Advanced Programming in
the UNIX Environment
Week 04, Segment 1: The Unix Filesystem
Department of Computer Science
Stevens Institute of Technology
Jan Schaumann
jschauma@stevens.edu
https://stevens.netmeister.org/631/
CS631 - Advanced Programming in the UNIX Environment
The Unix Filesystem
• a disk can be divided into logical partitions
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
The Unix Filesystem
physical blocksize
• a disk can be divided into logical partitions
first NetBSD partition
second NetBSD partition
NetBSD portion of disk
3 entire disk
Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
The Unix Filesystem
• on each logical partition you may create a file system containing the cylinder groups
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
The Unix Filesystem
• each cylinder group contains a list of inodes (i-list) as well as the actual directory-
and data blocks
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
The Unix Filesystem
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
The Unix Filesystem
#123
• data blocks containing the actual data (i.e., contents of the file) are referenced from
the inode
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
The Unix Filesystem
#123
#123
• a directory entry is really just a hard link mapping a “filename” to an inode
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
The Unix Filesystem
#123
#123
#123
• a directory entry is really just a hard link mapping a “filename” to an inode
• you can have many such mappings to the same inode
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
The Unix Filesystem
• directories are special "files" containing a list of hard links
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
The Unix Filesystem
• each directory contains at least two entries:
• "." -- this directory
• ".." --
Jan Schaumann the parent directory 11
2020-09-17
CS631 - Advanced Programming in the UNIX Environment
The Unix Filesystem
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
Inodes
• The inode number in a directory entry must point to an inode on the same file system
(no hardlinks across filesystems).
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
Inodes
• The inode number in a directory entry must point to an inode on the same file system
(no hardlinks across filesystems).
• The inode contains most of the information found in the struct stat.
• Every inode has a link count (st_nlink): it shows how many “things” point to this
inode. Only if this link count is 0 (and no process has the file open) are the data
blocks freed.
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
Inodes
• To move a file within a single filesystem, we can just ”move” the directory entry
(actually done by creating a new entry, and deleting the old one).
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
Inodes
• To move a file within a single filesystem, we can just ”move” the directory entry
(actually done by creating a new entry, and deleting the old one).
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
Inodes
• To move a file within a single filesystem, we can just ”move” the directory entry
(actually done by creating a new entry, and deleting the old one).
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Jan Schaumann 2020-09-17
CS631 - Advanced Programming in the UNIX Environment
The Unix Filesystem
Visualizing the Unix Filesystem helps us understand the concept of hard links, what
directories "look like", and how operations on a directory are independent of the files
and their data.
Coming up: creating, removing, and renaming links (hard and symbolic)
link(2) unlink(2) rename(2) symlink(2) / readlink(2)
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