/           # Root - the top of the entire hierarchy, everything branches from here
/home       # User home directories (/home/alice, /home/bob) -- personal files and configs
/etc        # System-wide configuration files (nginx.conf, ssh config, hostname, etc.)
/var        # Variable data that changes at runtime (logs in /var/log, databases, web content)
/usr        # User-installed programs and libraries (most of your installed software lives here)
/bin        # Essential command binaries (ls, cp, mv, cat) -- available even in single-user mode
/sbin       # System binaries that typically require root (fdisk, iptables, reboot)
/tmp        # Temporary files -- cleared on reboot, writable by everyone (be careful what you store here)
/opt        # Optional/third-party software installed outside the package manager
/dev        # Device files -- hardware represented as files (/dev/sda = first disk, /dev/null = black hole)
/proc       # Virtual filesystem exposing kernel and process info (/proc/cpuinfo, /proc/meminfo)

# Print working directory
pwd

# List files
ls              # Basic listing
ls -l           # Long format (permissions, size, date)
ls -a           # Show hidden files
ls -lh          # Human-readable sizes
ls -lah         # All options combined

# Change directory
cd /var/log     # Absolute path
cd logs         # Relative path
cd ..           # Parent directory
cd ~            # Home directory
cd -            # Previous directory

# Show directory tree
tree            # Install with: apt install tree
tree -L 2       # Limit depth to 2 levels

# Create an empty file (or update the timestamp of an existing file)
# touch is like tapping a file to say "I was here" -- it creates it if missing
touch file.txt

# Create a directory
mkdir mydir

# Create nested directories (the -p flag creates all missing parents)
# Without -p, this fails if "path" or "path/to" do not already exist
mkdir -p path/to/nested/dir

# Create multiple directories at once
mkdir dir1 dir2 dir3

# Brace expansion lets you create a project structure in one command
mkdir -p project/{src,tests,docs,config}

# Copy files
cp source.txt dest.txt
cp -r sourcedir/ destdir/    # Copy directory recursively
cp -i file.txt backup.txt    # Interactive (prompt before overwrite)

# Move/rename
mv oldname.txt newname.txt
mv file.txt /path/to/destination/
mv *.txt documents/          # Move all .txt files

# Remove a single file
rm file.txt
rm -i file.txt    # Interactive -- asks "are you sure?" before each deletion
rm -f file.txt    # Force -- no confirmation prompt, ignores missing files

# Remove directories and everything inside them
rm -r directory/  # Recursive -- required for non-empty directories
rm -rf directory/ # Force recursive -- deletes everything without asking (DANGEROUS)

# Remove only empty directories (safe alternative when you expect them to be empty)
rmdir emptydir/

# Display entire file
cat file.txt

# Display with line numbers
cat -n file.txt

# Concatenate multiple files
cat file1.txt file2.txt > combined.txt

# Page through file
less file.txt    # Use arrows, q to quit
more file.txt    # Older pager

# First/last lines
head file.txt         # First 10 lines
head -n 20 file.txt   # First 20 lines
tail file.txt         # Last 10 lines
tail -n 50 file.txt   # Last 50 lines
tail -f /var/log/syslog  # Follow file updates (live)

# Word count
wc file.txt           # Lines, words, bytes
wc -l file.txt        # Just line count

# Find by name
find /path -name "*.txt"
find . -name "config.json"

# Find by type
find /var -type f    # Files only
find /var -type d    # Directories only

# Find by size
find . -size +100M   # Larger than 100MB
find . -size -1k     # Smaller than 1KB

# Find and execute
find . -name "*.log" -delete
find . -name "*.txt" -exec cat {} \;

# Find modified in last 7 days
find . -mtime -7

# Fast file search (uses a pre-built database -- instant results even on huge filesystems)
# The trade-off: results may be stale because the database is only updated periodically
locate filename

# Update the database manually (usually runs nightly via cron)
# Run this after creating new files if you need locate to find them immediately
sudo updatedb

# Case-insensitive search
locate -i filename

# Search for text in file
grep "error" logfile.txt

# Case-insensitive
grep -i "error" logfile.txt

# Recursive search in directory
grep -r "TODO" /path/to/code/

# Show line numbers
grep -n "error" logfile.txt

# Count matches
grep -c "error" logfile.txt

# Invert match (lines NOT containing pattern)
grep -v "success" logfile.txt

# View permissions
ls -l

# Output format:
# -rw-r--r-- 1 user group 1234 Dec 2 10:00 file.txt
# │││││││││
# │││││││└└ Other permissions (read, write, execute)
# ││││││└└└ Group permissions
# │││└└└└└└ Owner permissions
# ││└─────── Number of links
# │└──────── File type (- = file, d = directory)

# Create project structure
mkdir -p project/{src,docs,tests}
cd project

# Create files
touch src/main.py src/utils.py
touch docs/README.md
touch tests/test_main.py

# View structure
tree

# Find files larger than 100MB
find /home -type f -size +100M -exec ls -lh {} \; 2>/dev/null

# Or use du
du -h /home | sort -rh | head -20

# Find errors in log file
grep -i "error" /var/log/syslog

# Count error occurrences
grep -c "error" /var/log/syslog

# Show errors with context
grep -C 3 "error" /var/log/syslog  # 3 lines before and after

# Tab completion
cd /var/l[TAB]  # Completes to /var/log

# Command history
history         # Show command history
!123            # Run command #123 from history
!!              # Run last command
!$              # Last argument of previous command

# Ctrl shortcuts
Ctrl+C          # Cancel current command
Ctrl+D          # Exit shell
Ctrl+L          # Clear screen
Ctrl+R          # Search command history
Ctrl+A          # Move to beginning of line
Ctrl+E          # Move to end of line

# * matches any characters
ls *.txt        # All .txt files
rm file*        # All files starting with "file"

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

# [] matches character range
ls file[1-3].txt    # file1.txt, file2.txt, file3.txt
ls [A-Z]*.txt       # Files starting with capital letter

# {} for multiple patterns
cp file.{txt,md} backup/  # Copy file.txt and file.md

# Output redirection
echo "Hello" > file.txt     # Write to file (overwrites existing content)
echo "World" >> file.txt    # Append to file (preserves existing content)

# Input redirection -- feed a file's contents as stdin to a command
wc -l < file.txt

# Pipes -- connect stdout of one command to stdin of the next
# Think of it as an assembly line: each command processes the data and passes it along
cat file.txt | grep "error" | wc -l   # Count lines containing "error"
ls -l | less                            # Page through a long file listing
ps aux | grep python                    # Find running Python processes

# Redirect stderr (error messages) to a file
command 2> error.log                   # Errors go to file, normal output still prints to screen
command > output.log 2>&1              # Both stdout AND stderr go to the same file
command > /dev/null 2>&1               # Discard all output entirely (useful in cron jobs)