibility by updating to a compatible kernel.

16. Failed User Login

Symptoms: Users unable to log in with correct credentials, sometimes due to permissions or lockouts.

Causes:

• Account lockout or disabled account.
• Permission issues in home directory or .bashrc errors.

Solutions:

• Unlock account: Use sudo usermod -U <username> or passwd -u <username>.
• Verify permissions: Check home directory permissions with ls -ld /home/username, ensuring ownership and permissions are correct.
• Check login shell: If login is blocked, ensure the shell is valid by checking /etc/passwd.