kb/filesystem ── File system ── beginner

Hard link

Hard link is a second name for the same [[inode]]. Both names are equal: the file lives as long as at least one of them exists.

view as markdownaka: hardlink, hard-links

What a hard link is

The command ln src dst (no flags) creates a hard link, a new entry in the directory that points to the same inode as src. After that:

src and dst have the same inode number (ls -i shows it)
stat on both reports Links: 2 (the number of names went up)
changes made through either name are visible through the other; it is one file
rm src leaves the file intact: the blocks are freed only when the last name is gone

Limits

A hard link can only be made within one filesystem, since an inode is local to a specific FS. And only to regular files: a hard link to a directory is forbidden (it would break the acyclic structure of the file tree; . and .. are the exception).

For cross-filesystem references and links to directories, use symbolic-link.

Where you meet it in practice

Snapshot backups (rsync --link-dest): between backups, unchanged files keep hard links to each other, which saves a huge amount of space byte for byte
Deduplication in /usr/share/doc and Docker layer images (on ext4)
Atomic publishing: ln new staging followed by mv -T staging old guarantees that at any moment old points to either the old or the new content, with no in-between state

§ команды

bash

ln a.txt b.txt && ls -li a.txt b.txt

Create a hard link and confirm both share one inode

bash

stat a.txt

The Links field shows how many names point to this inode

bash

find . -inum 12345

Find every name that points to a specific inode

§ см. также

§ упоминается в уроках

kb/filesystem ── File system ── beginner

Hard link

Hard link is a second name for the same [[inode]]. Both names are equal: the file lives as long as at least one of them exists.

view as markdownaka: hardlink, hard-links

What a hard link is

The command ln src dst (no flags) creates a hard link, a new entry in the directory that points to the same inode as src. After that:

src and dst have the same inode number (ls -i shows it)
stat on both reports Links: 2 (the number of names went up)
changes made through either name are visible through the other; it is one file
rm src leaves the file intact: the blocks are freed only when the last name is gone

Limits

For cross-filesystem references and links to directories, use symbolic-link.

Where you meet it in practice

Snapshot backups (rsync --link-dest): between backups, unchanged files keep hard links to each other, which saves a huge amount of space byte for byte
Deduplication in /usr/share/doc and Docker layer images (on ext4)
Atomic publishing: ln new staging followed by mv -T staging old guarantees that at any moment old points to either the old or the new content, with no in-between state

§ команды

bash

ln a.txt b.txt && ls -li a.txt b.txt

Create a hard link and confirm both share one inode

bash

stat a.txt

The Links field shows how many names point to this inode

bash

find . -inum 12345

Find every name that points to a specific inode