Linux

On Linux, everything is a file.

• It's the OS that's in basically everything, so get used to it.
• It comes in a variety of distributions or "distros"
  • Don't get distracted with the distros
    • Most of them are built on a core version (like Debian or Arch) and have various quality-of-life or customization features added (like Ubuntu, Mint, SteamOS, or Kali.)
  • There are many Unix-like OS's, like Linux, macOS, and FreeBSD
    • They often function similarly, but will have different command structures for the same task, like app installation or network interface configuration
• Users and privileges
  • Becoming SU
    • sudo su -
      • This changes the terminal environment from running as user AS root, to actually BEING root

Resources if you get lost

WHEN IN DOUBT: You can always learn about a command with by running man <command>, like man pwd

• die.net
  • Various Linux manual pages
• explainshell.com
  • Copy and paste an entire line of shell commands to have each section broken down and identified clearly

File directories

Root Directory	Description	Notes
/boot	Boot loader files
/dev	Device files
/etc	System configuration files
/home	User home directories
/lib	Libraries and kernel modules
/media	Removeable media mount point	Should have just been /mnt, but HERE WE ARE
/mnt	Mount point for temporary mounted file systems
/opt	Add-in application software packages
/bin	User binaries
/sbin	System binaries
/srv	Data for services
/tmp	Temporary files
/usr	User utilities and applications
/var	Variable files
/root	Root user home directory
/proc	Virtual file system

Navigating Linux and directories

• cd
  • Change directory
  • Running without any changes will send you back to the home directory
  • cd /proc
    • /proc is the virtual file system, and contains information about running processes
• ls
  • Show any visible (non-hidden) files
  • ls -lrta
    • Shows hidden files
• mkdir
  • create directory
• locate [string]
  • It will search a file index to look for and list any locations
  • sudo update db
    • Update the index
• ps aux
  • List processes

Running Processes

1. ps aux
   1. Shows all processes
      1. a - all processes
      2. u - sorted by user
      3. x - include the processes using a teletype terminal
   2. How to Use the ps aux Command in Linux | Linode Docs
2. top
   1. Shows live processes

Networking and file management

1. ip a
   1. New version of ifconfig, shows network interface configuration and various stats
      1. If it's not installed, you can install the iproute2 package from apt
   2. ifconfig is no longer installed by default on newer versions of Linux, so you should get used to running ip a
      1. However, if necessary, you can install net-tools (sudo apt install net-tools) to get it back
2. netstat
   1. Displays active network connections
      1. a - all listening and non-listening (e.g. established) connections
         1. Listening sockets are waiting for a connection
         2. Established sockets have an active connection
      2. t - TCP connections
      3. u - UDP connections
3. lsof
   1. List open files
   2. lsof -i -P
      1. Shows all open files and their connected devices
      2. i - Shows all open network files
         1. You can also specify an IP address to narrow it down to a specific host
      3. P - Prohibits convert port numbers to names
         1. Can make lsof run faster,
   3. pipe, backpipe, what port it's listening to, etc.

Nmap

Linux Terminal History

1. history
   1. Shows a complete history of Bash commands
   2. Relatively easy to evade, add a space before the command
      1. ~$ ps aux
         2. Will appear in the history
      2. ~$ ps aux
         1. Will NOT appear in the history