Linux Basic Guide (Ubuntu/Kali)

· · tutorial linux

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}