Paths, Links, and Inodes#

Concepts#

How Linux Stores Files — Inodes#

To understand links, you first need to understand how Linux actually stores files.

Every file on a Linux filesystem has two parts:

1. The data — the actual content of the file (bytes on disk).
2. The inode (index node) — a data structure that stores metadata about the file:
   • File type (regular file, directory, symlink, etc.)
   • Permissions (read, write, execute)
   • Owner and group
   • File size
   • Timestamps (created, modified, accessed)
   • Pointers to the disk blocks where the data is stored
   • Link count — how many names point to this inode

Notice what the inode does not contain: the filename. The filename is stored in the directory that contains the file. A directory is essentially a table mapping names to inode numbers.

Directory entry:     filename → inode number
Inode:               metadata + pointers to data blocks
Data blocks:         actual file content

You can see a file’s inode number with:

ls -i filename
stat filename

Hard Links#

A hard link is an additional name (directory entry) that points to the same inode. The file has one set of data and one inode, but multiple names.

# Create a file
echo "Hello" > original.txt

# Create a hard link
ln original.txt hardlink.txt

Now original.txt and hardlink.txt are two names for the exact same file:

ls -li original.txt hardlink.txt
# Same inode number, same size, link count = 2

Directory:
  original.txt  → inode 12345
  hardlink.txt  → inode 12345   (same inode!)

Inode 12345:
  data: "Hello"
  link count: 2

Key properties of hard links:

• Same inode: both names point to the same data. Editing one changes both.
• No “original”: after creating a hard link, neither name is more “real” than the other. Deleting one does not affect the other.
• Link count: the inode tracks how many names point to it. Data is only deleted when the link count reaches 0 (all names are removed).
• Cannot cross filesystems: a hard link must be on the same filesystem as the target (because inode numbers are per-filesystem).
• Cannot link to directories: hard links to directories are not allowed (to prevent loops in the filesystem tree).

Symbolic Links (Symlinks)#

A symbolic link (symlink or soft link) is a special file that contains the path to another file. It is a pointer by name, not by inode.

# Create a symlink
ln -s /home/kmiguel/Documents/report.txt ~/Desktop/report-link

Directory:
  report-link  → inode 99999 (a different inode!)

Inode 99999:
  type: symlink
  data: "/home/kmiguel/Documents/report.txt"   (the path, stored as text)

Key properties of symlinks:

• Different inode: the symlink is its own file, containing the path to the target.
• Can cross filesystems: since they store a path (text), they work across different disks and partitions.
• Can link to directories: no restriction.
• Can break: if the target file is moved or deleted, the symlink becomes a “dangling” link (points to nothing). It still exists but cannot be followed.
• Permissions are irrelevant: symlink permissions are always lrwxrwxrwx. Access depends on the target’s permissions.
• Visible in ls -l: shown with -> pointing to the target.

ls -l ~/Desktop/report-link
# lrwxrwxrwx 1 kmiguel kmiguel 42 Oct 15 12:00 report-link -> /home/kmiguel/Documents/report.txt

Hard Links vs Symlinks — Comparison#

In practice, symlinks are far more common because of their flexibility. Hard links are used in specific situations (backups, package management internals).

Seeing Inodes and Links in Practice#

# Show inode numbers
ls -i

# Detailed file information including inode
stat filename

# Find all hard links to a file (by inode number)
find / -inum 12345 2>/dev/null

# Find broken symlinks
find /path -xtype l

Absolute vs Relative Symlinks#

Symlinks can store either absolute or relative paths:

# Absolute symlink — always works regardless of the symlink's location
ln -s /home/kmiguel/Documents/report.txt ~/Desktop/report-link

# Relative symlink — relative to the symlink's location
cd ~/Desktop
ln -s ../Documents/report.txt report-link

Relative symlinks are portable (they still work if the whole directory tree is moved), but absolute symlinks are easier to understand and debug.

Lab#

Exercise 1: Examine Inodes#

mkdir -p ~/lab/links
cd ~/lab/links

# Create a file
echo "This is the original file" > original.txt

# See its inode number
ls -i original.txt

# Get detailed inode information
stat original.txt
# Note: Inode number, Links count (should be 1), Size, Timestamps

Exercise 2: Hard Links#

# Create a hard link
ln original.txt hardlink.txt

# Compare inode numbers — they should be identical
ls -li original.txt hardlink.txt

# Check link count — should be 2 for both
stat original.txt | grep Links
stat hardlink.txt | grep Links

# Modify through one name, see the change through the other
echo "Added a line" >> hardlink.txt
cat original.txt
# You should see "Added a line" — both names share the same data

# Delete the original — the data survives
rm original.txt
cat hardlink.txt
# Still works! Link count is now 1.

# Verify
stat hardlink.txt | grep Links

Exercise 3: Symbolic Links#

# Create a new file
echo "Symlink target content" > target.txt

# Create a symlink
ln -s target.txt mylink

# Examine the symlink
ls -l mylink
# Shows: mylink -> target.txt

# Different inode numbers
ls -i target.txt mylink

# Read through the symlink
cat mylink
# Shows the content of target.txt

# Delete the target — the symlink breaks
rm target.txt
ls -l mylink
# Still shows mylink -> target.txt, but in red (broken link)

cat mylink
# Error: No such file or directory

# Recreate the target — the symlink works again
echo "Recreated content" > target.txt
cat mylink
# Works!

Exercise 4: Symlinks to Directories#

# Create a directory with files
mkdir -p ~/lab/links/mydata
echo "file inside mydata" > ~/lab/links/mydata/info.txt

# Create a symlink to the directory
ln -s ~/lab/links/mydata ~/lab/links/data-shortcut

# Navigate through the symlink
ls ~/lab/links/data-shortcut/
cat ~/lab/links/data-shortcut/info.txt

# The symlink behaves like the directory itself
cd ~/lab/links/data-shortcut
pwd
# Note: might show the symlink path or the real path depending on the shell
pwd -P
# Shows the real (physical) path, resolving all symlinks

Exercise 5: Observe System Symlinks#

# The usr-merge symlinks
ls -la /bin
ls -la /sbin
ls -la /lib

# Common symlinks in /etc
ls -la /etc/alternatives/ | head -10
# The alternatives system uses symlinks to manage default commands

# Python symlinks
ls -la /usr/bin/python3

Exercise 6: Hard Link Limitations#

cd ~/lab/links

# Try to hard link to a directory (fails)
mkdir testdir
ln testdir testdir-hardlink
# Error: hard link not allowed for directory

# Try to hard link across filesystems (fails if /tmp is a separate filesystem)
echo "test" > /tmp/crossfs.txt
ln /tmp/crossfs.txt ~/lab/links/crossfs-hardlink
# May fail with: Invalid cross-device link

# Clean up
rm -f /tmp/crossfs.txt
rmdir testdir

Review#

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