Two Primary Kernel Subsystems

1. File System: Deals with all input and output
   • Includes files and terminals
   • Integration of storage devices
2. Process Management: Deals with programs and program interaction
   • How processes share CPU, memory, and signals
   • Scheduling
   • IPC
   • Memory management

Note: UNIX variants have slightly different implementations of various subsystems.

Unix File Types

The UNIX filesystem contains several different types of files:

• Ordinary Files: Store your information.
  • E.g., documents, images, etc.
  • Always located under a directory file
  • Don’t contain other files
• Directories: Organize groups of files
  • Branching points in the hierarchical tree
  • Can contain ordinary files, special files, and other directories.
  • Never contain “real” information, only organizes it.
  • All files are descendants of the root directory (/) located at the top of the tree.
• Special Files: Represent a real physical device.
  • E.g., printer, SSD, keyboard, etc.
  • UNIX considers any device attached to the system to be a file, including your terminal.
    • By default, your terminal is the standard input file, standard output file, and standard error file!
  • Two types of I/O (usually found under /dev):
    1. Character
    2. Block

Three Default I/O Channels:

• Standard Input (stdin): Where a program expects to find input
• Standard Output (stdout): Where a program writes its output
• Standard Error (stderr): Where a program writes error messages

Interprocess Communication:

• Pipe: One-way data channel that allows two processes to be linked.
  • Acts as a temporary file which exists to hold data from one command until it’s read by another.
• Socket: Two-way high-speed data channel that allows two processes on two machines to talk.

Remember: File types aren’t determined by extension on UNIX.

• You can use the file command to find a file’s type.

UNIX File Names

On File Names:

• Most characters are permitted. Avoid spaces, tabs, and metacharacters.
• Case sensitive.
  • Which is why lowercase is recommended.
• Can be up to 256 characters long.
• Extensions may be used to identify file types.
  • Even though UNIX doesn’t need file extensions, some applications might.
• All files in same parent directory must have unique names
  • Files in separate directories can have identical names.

Hidden Files: Files that begin with a dot (.)

• E.g., .zshrc, .mailrc
• Most resource configuration files are hidden files.

Reserved Filenames:

• Slash (/): Root Directory
• Dot (.): Current directory
• Double Dot (..): Parent directory
• Tilde (~): Home directory

UNIX Pathnames

Pathnames: Specify where a file is located in the hierarchically organized file system.

• Each directory or filename is separated from the previous one by a /

Two Kinds of Pathnames:

• Absolute Pathname: Tells how to reach a file starting from root.
  • Always starts with slash (/)
  • Examples: /usr/bin/bash, /etc/portage/package.use/blender
• Relative Pathname: Tells how to reach a directory from your current working directory.
  • Examples: ../../daemon.sh, ./markdown/06 - UNIX Files.md

Linux File System Hierarchy Standard (FHS)

Directory Tree: All the files in the UNIX file system are organized into a multi-level hierarchy.

• The UNIX file system can be envisioned as an inverted tree.
• The very top of the file system is root (/), all other files are descendants of root.
• The number of levels is largely arbitrary, but most systems have some organizational similarities.

Linux File Hierarchy Structure (FHS): Defines the directory structure and contents in UNIX-like operating systems.

• Maintained by the Linux Foundation.
• All files and directories appear under root, even if they’re on different physical or virtual devices.
• Some of these directories only exist on a particular system if certain subsystems are installed.
  • E.g., /etc/X11 only exists if the X Window System is installed, etc.
• Most of these directories exist in all UNIX operating systems and are generally used in the same way.
  • Though, the FHS is not considered authoritative for platforms other than Linux.

General FHS for Linux

• /: Root
  • Every single file and directory starts here.
  • Only root user has write privileges for this directory.
  • Note: /root is the root user’s home directory, not root itself.
• /bin: User binaries
  • Contains essential low-level binary executables
  • Stores common commands and programs that are available to all users.
    • e.g., ps, ls, ping, grep, cp
• /sbin: System binaries
  • Contains binary executables typically used by the system administrator.
    • e.g., iptables, reboot, fdisk, ifconfig, swapon
• /etc: Configuration files
  • Contains configuration files requires by all programs.
    • Also contains startup and shutdown shell scripts used to start and stop individual programs.
    • e.g., /etc/resolve.conf, /etc/hostname
• /dev: Device files
  • Contains device files.
    • Includes any device attached to the system (gamepads, terminal devices, USBs, etc.)
    • e.g., /dev/tty1, /dev/input/mouse0, /dev/nvme0
• /proc: Process information
  • Contains information about running system processes.
    • Is a pseudo/virtual filesystem.
    • e.g., /proc/{pid} stores information about the process with that particular PID, /proc/uptime stores information about system uptime
• /var: Variable files
  • Contains files that are expected to grow/shrink (log files, database files, emails, etc.)
    • e.g., /var/log /var/lib, /var/mail
• /tmp: Temporary files
  • Contains temporary files created by system and users.
  • Files in here are deleted when the system is rebooted.
• /usr: User programs
  • Contains binaries, libraries, documentation, and source-code for second-level programs.
  • /usr/bin stores binary files for higher-level user programs
    • e.g., at, awk, cc, less, scp
  • /usr/sbin stores binary files for system administrators
    • e.g., atd, cron, sshd, useradd, userdel
  • /usr/lib stores libraries for /usr/bin and /usr/sbin
  • /usr/local stores user’s programs installed from source
    • e.g., /usr/local/apache2
• /home: Home directories
  • Contains user’s personal files.
    • e.g., /home/inevitabby, /home/larry, /home/tkaczynski
• /boot: Boot loader files
  • Contains boot loader-related files.
    • Kernel initrd, vmlinux, and grub files are stored here.
    • e.g., initramfs-6.1.67-gentoo-dist.img, vmlinuz-6.1.67-gentoo-dist
• /lib: System libraries
  • Contains library files for the binaries in /bin and /sbin
    • Are either ld* or lib*.so.*
    • e.g., ld-2.11.1.so, libncurses.so.5.7
• /opt: Optional add-on apps
  • Contains add-on applications from individual vendors.
• /mnt: Mount directory
  • Temporary mount directory where sysadmins can mount filesystems.
• /media: Removable devices
  • Temporary mount directory for removable devices
    • e.g., /media/cdrom, media/floppy. media/cdrecorder
• /srv: Service data
  • Contains server-specific services-related data
    • e.g., /src/cvs contains CVS-related data.

Remember: The /usr variants of folders that can be found under root contain higher-level versions of their counterparts!

• For example, /usr/bin contains higher-level system utilities while /bin contains essential lower-level utilities.

Fundamentals of Security (Groups)

UNIX is a multi-user system.

• A set of users can form a group, and
• A user can be a member of multiple groups.

UNIX systems have one or more users, identified by numeric uid and username.

• There is a special user with complete control (uid: 0, username: root)
• Every user has a username, numeric uid, and default group association.
  • Associations with other groups are also possible.

Command: id lets you view your uid, default groups, and which groups your belong to.

Unix File/Directory Permissions

Every file/directory has:

1. A single owner.
2. An association with a single group.
3. A set of access permissions associated with it.
   • For a file, permissions control what can be done to its contents.
   • For a directory, permissions control whether files inside it can be listed, searched, renamed, or removed.

Permissions lets us answer questions like:

• Can this file be read or written?
• Can this program be run?
• Can this hardware be used?
• Can this process be stopped?

On Access Permissions

Three Types of Permissions:

1. Read (r): Process may read contents of file, or list directory contents.
2. Write (w): Process may write contents of file, or add/remove directory contents.
3. Execute (x): Process may execute file, or open files in directory or subdirectories.

Three Sets of Permissions:

1. User (u): Permissions for owner
2. Group (g): Permissions for group (1 group per file)
3. Other (o): Permissions for everyone on the system.

Note: Every user is responsible for controlling access to their files and directories.

• Use this power with care!

Viewing File Permissions

When using ls -l (long listing), file permissions are given in the following format—

• owner read (r)
• owner write (w)
• owner execute (x)
  • group read (r)
  • group write (w)
  • group execute (x)
    • public read (r)
    • public write (w)
    • public execute (x)

—which are displayed as: -rwxrwxrwx - The first character shows file type: 1. File: - 2. Directory: d 3. Link: l - If a r, w, or x is replaced by a -, that permission is denied for that respective set. - Starting after the file type character, the: * First RWX triplet are for the user owner, * Middle RWX triplet are for the group, and * Last RWX triplet are for others.

Examples: Reading ls -a

-rwxr-xr-x 1 larry larry 15480 Dec 28 23:16 a.out

• A file that everyone has execute and read permissions for, but only the owner has write.

drwxr-xr-x 1 larry larry 0 Dec 28 23:17 directory

• A folder that everyone has execute and read permissions for, but only the owner has write.

-rw-r--r-- 1 larry larry 90 Dec 28 23:16 source.c

• A file that everyone has read permissions for, but only the owner has write.

Utilities for Manipulation File Attributes

• chmod: Change file permissions
  • Format: chmod [permission] [filename]
• chown: Change file owner
  • Format: chown [username] [filename]
• chgrp: Change file group
  • Format: chgrp [groupname] [filename]
• umask: User file creation mode mask
  • Only the owner or superuser can change file attributes.
  • Upon creation, default permissions given to file modified by process umask value.

Permission Settings for Octal Mode

Permission settings use octal numbers:

• Each r, w, and x is actually an on/off toggle, and the math is pretty simple.
  • For example, \text{rx} = 4 + 1 = 5, or \text{rwx} = 4 + 2 + 1 = 7

Example: Converting permissions to octal equivalents

Permission	Binary	Octal
rwx rwx rwx	111 111 111	7 7 7
rwx r-x r-x	111 101 101	7 5 5
rw- r-- r--	110 100 100	6 4 4
r-- --- ---	100 000 000	4 0 0

• These codes (777, 755, 644, and 400) are the most-common codes.

Example: Using chmod with symbolic access mode ({u,g,o}/{r,w,x}) and octal access mode

Format: chmod [ugoa] [+-=] [rwx] [file/dir]

• [ugoa]: User, group, other, and all, respectively. (Defaults to user)
• [+-=]: Add, remove, and set permissions, respectively.
• [rwx]: Permission code.
• [file/dir]: File or directory to apply it to.

$ chmod +x daemon.sh

• Give execute permission to daemon.sh for current owner.

$ chmod o -r unreadable.txt

• Remove read permission from unreadable.txt

$ chmod a=r-x file

• Set permissions to r-x for unreadable.txt for all.
  • Notice how when using = we need to specify all three permission codes.

$ chmod go-w unwriteable.txt

• Remove write permission from unwriteable.txt for group and other.

$ chmod g:cs2600-x unexecutable

• Remove execute permissions from unexecutable from the cs2600 group.
  • Note: When a specific group isn’t given, your default group is the one that’s modified by chmod.

$ chmod 600 my_file

• Set permissions for owner to be able to read and write, give no permissions to group and other.

$ chmod 755 our_file

• Set permissions for owner to be able to read, write, and execute; give only read and write permissions to group and other.