§39.15–39.16Symbolic Links, Permissions, and Access Control

Hard links are powerful but limited — you can’t hard-link a directory (cycle risk), and you can’t hard-link across file systems (inode numbers are only unique within one file system). A second kind of link lifts those limits.

39.15 Symbolic Links

A symbolic link symbolic link A soft link: a distinct file type whose data is the pathname of another file (ln -s). Unlike a hard link it can cross file systems and point to directories, but if the target is removed it becomes a dangling reference (points to a nonexistent path). Its size equals the length of the stored pathname. defined in ch. 39 — open in glossary (or soft link) is made with the same ln program plus -s. On the surface it looks like a hard link, but it is fundamentally different: a symlink is its own file, of a distinct type (the third type the file system knows, after regular files and directories). Its data is simply the pathname of the file it points to — so reading the link resolves through that path.

That path-based design is exactly why removing the target leaves a dangling reference — the link still holds the text of a path that no longer resolves. A hard link can’t dangle, because it keeps the inode alive through the link count. Here’s the full contrast:

39.16 Permission Bits And Access Control Lists

The process abstraction gave each program its own private CPU and memory. Files are different — they’re commonly shared among users and processes — so file systems need mechanisms to control that sharing. The classic one is the UNIX permission bits permission bits The classic UNIX access control: nine bits shown as rwxrwxrwx giving read/write/execute for owner, group, and other. Changed with chmod. For a regular file, execute means runnable; for a directory, execute allows cd/traversal into it. A coarser model than access control lists. defined in ch. 39 — open in glossary , the rwxrwxrwx you see in ls -l:

prompt> ls -l foo.txt
-rw-r--r-- 1 remzi wheel 0 Aug 24 16:29 foo.txt

The first character is the file type (- regular, d directory, l symlink). The next nine are three groups of read / write / execute — for the owner, the group, and other (everyone else). You change them with chmod, whose octal digits add read = 4, write = 2, execute = 1:

The execute bit is special. On a regular file it decides whether the file can be run as a program; on a directory it instead grants the right to enter (cd into) and traverse it.

Permission bits are coarse — just owner/group/other. Some systems (e.g. the distributed file system AFS) add an access control list access control list An ACL: a more general, precise way than permission bits to say exactly who may access a resource and how, used by systems like AFS (per-directory). Where permission bits offer only owner/group/other, an ACL can list specific users and rights (read, write, lookup, insert, delete, administer, lock). defined in ch. 39 — open in glossary (ACL): a precise, per-directory list of exactly which users have which rights (read, write, lookup, insert, delete, administer, lock) — far more expressive than three groupings.