Linux Basic Guide (Ubuntu/Kali)

Commands, concepts, and programs tutorial

Installing WSL

dism.exe /online /enable-feature /featurename:Microsoft-Windows-Subsystem-Linux /all /norestart

After restart:

wsl --install

After restart, go to Microsoft Store, pick whatever distribution you like & install it.

General knowledge

First word of any command is the program name, and anything that comes after it are the parameters, Examples:
- curl google.com: Program curl, Paramaters: google.com
- g++ file.cpp -o ahmad -O2: Program g++, Paramaters: file.cpp, -o, ahmad, -O2
Every program has its own way to handle parameters
Usually every program will response to --version and --help, but sometimes they won’t so you must read manual
Program help might show you some commands, mostly useless
Manual man {command}, will give you a full manual
If a program run infinitly, you can shut it down using Ctrl + C
Don’t enter vim without minimum 2 years of experience, because you won’t be able to get out of it.

Working Directory (WD)

To check the working directory use pwd
cd (Change Directory)

Usage: cd {PATH}
To get to parent folder use cd ..
To get to grandparent folder use cd ../..
To get to root cd /
To get to home cd ~
To get to a child directory cd ./child/childofchild
The concept here is about Relative and Static paths:
- Relative: describing a path based on the working directory
- Static: describing a path fully, it must start with / which indicates the root, like /usr/bin/g++

Root Permissions

Append sudo before any command to run as root
To login as root use sudo su -
Usually running with sudo solves any “Permission Denied” problem

Files/Directories

Related commands: touch, mv, cp, rm, mkdir, ls
Creating file: touch file.txt
Creating directory: mkdir dir
Moving file: mv file.txt /path/to/anothername.txt
Renaming file: mv file.txt another.txt
Removing file: rm file.txt
Removing directory: rm -d somedirectory
Removing directory and removing contents recursively: rm -d -r somedirectory
Listing all contents of WD: ls
Listing all contents of WD with extra details: ls -la
Copying a file: cp old_code.cpp new_code.cpp

Editing text files

To append to a file: echo "Added this text" >> file.txt
To overwrite a file: echo "Added this text" > file.txt
To write to a file: cat >file.txt and when you finish just interrupt it with Ctrl+C
To output a file: cat file.txt
To output first 5 lines of a file: head -n 5 file.txt
To edit a file: sudo nano file.txt

Running executables

About `$PATH`

There is an environment variable called PATH, you can see it by echo $PATH
This have a list of directory paths seperated by a colon (:)
If you run a command, without specifing a path for it (either static or relative) it will search on these directories by their order
If not found you will get command not found with error 127

Running

Running a local executable (in your directory): ./app
Running a global executable: app
Running a global executable that matchs some command in help list: \app

Multiple commands

mkdir child; cd child - Using ; it will run these two commands
mkdir child && cd child- Using && it will run the second command if the first have a zero exit code

File Input/Output pipe redirection

Every program have 3 pipes (file descriptors):
- 0: stdin
- 1: stdout
- 2: stderr
stderr/stdout will be shown togother in the same terminal
Redirecting stdin to a file: ./app < file.in
Redirecting stdout to a file: ./app 1> file.in
Redirecting stderr to a file: ./app 2> file.in
Run without output (redirect to empty): ./app >/dev/null
Read from random input: ./app </dev/urandom
Append to a file: ./app >> append.txt
./code | ./checker - Using | it will run first app and redirect its output to second app input

Environment Variables

The system has some variables with assigned values, so programs can use it

Checking environment variables: env
Writing an environment variable in current session: export VAR="value"
Writing a permanent environment variable: echo 'export VAR="value"' >> ~/.bashrc
Outputting an environment variable: echo $VAR
Outputting last command exit code: echo $?

Processes management

Every process have a Process ID (PID)
Check current processes: ps, for all use ps -a
Kill a process by PID: kill 7522, force kill kill -9 7522

File Permissions

When doing ls -la, here is an expected output:

drwxrwxrwx root root somedir
lrwxr-xr-- root root somelnk
-rw-rw-r-- user user somefile

First character: file type d=directory, l=link, -=file
Next 3 characters: Permissions of owner
Next 3 characters: Permissions of group
Next 3 characters: Permissions of others
Permissions rwx means read, write, execute.
After it the owner
After it the group
Changing owner: chown
Changing group: chgrp
Changing permissions: chmod

For more, check this page

About shells

There are multiple shell prgrams
Most common ones are bash, sh, zsh
Each one might have slight differences
To know which shell are you using: run echo $0

Shell execuatables

If you had a text file that is executable, it will run it based on first line of it
Usually it’s on this format #!/bin/bash, After #! is the static path of running this file
Using this you might write any interpreter programs as executables\
It could be #!/usr/bin/env python3 and the script is a Python code

Example

#!/bin/bash

g++ --version > script_temp.txt
head -n 1 script_temp.txt
rm script_temp.txt

This script will show you the first line of g++ --version output

Example 2

Helping Programs

time {COMMAND} - Measures execution time of a command
which {PROGRAM_NAME} - Shows you the path of the program
timeout {SECONDS} {COMMAND} - Exits with error code 127 in case it exceeds time
curl - Client for sending web requests

Package Manager

Almost every linux-based system has a package manager
The package manager installs programs/compilers/tools easily

Different Systems Package Manager

Debian: dpkg
Ubuntu: apt
Arch: pacman
Alpine: apk
MacOS: homebrew (Not installed by default in MacOS)

`apt`

Installing: sudo apt install {package}
Updating: sudo apt update
Upgrading sudo apt upgrade
Removing sudo apt remove {package}