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(2) UNIX File System

The UNIX File System

Files and directories live in the file system and have a "location", the location is called the "path". Absolute pathnames are unique to a system.
When logged in you have a current working directory (pwd -- print working directory). The current directory is initially set to your home directory ($HOME).
A "path" can be absolute or relative
Absolute Path
- Absolute path is unique
- Starts with root directory (/), or
- Can start with a home directory (~) or (~username)
Relative Path
- Any pathname which is not an absolute path is interpreted as relative to the current directory.
- May start with the name of a subdirectory. E.g. cd classes
- May explicitly specify the current directory (./)
- May specify the parent directory using ../, or higher up the directory tree, by stacking the ".." (../../..)
Multiple file systems make up the complete directory hierarchy
- All Unix systems have a root filesystem, mounted on /
- A file system is physically a partition of a disk, a whole disk, a logical combination of disks, a removeable device (CDROM), a remotely mounted fileserver, or possibly a virtual construction made to resemble a directory hierarchy
- File systems are mounted and become a part of the hierarchy. They are not referred to as separate entities (unlike MacOS volumes and Windows drive letters).
- df shows mounted file systems
- For most purposes, you do not need to know where a file physically resides, only the pathname to it.
Symbolic Links
- Symbolic links are files that point to other files or directories, similar to aliases in MacOS or shortcuts in Windows. ls -li shows links. There is only one 'real' file.
- Good for sharing things while maintaining only one copy
- ln -s real_file link_name
  e.g.
  ln -s hosts1 h1
  ln -s ~richard/public/sendmail mail-notes
- Symbolic links can form loops, which is not usually what you wanted - be careful.
- Symbolic links make it possible to refer to the same file using many different pathnames - sometimes confusing.
Hard Links
It is possible to have multiple names for the same file. These are called hard links and unlike symbolic links, there is no distinction between the "real" file and other links to it - they are all equivalent. Hard links tend to be confusing.
Unix actually references all files internally using a numeric value (unique in a given filesystem). The directory itself is just a special file storing human-readable names and their equivalent identifiers (called inodes). Several features are made possible by this arrangement:
- File names can be almost unlimited length, since they aren't moved around internally
- File name lookups are very fast. Obtaining the additional information about a file takes time, (to retrieve the inode data) but simply looking up the names is fast.
- The "." and ".." entries in a directory are trivially implemented as hard links.
- Column 2 of ls -l output is the number of links to a file or directory
- e.g. ls -l /usr/bin/mc*
- e.g. ls -li /usr/bin/mc*
- Creating a hard link:
  The same command (ln) is used to create hard links and symbolic links, but with no option. ln existing-file-name new-file-name
- The system call to delete a file is unlink(). When the link count reaches zero, the filesystem actually deletes the file.

slide02.src last modified Feb 8, 2011

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