links

• Progress: Draft

Concepts

• Hard link - second directory entry pointing to the same inode as an existing file
  • No new inode, new data, or copying occurs
  • Example - ln report.txt backup.txt
  • Filenames report.txt and backup.txt map to the same inode number (example: 84213)
  • Increments the inode link count from 1 to 2
• Symbolic link (symlink) - separate file containing a path string pointing to another file
  • Has its own inode, size (path length), and permissions
  • Example - ln -s report.txt shortcut.txt
  • Allocates a new inode (example: 90517) of type symlink
  • Inode data stores the target path string report.txt
  • Creates a new directory entry mapping shortcut.txt to the new inode
  • Original file remains untouched and unaware of the symlink

How it works

• Inode link count tracks directory entries pointing to it (visible via ls -l or stat)
• Hard links are robust because data resides in the inode

File deletion process via rm

• Kernel invokes unlink() to remove the directory entry
• Kernel decrements the inode link count
• Filesystem frees the inode and data blocks when the link count reaches zero

Symbolic link resolution process (example: cat shortcut.txt)

• Kernel looks up shortcut.txt to find inode 90517
• Kernel detects inode type is symlink and reads target path report.txt
• Kernel restarts path lookup using the target path report.txt
• Kernel resolves report.txt to target inode 84213
• Kernel reads target inode data blocks

Comparison

Feature	Hard Links	Symbolic Links
Target Type	Files only	Files and directories
Scope	Single filesystem only	Cross-filesystem support
Inode usage	Share the same inode	Allocate a new inode
Visual representation	Same as standard file	Distinguishable (example: l permission flag)
Dangling vulnerability	Resilient to target deletion	Breaks if target moves or is deleted
Silent stale risk	No	Yes (if target is replaced)

• Symlinks can use absolute or relative paths
  • Example absolute - ln -s /etc/nginx/sites-available/blog enabled-blog
  • Example relative - ln -s ../sites-available/blog enabled-blog
• Dangling symlink - link pointing to a non-existent path (typically highlighted in red by ls -l)

Use Cases

• Incremental backups
  • Tools like rsnapshot or Apple Time Machine use hard links for unchanged files

backups/2026-05-28/home/alex/report.txt   (inode 84213)
backups/2026-05-29/home/alex/report.txt   (inode 84213)  ← unchanged, hard-linked
backups/2026-05-30/home/alex/report.txt   (inode 92841)  ← modified, new copy

• File deduplication
  • Replaces identical files with hard links to reclaim disk space
• Content-addressed package managers
  • pnpm hard-links package files from a global store into node_modules
• Git local clone optimization
  • git clone /path/to/local/repo hard-links object files instead of copying
• Versioned software redirection
  • Repointing a symlink enables fast software upgrades or rollbacks
  • Tools like nvm or pyenv use this pattern internally

/opt/node-18.17.0/
/opt/node-20.10.0/
/opt/node-current  →  /opt/node-20.10.0

• Shared library version chains

libssl.so       →  libssl.so.3
libssl.so.3     →  libssl.so.3.0.2
libssl.so.3.0.2    (the actual file)

• Dotfile management

~/.bashrc      →  ~/dotfiles/bashrc
~/.vimrc       →  ~/dotfiles/vimrc
~/.gitconfig   →  ~/dotfiles/gitconfig

• Service activation (Debian-style)
  • Link creation enables the service without modifying the original configuration file
  • systemd uses the same pattern inside .wants directories

/etc/nginx/sites-available/   ← all configs live here
/etc/nginx/sites-enabled/     ← symlinks to the active ones

• Cross-filesystem references
  • Moving directories (example: /var/log) to separate disks while retaining original paths
• System kernel interfaces
  • Special symlinks like /proc/self resolve to active calling process information
• Executable version selection
  • Debian update-alternatives system routes paths through intermediate symlinks

/usr/bin/python  →  /etc/alternatives/python  →  /usr/bin/python3.11

• Directory path shortening

ln -s /mnt/storage/clients/2026/acme-corp/engineering ~/work