Follows the Filesystem Hierarchy Standard (FHS).
/File.txt ≠ file.txt. (e.g., .bashrc)/
├── bin → Essential binaries (ls, cat, grep) — symlink to /usr/bin on modern distros
├── boot → Kernel and bootloader files
├── dev → Device files (block, char, pseudo-devices like /dev/null)
├── etc → System-wide config files
├── home → User home directories (/home/alice, /home/bob)
├── lib → Shared libraries for /bin and /sbin
├── media → Auto-mounted removable media (USB, CD)
├── mnt → Temporary manual mount point
├── opt → Optional third-party software
├── proc → Virtual FS — live kernel and process info (not on disk)
├── root → Root user's home directory
├── run → Runtime data in RAM (PIDs, sockets) — cleared on reboot
├── sbin → System admin binaries — symlink to /usr/sbin on modern distros
├── srv → Data served by web/FTP services
├── sys → Virtual FS — kernel device and driver interface
├── tmp → Temporary files, cleared on reboot
├── usr → User-installed apps, libraries, docs
│ ├── bin → Most user commands live here
│ ├── lib → Libraries for /usr/bin
│ └── share → Architecture-independent data
└── var → Variable data: logs (/var/log), mail, caches, spool
-rwxr-xr-- 1 alice staff 4096 May 1 file.txt
^^^ ^^^ ^^^
│ │ └── other: read only
│ └─────── group: read + execute
└──────────── owner: read + write + execute
First character: - file, d directory, l symlink, b block device, c char device.
Each permission set: r (4) w (2) x (1). Octal notation combines them: rwxr-xr-- = 754.
chmod 755 file # set permissions numerically
chmod u+x file # add execute for owner
chown alice:staff file # change owner and group
ls -la # list with hidden files and details
find / -name "*.conf" 2>/dev/null # find files by name
file <path> # identify file type
stat <path> # detailed metadata
lsof -p <pid> # files open by a process
df -h # disk space by mount point
du -sh * # size of each item in current dir