# 30 Essential Commands in Linux: Quick Guide for SREs

### 1\. System Observability & Performance

| **Command** | **Purpose** | **Real-World SRE Example** |
| --- | --- | --- |
| `top` / `htop` | Process monitoring. | `htop` (to visually identify which CPU core is pinned at 100%). |
| `uptime` | System load average. | `uptime` (to check if the load average exceeds the number of CPU cores). |
| `vmstat` | Memory & CPU stats. | `vmstat 1 5` (report every second to detect context switching or swapping). |
| `iostat` | Disk I/O utilization. | `iostat -xz 1` (monitor extended disk latency in real-time). |
| `free -h` | Available RAM & Swap. | `free -h` (quick check before deploying a high-memory container). |
| `sar` | Historical performance. | `sar -u -f /var/log/sa/sa10` (analyze CPU usage from the 10th of the month). |

### 2\. Networking & Connectivity

| **Command** | **Purpose** | **Real-World SRE Example** |
| --- | --- | --- |
| `ip addr` | IP configuration. | `ip addr show eth0` (validate the IP assigned to a specific interface). |
| `ss -tulpn` | Open ports & sockets. | `ss -tulpn` |
| `dig` | DNS lookups. | `dig +short` [`blog.uptodeploy.com`](http://blog.uptodeploy.com) (get only the A record IP for a domain). |
| `curl -Iv` | HTTP(S) debugging. | `curl -Iv` [`https://google.com`](https://google.com) (inspect TLS handshake and headers). |
| `traceroute` | Network path routing. | `traceroute -I 8.8.8.8` (use ICMP to find where the packet drop occurs). |
| `tcpdump` | Packet capturing. | `tcpdump -i eth0 port 443` (sniffing HTTPS traffic for deep debugging). |

### 3\. File System & Logs

| **Command** | **Purpose** | **Real-World SRE Example** |
| --- | --- | --- |
| `df -h` | Disk space usage. | `df -h /` (check if the root partition is at 100% capacity). |
| `du -sh *` | Directory size. | `du -sh /var/log/*` (find which log file is consuming the most space). |
| `lsof` | List open files. | `lsof -i :22` (see active users connected via SSH). |
| `tail -f` | Real-time log follow. | `tail -f /var/log/syslog` (monitor system events as they happen). |
| `grep -r` | Pattern searching. | `grep -r "error" /var/log/nginx/` (search for errors across all Nginx logs). |
| `find` | Locate files. | `find /etc -name "*.conf"` (locate all configuration files in /etc). |

### 4\. Process Management & Security

| **Command** | **Purpose** | **Real-World SRE Example** |
| --- | --- | --- |
| `ps aux` | List active processes. | `ps aux --sort=-%mem` (list processes by highest RAM consumption). |
| `kill -9` | Force termination. | `kill -9 1234` (kill a zombie or hung process with PID 1234). |
| `systemctl` | Manage services. | `systemctl restart docker` (restart the Docker daemon). |
| `journalctl -xe` | Systemd logs. | `journalctl -u nginx.service -f` (follow logs for a specific service). |
| `sudo` | Superuser privileges. | `sudo visudo` (safely edit the sudoers file to manage permissions). |
| `chmod` / `chown` | Permissions & Ownership. | `chown -R www-data:www-data /var/www/html` (fix web server permissions). |

### 5\. SRE Power Tools

| **Command** | **Purpose** | **Real-World SRE Example** |
| --- | --- | --- |
| `strace` | Trace system calls. | `strace -p 1234` (debug why a process is stuck or failing). |
| `dmesg -T` | Kernel ring buffer. | `dmesg -T` |
| `awk` | Text processing. | `awk '{print $1}' access.log` (extract only the IPs from an access log). |
| `rsync` | Efficient file sync. | `rsync -avz ./data/ remote:/backup/` (sync data with compression and delta). |
| `openssl` | SSL/TLS management. | `openssl x509 -in cert.crt -text -noout` (check certificate expiration/details). |