Question

[BushQ2] Consider the typical file structure of the C: (Windows) or / (Linux and macOS) drives. How is the hierarchy of this structure reflective of the permissions granted to various users within the system? Explain.

Answer 1

The Linux File Hierarchy Structure or the Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Unix-like operating systems.It is maintained by the Linux Foundation.

• In the FHS, all files and directories appear under the root directory /, even if they are stored on different physical or virtual devices.
• Some of these directories only exist on a particular system if certain subsystems, such as the X Window System, are installed.
• Most of these directories exist in all UNIX operating systems and are generally used in much the same way; however, the descriptions here are those used specifically for the FHS, and are not considered authoritative for platforms other than Linux.

1. / (Root) : Primary hierarchy root and root directory of the entire file system hierarchy.

• Every single file and directory starts from the root directory
• Only root user has the right to write under this directory
• /root is root user’s home directory, which is not same as /

2. /bin : Essential command binaries that need to be available in single user mode; for all users, e.g., cat, ls, cp.

• Contains binary executables
• Common linux commands you need to use in single-user modes are located under this directory.
• Commands used by all the users of the system are located here e.g. ps, ls, ping, grep, cp

3. /boot : Boot loader files, e.g., kernels, initrd.

• Kernel initrd, vmlinux, grub files are located under /boot
• Example: initrd.img-2.6.32-24-generic, vmlinuz-2.6.32-24-generic

4. /dev : Essential device files, e.g., /dev/null.

• These include terminal devices, usb, or any device attached to the system.
• Example: /dev/tty1, /dev/usbmon0

5. /etc : Host-specific system-wide configuration files.

• Contains configuration files required by all programs.
• This also contains startup and shutdown shell scripts used to start/stop individual programs.
• Example: /etc/resolv.conf, /etc/logrotate.conf.

6. /home : Users’ home directories, containing saved files, personal settings, etc.

• Home directories for all users to store their personal files.
• example: /home/kishlay, /home/kv

7. /lib : Libraries essential for the binaries in /bin/ and /sbin/.

• Library filenames are either ld* or lib*.so.*
• Example: ld-2.11.1.so, libncurses.so.5.7

8. /media : Mount points for removable media such as CD-ROMs (appeared in FHS-2.3).

• Temporary mount directory for removable devices.
• Examples, /media/cdrom for CD-ROM; /media/floppy for floppy drives; /media/cdrecorder for CD writer

9. /mnt : Temporarily mounted filesystems.

• Temporary mount directory where sysadmins can mount filesystems.

10. /opt : Optional application software packages.

• Contains add-on applications from individual vendors.
• Add-on applications should be installed under either /opt/ or /opt/ sub-directory.

11. /sbin : Essential system binaries, e.g., fsck, init, route.

• Just like /bin, /sbin also contains binary executables.
• The linux commands located under this directory are used typically by system aministrator, for system maintenance purpose.
• Example: iptables, reboot, fdisk, ifconfig, swapon

12. /srv : Site-specific data served by this system, such as data and scripts for web servers, data offered by FTP servers, and repositories for version control systems.

• srv stands for service.
• Contains server specific services related data.
• Example, /srv/cvs contains CVS related data.

13. /tmp : Temporary files. Often not preserved between system reboots, and may be severely size restricted.

• Directory that contains temporary files created by system and users.
• Files under this directory are deleted when system is rebooted.

14. /usr : Secondary hierarchy for read-only user data; contains the majority of (multi-)user utilities and applications.

• Contains binaries, libraries, documentation, and source-code for second level programs.
• /usr/bin contains binary files for user programs. If you can’t find a user binary under /bin, look under /usr/bin. For example: at, awk, cc, less, scp
• /usr/sbin contains binary files for system administrators. If you can’t find a system binary under /sbin, look under /usr/sbin. For example: atd, cron, sshd, useradd, userdel
• /usr/lib contains libraries for /usr/bin and /usr/sbin
• /usr/local contains users programs that you install from source. For example, when you install apache from source, it goes under /usr/local/apache2
• /usr/src holds the Linux kernel sources, header-files and documentation.

15. /proc : Virtual filesystem providing process and kernel information as files. In Linux, corresponds to a procfs mount. Generally automatically generated and populated by the system, on the fly.

• Contains information about system process.
• This is a pseudo filesystem contains information about running process. For example: /proc/{pid} directory contains information about the process with that particular pid.
• This is a virtual filesystem with text information about system resources. For example: /proc/uptime

Modern Linux distributions include a /run directory as a temporary filesystem (tmpfs) which stores volatile runtime data, following the FHS version 3.0. According to the FHS version 2.3, such data were stored in /var/run but this was a problem in some cases because this directory is not always available at early boot. As a result, these programs have had to resort to trickery, such as using /dev/.udev, /dev/.mdadm, /dev/.systemd or /dev/.mount directories, even though the device directory isn’t intended for such data.Among other advantages, this makes the system easier to use normally with the root filesystem mounted read-only. For example, below are the changes Debian made in its 2013 Wheezy release:

• /dev/.* ? /run/*
• /dev/shm ? /run/shm
• /dev/shm/* ? /run/*
• /etc/* (writeable files) ? /run/*
• /lib/init/rw ? /run
• /var/lock ? /run/lock
• /var/run ? /run
• /tmp ? /run/tmp