Linux filesystem

Linux file names are case-sensitive.

Folders are referred to as Directories (if you’re coming from Windows). There are no Local Disks in Linux, everything is stored in the root directory. However, you can mount different directories to different partitions or storage devices if you want.

The linux file system layout is defined in the FHS (Filesystem Hierarchy Standard). However, some linux distributions don’t really follow it exactly. Several directory structuring styles have also changed over the years.

/ - Root directory contains everything that is needed to run the system. If the linux kernel is the brain, the root directory is akin to the heart of the system.
/bin - Short for binary. Contains the most basic binaries necessary for the system, are present here such as ls, cat, etc.
/sbin - System binaries. These are binaries that a system administrator would use. A standard user won’t have access to this directory without proper permissions.

Both the /bin and /sbin directories contain the binaries that are necessary for running the system in single user mode.

/boot - Contains everything an os needs to boot. Better not touch anything here.
/cdrom - legacy mounting point for CD ROMS. Might not be present in all distros.
/dev - Devices live here. Hardware devices such as keyboard, mouse, hard disks, etc are present. Disks are referred to by the files sdx.
/etc - etcetera. This directory contains system wide configuration files.
/lib, /lib32, /lib64 - libraries are stored here. Required for different binaries.
/media, /mnt - Contains mounted drives such as hard disks, usb drives, hard drives, etc. The /media directory is used in recent distros to manage disk drives by the system. When manually mounting something, use the /mnt and let the /media directory to be managed by the os.
/opt - optional directory. Usually contains vendor provided packages. You can place the applications that you created here.
/proc - contains information about system processes and resources. You can also find information about cpu such as cat /proc/cpuinfo and find out the uptmie of the system cat /proc/uptime.
/root - Home directory for the root user. Unlike a typical user, it does not contain all the directories found inside a user’s home directory. Note that, this resides outside the /home directory. You need root permission to access files in this directory. Why is the root directory present in /root and not in /home/root? The answer is, so that the root user can access his/her home directory even if the /home directory is mounted on another partition or disk which may not be available.
/run - fairly new, found in recent distros. Its a tempfs (temporary file system), which means everything present here resides in the RAM. So everything is gone, when you shut down/reboot the system.
/snap - [not standard] contains the Snap package management related files and directories. This is found in Ubuntu and any system that utilizes the snap packaging system.
/srv - Service directory. Service data is stored here for example when you run a web server or a ftp server, you would want to place your files or directories that you want to serve, here for the users to access.
/sys - System directory. Present around for a long time. Its a way to interact with the kernel. Similar to the /run directory in that, this directory does not actually physically exist, it is created evertime the system boots up.
/tmp - Temporary directory - Used by applications to store temporary files here such as files in word processors that you are currently editing, etc. Should be safe to delete stuffs from this directory.
/usr - User application space. Applications used by the users are stored here as opposed to /bin and /sbin which contains applications for the system. Also known as “Unix System Resource”. Applications present here are considered non-essential for the os to function properly. Most program installed from source code will usually end up in /usr/local directory. Larger programs might install into the /usr/share directory. Installed source code will go into /usr/src.
/var - Variable directory. Contains files and directories that are expected to grow in size. For example /var/log contains the logs for different applications, /var/crash contains crash logs, /var/cache to store different cache items.
/home - Home directory for users. Each user has his/her own home directory in the format /home/username. Contains all user related data such as user specific configuration files, cache, etc. /home/username/.themes and /home/username/.icons contain themes and icons that are available to the current user. If you want to save all your settings, this is the directory that you want to backup. After upgrading or updating to a new system, restoring the /home directory means that all your settings will stay as is even if you reinstall applications.