# 1. 🐧 Linux Fundamentals for DevOps Engineers ### 🔹 Why Linux is Important for DevOps? Linux is the backbone of: * Cloud servers * Containers * CI/CD runners * Kubernetes nodes --- ## 🔹 Process vs Thread * **Process**: Independent execution unit with its own memory * **Thread**: Lightweight execution unit sharing process memory 👉 Example: One Java process running multiple threads. --- ## 🔹 Check CPU, Memory & Disk Usage ```bash top htop free -m df -h ``` --- ## 🔹 Find High Memory Consuming Process ```bash ps aux --sort=-%mem | head ``` --- ## 🔹 System Monitoring Commands Explained | Command | Use | | --- | --- | | top | Real-time stats | | vmstat | CPU, memory & IO | | free | RAM usage | | df | Disk usage | --- ## 🔹 Check and Kill a Process ```bash ps -ef | grep java kill -9 ``` --- ## 🔹 Find Which Service Uses a Port ```bash netstat -tulnp | grep 8080 ss -tulnp lsof -i :8080 ``` --- ## 🔹 Linux File Permissions ```bash chmod 755 script.sh chown user:group script.sh ``` * r=4, w=2, x=1 --- ## 🔹 Linux Logs ```bash tail -f /var/log/syslog journalctl -u docker journalctl -xe ``` --- ## 🔹 Soft Link vs Hard Link ```bash ln -s file softlink ln file hardlink ``` | Soft | Hard | | --- | --- | | Cross FS | Same FS | | Breaks | Safe | --- ## 🔹 Troubleshooting Tips * High CPU → `top`, `htop` * Low Disk → `du -sh`, `df -h` --- ## 🔹 Uptime & Reboot History ```bash uptime last reboot ``` --- ## 🔹 Scheduling Jobs ```bash crontab -e 0 2 * * * backup.sh ``` --- ## 🔹 find, grep, awk, sed ```bash find /var -name "*.log" grep ERROR app.log awk '{print $1}' file.txt sed 's/dev/prod/g' file.txt ``` --- ## 🎯 Key Takeaways * Linux is non-negotiable for DevOps * Monitoring & troubleshooting are daily tasks * These commands are **interview favorites**