Linux Commands - 1

gistfile1.txt

# Linux Commands Cheat Sheet

| **Category**         | **Command**      | **Description**                                      | **Example**                                   |
|-----------------------|------------------|------------------------------------------------------|-----------------------------------------------|
| **Process Monitoring**| `top`            | Display real-time CPU and memory usage of processes. | `top`                                         |
|                       | `htop`           | Interactive process viewer.                         | `htop`                                        |
| **System Metrics**    | `vmstat`         | Report system performance (CPU, memory, I/O, etc.). | `vmstat 5 10` (Updates every 5 seconds, 10 times) |
|                       | `iostat`         | Show CPU and disk I/O statistics.                   | `iostat -x 1` (Extended stats every second)   |
| **Logs & Kernel**     | `dmesg`          | Display boot and kernel messages.                   | `dmesg | tail -n 20` (Last 20 messages)       |
|                       | `journalctl`     | Query and display system logs.                      | `journalctl -xe` (View recent errors)         |
| **Disk Metrics**      | `df`             | Show disk usage for all filesystems.                | `df -h` (Human-readable format)               |
|                       | `du`             | Estimate file/directory space usage.                | `du -sh /var/log` (Summarized for directory)  |
| **I/O Analysis**      | `iotop`          | Show real-time I/O usage by processes.              | `sudo iotop`                                  |
|                       | `lsblk`          | List information about block devices.               | `lsblk -f`                                    |
|                       | `blkid`          | Show block device attributes (UUID, type, etc.).    | `sudo blkid`                                  |
| **Networking**        | `traceroute`     | Display route packets take to reach a host.         | `traceroute example.com`                      |
|                       | `ping`           | Check connectivity to a host.                       | `ping -c 4 google.com` (Send 4 packets)       |
|                       | `netstat`        | Show active connections and listening ports.        | `netstat -tuln`                               |
|                       | `ss`             | Modern replacement for `netstat`.                   | `ss -tuln` (TCP/UDP listening ports)          |
|                       | `tcpdump`        | Capture network packets for analysis.               | `sudo tcpdump -i eth0 port 80` (Capture HTTP traffic) |
|                       | `ip`             | Manage IP addresses and routing.                    | `ip addr show`                                |
| **Troubleshooting**   | `strace`         | Trace system calls and signals.                     | `strace -p <pid>`                             |
|                       | `lsof`           | List open files by processes.                       | `lsof -i :80` (Processes using port 80)       |
|                       | `ps`             | Show details of running processes.                  | `ps aux | grep nginx`                         |
| **File & Disk**       | `mount`          | Mount a filesystem.                                 | `mount /dev/sda1 /mnt`                        |
|                       | `umount`         | Unmount a filesystem.                               | `umount /mnt`                                 |
|                       | `fsck`           | Check and repair filesystem issues.                 | `sudo fsck /dev/sda1`                         |
|                       | `tune2fs`        | Adjust filesystem parameters.                       | `sudo tune2fs -l /dev/sda1`                   |
| **Packet Sniffing**   | `tcpdump`        | Capture and analyze packets on a network interface. | `sudo tcpdump -i eth0 port 443` (Capture HTTPS traffic) |
|                       | `wireshark`      | Graphical packet capture and analysis tool.         | `wireshark`                                   |
| **TCP/IP Stack**      | `sysctl`         | View and configure kernel parameters.               | `sysctl -a | grep tcp` (View TCP settings)    |
|                       | `iptables`       | Configure Linux firewall rules.                     | `iptables -L -v -n` (List rules)              |
|                       | `ping`           | Send ICMP echo requests to a host.                  | `ping -c 4 8.8.8.8`                           |
| **System Analysis**   | `sar`            | Collect and report system activity metrics.         | `sar -u 1 5` (CPU usage every 1 sec for 5 iterations) |
|                       | `uptime`         | Show system uptime and load averages.               | `uptime`                                      |
|                       | `free`           | Display memory usage.                               | `free -h` (Human-readable format)             |

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### **Notes:**
1. Commands requiring root permissions should be prefixed with `sudo`.
2. Use the `man` command for detailed documentation on each command, e.g., `man top`.
3. For `tcpdump` and similar tools, ensure you have proper permissions to capture packets.