Linux

Two main flavors in use - Debian-based (Ubuntu, Debian, Mint, etc) & RHEL-based (Red Hat, CentOS, Fedora etc)
- Debian uses apt-get to install packages whereas RHEL uses yum
- For Debian, apt-get update, apt-get upgrade and apt-get install <app> are all that’s needed
- /etc/network/interfaces used for permanent interface modification in Debian distros
- /etc/resolvconf/resolv.conf.d/ stores files for generating resolv.conf file in Debian distros, which determines DNS resolution
- To restart Debian networking after making changes, use sudo systemctl restart networking
Bash script files start with #!/bin/bash, called a shebang which points to the location of the bash interpreter
- Similarly, Python scripts would have a shebang of #!/usr/bin/env python3

Dynamic variables stored in the system and used by the shell and its applications
.env extension files typically used to store variables, like below:

#.env file
# IP Address
export HOST="192.168.1.10"

export is used to set environment variables, and set is used to view them
By storing credentials in environment variables and not including these variables in your git repo, you can prevent accidentally pushing credentials to git in a script

/bin - contains binaries, applications and programs such as ‘ls’
/boot - system boot files - do not modify
/dev - device files such as USB drives, webcams etc
/etc - mostly contains configuration files
/home - user personal directories
/lib - contains libraries, kernel modules, drivers
/media - mount point for external storage
/mnt - mostly legacy, for manually mounting storage partitions
/opt - compiled software lives here
/proc - contains files for the core operating system
/root - superuser home directory
/sbin - like bin but for programs only superusers can run
/usr - original home directory, now used for applications, documents, libraries and other files
/srv - configuration files for Linux servers
/sys - similar to /proc and /dev, contains information about connected devices
/tmp - temporary files
/var - mostly log files

ls, cd, pwd, mkdir, rmdir, rm, cp, mv, uptime - Basic commands
cd ~ will bring you to your home directory, cd / for the root directory
find <directory> -name <name> can be used to find files, accepts wildcards
grep can be used for searching within files
less can be used to view files
chmod - Used for modifying file permissions
echo will print a command to the terminal with stdout, can be redirected to a file with >
read can take user input from the shell, stdin
htop, ps aux - Different commands for displaying system processes, can use kill <pid> to stop process
df -h - Shows disk usage for each mounted filesystem
free -h - Displays memory usage
route - Displays route table
ifconfig - Displays interfaces & IP addresses
history - Displays past commands used - can also use CTRL+R to search history
set - Displays shell and environment variables
export - Set an environment variable
vi ~/.bashrc, source ~/.bashrc can be used to modify aliases - alias will show all existing aliases