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Unionfs is a filesystem service for Linux, FreeBSD and NetBSD which implements a union mount for other file systems.It allows files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system.
It combines multiple different underlying mount points into one, resulting in single directory structure that contains underlying files and sub-directories from all sources. Common applications overlay a read/write partition over a read-only partition, such as with LiveCDs and IoT devices with limited flash memory write cycles.
Squashfs is a compressed read-only file system for Linux. Squashfs compresses files, inodes and directories, and supports block sizes from 4 KiB up to 1 MiB for greater compression. Several compression algorithms are supported. Squashfs is also the name of free software, licensed under the GPL, for accessing Squashfs filesystems.
All of the Linux filesystem drivers support all three FAT types, namely FAT12, FAT16 and FAT32.Where they differ is in the provision of support for long filenames, beyond the 8.3 filename structure of the original FAT filesystem format, and in the provision of Unix file semantics that do not exist as standard in the FAT filesystem format such as file permissions. [1]
For example, overlay data can be stored on a USB flash drive, while a compressed SquashFS read-only image stored on a live CD acts as a root filesystem. [ 7 ] [ 8 ] Depending on which algorithms were compiled statically into it, the kernel can unpack initrd/initramfs images compressed with gzip , bzip2 , LZMA , XZ , LZO , LZ4 , [ 9 ] and zstd .
Most file systems include attributes of files and directories that control the ability of users to read, change, navigate, and execute the contents of the file system. In some cases, menu options or functions may be made visible or hidden depending on a user's permission level; this kind of user interface is referred to as permission-driven.
Generally, the mask only affects file permissions during the creation of new files and has no effect when file permissions are changed in existing files; however, the chmod command will check the mask when the mode options are specified using symbolic mode and a reference to a class of users is not specified.
The mount command instructs the operating system that a file system is ready to use, and associates it with a particular point in the overall file system hierarchy (its mount point) and sets options relating to its access. Mounting makes file systems, files, directories, devices and special files available for use and available to the user.