#Shell Introduction

$ ls -la /home
# Shell parses this command, calls the ls program, and passes the arguments -la and /home

#!/bin/bash
for file in *.txt; do
    echo "Processing file: $file"
done

$ export PATH=$PATH:/new/path
$ echo $HOME

$ command > output.txt     # Output redirection
$ command < input.txt      # Input redirection
$ command1 | command2      # Pipe

$ command &                # Run in background
$ jobs                     # View jobs
$ fg                       # Bring to foreground

# Check if using Bash
$ echo $BASH_VERSION
5.1.16(1)-release

# Install Zsh
$ sudo apt install zsh

# Set as default Shell
$ chsh -s /bin/zsh

# Install Fish
$ sudo apt install fish

# Run Fish
$ fish

# Method 1: View $SHELL variable
$ echo $SHELL
/bin/bash

# Method 2: View current process
$ ps -p $$
  PID TTY          TIME CMD
 1234 pts/0    00:00:00 bash

# Method 3: View $0
$ echo $0
-bash

$ cat /etc/shells
/bin/sh
/bin/bash
/bin/zsh
/usr/bin/zsh
/bin/fish

# Start Zsh
$ zsh

# Start Fish
$ fish

# Exit back to original Shell
$ exit

# Use chsh command
$ chsh -s /bin/zsh

# Or edit /etc/passwd (requires root)
$ sudo usermod -s /bin/zsh username

# Check if it's a login Shell
$ shopt -q login_shell && echo "Login Shell" || echo "Non-login Shell"

# Force login Shell
$ bash --login
$ bash -l

# System-wide environment variable settings
# Executed when all users log in

# User's login configuration
# Usually contains:
if [ -f ~/.bashrc ]; then
    . ~/.bashrc
fi

# User's Shell configuration
# Aliases, functions, prompts, etc.

# Aliases
alias ll='ls -la'
alias grep='grep --color=auto'

# Environment variables
export EDITOR=vim

# Custom functions
mkcd() {
    mkdir -p "$1" && cd "$1"
}

# Prompt
PS1='\u@\h:\w\$ '

# Commands to execute on logout
# For example, cleaning temporary files

# Reload .bashrc
$ source ~/.bashrc
$ . ~/.bashrc

# Reload .profile
$ source ~/.profile

# View current PS1
$ echo $PS1
\u@\h:\w\$

# Customize prompt
$ PS1="[\t] \u@\h:\w\$ "
[10:30:00] maxwell@ubuntu:~$

# Color codes
# \e[color code]m start, \e[0m end

# Example: Green username, blue path
PS1='\[\e[32m\]\u@\h\[\e[0m\]:\[\e[34m\]\w\[\e[0m\]\$ '

# Displayed when command is incomplete
$ echo "hello
> world"

# PS3 - Prompt for select statement
# PS4 - Prompt for debug mode

# View history
$ history

# History configuration
HISTSIZE=1000           # Number of commands saved in memory
HISTFILESIZE=2000       # Number of commands saved in history file
HISTCONTROL=ignoreboth  # Ignore duplicate and space-prefixed commands
HISTIGNORE="ls:cd:pwd"  # Ignore specific commands

# Tab completion for files and commands
$ cd Doc<Tab>
$ cd Documents/

# Install enhanced completion
$ sudo apt install bash-completion

# Define alias
$ alias ll='ls -la'

# View aliases
$ alias

# Delete alias
$ unalias ll

# Temporarily skip alias
$ \ls
$ command ls

# Built-in commands: Commands built into Shell
$ type cd
cd is a shell builtin

# External commands: Independent executable files
$ type ls
ls is /usr/bin/ls

# Aliases
$ type ll
ll is aliased to 'ls -la'

# Functions
$ type mkcd
mkcd is a function

$ bash -c "$(curl -fsSL https://raw.githubusercontent.com/ohmybash/oh-my-bash/master/tools/install.sh)"

# Theme
OSH_THEME="powerline"

# Plugins
plugins=(git bashmarks)