Command Line Interface

In the beginning… was the command line.

Neal Stephenson (1999)

What is a Command Interpreter?

A command interpreter, also known as a Terminal, is the most basic interface between the user and the operating system and programs. It’s a program that translates commands entered by the user (typically via keyboard) using a set of commands that must be known by both the system and the user.

The user must enter commands with different parameters, and when pressing Enter, the operating system interprets the character sequence and if there are no errors, executes it.

This enables interactive communication:

User → Command → Operating System → User → Command ...

What is a Script?

A script is a file that includes a series of commands that will be executed by a command interpreter. The word script means “screenplay,” and each script can be interpreted by different actors (interpreters). In this module, we’ll work extensively with:

PowerShell (Windows)
BASH (GNU/Linux)

Creating Scripts

A script is created using a plain text editor:

Windows: Notepad.exe or PowerShell ISE
GNU/Linux: nano, vim, or other text editors

Script Structure

First Line (Shebang)

On UNIX systems, the first line contains the path to the interpreter:

#!/bin/bash
echo "Hello ASIR!"
exit 0

For PowerShell:

#!/usr/bin/env pwsh
Write-Host "Hello ASIR from PowerShell"

Script Body

Commands are executed from top to bottom:

#!/bin/bash

echo "1"
echo "2"
echo "3"

exit 0

Script Termination

It’s good practice to end with exit 0 to return a success code:

exit 0

PowerShell

PowerShell is Microsoft’s modern command interpreter designed for system administrators.

Key Features

Object-oriented: Everything is an object, not just text strings
Uses .NET Framework: Rich programming capabilities
Open Source: MIT license since version 6
Cross-platform: Available on Linux and macOS

cmdlets

Basic commands in PowerShell follow the <Verb>-<Noun> pattern:

Verb	Action
Enable	Enable the object
Disable	Disable the object
Install	Install the object
Uninstall	Uninstall the object
Move	Move the object
New	Create information
Get	Obtain information
Set	Modify information
Test	Test information
Remove	Delete information

Parameters

Get-ChildItem -Name w*

Returns directories and files starting with ‘w’.

Pipelines

One of the most powerful concepts - pass output from one command as input to another:

Get-Process | sort -Descending id

Help System

get-help command              # Standard help
get-help command -detailed    # Detailed help with examples
get-help command -full        # Complete help with technical notes

Execution Policy

To enable script execution:

Set-ExecutionPolicy -ExecutionPolicy Unrestricted

Run PowerShell as administrator to change execution policy.

BASH Shell in GNU/Linux

When opening a terminal in Linux, you see a shell prompt:

violin@xubuntu:~$

This tells us:

User: violin
Hostname: xubuntu
Current directory: ~ (home)
User type: $ (regular user, # for root)

BASH Features

Auto-completion

Press TAB to auto-complete commands, filenames, or variables. Press TAB twice to see all possibilities.

Command History

Use arrow keys to navigate through previously executed commands (up to 1000 by default).

Control Structures

Use if, for, while, select, case statements for powerful scripting.

Functions and Aliases

Define custom functions and create command aliases for frequently used operations.

Process Management

What is a Process?

According to UPV’s Operating Systems department: “A process is a program in execution.” From the OS perspective, a process is an object that must be managed and serviced.

Process Lifecycle

When a program executes:

OS checks permissions
Allocates RAM space
Assigns CPU time (priority)
Process accesses instructions and performs tasks

As administrators, we need to know how to obtain process information for monitoring, security audits, network usage analysis, etc.

Processes in Windows

Task Manager

Access via:

Ctrl+Alt+Del
Right-click Start button → Task Manager

In VirtualBox: Use Right Ctrl+Del or the VirtualBox menu.

Key Tabs

Processes

Shows applications and background processes
Resource usage: CPU, Memory, Disk, Network

Performance

System performance graphs
Uptime
Active processes count
CPU and memory statistics

Startup

Programs that launch at startup
Impact on boot time
Good place to check for malware

Details

Column	Meaning
Name	Executable name
PID	Process identifier
Status	Process status
User name	User who launched the process
CPU	CPU usage percentage
Memory	RAM usage
Description	Executable description

Services

Running system services
Background processes (Windows Defender, Print Spooler, etc.)

Other Tools

Process Explorer: Advanced process monitoring
PowerShell: Command-line process management

Processes in GNU/Linux

Command: ps

Shows active processes:

ps -e

Output:

  PID TTY          TIME CMD
?        00:00:02 systemd
?        00:00:00 kthreadd
?        00:00:01 xfce4-terminal
pts/0    00:00:00 bash

Filter with grep:

ps -e | grep firefox
ps -e | grep mousepad

Command: pstree

Shows process tree:

ps -e | grep xfce4-panel
pstree 1907

Command: top

Real-time process monitoring:

top

Shows:

CPU usage
Memory available/consumed
Uptime
Process details

Command: kill

Send signals to processes:

kill -L  # List all signals

Terminate a process:

ps -e | grep mousepad
kill -9 23895  # SIGKILL

Command: killall

Stop all instances by name:

killall firefox

Command: nice

Set process priority (-20 to 19):

nice          # Show current niceness
nice 10 bash  # Run bash with niceness 10

-20: Highest priority
0: Default priority
19: Lowest priority

Network Configuration

MAC Address

Physical address identifying network adapter (48 bits):

74:e6:e2:35:6a:a1

MAC addresses can be spoofed, which can be a security vulnerability.

IP Address (IPv4)

Logical and hierarchical identifier for network interfaces.

Static IP

Manual configuration requires:

Address: IP address
Subnet Mask: Network mask
Gateway: Default gateway
DNS Servers: Name resolution

Dynamic IP (DHCP)

Automatic configuration via DHCP server.

Network Configuration in Windows

Access via:

Control Panel → Network and Sharing Center
Network adapter properties

Connection Details

IPv4 Connectivity: IPv4 network connection status
IPv6 Connectivity: IPv6 network connection status
Media State: Device enabled status
Duration: Time device has been enabled
Speed: Transfer rate

Activity Monitoring

Bytes sent/received
Useful indicator of connection health

Workgroups and Domains

Domain

Group of computers sharing database and security policy
Centrally managed
Requires password/credentials for access

Workgroup

Peer-to-peer network
No central control
Each computer has own user accounts
Typically ≤20 computers

Network Configuration in GNU/Linux

netplan (Ubuntu 18.04+)

Configuration files in /etc/netplan/*.yaml

# /etc/netplan/01-netcfg.yaml
network:
    version: 2
    renderer: networkd
    ethernets:
        enp0s3:
            dhcp4: yes

Respect indentation in YAML files - incorrect spacing causes configuration errors.

netplan Commands

netplan generate  # Generate configuration
netplan apply     # Apply configuration

Network Commands

tracepath

tracepath lliurex.net

ping

ping www.google.es
ping -c 5 www.google.es  # Send 5 packets
ping -i 2 www.google.es  # Wait 2 seconds between packets

ifconfig (legacy)

ifconfig                    # Show all interfaces
ifconfig eth0               # Show specific interface
ifconfig eth0 up            # Enable interface
ifconfig eth0 192.168.1.10  # Set IP address

ip (modern)

ip addr                          # Show all addresses
ip -4 addr                       # Show IPv4 only
ip address add 192.168.4.177 dev enp0s3
ip link set enp0s3 up           # Enable interface
ip route                         # Show routing table
ip route add 192.168.3.0/24 dev enp0s3

hostname

hostname                    # Show hostname
hostname -F /etc/hostname   # Set hostname from file

Be careful when changing hostname - programs like sudo may rely on it.

UD01 - Operating Systems Introduction

UD02 - Virtualization

UD03 - Installation & Boot

UD04 - Processes & Networking

UD05 - Users, Groups & Permissions

UD06 - File Systems

UD07 - Software & Updates

UD08 - Security

UD09 - Domain Administration

UD10 - Domain Access

UD11 - Performance & Auditing

UD12 - Incident Management

​Command Line Interface

​What is a Command Interpreter?

​What is a Script?

​Creating Scripts

​Script Structure

​PowerShell

​Key Features

​cmdlets

​Parameters

​Pipelines

​Help System

​Execution Policy

​BASH Shell in GNU/Linux

​BASH Features

​Process Management

​What is a Process?

​Process Lifecycle

​Processes in Windows

​Task Manager

​Key Tabs

​Other Tools

​Processes in GNU/Linux

​Command: ps

​Command: pstree

​Command: top

​Command: kill

​Command: killall

​Command: nice

​Network Configuration

​MAC Address

​IP Address (IPv4)

​Static IP

​Dynamic IP (DHCP)

​Network Configuration in Windows

​Connection Details

​Activity Monitoring

​Workgroups and Domains

​Network Configuration in GNU/Linux

​netplan (Ubuntu 18.04+)

​netplan Commands

​Network Commands

Build docs developers (and LLMs) love

Command Line Interface

What is a Command Interpreter?

What is a Script?

Creating Scripts

Script Structure

PowerShell

Key Features

cmdlets

Parameters

Pipelines

Help System

Execution Policy

BASH Shell in GNU/Linux

BASH Features

Process Management

What is a Process?

Process Lifecycle

Processes in Windows

Task Manager

Key Tabs

Other Tools

Processes in GNU/Linux

Command: ps

Command: pstree

Command: top

Command: kill

Command: killall

Command: nice

Network Configuration

MAC Address

IP Address (IPv4)

Static IP

Dynamic IP (DHCP)

Network Configuration in Windows

Connection Details

Activity Monitoring

Workgroups and Domains

Network Configuration in GNU/Linux

netplan (Ubuntu 18.04+)

netplan Commands

Network Commands