Shell Introduction

What is Shell?

Shell is the interface between the user and the Linux kernel. It receives commands entered by the user, interprets them, and passes them to the kernel for execution, then returns the results to the user.

┌──────────────────────────────────────────────────────────┐
│                        User                            │
│                         │                             │
│                         ▼                             │
│  ┌─────────────────────────────────────────────────┐    │
│  │                    Shell                         │    │
│  │   • Command interpreter                      │    │
│  │   • Script programming environment            │    │
│  │   • User interaction interface                │    │
│  └─────────────────────────────────────────────────┘    │
│                         │                             │
│                         ▼                             │
│  ┌─────────────────────────────────────────────────┐    │
│  │                  Linux Kernel                   │    │
│  │   • Process management                     │    │
│  │   • Memory management                      │    │
│  │   • File system                           │    │
│  │   • Device drivers                        │    │
│  └─────────────────────────────────────────────────┘    │
│                         │                             │
│                         ▼                             │
│                      Hardware                             │
└──────────────────────────────────────────────────────────┘

Shell Functions

1. Command Interpretation

Shell reads and executes user-entered commands:

bash

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

2. Script Programming

Shell is a script programming language that can write automation scripts:

bash

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

3. Environment Management

Shell manages environment variables and working environment:

bash

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

4. Input/Output Redirection

Shell can redirect command input and output:

bash

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

5. Job Control

Shell can manage background processes:

bash

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

Common Shells

Bash (Bourne Again Shell)

The most popular Shell, the default Shell for most Linux distributions.

Features

Part of the GNU project
Compatible with Bourne Shell (sh)
Rich features: command completion, history, aliases
Powerful scripting capabilities

bash

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

Zsh (Z Shell)

A more feature-rich Shell, the default Shell for macOS.

Features

Powerful auto-completion
Theme and plugin support (Oh My Zsh)
Spell correction
Shared history

bash

# Install Zsh
$ sudo apt install zsh

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

Fish (Friendly Interactive Shell)

A user-friendly modern Shell.

Features

Out-of-the-box syntax highlighting
Intelligent auto-completion
Web-based configuration
Not POSIX compatible

bash

# Install Fish
$ sudo apt install fish

# Run Fish
$ fish

Other Shells

Shell	Description
sh (Bourne Shell)	The earliest Unix Shell
csh (C Shell)	C-like syntax
tcsh	Enhanced version of csh
ksh (Korn Shell)	Commonly used in commercial Unix
dash	Default Shell for Debian system scripts

Viewing and Switching Shells

View Current Shell

bash

# 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

View Available Shells

bash

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

Temporary Shell Switch

bash

# Start Zsh
$ zsh

# Start Fish
$ fish

# Exit back to original Shell
$ exit

Change Default Shell

bash

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

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

Bash Configuration Files

Bash uses multiple configuration files that are loaded in different scenarios.

Configuration File Overview

┌─────────────────────────────────────────────────────────────┐
│                    Shell Startup Type                     │
├─────────────────────────────────────────────────────────────┤
│                                                             │
│   Login Shell                      Non-Login Shell        │
│   (Login Shell)                   (Non-login Shell)       │
│        │                                │               │
│        ▼                                ▼               │
│   /etc/profile                    /etc/bash.bashrc          │
│        │                                │               │
│        ▼                                ▼               │
│   ~/.bash_profile                 ~/.bashrc               │
│   or ~/.bash_login                                          │
│   or ~/.profile                                             │
│                                                             │
└─────────────────────────────────────────────────────────────┘

Login Shell

Starts when user logs into the system (TTY login, SSH login)
Executes /etc/profile and ~/.bash_profile

Non-Login Shell

Starts when opening a terminal emulator
Executes /etc/bash.bashrc and ~/.bashrc

bash

# 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

Main Configuration Files

bash

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

bash

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

~/.bashrc (User-level, each startup)

bash

# 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\$ '

~/.bash_logout (at exit)

bash

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

Loading Configuration Files

bash

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

# Reload .profile
$ source ~/.profile

Shell Prompt

PS1 - Main Prompt

bash

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

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

Special Characters

Character	Meaning
`\u`	Username
`\h`	Hostname
`\H`	Full hostname
`\w`	Current directory
`\W`	Current directory name (without path)
`\d`	Date
`\t`	Time (24-hour)
`\T`	Time (12-hour)
`\n`	Newline
`\$`	Prompt (regular user $, root #)
`\[...\]`	Non-printing character wrapper (for colors)

Colored Prompt

bash

# 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\[\e[0m\]\$ '

Common Color Codes

Code	Color
30	Black
31	Red
32	Green
33	Yellow
34	Blue
35	Magenta
36	Cyan
37	White

PS2 - Continuation Prompt

bash

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

PS3 and PS4

bash

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

Bash Features

Command History

bash

# 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

Command Completion

bash

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

# Install enhanced completion
$ sudo apt install bash-completion

Aliases

bash

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

# View aliases
$ alias

# Delete alias
$ unalias ll

# Temporarily skip alias
$ \ls
$ command ls

Command Types

bash

# 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

Oh My Bash

Oh My Bash is a Bash configuration framework that provides themes and plugins.

Installation

bash

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

Configuration

Edit ~/.bashrc:

bash

# Theme
OSH_THEME="powerline"

# Plugins
plugins=(git bashmarks)

Summary

This chapter introduced the basic concepts of Shell:

Shell's role: Command interpretation, script programming, environment management
Common Shells: Bash, Zsh, Fish
Configuration files: .bashrc, .bash_profile, etc.
Prompt customization: PS1 and color settings
Bash features: History, completion, aliases

Shell is a core tool in Linux; understanding it deeply will greatly improve your work efficiency. Next, we'll learn about input/output redirection.

Previous chapter: File Search

Next chapter: Input/Output Redirection

Shell Introduction ​

What is Shell? ​

Shell Functions ​

1. Command Interpretation ​

2. Script Programming ​

3. Environment Management ​

4. Input/Output Redirection ​

5. Job Control ​

Common Shells ​

Bash (Bourne Again Shell) ​

Zsh (Z Shell) ​

Fish (Friendly Interactive Shell) ​

Other Shells ​

Viewing and Switching Shells ​

View Current Shell ​

View Available Shells ​

Temporary Shell Switch ​

Change Default Shell ​

Bash Configuration Files ​

Configuration File Overview ​

Login Shell vs Non-Login Shell ​

Main Configuration Files ​

/etc/profile (System-level, at login) ​

~/.bash_profile (User-level, at login) ​

~/.bashrc (User-level, each startup) ​

~/.bash_logout (at exit) ​

Loading Configuration Files ​

Shell Prompt ​

PS1 - Main Prompt ​

Special Characters ​

Colored Prompt ​

PS2 - Continuation Prompt ​

PS3 and PS4 ​

Bash Features ​

Command History ​

Command Completion ​

Aliases ​

Command Types ​

Oh My Bash ​

Installation ​

Configuration ​

Summary ​

Shell Introduction

What is Shell?

Shell Functions

1. Command Interpretation

2. Script Programming

3. Environment Management

4. Input/Output Redirection

5. Job Control

Common Shells

Bash (Bourne Again Shell)

Zsh (Z Shell)

Fish (Friendly Interactive Shell)

Other Shells

Viewing and Switching Shells

View Current Shell

View Available Shells

Temporary Shell Switch

Change Default Shell

Bash Configuration Files

Configuration File Overview

Login Shell vs Non-Login Shell

Main Configuration Files

/etc/profile (System-level, at login)

~/.bash_profile (User-level, at login)

~/.bashrc (User-level, each startup)

~/.bash_logout (at exit)

Loading Configuration Files

Shell Prompt

PS1 - Main Prompt

Special Characters

Colored Prompt

PS2 - Continuation Prompt

PS3 and PS4

Bash Features

Command History

Command Completion

Aliases

Command Types

Oh My Bash

Installation

Configuration

Summary