Hard links and symbolic links with ln

In lesson 19 you saw that a file is really an inode, a numbered record holding the data, the size and the permissions, and that the name you see in a directory is just a label pointing at that inode. Once you know that, links are easy. A hard link is a second label for the same inode. A symbolic link (symlink) is a tiny separate file whose only content is a path to somewhere else.

Hard links: one file, two names

~% mkdir -p lab/l91 && cd lab/l91
l91% echo "version one" > data.txt
l91% ln data.txt backup.txt
l91% ls -li
   6985 -rw-r--r--    2 root     root            12 Oct  7 19:41 backup.txt
   6985 -rw-r--r--    2 root     root            12 Oct  7 19:41 data.txt
l91% rm data.txt
l91% cat backup.txt
version one

ln TARGET NAME creates the hard link. ls -i prints the inode number first, and both names show 6985: they are the same file. The number after the permissions, 2, is the link count; it was 1 before. Neither name is the original: edit through one and the other sees the change, and the data only disappears when the last name is removed, which is why cat backup.txt still works after rm data.txt. Hard links have two limits: they cannot point at a directory (ln: hd: Operation not permitted) and they cannot cross from one filesystem to another, because inode numbers are only unique inside one filesystem.

Symbolic links: a signpost

l91% mkdir -p releases/v1 releases/v2
l91% ln -s releases/v2 current
l91% ln -s nowhere.txt broken
l91% ls -l
lrwxrwxrwx    1 root     root            11 Oct  7 19:41 broken -> nowhere.txt
lrwxrwxrwx    1 root     root            11 Oct  7 19:41 current -> releases/v2
drwxr-xr-x    4 root     root             0 Oct  7 19:41 releases
l91% readlink current
releases/v2
l91% readlink -f current
/root/lab/l91/releases/v2
l91% cat broken
cat: can't open 'broken': No such file or directory

ln -s TARGET NAME writes the path TARGET into a new link called NAME. ls -l marks it with an l and shows the arrow. Symlinks can point at directories and at other filesystems, which is why they are used far more often than hard links: /var/spool in this VM is itself a symlink to ../tmp. The price is fragility. The link stores a path, not an inode, so if the target is moved or deleted the link becomes dangling: it still exists, but opening it fails. readlink prints the stored path; readlink -f resolves it all the way to an absolute path.

Repointing a link

ln -s refuses to overwrite an existing name (ln: current: File exists). Add -f to replace it: ln -sf releases/v1 current. When the old link points at a directory, also add -n, so ln treats the link itself as the destination instead of creating a new link inside the directory it points to: ln -sfn releases/v1 current. This one-line switch is how many deployments flip between versions. Removing a link is just rm current; it deletes the link, never the target. Do not add a trailing slash (rm current/), which refers to the directory behind it.

In scripts, [ -L path ] is true for a symlink (even a dangling one), while [ -e path ] follows the link and is false when it dangles; together they detect broken links. find /dir -type l lists every symlink below a directory.