__linux

Books

• https://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/index.html
  • PDF: Linux-Filesystem-Hierarchy

File System Hierarchy

• /boot
  • stores data that is used before the kernel begins executing user−mode program
  • contains backup MBR data, boot data etc.
• /dev
  • contains device as files
  • sda, sda1, stdin, stdout, stderr, tty0, tty1 etc.
• /proc
  • virtual filesystem containing system information
  • system memory, devices mounted, hardware configuration etc.
• /etc
  • contains all system related configuration files
  • /etc/hosts, /etc/sudoers, /etc/init.d, /etc/httpd etc.
• /mnt
  • mount point for filesystems or devices
• /media
  • mount point for removable media like CD-ROM, External HDD
• /home
  • contains user directories
  • /home/$USER
• /root
  • home directory for administrator “root”
• /opt
  • Reserved for all the software and add−on packages that are not part of the default installation
• /bin
  • commands for both system admin and non-admin users
  • shell: bash, csh etc.
  • commands: cp, mv, echo, ls, mkdir, cat etc.
• /sbin
  • executables for system maintenance, not for regular users
  • fdisk, ifconfig, init, route, reboot etc.
• /usr
  • contains by far the largest share of data on a system
  • user programs like telnet, ftp resides here
  • large software packages must not use a direct subdirectory under the /usr hierarchy
  • /usr/lib: program libraries
  • /usr/bin: vast majority of binaries like vi, gcc etc.
  • /usr/sbin: system administrator binaries like chroot, chown, adduser etc.
  • /usr/local:
    • /usr/local/bin: Locally compiled (installed) normal programs like node, python3
    • /usr/local/sbin: Locally compiled (installed) system programs like nginx, openvpn
  • /usr/share: shareable, architecture−independent files (docs, icons, fonts etc)
    • /usr/share/doc
    • /usr/share/man
  • /usr/include: header files for user space source code
  • /usr/src: linux source code
• /var
  • variable data like logging files, mail and printer spool, temporary files, lock files
  • can be mounted on separate device for backup purposes
• Additional
  • /initrd: initial RAM Disk
  • /lost+found: rebooting after system crash, fsck tries to recover files and save here
  • /lib: kernel modules + shared library images (*.so) to boot system
  • /srv: site−specific data which is served by this system
  • /tmp: temporary files, should not delete data, can cause system crash

Software Installation

• https://askubuntu.com/questions/1148/when-installing-user-applications-where-do-best-practices-suggest-they-be-loc
• https://unix.stackexchange.com/questions/8656/usr-bin-vs-usr-local-bin-on-linux
• /bin and /sbin are loaded before larger /usr is mounted
  • These are required during startup
  • /bin: system programs
  • /sbin: system management programs, not for normal users
• /usr and /usr/local are mostly same hierarchy
  • /usr: distribution managed (apt) programs
    • /usr/bin
    • /usr/sbin
  • /usr/local: locally compiled packages
    • /usr/local/bin
    • /usr/local/sbin
• /opt: installation of pre-compiled (binary) packages like tarball of Firefox

/proc

• virtual filesystem
• aka process information pseudo−file system
• all files have zero size
• all files contain runtime system information
  • system memory, devices mounted, hardware configuration etc.
• many commands use this to display data
• https://www.reddit.com/r/linuxadmin/comments/1890gvk/why_does_the_proc_directory_exist/

/etc

• User management
  • /etc/passwd: Stores core user account details like usernames, User IDs (UID), Group IDs (GID), home directories, and default shells
  • /etc/shadow: Holds securely hashed user passwords and password expiration policies, accessible only by root
  • /etc/group: Defines the system groups and lists which users belong to each group
  • /etc/sudoers: Controls which users or groups are granted administrative privileges via the sudo command
• Networking & DNS
  • /etc/hosts: Maps hostnames to IP addresses locally before DNS lookup
  • /etc/resolv.conf: Defines DNS resolvers (IP addresses of name servers)
  • /etc/hostname: Sets the system’s unique network hostname
• Core System Configuration
  • /etc/fstab: Lists filesystems and devices to mount automatically during boot
  • /etc/environment: Global environment variables available to all users/processes
  • /etc/os-release: Linux distribution and OS version information
• Automation & Services
  • /etc/crontab: Scheduled system-wide tasks
  • /etc/ssh/sshd_config: SSH server settings, including port numbers and remote login access rules
  • /etc/default/grub: Boot loader configuration (default OS selection, boot countdown timers, etc.)

Initial RAM Disk

• https://en.wikipedia.org/wiki/Initial_ramdisk
• Load temporary root file system into RAM
• make preparations before the real root file system can be mounted
• Ways to achieve:
  • initrd: initial RAM Disk
    • stored in /initrd
  • initramfs: Initial RAM Filesystem
    • stored in /boot

man

• /usr/share/man
  • man1: User programs
  • man2: System calls
  • man3: Library functions and subroutines
  • man4: Special files like /dev
  • man5: File formats
  • man6: Games
  • man7: Miscellaneous
  • man8: System administration Programs

# see man page of netstat
man netstat
 
# bind(2): see section 2 of bind
man 2 bind
 
# see man page of man
man man

Multiple man pages

• https://stackoverflow.com/questions/36789974/what-is-the-difference-between-multiple-man-pages-of-the-same-command
• socket(2) --- System call
• socket(3) --- POSIX interface
• socket(7) --- General topics on sockets