Command Line Basics

sudo apt install ghostty

echo $SHELL

pwd
# Example output: /home/username

# List current directory contents
ls
 
# Show detailed information (permissions, size, modification time, etc.)
ls -l
 
# Show hidden files (files starting with .)
ls -a
 
# Combined usage: detailed info + hidden files + human-readable file sizes
ls -lah
 
# List contents of a specific directory
ls /etc
 
# Sort by modification time (newest first)
ls -lt

# Enter a specific directory
cd /var/log
 
# Go up one level
cd ..
 
# Return to the home directory
cd ~
# Or simply type
cd
 
# Return to the previous directory
cd -
 
# Enter a multi-level directory
cd /usr/share/doc

# Create a single directory
mkdir myproject
 
# Create nested directories (-p auto-creates parent directories)
mkdir -p projects/web/frontend
 
# Create multiple directories at once
mkdir dir1 dir2 dir3

# Copy a file
cp file.txt backup.txt
 
# Copy a file to a specific directory
cp file.txt /home/username/Documents/
 
# Copy an entire directory (requires -r for recursive copy)
cp -r myproject myproject-backup
 
# Copy while preserving file attributes (permissions, timestamps, etc.)
cp -a source/ destination/

# Rename a file
mv oldname.txt newname.txt
 
# Move a file to another directory
mv file.txt /home/username/Documents/
 
# Move and rename
mv file.txt /home/username/Documents/newname.txt
 
# Move an entire directory
mv myproject /opt/

# Remove a file
rm file.txt
 
# Remove a directory and its contents (recursive deletion)
rm -r myproject
 
# Force removal (no confirmation prompt)
rm -f file.txt
 
# Confirm before each deletion
rm -i *.txt

# View the complete contents of a file
cat file.txt
 
# Display line numbers
cat -n file.txt
 
# Concatenate multiple files
cat file1.txt file2.txt
 
# Create a new file (Ctrl+D to finish input)
cat > newfile.txt

less /var/log/syslog

# View the first 10 lines (default)
head file.txt
 
# View the first 20 lines
head -n 20 file.txt
 
# View the last 10 lines
tail file.txt
 
# View the last 50 lines
tail -n 50 file.txt
 
# Follow file changes in real time (commonly used for viewing logs)
tail -f /var/log/syslog

# Absolute path examples
/home/username/Documents/file.txt
/etc/nginx/nginx.conf
/var/log/syslog

# Assuming you are in /home/username
# The following two are equivalent:
cat /home/username/Documents/file.txt   # Absolute path
cat Documents/file.txt                   # Relative path

# Practical examples
# Return to the home directory from anywhere
cd ~
 
# View the contents of the parent directory
ls ..
 
# Run a script in the current directory
./my-script.sh
 
# Copy a file from two levels up to the current directory
cp ../../config.txt .

# View all processes and search for lines containing "nginx"
ps aux | grep nginx
 
# List directory contents and display them page by page
ls -la /etc | less
 
# Count the number of files in the current directory
ls | wc -l
 
# View disk usage, sorted by size
du -sh * | sort -h
 
# Multi-level pipe: find errors in logs, deduplicate, and sort
cat /var/log/syslog | grep "error" | sort | uniq -c | sort -rn

# Write command output to a file (overwrites existing content)
echo "Hello, Ubuntu!" > hello.txt
 
# Append command output to the end of a file
echo "Another line" >> hello.txt
 
# Save a command listing to a file
ls -la > filelist.txt
 
# Redirect error output to a file
command 2> errors.log
 
# Redirect both standard output and error output
command > output.log 2>&1
# Or the shorter version
command &> output.log

# Read input from a file
wc -l < file.txt
 
# Common usage: import an SQL file into a database
mysql -u root -p database_name < backup.sql

# Install software
sudo apt install vim
 
# Edit a system configuration file
sudo nano /etc/hosts
 
# Open a shell as root (use with caution)
sudo -i
 
# Check sudo privileges
sudo -l

# View file permissions
ls -l file.txt
# Example output: -rw-r--r-- 1 user group 1024 Jan 1 12:00 file.txt
 
# Add execute permission to a file
chmod +x script.sh
 
# Set exact permissions (owner: rwx, group: rx, others: rx)
chmod 755 script.sh
 
# Common permission combinations
chmod 644 file.txt     # Owner read/write, others read-only
chmod 600 secret.key   # Owner read/write only
chmod 777 shared.txt   # Full permissions for everyone (not recommended)
 
# Recursively modify directory permissions
chmod -R 755 /var/www/html

# Tab completion example
cd /etc/ng[Tab]    # Auto-completes to cd /etc/nginx/
 
# Typical usage of !!: forgot to add sudo
apt install vim
# Permission error, then:
sudo !!
# Equivalent to sudo apt install vim
 
# Ctrl+R search history
# Press Ctrl+R, type "docker", and the most recent command containing "docker" will appear

# View the manual for the ls command
man ls
 
# Search manuals (by keyword)
man -k "copy file"
 
# View a specific section of the manual
man 5 crontab   # View the crontab file format (section 5)

ls --help
cp --help
systemctl --help

# Install tldr
sudo apt install tldr
 
# Update the database
tldr --update
 
# View command usage
tldr tar
tldr rsync
tldr systemctl

# View a brief description of a command
whatis ls
 
# Find the path of a command
which python3
whereis nginx
 
# Check the command type (built-in, external, alias, etc.)
type cd
type ls

# Temporary alias (only valid for the current session)
alias ll='ls -lah'
alias gs='git status'
 
# Permanent alias (written to ~/.bashrc)
echo "alias ll='ls -lah'" >> ~/.bashrc
echo "alias update='sudo apt update && sudo apt upgrade -y'" >> ~/.bashrc
 
# Apply aliases immediately
source ~/.bashrc

# View command history
history
 
# Search command history
history | grep "apt install"
 
# Execute command #42 from history
!42
 
# Clear history
history -c

# * matches any characters
ls *.txt          # All .txt files
cp *.jpg photos/  # Copy all .jpg files
 
# ? matches a single character
ls file?.txt      # Matches file1.txt, fileA.txt, etc.
 
# [] matches a specified range
ls file[1-3].txt  # Matches file1.txt, file2.txt, file3.txt