🐧 Linux Common Errors and Troubleshooting Guide

🐧 Linux Common Errors and Troubleshooting Guide

Even experienced Linux administrators occasionally face issues — failed services, denied permissions, blocked ports, or SELinux restrictions. This guide summarizes the 10 most common Linux errors and provides quick diagnostic commands and recovery solutions.

1️⃣ Permission Denied

Appears when accessing protected files or running scripts without proper privileges.

sudo chown -R user:user /path/to/file
sudo chmod 755 /usr/local/bin/script.sh
getenforce        # Check SELinux mode
sudo restorecon -Rv /var/www/html

Tip: Verify file ownership, permission bits, and SELinux labels.

2️⃣ systemd Service Failed to Start

Analyze service startup failures using:

sudo systemctl status nginx
sudo journalctl -xeu nginx

Common causes: syntax errors, permission issues, or missing dependencies.

3️⃣ Network Connection Problems

When ping or curl cannot connect, check:

ip a
sudo systemctl restart NetworkManager
curl -I https://example.com
ss -tulnp | grep 80

4️⃣ Firewall Blocking (Firewalld / UFW)

After installing a service, ports might still be blocked:

# CentOS / Rocky Linux
sudo firewall-cmd --permanent --add-port=80/tcp
sudo firewall-cmd --reload

# Ubuntu / Debian
sudo ufw allow 80/tcp
sudo ufw reload

5️⃣ Package Dependency Errors

sudo dnf clean all && sudo dnf makecache
sudo apt --fix-broken install

If still failing, inspect repository configurations or use dnf repoquery to find missing packages.

6️⃣ No Space Left on Device

Disk full errors can stop services and updates.

df -h
du -sh /* | sort -h

Clean logs with journalctl --vacuum-time=7d or remove old backups under /var/log.

7️⃣ Out of Memory (OOM)

Add swap space when memory is exhausted:

sudo fallocate -l 2G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

8️⃣ SSH Login Failed

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys
sudo systemctl restart sshd

Ensure PubkeyAuthentication yes is enabled in /etc/ssh/sshd_config.

9️⃣ Update or Repository Errors

For GPG key or repository issues:

sudo rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY-rockyofficial
sudo dnf update -y

For Ubuntu, verify mirror URLs in /etc/apt/sources.list.

🔟 Auto-start Issues After Reboot

sudo systemctl enable squid
sudo systemctl is-enabled squid
sudo systemctl list-dependencies multi-user.target

Define startup order using After=network.target if needed.

📘 Conclusion

Linux is stable yet highly configurable — every error message is a clue. Mastering tools like systemctl, journalctl, firewall-cmd, and ss/netstat will help you troubleshoot faster and bring systems back online efficiently.

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🔗 Related Articles

• 🐧🧱 Linux Proxy Server Setup (Squid Installation & Configuration)
• 🐧 Linux systemd Deep Dive & Boot Process Management
• 🔥 Linux Firewall + Fail2Ban Security Implementation

— WWFandy・System & Network Notes