What Is a Symbolic Link?

It is a special file that contains a reference to a specific file or directory. You can think of it as similar to a shortcut in Windows.

Usage: Create a Symbolic Link

Usage is simple. Use the ln command, which uses the link system call. If you pass -s, it uses the symlink system call and creates a symbolic link.

$ ln -s [target path] [link path]

Target path can be relative or absolute. It is what the symlink points to. Although uncommon, you can also specify a non-existing target. See example:

# Create test file `test.txt`
$ echo "hi" > test.txt

# Create symlink `link_a` to test file
$ ln -s test.txt link_a
$ ls -l
lrwxrwxrwx  1 kimtaeng other    8 2020-09-02 18:39 link_a -> test.txt
-rw-r--r--  1 kimtaeng other    3 2020-09-02 18:38 test.txt

Usage: Delete a Symbolic Link

Use rm to remove a symbolic link. If you append / at the end of symlink name, deletion fails.

# Create test directory `test`
$ mkdir test

# Create symlink `link_sym` to test directory
$ ln -s test link_sym

# Run delete command
$ rm link_sym/
rm: cannot remove `link_sym/': Is a directory

# Remove `/` and run delete command
$ rm link_sym

Important caution: when deleting symlinks to source directories with options like -r and -f, files inside original directory can be removed in some cases.

# Create test directory `test`
$ mkdir test

# Create `test.txt` under `test`
$ echo "hi" > test/test.txt

# Create symlink `link_sym` to test directory
$ ln -s test link_sym

# Run delete command
$ rm -r link_sym/
rm: cannot remove `link_sym': Not a directory

# `test` directory remains, but files inside are deleted

Usage: Change Symbolic Link Target

When you want to retarget a symbolic link, you can delete and recreate it, but there is a more efficient method that keeps the link name.

$ ln -Tfs [new target path] [symbolic link to change]

Here, options except -s are new. -T treats destination as normal file path. -f overwrites existing symbolic link. Without -f, existing symlink target is not replaced.

Example:

# Create test directories `test1`, `test2`
$ mkdir test1 && mkdir test2

# Create symlink `link_a` to `test1`
$ ln -s test1 link_a

$ ls -l
lrwxrwxrwx 1 kimtaeng other    5 2020-09-02 18:40 link_a -> test1
drwxr-xr-x 2 kimtaeng other 4096 2020-09-02 18:40 test1
drwxr-xr-x 2 kimtaeng other 4096 2020-09-02 18:40 test2

# Try changing `link_a` to `test2` without overwrite, target remains unchanged
$ ln -s test2 link_a
$ ls -l
lrwxrwxrwx 1 kimtaeng other    5 2020-09-02 18:40 link_a -> test1
drwxr-xr-x 2 kimtaeng other 4096 2020-09-02 18:40 test1
drwxr-xr-x 2 kimtaeng other 4096 2020-09-02 18:40 test2

# Change target using `-Tfs`
$ ln -Tfs test2 link_a
$ ls -l
lrwxrwxrwx 1 kimtaeng other    5 2020-09-02 18:41 link_a -> test2
drwxr-xr-x 2 kimtaeng other 4096 2020-09-02 18:40 test1
drwxr-xr-x 2 kimtaeng other 4096 2020-09-02 18:40 test2

Hard Link vs Symbolic Link

If you run ln without -s, it creates a hard link. A hard link has the same inode as the original, so it remains usable even when original path is deleted.

You can verify this as below. Hard link shares inode with original. Symbolic link has different inode.

# Create test file `test.txt`
$ echo "hi" > test.txt

# Create symbolic link `link_symbolic` to test file
$ ln -s test.txt link_symbolic

# Create hard link `link_hard` to test file
$ ln text.txt link_ahrd

# Print current directory (including inode)
$ ls -li
164888621 -rw-r--r-- 2 kimtaeng other 3 2020-09-02 18:42 link_hard
164888626 lrwxrwxrwx 1 kimtaeng other 8 2020-09-02 18:42 link_symbolic -> text.txt
164888621 -rw-r--r-- 2 kimtaeng other 3 2020-09-02 18:42 test.txt

Now delete original file and compare behavior.

$ rm test.txt

# Symbolic link
$ cat link_symbolic
cat: link_symbolic: No such file or directory

# Hard link
$ cat link_hard
hi

Once a hard link is created, it is effectively the same file on disk. In this example we call one side original and the other link, but since original path is also a hard link to inode, this distinction can be conceptually weak.