Linux Basics/The structure of Linux

Basically it's CLI like DOS and Windows's command prompt There are graphical user interfaces too (GUI) Filesystem thumb|FAT structure There are no letters at disks, like in DOS/Windows -> Everything is under / It mounts (connects) the external storages Nowadays Windows is also capable of it The /etc/fstab file contains the automatical mount point of partitions and settings associated with them. fdisk or fdisk -l commands, latter one lists the partitions. Under /mnt we can find the mounted mediums (but that's not true mostly because in modern partitions the automatically mounted mediums like flash drives usually get mounted to /media, but that's true that /mnt is created to mount partitions there) thumb|EXT structure FAT-table contains entries for every cluster on a continuous area of the medium. Every entry contains the next cluster's number in a file or the EOF (end of file), unused disk space and the specially reserved space of the disk. It only can do soft-link. EXT filesystem: It uses inode(Index node) table. In inode there are the files' description and the place of the data blocks, without filename. There is only one inode for each file. Files with bigger size may contain multiple catalog-entries, which contains the filename. Inode contains the name, description, creation date, permissions, owner(UID), group(GID), and how many links are attached to it. (so it can do much more than FAT) It can do soft-link and hard-link, which is one of the biggest advantage of EXT file system to FAT. Soft-link: shortcut, if the original gets deleted, then it deletes itself, or it says error that it doesn't find the executable. Hard-link: A copy of the file, but if we change the original one, then the content of the copy changes, but the file will only disappear eventually, if all the copies are deleted. Properties of filesystem It attaches permissions separately to every file It limits permission to the owner or groups Read, write, execute permissions File managers thumb|Midnight Commander under Fedora 29. mc (Midnight Commander) - CLI - it's based on Norton Commander Gnome Commander - GUI and the graphical file managers that you get with the Desktop Environment you use: thunar, nautilus, rox-filer, dolphin, pcmanfm, stb. (https://www.tecmint.com/top-best-lightweight-linux-file-managers/) File managing: operations with files Kernel Kernel = system core, the "soul" of the OS Tasks of it: handling peripherals, handling filesystem, implementing directory structure, taking control of processor Types of kernels: Monolithic: The kernel consists of one unbreakable unit module. It's common for older Linuxes (older than 2.0) and UNIX systems. Hybrid: like Windows – permissions can be set. Hybrid kernels are basically microkernels in which some "not important" code parts have been moved from user level (userspace) to kernel level (kernelspace) using fewer abstractions to run faster. Microkernel: Kernel is in multiple small programs, not in one big module. Modular: The modular kernel tries to fix the monolithic kernel's problems. Parts of the kernel - physical and logical - and modules created by this, which are mostly drivers can be loaded into memory. It saves the size of the kernel, but both together becomes bigger and bigger (its size is relative though, it depends how many drivers do you want to add, complete kernels are mostly around 100 MB or above, but if you are minimalist, it can be under 10 MB). We put modern Linux systems under the modular term (to be punctual, Linux kernel is monolithic modular). The kernel modules can be found in /lib/modules/kernel-version folder. Place of kernel: It's in the /boot folder, and there are soft-links to vmlinuz and initrd in the root. Filesystems At filesystems arrays have huge role in Linux. Virtual filesystems (for the system) SMBFS, SSHFS, NFS, etc. Supported filesystems: ext2, ext3, ext4 (own filesystem of Linux) ReiserFS, Btrfs, ZFS, XFS FAT32, NTFS stb. Directory structure Hierarchical directories (folders) files Single-level directory: Commodore floppy; magnetic tape Path Root: / e.g.: /home/joe/myfile.txt Connected (mounted) media e.g. /mnt/usb Mounting: mount /dev/scd -t ISO9660 /mnt/cdrom (if we want to use CD-ROM) mount /dev/sdb -ta /mnt/[name], where -ta indicates automatic detection of the media's type. The sda,sdb, sdc… indicates the HDD,SSDs, and modern distros mount them to /media/[partition name]. Unmount / disconnect: umount /dev/sdb Main folders /bin, /sbin executable, binary files (only admin can run sbin programs), no file extension needed /boot system startup files: GRUB, LILO, kernel (/boot/vmlinuz) /cdrom, /media, /mnt media mounting: USB, Floppy, CD/DVD /dev we can reach the devices through files e.g.: /dev/dsp (soundcard) /dev/cdrom (CD) /dev/sda2 (first HDD/SSD's 2nd partition) /etc configuration file sysadmin has only access to it global configuration folder /home users' folder user's files settings, configuration files, folders for eg.: /home/peter /home/peter/documents /lib shared objects, libraries like DLL files in Windows kernel modules /lost+found damaged files recoverables /proc currently running operations - numbered by files and information of the system(processor, memory etc) example command (processor information): cat /proc/cpuinfo /tmp location of temporary files both system and user level /usr location of user files programok installed programs libraries sources (kernel): /usr/src documentations: /usr/share/doc, /usr/doc icons, and so on. /var collection of services cache(temporary storage): /var/cache log files: /var/log/maillog email accounts (mailbox): /var/mail/root www folder: the default webpage folder of apache (/var/www) Users system administrator: root users: „root” user creates them basically users can be grouped permissions can be given in the filesystem e.g.: /home/joe/myfile.txt 26 joe joe -rwxrwxrwx The first user has also root permissions (which you set in the installation) - but this depends on distribution, e.g. Debian users have no root permission by default, and there is no sudo.