How can I edit symlinks?

Question

My basic understanding of a symlink is as a special file, a file that contains a string path to another file. The kernel's VFS abstracts a lot of that away but is there any reason why symlinks seem to be impossible to edit?

In other words: Can I edit a symlink? If not, why not?

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I do understand that there are various ways of replacing symlinks (two alternatives are currently in the answers section) but it would be interesting to get an explanation on why replacement seems to be the only way to deal with symlinks. Why can't you just change where they point?

Answer 1

Given that -f just does a silent replacement, you can do an atomic replacement with mv -T (-T makes it work even if /loc.../link is a directory):

ln -s /location/to/link linkname
# ... 
ln -s /location/to/link2 newlink
mv -T newlink linkname

linkname is accessible throughout the process.

Answer 2

If by edit, you mean to change the file it points to, then yes you can:

$ 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

The -f parameter (--force) when passed to ln it causes it to call the unlink() system call right before symlink()

Taken from the following stack overflow answer.

Answer 3

Symbolic links need to be modified atomically. If you're halfway through writing them, they won't work. The content of a symbolic link is pretty small (at most 4095 characters on Linux: the maximum length of a path to a file), so there would be little point in editing part of a symbolic link at the kernel level. Therefore the kernel does not offer any interface to edit a symbolic link, only an interface to create a new one, the symlink system call (plus the generic interface unlink to remove any file).

The symlink system call only creates a new symbolic link, it does not remove any existing file. This is annoying, but consistent with other system calls to create files such as open (which can create a new file or truncate an existing file, but not replace an existing file by a newly-created file) and mkdir.

In the shell, as you've discovered, while you can't replace a symbolic link atomically with the ln command (ln -sf unlinks the previous file then creates the symbolic link), you can do it by first creating a symbolic link under a temporary name and then moving it into place.

tmp=$(TMPDIR=$(dirname -- "$link") mktemp)
ln -sf -- "$target" "$tmp"
mv -f "$tmp" "$link"

Answer 4

Assume linkname exists as the result of having done (in the past):

 ln -s   /the/path/to/a/file   linkname

Then, there are three ways to change the symlink:

• Use ln with -f force and even for directories -n (inode could get reused):

  ln -sfn /some/new/path linkname
  

• Remove the symlink and create a new one (even for directories):

  rm linkname; ln -s /some/new/path linkname
  

• create a new symlink, then mv it (atomic change even for directories):

  ln -s  /some/new/path newlinkname
  mv -fT newlinkname linkname             # linkname remains after the command
  

Answer 5

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 6

I found a link file "gap" in my ~/bin folder, must have been placed there by some installation process (I never used the ln command myself). I wanted to upgrade from gap-4.10.2 to gap-4.11.0, so I opened the link file using

vim gap

but it opened in read-only mode, so I opened it with

sudo vim gap

and then I could change it with insert mode.

I just edited the content of the link file from

#!/bin/sh

GAP_EXE=$GAP_DIR
if [ "x$GAP_DIR" = "x" ]; then
  GAP_DIR=$(cd "/home/user/gap-4.10.2" && pwd)
  GAP_EXE=/home/user/gap-4.10.2
fi

exec "$GAP_EXE/gap" -l "$GAP_DIR;/home/user/.gap" "$@"

by replacing both numbers by 4.11.0 respectively (a path that already existed), so that it looked like

#!/bin/sh

GAP_EXE=$GAP_DIR
if [ "x$GAP_DIR" = "x" ]; then
  GAP_DIR=$(cd "/home/user/gap-4.11.0" && pwd)
  GAP_EXE=/home/user/gap-4.11.0
fi

exec "$GAP_EXE/gap" -l "$GAP_DIR;/home/user/.gap" "$@"

Then save&exit with

:wq

So, YES, you CAN edit a link file.

One problem still remains: in /bin with

ls -l

the name is still

gap -> ../gap-4.10.2/bin/gap.sh

so the long name of the link file still has the old path in it, but it points to the new path. Is there any way to change that identifyer? Not that it's really bothering me. The link works fine now.