Summary
This lesson presents a summary of this chapter on files and directories along with some key file system terms.
The file system interface in UNIX systems (and indeed, in any system) is seemingly quite rudimentary, but there is a lot to understand if you wish to master it. Nothing is better, of course than simply using it (a lot). So please do so! Of course, read more; as always,
ASIDE: KEY FILE SYSTEM TERMS
A file is an array of bytes which can be created, read, written, and deleted. It has a low-level name (i.e., a number) that refers to it uniquely. The low-level name is often called an i-number.
A directory is a collection of tuples, each of which contains a human-readable name and low-level name to which it maps. Each entry refers either to another directory or to a file. Each directory also has a low-level name (i-number) itself. A directory always has two special entries: the
.
entry, which refers to itself, and the..
entry, which refers to its parent.A directory tree or directory hierarchy organizes all files and directories into a large tree, starting at the root.
To access a file, a process must use a system call (usually,
open()
) to request permission from the operating system. If permission is granted, the OS returns a file descriptor, which can then be used for read or write access, as permissions and intent allow.Each file descriptor is a private, per-process entity, which refers to an entry in the open file table. The entry therein tracks which file this access refers to, the current offset of the file (i.e., which part of the file the next read or write will access), and other relevant information.
Calls to
read()
andwrite()
naturally update the current offset. Otherwise, processes can uselseek()
to change its value, enabling random access to different parts of the file.To force updates to persistent media, a process must use
fsync()
or related calls. However,, so think carefully when doing so. doing so correctly while maintaining high performance is challenging “All File Systems Are Not Created Equal: On the Complexity of Crafting Crash-Consistent Applications” by Thanumalayan S. Pillai, Vijay Chidambaram, Ramnatthan Alagappan, Samer Al-Kiswany, Andrea C. Arpaci-Dusseau, and Remzi H. Arpaci-Dusseau. OSDI ’14, Broom- field, Colorado, October 2014. The full conference paper on this topic – with many more details and interesting tidbits than the first workshop paper above. To have multiple human-readable names in the file system refer to the same underlying file, use hard links or symbolic links. Each is useful in different circumstances, so consider their strengths and weaknesses before usage. And remember, deleting a file is just performing that one last
unlink()
of it from the directory hierarchy.Most file systems have mechanisms to enable and disable sharing. A rudimentary form of such controls are provided by permissions bits. Whereas, the more sophisticated access control lists allow for more precise control over exactly who can access and manipulate information.
Get hands-on with 1200+ tech skills courses.