/                        ← root directory
├── bin/                 ← essential programs (ls, cp, bash)
├── boot/                ← kernel and bootloader (vmlinuz)
├── etc/                 ← system configuration files
│   ├── passwd           ← user accounts
│   └── group            ← group definitions
├── home/                ← user home directories
│   └── alice/
│       └── .bashrc
├── usr/                 ← user programs and libraries
│   └── include/
│       └── stdio.h
└── var/                 ← logs, mail, databases

Ordinary data files — text, binaries, images, documents. The most common type.

Special files whose contents are a table of filename-to-inode mappings. Every directory contains at least two entries: . (link to itself) and .. (link to its parent).

A file containing the pathname of another file. The kernel transparently follows (dereferences) it when encountered in a path. Shown as linkname -> target in ls output.

Devices providing a byte-stream interface — terminals, serial ports. Example: /dev/tty.

Devices providing random-access block I/O — hard disks, SSDs, USB drives. Example: /dev/sda.

A pipe with a filesystem name. Used for IPC between unrelated processes. Created with mkfifo.

Used for IPC via network or local connections. Created by the socket() system call.

• A directory entry pointing directly to an inode (the kernel data structure holding file metadata and data pointers)
• Every file starts with link count = 1; creating a hard link raises it to 2
• Data is only freed when link count reaches zero
• Cannot span filesystems (inodes are filesystem-local)
• Cannot point to directories
• All hard links to the same file are equal — no “original” vs “copy”

ln original.txt hardlink.txt      # create hard link
ls -li original.txt hardlink.txt  # same inode number, link count = 2

• A separate file containing a pathname string
• Kernel dereferences it transparently when processing a path
• If the target is deleted, the symlink becomes dangling
• Can span filesystems
• Can point to directories

ln -s original.txt symlink.txt    # create symbolic link
ls -l symlink.txt
# lrwxrwxrwx 1 alice ... symlink.txt -> original.txt

Begin with / and specify the full location from the root directory. Always unambiguous regardless of where the process currently is.

Do not begin with /. Interpreted relative to the process’s current working directory (CWD). Each process inherits its CWD from its parent; the shell changes it with cd.

# Assuming CWD is /usr
include/stdio.h        → resolves to /usr/include/stdio.h
../home/alice          → resolves to /home/alice  (.. = parent)

ls -l myfile.sh
-rwxr-x---  1  alice  devs  2048  May 1  myfile.sh
 │ │││ │││  └── other: no permission
 │ │││ └──────── group: read + execute
 │ └──────────── owner: read + write + execute
 └────────────── file type: regular file

• r (read) — files: read content. Directories: list filenames inside.
• w (write) — files: modify content. Directories: add, remove, rename entries.
• x (execute) — files: run as a program. Directories: access files inside (also called search permission). Without x on a directory you cannot access any file inside it even if you know the name.

Shell I/O redirection works by closing and reopening these descriptors before exec() — the new program sees nothing unusual. In the C stdio library these correspond to the FILE* streams stdin, stdout, and stderr. Functions like printf(), fopen(), and fgets() are layered on top of the raw system call interface.

Answer: A hard link is a directory entry pointing directly to an inode. All hard links share the same inode number; the file data is freed only when all links are deleted (link count reaches zero). Hard links cannot cross filesystems and cannot point to directories. A symbolic link is a separate file containing a pathname string; the kernel follows it transparently. If the target is deleted, the symlink becomes dangling. Symlinks can cross filesystems and can point to directories. Use ls -li to distinguish them: hard links share the same inode number; symlinks show an arrow to their target.

Answer: On a directory, execute means search permission — the ability to access files inside it. Without execute on a directory, you cannot access any file inside even if you know its full name and have permission on the file itself. Read permission on a directory only allows listing filenames. The kernel checks execute permission on every directory component in a pathname: to access /home/alice/file.txt, it checks x on /, then on /home, then on /home/alice, then read permission on file.txt itself.

Answer: Universality of I/O means the same four system calls — open(), read(), write(), close() — work on all file types: regular files, terminals, pipes, sockets, and device files. A program does not need to know what type of resource it reads from or writes to. This enables powerful compositions: a program reading from stdin works identically whether stdin is a keyboard, a file, or a pipe. It also means one consistent API covers everything, simplifying both programming and system design.

Answer: 0 is stdin — default input source, normally the keyboard. 1 is stdout — default output destination. 2 is stderr — error and diagnostic messages, separate from stdout so they can be redirected independently. In an interactive shell all three are connected to the terminal. In the C library they correspond to FILE* streams stdin, stdout, and stderr. Shell redirection (> file, 2> err.log) works by closing and reopening these descriptors before exec(), transparently to the program.

Answer: An absolute pathname starts with / and specifies the complete location from the root directory — it is unambiguous regardless of the process’s current directory. A relative pathname does not start with / and is interpreted relative to the process’s current working directory. The component .. refers to the parent directory and . to the current directory. Each process inherits its CWD from its parent; the shell changes it with cd. The same file can be referred to by many different relative paths depending on where the process currently is.