Linux - Filesystem

Inode

On every Unix-style filesystem (ext4, XFS, APFS, etc.), each regular file, directory, FIFO, … is represented on disk by a tiny metadata record called an inode (“index node”). That’s like a file’s identity card

Inside the filesystem, a directory is just a table with rows (or directory entry) like

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<FILENAME>   →   <inode number>

• FILENAME is something like vacation.jpg
• The inode number is an integer key that the kernel uses to find the inode structure on disk; that inode holds the object’s metadata and the list of disk blocks where its data live.
  • The metadata inode has includes: type, size, timestamps, block pointers, …

This way you can have many hard links (filenames TODO?) all referring to the same directory inode. Also, this means renaming a file does not change the file’s data, only the directory data?

Soft and Hardlinks

Softlink

Symlinks are a.k.a softlinks. They are a separate inode whose data is a text path to the target. Deleting or moving the target leaves the link dangling. It can be:

• The pathname can point to a file on another mounted volume.
• It’s easily breakable, if the target’s filepath has changed.
• ls -l shows lrwxrwxrwx 1 … -> target
• Create a symlink: ln -s /path/tofile linkname

Hardlink

Because any directory entry can point to any inode on the same filesystem, you can create more than one entry that references the exact same inode number. Such an entry is a “hardlink”

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ln original.txt copy.txt      # hard-link

After that:

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2
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$ ls -li
12345 -rw-r--r-- 2 alice users … original.txt
12345 -rw-r--r-- 2 alice users … copy.txt

Both names map to inode 12345; the link-count field (the third column) is now 2. These two names are equal peers: delete either one (rm original.txt) and the file’s data stay on disk

Some notes are:

• Hard-linking a directory is disallowed on almost all Unix systems to prevent infinite directory loops; the only directory hard links that exist are the automatic “.” and “..” entries that every directory contains.
• It does not work across file systems.
• On most linux systems, they cannot point to directories.
• If the target’s filepath has changed, this wouldn’t be broken
• ls -l shows identical size/perm as original file

Example of cross-filesystem boundary

Imagine your laptop has:

• A primary root filesystem mounted at /
• An external USB drive auto-mounted at /media/usbdrive

You keep your photo library on the USB drive but want a convenient alias in your home directory:

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ln -s /media/usbdrive/Photos/2024/Vacation ~/Pictures/Vacation2024

Note that hard links cannot be created cross file-system boundaries

FIFO (a.k.a Named Pipe For Its Behavior)

A FIFO special file is a rendez-vous point in the filesystem that lets two or more processes exchange a byte stream, first-in/first-out, just like the anonymous pipe you get from pipe().

• You create one with mkfifo mypipe.
• The directory entry holds no data; the kernel moves the bytes directly between processes once both ends are open.
• Because it lives in the directory tree, unrelated processes can open it by name (unlike an anonymous pipe).

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