Essential Commands:

mount: Gives an overview of all mounted devices.
findmnt: Shows mounts and the relationship that exists between the different mounts.
df: Shows available disk space on mounted devices.

General Notes:

Linux consists of one big logical filesystem tree.
There maybe be many distinct filesystems mounted to the system at various points appearing as subdirectories.
Shareable data: data that can be shared between hosts.
Non-shareable data: data belongs exclusively to one host.
Static data: binaries, libraries, documentations, and anything that does not change without sys-admin making the changes.
Variable data: anything that maybe change, even without sys-admin making changes.
Filesystem Hierarchy Standard(FHS): specifies the main directories that need to be present, and describes their purposes.
Directory Tree: Use Quizlet link to practice.
Root Directory(/): Must contain all essential files required to boot the system and then mount all other filesystems. Root directory must have the files to:
- Boot the system.
- Restore the system from backups on external media.
- Recover and/or repair the system. Note: no application or package should create new subdirectories of the root directory.

Environment Configuration Files:

/etc/profile: This is the generic file that is processed by all users upon login.
/etc/bashrc: This file is processed when subshells are started.
~/.bash_profile: In this file, user-specific login shell variables can be defined.
~/.bashrc: In this user-specific file, subshell variables can be defined.

Essential Directories:

/bin Directory: Important for the following:
- Contains executable programs and scripts needed by users which are required when no other filesystems have yet been mounted.
- Contains executable used indirectly by scripts.
- May not include any subdirectories. /usr/bin are for binaries that are required only by non-root users.
/boot Directory: Contains everything required for the boot process. The following files are essential:
- vmlinuz: The compressed Linux kernel
- initramfs: The initial RAM filesystem, mounted before the real root filesystem becomes available. Can also be initrd Also includes the following files for debugging:
- config: Used to configure the kernel compilation.
- System.map: Kernel symbol table, used for debugging.
/dev Directory: Contains special device files which represent devices built into or connected to the system. The files represent byte-stream and block I/O devices
/etc Directory: Contains machine-local configuration files and some startup scripts. Should not contain executable binary programs. It has the following important subdirectories:
- /etc/skel: Contains skeleton files used to populate newly created home directories.
- /etc/sysstemd: Contains or points to configuration scripts for starting, stopping, and controlling system services when using systemd
- /etc/init.d: Contains startup and shutdown scripts for when using System V initialization.
/home Directory: All personal, configuration, data, and executable progarams are placed under this directory. Subdirectories contain content for various groups and users. You can use $HOME or ~ to get to an user’s home directory.
/lib and /lib64 Directories: Contain libarries needed to execute the binaries in /bin and /sbin. These libaries are used for booting system and executing commands within root filesystem. Note: Some newer distributions just have one directory for lib and use symbolic links to preserve the two directories view.
/media Directory: Used to mount filesystems on removable media.
/mnt Directory: Use for temporarily mount a filesystem. Use for network filesystems like: NFS; Samba; CIFS; AFS.
/opt Directory: Designed for software packages that wish to keep all or most of their files in one isolated place. Directories: /opt/bin, /opt/doc, /opt/include, /opt/info, /opt/lib, and /opt/man are reserved for local sys-admin use.
/proc Directory: The mount point for pseudo-filesystem, where all information resides only in memory and not disk. The entries are considered to be virtual files which can be listed as zero bytes, but contain large amount of information.
Note: Important pseudo-files: /proc/interrupts, /proc/meminfo, /proc/mounts, /proc/partitions, /proc/filesystems. and /proc/sys/.
- /proc/scsi/ contains information for all physical SCSI devices. Likewise, the process directories contain information about each running process on the system.
/sys Directory: The mount point for the sysfs pseudo-filesystem where all information resides only in memory. Empty for non-running system. sysfs is used to gather information about the system, and modify its behavior while running.
/root Directory: Home directory for root user.
/sbin Directory: Contains binaries essential for booting, restoring, recovering, and/or repairing to those in /bin directory.
/srv Directory: Contains site-specific data served by the system. Ubuntu and Red Hat has /srv empty by default.
/tmp Directory: Used to store temporary files, and can be accessed by any other user or application. Avoid creating large files on /tmp as they occupy space in memory rather than disk.
To cancel policy for storing /tmp on disk, run:
sudo systemctl mask tmp.mount
Then follow by a system reboot.
/usr Directory: Can be thought of as a secondary hierarchy. Used for files not needed for system booting. Does not need to reside in the same partition as the root directory, and can be shared across a network. Software packages should not create subdirectories under /usr. Use symbolic links to other locations for compability. Typically only contains read-only data
/var Directory: Contaions variable data files that change frequently during system operation. These includes: log files; spool directories and files; administrative data files; temporary files such as cache contents.
Note: best practice to mount /var as a separate filesystem. /var/spool is for local files for processes such as mail, printing, and cronjobs.
/run Directory: Store transient files, files that contain runtime information. Generally implemented as an empty mount point, with a tmpfs ram disk (like /dev/shm) mounted there at runtime.
sudo du -shxc --exclude=proc * to show data usage of the directories.
Directories that contain special entries which abstract system properties, instead of data stored on the disk: /dev; /proc; /sys.