POSIX is the Portable Operating System Interface standard, IEEE Std 1003.1-1988 and related. These standards, based on the Unix operating system, define a set of programming and command interfaces. Programs and scripts following these standards are supposed to be easily portable between operating system platforms providing these interfaces.
The POSIX standards imply a model for file system organization: POSIX file systems are organized as directories (folders) containing files (documents). The POSIX programming API defined a number of operations to work with files, directories, and entire file systems:
umountthe file system
readto/from file descriptor
fcntl(byte range locks, etc.)
- Path names are case sensitive, components are separated with forward slash (
Essentially, this is the API that evolved for Unix and adopted by Linux.
POSIX file systems are treated as a sequence of bytes. However, internally the data content of a file is stored as logical sequence of file system blocks:
- Each block is a fixed number of bytes. The last block might not be full.
- Within a block, all the bytes are sequential. However, within a file, the blocks might not reside sequentially on the disk.
- Underlying storage systems are usually organized as blocks, and ideally file system blocks are aligned with the storage system’s blocks.
File systems also contain metadata, i.e., “data about the data”:
- There is an inode for each file or directory, providing:
- Locations of the data blocks for the file
- Attributes about the file, like “time last accessed” or “owner of the file”
- The inode table records where each inode is located, indexed by number.
- Directories are special files listing the names and inode numbers of files under the folder.