Directory Layout

/
├── bin      → Essential user commands
├── boot     → Boot files
├── dev      → Device files
├── etc      → System configuration
├── home     → User home directories
├── lib      → System library files
├── media    → Auto-mount points
├── mnt      → Manual mount points
├── opt      → Third-party software
├── proc     → Process information (virtual)
├── root     → Root user home directory
├── run      → Runtime data
├── sbin     → System administration commands
├── srv      → Service data
├── sys      → Kernel device information (virtual)
├── tmp      → Temporary files
├── usr      → User-level programs and data
└── var      → Variable data

/etc/
├── apt/                 # APT package manager configuration
│   └── sources.list.d/  # Software source configuration
├── default/grub         # GRUB bootloader configuration
├── fstab                # Disk mount configuration
├── hostname             # Hostname
├── hosts                # Local DNS mapping
├── NetworkManager/      # Network management configuration
├── nginx/               # Nginx configuration (if installed)
├── passwd               # User account information
├── shadow               # User passwords (encrypted)
├── ssh/                 # SSH server configuration
├── sudoers              # sudo permissions configuration
├── systemd/             # systemd configuration overrides
└── environment          # Global environment variables

/var/
├── cache/apt/archives/  # APT download cache
├── lib/                 # Program runtime data
│   ├── dpkg/            # dpkg package database
│   └── snapd/           # Snap data
├── log/                 # System logs
│   ├── syslog           # System log
│   ├── auth.log         # Authentication log
│   └── kern.log         # Kernel log
├── mail/                # Mail
├── spool/cron/          # Cron jobs
└── www/html/            # Default web directory

/home/user/
├── .bashrc              # Bash configuration
├── .config/             # User-level application configuration (XDG)
├── .local/              # User-level data and programs
│   ├── bin/             # User-level executables
│   └── share/           # User-level data
├── .ssh/                # SSH keys and configuration
├── Desktop/             # Desktop
├── Documents/           # Documents
├── Downloads/           # Downloads
└── snap/                # Snap user data

/usr/
├── bin/                 # User commands (ls, cp, vim, etc.)
├── sbin/                # System administration commands
├── lib/                 # Library files
├── lib64/               # 64-bit library files
├── include/             # C/C++ header files
├── share/               # Architecture-independent data
│   ├── applications/    # .desktop files
│   ├── doc/             # Documentation
│   ├── fonts/           # Fonts
│   └── man/             # Man pages
└── local/               # Locally compiled programs
    ├── bin/
    ├── lib/
    └── share/

/opt/
├── google/chrome/       # Google Chrome
├── visual-studio-code/  # VS Code (when not installed as Snap)
└── ...

# Check /tmp usage
du -sh /tmp
 
# Permissions: drwxrwxrwt (sticky bit)
# Anyone can write, but only the owner can delete their own files

/boot/
├── vmlinuz-*            # Linux kernel
├── initrd.img-*         # Initial RAM disk
├── config-*             # Kernel configuration
├── grub/                # GRUB bootloader
│   └── grub.cfg         # GRUB configuration (auto-generated)
└── efi/                 # EFI partition mount point
    └── EFI/
        ├── ubuntu/      # Ubuntu boot files
        └── BOOT/        # Default boot files

/dev/
├── sda, sdb...          # SATA/SCSI hard drives
├── nvme0n1              # NVMe hard drive
├── tty*                 # Terminal devices
├── video0               # Webcam
├── null                 # Null device (discards output)
├── zero                 # Zero device (produces zero bytes)
├── random, urandom      # Random number generators
└── loop*                # Loop devices (used by Snap)

# Common /proc files
cat /proc/cpuinfo        # CPU information
cat /proc/meminfo        # Memory information
cat /proc/version        # Kernel version
cat /proc/partitions     # Partition information
 
# Common /sys files
cat /sys/class/power_supply/BAT0/capacity  # Battery level
cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor  # CPU frequency governor

/run/
├── user/1000/           # Current user runtime data
├── snapd/               # Snap runtime
├── lock/                # Lock files
└── systemd/             # systemd runtime

# Find the path of a command
which command-name
type command-name
 
# Find which package owns a file
dpkg -S /path/to/file
 
# List files installed by a package
dpkg -L package-name