Q: What should I do when a server runs out of disk space?

First, identify what's consuming space: du -sh /* --exclude=proc | sort -h. Common culprits: /var/log (rotate with logrotate), /var/cache (clean with apt clean or yum clean all), /tmp (clear old temp files), and Docker (/var/lib/docker — run docker system prune). For immediate relief, find and remove large files. Long-term, set up log rotation, disk usage monitoring with alerts at 85%, and automatic cleanup cron jobs.

Q: Why does my cron job run manually but not from crontab?

Cron runs with a minimal environment — it doesn't source your .bashrc or .profile. The most common fix is to specify full paths to all commands in the crontab entry, or set PATH at the top of the crontab. Other causes: the crontab has wrong syntax (use crontab -e to validate), the cron service isn't running (systemctl status crond/cron), output is being discarded (redirect stderr: 2>&1), or the script has Windows line endings (fix with dos2unix).

Question 1

How do I find out why a systemd service failed to start?

Accepted Answer

Run systemctl status to see the last few log lines and the exit code. For full details, run journalctl -xeu to see the complete journal output with explanations. Most services also have a config test command (nginx -t, apachectl configtest, named-checkconf) that catches syntax errors before you try to start. Check for port conflicts with ss -tlnp and permission issues with ls -la on the service's config and data files.

Question 2

How do I troubleshoot SELinux denials without disabling SELinux?

Accepted Answer

Check /var/log/audit/audit.log for 'avc: denied' entries. Run audit2why < /var/log/audit/audit.log to get human-readable explanations. Most issues are fixed by setting the correct file context: use semanage fcontext -a -t '' followed by restorecon -Rv . For services accessing non-standard ports, use semanage port. For boolean-controlled features, use setsebool -P. Only as a last resort, generate a custom policy module with audit2allow.

Question 3

What should I do when a server runs out of disk space?

Accepted Answer

First, identify what's consuming space: du -sh /* --exclude=proc | sort -h. Common culprits: /var/log (rotate with logrotate), /var/cache (clean with apt clean or yum clean all), /tmp (clear old temp files), and Docker (/var/lib/docker — run docker system prune). For immediate relief, find and remove large files. Long-term, set up log rotation, disk usage monitoring with alerts at 85%, and automatic cleanup cron jobs.

Question 4

Why does my cron job run manually but not from crontab?

Accepted Answer

Cron runs with a minimal environment — it doesn't source your .bashrc or .profile. The most common fix is to specify full paths to all commands in the crontab entry, or set PATH at the top of the crontab. Other causes: the crontab has wrong syntax (use crontab -e to validate), the cron service isn't running (systemctl status crond/cron), output is being discarded (redirect stderr: 2>&1), or the script has Windows line endings (fix with dos2unix).

Question 5

How do I diagnose high CPU usage or load average on Linux?

Accepted Answer

Run top or htop to identify which process is consuming CPU. Check the load average (uptime) — values above your CPU core count indicate saturation. If a specific process is consuming 100% CPU, it might be stuck in a loop — check its logs and consider strace -p <pid> to see what it's doing. If load is high but CPU usage is moderate, the bottleneck is likely I/O — check iostat and iotop for disk-bound processes. For sudden spikes, check for fork bombs or runaway child processes.

Question 6

How do I fix SSH connection issues?

Accepted Answer

For 'connection refused': verify sshd is running (systemctl status sshd) and check which port it's on (grep Port /etc/ssh/sshd_config). For 'connection timed out': check firewalls (iptables -L, firewall-cmd --list-all) and cloud security groups. For 'permission denied (publickey)': ensure your public key is in ~/.ssh/authorized_keys on the server, with permissions 600 on the file and 700 on the .ssh directory. Use ssh -vvv to see verbose connection debugging output.

Question 7

How do I manage and troubleshoot Docker containers on Linux?

Accepted Answer

Check container status with docker ps -a. View logs with docker logs . For containers that won't start, check the exit code (docker inspect | grep ExitCode) and logs. Common issues: port conflicts (another service using the port), volume mount permission problems (the container user can't write to the host directory), and out-of-disk errors in /var/lib/docker. Use docker system df to check Docker's disk usage and docker system prune to clean up unused resources.

Symptom	Likely Cause	First Step
Service failed to start	Config syntax error or port conflict	Run config test (nginx -t, apachectl configtest); check journalctl -xeu <service>
Permission denied (not SELinux)	Wrong file ownership or mode	Check ls -la; fix with chown/chmod; verify parent directory permissions
Permission denied (SELinux)	Wrong SELinux context on files	Check audit.log for 'avc: denied'; fix with semanage fcontext + restorecon
No space left on device	Disk full or inode exhaustion	Check df -h and df -i; clean logs, caches, old packages; extend volume if possible
SSH connection refused	sshd not running or wrong port	Verify sshd status; check /etc/ssh/sshd_config for Port; check firewall
SSH permission denied (publickey)	Key not in authorized_keys or wrong permissions	Check ~/.ssh/authorized_keys; ensure 600 on key files, 700 on .ssh dir
OOM killer terminated process	Server out of memory	Check dmesg for OOM messages; reduce service memory usage or add swap/RAM
Cron job not running	Bad crontab syntax, wrong PATH, or service disabled	Check crontab -l; verify PATH in crontab; check /var/log/cron or journalctl
NFS mount hanging or timing out	Firewall blocking NFS ports or server unreachable	Check NFS server status; verify firewall allows ports 111, 2049; test with showmount
High CPU / load average	Runaway process or resource contention	Run top/htop; identify the process; check for infinite loops or fork bombs

Linux System Administration Errors: Complete Troubleshooting Guide

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