Hard links and symbolic links

Two “link” concepts live in Linux. Both make a file reachable from more than one path, but they do it in fundamentally different ways. Choosing the wrong one produces production surprises.

Hard links

A hard link is a directory entry that points at the same inode as another directory entry. Both paths resolve to the same file; they are the same file. Deleting one does not affect the other as long as the inode’s link count stays above zero.

$ echo hello > original.txt; ln original.txt hardlink.txt; ls -li original.txt hardlink.txt

12345 -rw-r--r-- 2 root root 6 Aug  9 11:11 original.txt
12345 -rw-r--r-- 2 root root 6 Aug  9 11:11 hardlink.txt

Symbolic links

A symbolic link (symlink) is a special file whose contents are a path string. When the kernel resolves a symlink, it reads the string and follows it. If the target does not exist, the symlink is dangling.

$ echo hello > original.txt; ln -s original.txt symlink.txt; ls -li original.txt symlink.txt

12345 -rw-r--r-- 1 root root 6 Aug  9 11:11 original.txt
12346 lrwxrwxrwx 1 root root 12 Aug  9 11:11 symlink.txt -> original.txt

Properties compared

Dangling symlinks: the operational risk

A dangling symlink points at a path that does not exist. Reading from or writing to the symlink returns ENOENT (no such file or directory). Many production failures are caused by symlinks that were valid at install time and became dangling later.

$ ln -s /opt/myapp/current/bin/run run-current; rm -rf /opt/myapp/current; ls -l run-current; ./run-current

lrwxrwxrwx 1 root root 23 Aug  9 11:11 run-current -> /opt/myapp/current/bin/run
bash: ./run-current: No such file or directory

1. Audit dangling symlinks regularly. find /etc /usr/local /opt -xtype l 2>/dev/null lists every dangling symlink on the searched paths
2. Prefer absolute target paths in production. A relative symlink depends on the directory it is in; an absolute path is unambiguous regardless of where you resolve from
3. Test symlinks before relying on them. A small CI step that checks every symlink expected to exist is cheap insurance
4. Prefer hard links for the same data at two paths use cases. They survive target deletion and have no dangling failure mode.

Atomic swap with symlinks

Symlinks are the canonical “atomic swap into place” tool in deployment scripts:

ln -s releases/v2 bin/myapp-current
mv -T bin/myapp-current bin/myapp   # atomic rename of the symlink

Renaming a symlink is atomic at the filesystem level; readers either see the old target or the new one, never a half-state. This is the basis of every blue/green deploy and every capistrano-style release directory layout.

Knowledge check