Troubleshooting Common Issues

1. Diagnosing Boot Failures

Issue	Description	Solution
Black screen after boot	System starts but displays only a black screen	Check hardware connections, try `nomodeset` in GRUB, or boot into recovery mode
Stuck at "Starting kernel..."	Boot process halts during kernel initialization	Boot into recovery mode or single-user mode to check logs in `/var/log`
GRUB menu not appearing	System boots directly to OS without showing GRUB menu	Reinstall GRUB using `grub-install` or edit `/etc/default/grub`
Filesystem errors during boot	Boot fails due to filesystem corruption or mount issues	Use `fsck` to repair filesystem or boot from live USB

Example: Check boot logs for errors

sudo dmesg | grep -i error
journalctl -b | grep -i boot

Note: Use journalctl -b to view logs from the last boot session.

Warning: Avoid modifying /boot or /etc/fstab without a backup.

Boot Failure Troubleshooting Steps

Reboot and enter GRUB menu (usually by pressing Shift or Esc)
Select "Advanced options for Ubuntu" and try a previous kernel version
Boot into recovery mode using root or single in kernel parameters
Run fsck on the root filesystem if prompted or accessible
Check /var/log/syslog and /var/log/kern.log for error messages

2. Recovering from Kernel Panics

Recovery Method	Description	Notes
Emergency Boot	Reboot system with `emergency` or `single` mode	Accesses single-user shell; useful for filesystem repairs
Kernel Logs	Check `/var/log/kern.log` or `dmesg`	Identifies root cause of panic (e.g., hardware failure, driver issues)
Systemd Recovery	Use `systemctl rescue` or `systemctl emergency`	Enters rescue mode to fix critical issues without full reboot
GRUB Recovery	Access GRUB menu and select recovery kernel or options	Allows booting with different kernel parameters or modules disabled

Example: Emergency Boot via GRUB

Press 'e' during boot to edit GRUB entry
Add 'init=/bin/bash' or 'single' to kernel line
Press Ctrl+X or F10 to boot

Note: Kernel panics often indicate hardware or driver issues. Ensure backups are in place before attempting recovery.

Warning: Using init=/bin/bash gives root access but may leave system in an inconsistent state.

Recovery Steps Summary

Identify panic logs using dmesg or /var/log/kern.log
Reboot into single-user or emergency mode
Run filesystem checks (fsck) if needed
Reinstall or update problematic drivers
Check hardware for faults (RAM, CPU, disk)

3. Fixing File System Errors

Command	Description	Usage
fsck	File system checker and repair tool	`sudo fsck /dev/sda1`
dmesg	Display or control the kernel ring buffer	`dmesg \| grep -i error`
journalctl	Control the systemd journal	`journalctl -b \| grep -i error`

Example: Checking for File System Errors

sudo fsck -f /dev/sda1

Note: Always backup critical data before running fsck.

Warning: Running fsck on a mounted filesystem can cause data loss.

Error Type	Description	Solution
No space left on device	File system is full	`df -h` to check space, remove unnecessary files
Permission denied	Insufficient permissions to access file	`sudo chmod` or `sudo chown`
Corrupted file system	File system structure is damaged	`fsck` to repair

4. Resolving Network Connectivity Issues

Issue	Symptoms	Command/Tool	Solution
No Internet Access	Cannot ping external IPs or resolve domain names	`ping 8.8.8.8`	Check physical connection, verify network interface is up
DNS Resolution Failure	Cannot resolve hostnames (e.g., `ping google.com` fails)	`nslookup google.com`	Check `/etc/resolv.conf`, test with `dig google.com`
Firewall Blocking	Connection times out or gets reset	`iptables -L`	Allow traffic on required ports (e.g., `iptables -A INPUT -p tcp --dport 80 -j ACCEPT`)
Incorrect Gateway	Local network works but no external access	`ip route`	Verify default gateway IP in output; update if needed via `ip route add default via <gateway>`

Example: Testing DNS Resolution

nslookup example.com
dig example.com

Example: Checking Firewall Rules

sudo iptables -L
sudo ufw status

Note: Ensure that firewalld or ufw is not blocking essential services like SSH or HTTP.

Warning: Modifying firewall rules without knowing the impact can lock you out of your system.

5. Debugging Service Failures

Command	Description	Usage
systemctl status `service-name`	Check the status of a specific service	`systemctl status nginx`
journalctl -u `service-name`	View logs for a specific service	`journalctl -u nginx.service`
journalctl -u `service-name` --since "1 hour ago"	View logs for a specific service within a time range	`journalctl -u nginx.service --since "1 hour ago"`
systemctl restart `service-name`	Restart a service	`systemctl restart nginx`
systemctl start `service-name`	Start a service	`systemctl start nginx`
systemctl stop `service-name`	Stop a service	`systemctl stop nginx`

Example: Check service status and logs

systemctl status nginx
journalctl -u nginx.service --since "1 hour ago"

Note: If the service is not starting, check its configuration file for syntax errors.

Warning: Ensure you have sufficient permissions to restart services.

Common Service Issues and Solutions

Service not starting: Check logs using journalctl
Service crashes: Review logs for error messages or stack traces
Permission denied: Ensure proper user permissions for service files

6. Investigating High CPU Usage

Command	Description	Usage
top	Real-time system monitor showing processes	`top` - shows processes sorted by CPU usage
htop	Enhanced version of top with better UI	`htop` - interactive process viewer
ps aux	Report process status with full details	`ps aux \| sort -nrk 3,3` - sort by CPU usage
pidstat	Per-process CPU statistics	`pidstat -u 1 5` - report every second for 5 times

Example: Identify top CPU-consuming processes

ps aux | sort -nrk 3,3 | head -n 5

Example: Monitor CPU usage over time

pidstat -u 1 10

Note: Use top or htop for real-time monitoring. The -u flag in pidstat shows CPU usage statistics.

Tool	Functionality	Best For
`top`	Live process list sorted by resource usage	Quick real-time checks
`htop`	Enhanced `top` with color and navigation	Interactive process management
`ps aux`	Full process listing with detailed stats	Scripting and automation
`pidstat`	Per-process CPU and I/O statistics	Detailed performance analysis

7. Diagnosing Memory Issues

Command	Description	Usage
free -h	Displays memory usage in human-readable format	`free -h`
top	Real-time view of system processes and memory usage	`top`
htop	Enhanced version of top with better UI and features	`htop`
vmstat	Reports virtual memory statistics	`vmstat 1 5`
ps aux	Lists all running processes with memory usage	`ps aux \| grep -E 'MEM\|PID'`

Example: Check memory usage with free

free -h
              total        used        free      shared  buff/cache   available
Mem:           7.7G        2.1G        3.2G        156M        2.4G        5.2G
Swap:          2.0G          0B        2.0G

Example: Identify memory-hungry processes with ps

ps aux --sort=-%mem | head -n 5
USER       PID %MEM     VSZ    RSS    TTY      STAT   START   TIME   COMMAND
user       1234 15.2     200000  1500000  ?        Sl     12:34   0:05   firefox
user       5678 8.3      120000   800000  ?        Sl     12:35   0:03   chrome

Note: RSS (Resident Set Size) indicates physical memory currently used by a process. VSZ (Virtual Size) includes all memory including swapped and shared pages.

Memory Diagnosis Workflow

Run free -h to check overall memory and swap usage.
Use top or htop to identify processes consuming high memory.
Filter output with ps aux --sort=-%mem to list top memory consumers.
Check for memory leaks or inefficient applications.

8. Resolving Disk Space Issues

Command	Description	Usage
df -h	Display disk space usage in human-readable format	`df -h`
du -sh *	Check disk usage of files and directories in current folder	`du -sh *`
ls -l /var/log	List log files to identify large logs	`ls -l /var/log`
find / -type f -size +100M	Find files larger than 100MB	`find / -type f -size +100M`

Example: Cleaning up old logs

# Clear old logs to free space
sudo journalctl --vacuum-size=100M
sudo find /var/log -name "*.log" -mtime +30 -delete

Example: Identifying large directories

# Show top 10 largest directories
du -sh /* | sort -hr | head -n 10

Note: Always double-check before deleting files or directories to avoid data loss.

Warning: Be cautious when deleting system logs or files in use.

9. Fixing Permission Problems

Problem	Solution	Notes
Permission denied (e.g., `Permission denied: '/usr/local/bin'`)	`sudo chown -R $USER:$USER /usr/local/bin`	Use `chown` to change ownership of files or directories
Read-only filesystem error	`sudo mount -o remount,rw /`	Remount root filesystem as read-write if needed
File or directory not accessible due to permissions	`ls -l filename`	Check file permissions using `ls -l`

Example: Changing Ownership of a Directory

sudo chown -R user:group /path/to/directory

Example: Setting Read/Write Permissions

chmod 755 /path/to/script.sh

Note: Use chmod to set file permissions. Common values: 755 (rwxr-xr-x), 644 (rw-r--r--).

Warning: Be cautious when using sudo chown -R on system directories like /usr/local/bin. It can cause unexpected behavior or break applications.

10. Debugging DNS Issues

Command	Description	Usage Example
nslookup	Query DNS to find IP addresses or hostnames	`nslookup example.com`
dig	Advanced DNS lookup tool with detailed output	`dig example.com`
host	Simple DNS lookup utility	`host example.com`
ping	Test network connectivity and DNS resolution	`ping example.com`

Example: Check DNS resolution for a domain

dig example.com

Example: Test DNS resolution with ping

ping -c 4 example.com

Note: If dig or nslookup return NXDOMAIN, the domain does not exist.

Warning: Ensure your DNS server is correctly configured in /etc/resolv.conf.

Problem	Possible Cause	Solution
DNS resolution fails	Incorrect DNS server config	Check `/etc/resolv.conf`
DNS resolution slow	Network issues or DNS server overload	Use `dig +trace` to trace resolution path
Host not found	Domain does not exist or typo in name	Verify domain name and check DNS records

11. Troubleshooting SSH Access

Issue	Possible Cause	Solution
Connection times out	Firewall blocking port 22, SSH service not running, incorrect host	Check `ufw status` or `iptables -L`, ensure `sshd` is running with `systemctl status sshd`
Permission denied (publickey)	Incorrect key, wrong user, permissions on keys	Verify `~/.ssh/id_rsa` exists and has correct permissions (600), check `ssh-add -l`, ensure user matches server config
Host key verification failed	Host key changed or unknown host in `~/.ssh/known_hosts`	Remove line from `known_hosts` or run `ssh-keygen -R [hostname]`
Too many authentication failures	Too many keys loaded in SSH agent	Run `ssh-add -D` to clear all keys, then re-add only needed ones

Example: Check SSH service status

systemctl status sshd

Example: Test SSH connection with verbose logging

ssh -v user@hostname

Note: Ensure the SSH daemon (sshd) is enabled and running on the target machine.

Warning: Do not disable SSH access without another way to manage the system (e.g., console or KVM).