Can you change what a symlink points to after it is created?

Question

Does any operating system provide a mechanism (system call — not command line program) to change the pathname referenced by a symbolic link (symlink) — other than by unlinking the old one and creating a new one?

The POSIX standard does not. Solaris 10 does not. MacOS X 10.5 (Leopard) does not. (I'm tolerably certain neither AIX nor HP-UX does either. Judging from this list of Linux system calls, Linux does not have such a system call either.)

Is there anything that does?

(I'm expecting that the answer is "No".)

---

Since proving a negative is hard, let's reorganize the question.

If you know that some (Unix-like) operating system not already listed has no system call for rewriting the value of a symlink (the string returned by readlink()) without removing the old symlink and creating a new one, please add it — or them — in an answer.

Answer 1

Yes, you can!

$ ln -sfn source_file_or_directory_name softlink_name

Answer 2

AFAIK, no, you can't. You have to remove it and recreate it. Actually, you can overwrite a symlink and thus update the pathname referenced by it:

$ ln -s .bashrc test
$ ls -al test
lrwxrwxrwx 1 pascal pascal 7 2009-09-23 17:12 test -> .bashrc
$ ln -s .profile test
ln: creating symbolic link `test': File exists
$ ln -s -f .profile test
$ ls -al test
lrwxrwxrwx 1 pascal pascal 8 2009-09-23 17:12 test -> .profile

EDIT: As the OP pointed out in a comment, using the --force option will make ln perform a system call to unlink() before symlink(). Below, the output of strace on my linux box proving it:

$ strace -o /tmp/output.txt ln -s -f .bash_aliases test
$ grep -C3 ^unlink /tmp/output.txt 
lstat64("test", {st_mode=S_IFLNK|0777, st_size=7, ...}) = 0
stat64(".bash_aliases", {st_mode=S_IFREG|0644, st_size=2043, ...}) = 0
symlink(".bash_aliases", "test")        = -1 EEXIST (File exists)
unlink("test")                          = 0
symlink(".bash_aliases", "test")        = 0
close(0)                                = 0
close(1)                                = 0

So I guess the final answer is "no".

EDIT: The following is copied from Arto Bendiken's answer over on unix.stackexchange.com, circa 2016.

This can indeed be done atomically with rename(2), by first creating the new symlink under a temporary name and then cleanly overwriting the old symlink in one go. As the man page states:

If newpath refers to a symbolic link the link will be overwritten.

In the shell, you would do this with mv -T as follows:

$ mkdir a b
$ ln -s a z
$ ln -s b z.new
$ mv -T z.new z

You can strace that last command to make sure it is indeed using rename(2) under the hood:

$ strace mv -T z.new z
lstat64("z.new", {st_mode=S_IFLNK|0777, st_size=1, ...}) = 0
lstat64("z", {st_mode=S_IFLNK|0777, st_size=1, ...}) = 0
rename("z.new", "z")                    = 0

Note that in the above, both mv -T and strace are Linux-specific.

On FreeBSD, use mv -h alternately.

Editor's note: This is how Capistrano has done it for years now, ever since ~2.15. See this pull request.

Answer 3

It is not necessary to explicitly unlink the old symlink. You can do this:

ln -s newtarget temp
mv temp mylink

(or use the equivalent symlink and rename calls). This is better than explicitly unlinking because rename is atomic, so you can be assured that the link will always point to either the old or new target. However this will not reuse the original inode.

On some filesystems, the target of the symlink is stored in the inode itself (in place of the block list) if it is short enough; this is determined at the time it is created.

Regarding the assertion that the actual owner and group are immaterial, symlink(7) on Linux says that there is a case where it is significant:

The owner and group of an existing symbolic link can be changed using lchown(2). The only time that the ownership of a symbolic link matters is when the link is being removed or renamed in a directory that has the sticky bit set (see stat(2)).

The last access and last modification timestamps of a symbolic link can be changed using utimensat(2) or lutimes(3).

On Linux, the permissions of a symbolic link are not used in any operations; the permissions are always 0777 (read, write, and execute for all user categories), and can't be changed.

Answer 4

Just a warning to the correct answers above:

Using the -f / --force Method provides a risk to lose the file if you mix up source and target:

mbucher@server2:~/test$ ls -la
total 11448
drwxr-xr-x  2 mbucher www-data    4096 May 25 15:27 .
drwxr-xr-x 18 mbucher www-data    4096 May 25 15:13 ..
-rw-r--r--  1 mbucher www-data 4109466 May 25 15:26 data.tar.gz
-rw-r--r--  1 mbucher www-data 7582480 May 25 15:27 otherdata.tar.gz
lrwxrwxrwx  1 mbucher www-data      11 May 25 15:26 thesymlink -> data.tar.gz
mbucher@server2:~/test$ 
mbucher@server2:~/test$ ln -s -f thesymlink otherdata.tar.gz 
mbucher@server2:~/test$ 
mbucher@server2:~/test$ ls -la
total 4028
drwxr-xr-x  2 mbucher www-data    4096 May 25 15:28 .
drwxr-xr-x 18 mbucher www-data    4096 May 25 15:13 ..
-rw-r--r--  1 mbucher www-data 4109466 May 25 15:26 data.tar.gz
lrwxrwxrwx  1 mbucher www-data      10 May 25 15:28 otherdata.tar.gz -> thesymlink
lrwxrwxrwx  1 mbucher www-data      11 May 25 15:26 thesymlink -> data.tar.gz

Of course this is intended, but usually mistakes occur. So, deleting and rebuilding the symlink is a bit more work but also a bit saver:

mbucher@server2:~/test$ rm thesymlink && ln -s thesymlink otherdata.tar.gz 
ln: creating symbolic link `otherdata.tar.gz': File exists

which at least keeps my file.

Answer 5

Wouldn't unlinking it and creating the new one do the same thing in the end anyway?

Answer 6

Technically, there's no built-in command to edit an existing symbolic link. It can be easily achieved with a few short commands.

Here's a little bash/zsh function I wrote to update an existing symbolic link:

# -----------------------------------------
# Edit an existing symbolic link
#
# @1 = Name of symbolic link to edit
# @2 = Full destination path to update existing symlink with 
# -----------------------------------------
function edit-symlink () {
    if [ -z "$1" ]; then
        echo "Name of symbolic link you would like to edit:"
        read LINK
    else
        LINK="$1"
    fi
    LINKTMP="$LINK-tmp"
    if [ -z "$2" ]; then
        echo "Full destination path to update existing symlink with:"
        read DEST
    else
        DEST="$2"
    fi
    ln -s $DEST $LINKTMP
    rm $LINK
    mv $LINKTMP $LINK
    printf "Updated $LINK to point to new destination -> $DEST"
}

Answer 7

Just in case it helps: there is a way to edit a symlink with midnight commander (mc). The menu command is (in French on my mc interface):

Fichier / Éditer le lien symbolique

which may be translated to:

File / Edit symbolic link

The shortcut is C-x C-s

Maybe it internally uses the ln --force command, I don't know.

Now, I'm trying to find a way to edit a whole lot of symlinks at once (that's how I arrived here).

Answer 8

You can modify the softlink created once in one of the two ways as below in Linux

1. one is where you can remove existing softlink with rm and again create new softlink with ln -s command .
2. However this can be done in one step , you can replace existing softlink with updated path with "ln -vfns Source_path Destination_path" command.

Listing initial all files in directory

$ ls -lrt 
drwxrwxr-x. 3 root    root      110 Feb 27 18:58 test_script
$

Create softlink test for test_script with ln -s command.

$ ln -s test_script test
$ ls -lrt
drwxrwxr-x. 3 root    root      110 Feb 27 18:58 test_script
lrwxrwxrwx. 1 root    root       11 Feb 27 18:58 test -> test_script
$

Update softlink test with new directory test_script/softlink with single command

$ ln -vfns test_script/softlink/ test
'test' -> 'test_script/softlink/'
$

List new softlink location

$ ls -lrt
lrwxrwxrwx. 1 root    root       21 Feb 27 18:59 test -> test_script/softlink/
$

ln --help

-v, --verbose print name of each linked file

-f, --force remove existing destination files

-n, --no-dereference treat LINK_NAME as a normal file if it is a symbol

-s, --symbolic make symbolic links instead of hard links