Shell and Command Line Basics

The command line is the most powerful tool in a Linux system. Mastering basic command line operations enables you to efficiently manage your system, install software, and troubleshoot problems. This chapter will take you from zero to comfortable in Debian's command line world.

🖥️ What is the Command Line

The Difference Between Terminal and Shell

In Linux, "terminal" and "Shell" are two distinct concepts:

Concept	Description	Analogy
Terminal	A window program that displays a text interface	TV screen
Shell	A program that interprets and executes commands	TV show
CLI (Command Line Interface)	A way of operating the system by typing text	Remote control

Debian 13 uses Bash (Bourne Again Shell) as the default Shell.

Why Learn the Command Line

• Greater Efficiency — Batch file operations and task automation are far faster than using a graphical interface
• Remote Management — Servers typically have no graphical interface and can only be managed via the command line
• Troubleshooting — When the system has problems, the command line is often the only way to fix things
• Deeper Understanding — The command line helps you truly understand how the system works

💡 Beginner Tip

You don't need to memorize every command at once. Start by learning the 10 most commonly used commands, and look up the rest when you need them. Learning the command line is a gradual process.

🚀 Opening the Terminal

Opening from the Graphical Interface

Desktop Environment	How to Open	Default Terminal Program
GNOME	Activities → Search for "Terminal"	GNOME Terminal
KDE Plasma	Application Menu → System → Konsole	Konsole
Xfce	Application Menu → System → Xfce Terminal	Xfce Terminal
MATE	Applications → System Tools → Terminal	MATE Terminal

Keyboard Shortcut

Most desktop environments support a keyboard shortcut to open the terminal:

Ctrl + Alt + T

💡 Beginner Tip

Remember the Ctrl + Alt + T shortcut — it works in virtually every Linux desktop environment.

Getting Familiar with the Terminal Interface

After opening the terminal, you'll see a prompt that looks something like this:

user@debian:~$

Part	Meaning	Example
user	The currently logged-in username	ivmm, root
debian	The computer's hostname	mypc, server1
~	The current directory (~ represents the home directory)	/home/user
$	Regular user prompt (root uses #)	$ or #

📂 Filesystem Basics

Linux Filesystem Structure

The Linux filesystem is a tree structure rooted at / (the root directory), unlike Windows' C:, D: drives:

Most Important Directories:

Directory	Purpose	Will You Use It?
/home/yourusername/	Where your personal files are stored	✅ Frequently
/etc/	System and software configuration files	✅ Frequently
/var/log/	System log files	✅ When troubleshooting
/usr/bin/	User executable programs	Occasionally
/tmp/	Temporary files (cleared on reboot)	Occasionally
/boot/	System boot files	Rarely
/dev/	Hardware device files	Rarely

Absolute Paths vs Relative Paths

# Absolute path — the full path starting from the root directory /
/home/user/Documents/report.txt

# Relative path — a path relative to the current directory
Documents/report.txt        # Documents under the current directory
./Documents/report.txt      # Same as above, ./ means current directory
../Downloads/file.zip       # Downloads under the parent directory

Special Directory Symbols

Symbol	Meaning	Example
/	Root directory	cd /
~	Current user's home directory	cd ~ is the same as cd /home/user
.	Current directory	./script.sh
..	Parent directory	cd ..
-	Previous directory	cd -

🗂️ Basic File Operation Commands

Browsing Directories

# Display the current directory
pwd
# Output: /home/user

# List files in the current directory
ls

# List with detailed information (permissions, size, date)
ls -l

# Show hidden files (files starting with .)
ls -la

# Display file sizes in human-readable format
ls -lh

# Change to a specific directory
cd /home/user/Documents

# Go back to the home directory
cd ~
# Or simply
cd

# Go up to the parent directory
cd ..

# Go back to the previous directory
cd -

💡 Beginner Tip

ls -lah is the most commonly used combination — it shows detailed information for all files (including hidden files) with file sizes displayed in KB/MB/GB.

Creating and Deleting

# Create a directory
mkdir my-project

# Create nested directories (automatically creates parent directories if they don't exist)
mkdir -p projects/web/css

# Create an empty file
touch readme.txt

# Delete a file
rm readme.txt

# Delete a directory (must be empty)
rmdir my-project

# Delete a directory and all its contents (recursive deletion)
rm -r my-project

# Confirm each deletion before proceeding
rm -ri my-project

⚠️ Dangerous Operation

rm -rf / will delete your entire system! Never run this command. Always double-check the path when using rm -r. Beginners should always add the -i flag to confirm each deletion.

Copying and Moving

# Copy a file
cp source.txt destination.txt

# Copy a file to a specific directory
cp report.txt /home/user/Documents/

# Copy an entire directory (recursive copy)
cp -r my-project/ backup-project/

# Move a file (also used for renaming)
mv old-name.txt new-name.txt

# Move a file to another directory
mv report.txt /home/user/Documents/

# Move an entire directory
mv my-project/ /home/user/workspace/

Viewing File Contents

# Display the entire file contents (suitable for short files)
cat config.txt

# View a file page by page (suitable for long files, press q to quit)
less /var/log/syslog

# Display the first 10 lines of a file
head config.txt

# Display the first 20 lines of a file
head -n 20 config.txt

# Display the last 10 lines of a file
tail config.txt

# Follow file changes in real time (commonly used for viewing logs)
tail -f /var/log/syslog

# Count the number of lines, words, and characters in a file
wc readme.txt

👤 File Permissions Explained

Permissions Overview

Every file in Linux has three sets of permissions, corresponding to the file's owner, group, and other users:

Permission	Letter	Number	Meaning for Files	Meaning for Directories
Read	r	4	View file contents	List directory contents
Write	w	2	Modify file contents	Create/delete files in the directory
Execute	x	1	Run the executable file	Enter the directory
None	-	0	No permission	No permission

Viewing Permissions

ls -l
# Example output:
# -rw-r--r-- 1 user group 4096 Jan 15 10:30 readme.txt
# drwxr-xr-x 2 user group 4096 Jan 15 10:30 Documents/

Interpreting -rw-r--r--:

Position	Value	Meaning
1st character	-	File type (- regular file, d directory, l symlink)
Characters 2-4	rw-	Owner permissions (read, write, no execute)
Characters 5-7	r--	Group permissions (read only)
Characters 8-10	r--	Others permissions (read only)

Modifying Permissions

# Modify permissions using numeric mode
chmod 755 script.sh    # Owner rwx, group and others r-x
chmod 644 config.txt   # Owner rw-, group and others r--
chmod 600 secret.key   # Only owner can read and write

# Modify permissions using symbolic mode
chmod +x script.sh     # Add execute permission for everyone
chmod u+x script.sh    # Add execute permission for owner only
chmod go-w file.txt    # Remove write permission for group and others

# Change file ownership
sudo chown user:group file.txt

# Recursively modify directory permissions
chmod -R 755 my-project/
sudo chown -R user:group my-project/

💡 Common Permission Quick Reference

• 755 — Directories and scripts (owner has full control, others can read and execute)
• 644 — Regular files (owner can read and write, others read only)
• 600 — Sensitive files (only owner can read and write)
• 700 — Private directories (only owner can access)

🔍 Searching and Editing

Searching for Files

# Search by filename
find /home/user -name "*.txt"

# Search by filename (case-insensitive)
find /home/user -iname "readme*"

# Search for files larger than 100MB
find / -size +100M

# Search for files modified in the last 7 days
find /home/user -mtime -7

# Quick search using locate (database must be updated first)
sudo updatedb
locate readme.txt

Searching File Contents

# Search for a keyword in a file
grep "error" /var/log/syslog

# Recursively search all files in a directory
grep -r "password" /etc/

# Case-insensitive search
grep -i "warning" /var/log/syslog

# Show line numbers
grep -n "error" /var/log/syslog

# Search for lines that don't contain a keyword
grep -v "debug" /var/log/syslog

Nano Editor (Recommended for Beginners)

Nano is the most beginner-friendly text editor, with simple and intuitive controls:

# Open or create a file
nano myfile.txt

Common Nano Shortcuts:

Shortcut	Function
Ctrl + O	Save the file
Ctrl + X	Exit the editor
Ctrl + K	Cut the current line
Ctrl + U	Paste
Ctrl + W	Search for text
Ctrl + G	View help
Alt + U	Undo

Vim Basics (Advanced)

Vim is powerful but has a steep learning curve — knowing the basics is enough to get started:

# Open a file
vim myfile.txt

Action	Key	Description
Enter insert mode	i	Start typing text
Exit insert mode	Esc	Return to command mode
Save and quit	:wq	Type in command mode
Quit without saving	:q!	Discard changes and exit
Delete a line	dd	In command mode
Search	/keyword	Press n to find next

💡 Beginner Tip

If you accidentally entered Vim and don't know how to exit, press Esc, then type :q! and press Enter to quit.

🔗 Pipes and Redirection

Pipes and redirection are among the most powerful features of the Linux command line — they allow you to combine multiple commands to accomplish complex tasks.

Output Redirection

# Write command output to a file (overwrites existing contents)
ls -l > filelist.txt

# Append command output to the end of a file
echo "new line" >> filelist.txt

# Redirect error messages to a file
command 2> errors.log

# Redirect both standard output and error output
command > output.log 2>&1

Pipe Operations

The pipe operator | passes the output of one command as input to the next:

# Search for errors in logs and count them
cat /var/log/syslog | grep "error" | wc -l

# View disk usage sorted by size
du -sh /home/user/* | sort -rh

# View the top 10 processes by memory usage
ps aux | sort -k4 -rn | head -10

# Filter logs in real time
tail -f /var/log/syslog | grep --color "error"

Practical Combination Examples

# Find and delete log files older than 7 days
find /var/log -name "*.log" -mtime +7 -exec rm {} \;

# Count file types in the current directory
find . -type f | sed 's/.*\.//' | sort | uniq -c | sort -rn

# Check which ports are currently listening
ss -tlnp | grep LISTEN

🔐 Sudo and Privilege Escalation

What is sudo

sudo (Super User DO) allows a regular user to execute commands as the administrator (root). It is a core tool for Debian system administration.

# Execute a command with root privileges
sudo apt update

# Open a file editor as root
sudo nano /etc/fstab

# Check your sudo permissions
sudo -l

# Switch to the root user (not recommended for daily use)
sudo -i
# Or
su -

⚠️ Security Reminder

• Only use sudo when necessary — don't add sudo to every command
• Never use sudo to run scripts from untrusted sources
• After switching to the root user, remember to exit promptly with exit

Difference Between sudo and su

Feature	sudo	su
Purpose	Execute a single command as root	Switch to the root user
Password	Enter the current user's password	Enter the root password
Security	More secure, with audit logging	Relatively less secure
Recommendation	✅ Recommended	⚠️ Use only when necessary

🌍 Environment Variables

Viewing Environment Variables

# View all environment variables
env

# View a specific environment variable
echo $HOME
echo $PATH
echo $USER
echo $SHELL

Important Environment Variables

Variable	Description	Example Value
$HOME	User's home directory	/home/user
$PATH	Search path for executable programs	/usr/local/bin:/usr/bin:/bin
$USER	Current username	user
$SHELL	Current Shell in use	/bin/bash
$LANG	System language	zh_CN.UTF-8
$PWD	Current working directory	/home/user/Documents

Setting Environment Variables

# Set temporarily (only valid for the current terminal session)
export MY_VAR="hello"

# Set permanently — add to ~/.bashrc
echo 'export MY_VAR="hello"' >> ~/.bashrc

# Modify PATH (add a new path)
export PATH="$HOME/.local/bin:$PATH"

# Apply changes immediately
source ~/.bashrc

Configuration File Loading Order

File	When Loaded	Purpose
/etc/profile	At login	Global configuration
~/.profile	At login	User's personal login configuration
~/.bashrc	Every time a terminal is opened	User's personal Shell configuration
~/.bash_aliases	Loaded by .bashrc	User command aliases

💡 Practical Tips

Tab Auto-Completion

The Tab key is the most useful key on the command line:

# Type the first few characters of a filename, press Tab to auto-complete
cd Doc<Tab>        # Auto-completes to cd Documents/

# Press Tab twice to show all possible completions
cd D<Tab><Tab>     # Shows Desktop/ Documents/ Downloads/

# Commands can also be auto-completed
apt in<Tab>        # Auto-completes to apt install

Wildcards

# * matches any number of characters
ls *.txt           # List all .txt files
rm *.log           # Delete all .log files

# ? matches a single character
ls file?.txt       # Matches file1.txt, fileA.txt, etc.

# [] matches any single character inside the brackets
ls file[123].txt   # Matches file1.txt, file2.txt, file3.txt

Command Aliases

# Create an alias (temporary)
alias ll='ls -lah'
alias update='sudo apt update && sudo apt upgrade'
alias ..='cd ..'

# Save aliases permanently — add to ~/.bashrc
echo "alias ll='ls -lah'" >> ~/.bashrc
source ~/.bashrc

Command History

# View command history
history

# View the last 20 commands
history 20

# Execute command number N from history
!42

# Execute the previous command
!!

# Re-run the previous command with sudo (super useful)
sudo !!

Shortcut Quick Reference:

Shortcut	Function
Ctrl + R	Reverse search command history
Ctrl + A	Move cursor to the beginning of the line
Ctrl + E	Move cursor to the end of the line
Ctrl + U	Delete everything before the cursor
Ctrl + K	Delete everything after the cursor
Ctrl + W	Delete the word before the cursor
Ctrl + L	Clear the screen (same as clear)
Ctrl + C	Terminate the current command
Ctrl + D	Exit the current terminal
↑ / ↓	Browse previous/next command in history

Multiple Command Execution

# Execute sequentially (regardless of success or failure)
command1 ; command2

# Execute the next command only if the previous one succeeds
command1 && command2

# Execute the next command only if the previous one fails
command1 || command2

# Example: Update the system and clean up
sudo apt update && sudo apt upgrade -y && sudo apt autoremove -y

📊 Viewing System Information

# View system information
uname -a           # Kernel version, architecture, etc.
lsb_release -a     # Debian version information
hostnamectl        # Hostname and system information

# View processes
ps aux             # List all processes
top                # Real-time process monitor (press q to quit)
htop               # Better process monitor (requires installation)

# View disk usage
df -h              # Usage of each partition
du -sh ~/Documents # Size of a specific directory

# View memory usage
free -h            # Memory and swap usage

# View network information
ip addr            # IP address information
ip route           # Routing table
ss -tlnp           # Listening ports

🆘 Common Issues

Command Not Found

# Error message: bash: xxx: command not found

# Solution 1: Install the corresponding package
sudo apt install xxx

# Solution 2: Check if PATH is correct
echo $PATH

# Solution 3: Use the full path to execute
/usr/bin/xxx

Permission Denied

# Error message: Permission denied

# Solution 1: Use sudo
sudo command

# Solution 2: Modify file permissions
chmod +x script.sh

# Solution 3: Change file ownership
sudo chown $USER:$USER file.txt

How to Stop a Running Command

Action	Shortcut	Description
Terminate command	Ctrl + C	Immediately stop the current command
Suspend command	Ctrl + Z	Suspend to background
Resume command	fg	Bring background task to foreground
Run in background	bg	Continue the suspended task in background

📝 Quick Reference Tables

Files and Directory Operations

Command	Function	Example
ls	List files	ls -lah
cd	Change directory	cd /home/user
pwd	Show current directory	pwd
mkdir	Create directory	mkdir -p a/b/c
rm	Delete files/directories	rm -r dir/
cp	Copy	cp -r src/ dst/
mv	Move/rename	mv old.txt new.txt
touch	Create empty file	touch file.txt
cat	View file contents	cat config.txt
less	View page by page	less /var/log/syslog
head	View first N lines	head -n 20 file
tail	View last N lines	tail -f /var/log/syslog

Searching and Processing

Command	Function	Example
find	Search for files	find / -name "*.conf"
grep	Search contents	grep -r "error" /var/log/
wc	Count lines/words/chars	wc -l file.txt
sort	Sort	sort -rn data.txt
uniq	Remove duplicates	sort file | uniq -c
diff	Compare files	diff file1 file2

System Administration

Command	Function	Example
sudo	Elevate privileges	sudo apt update
chmod	Modify permissions	chmod 755 script.sh
chown	Change ownership	sudo chown user:group file
ps	View processes	ps aux
top	Real-time monitoring	top
kill	Terminate process	kill -9 PID
df	Disk usage	df -h
free	Memory usage	free -h
uname	System information	uname -a

Network Related

Command	Function	Example
ip addr	View IP addresses	ip addr
ping	Test connectivity	ping debian.org
ss	View ports	ss -tlnp
curl	Download/request	curl -O https://url/file
wget	Download files	wget https://url/file

Next Steps

Congratulations on mastering the basics of the Linux command line! Next, you can continue learning:

1. System Configuration - Further configure and optimize your Debian system
2. APT Package Management - Learn software installation and management
3. Users and Permissions - Dive deeper into user management and access control
4. Disk and Storage Management - Manage disks and storage devices

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Getting more comfortable with the command line? Start learning system configuration -->