Hard Links – How to Break a Hard-Link In-Place

hard link

I am keeping my dotfiles under version control and the script deploying them creates hard links. I also use etckeeper to put my /etc under version control. Recently I have gotten warnings like this:

warning: hard-linked files could cause problems with bzr

A simple copy (cp filename.ext filename.ext) will not work:

cp: `filename.ext' and `filename.ext' are the same file

Renaming/moving a file – except across volumes – also doesn't break the hard-link.

So my question is: is there a way to break a hard-link to a file without actually having to know where the other hard-link/s to that file is/are?

Best Answer

cp -p filename filename.tmp
mv -f filename.tmp filename

Making it scriptable:

dir=$(dirname -- "$filename")
tmp=$(TMPDIR=$dir mktemp)
cp -p -- "$filename" "$tmp"
mv -f -- "$tmp" "$filename"

Doing the copy first, then moving it into place, has the advantage that the file atomically changes from being a hard link to being a separate copy (there is no point in time where filename is partial or missing).

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Symbolic link and hard link questions

. is actually the current working directory in either case; it has nothing to do with the directory holding the script:

[/tmp] $ echo "realpath ." > test.sh && chmod +x test.sh
[/tmp] $ /tmp/test.sh
/tmp
[/tmp] $ cd /usr/bin
[/usr/bin] $ /tmp/test.sh
/usr/bin

Symbolic Links – How to Keep Track of Symbolic Links

Concering your second question, if you make the symlink using a relative path and then move the whole directory structure, it still should work. Consider the following terminal session:

~$ mkdir test
~$ cd test/
~/test$ mkdir test2
~/test$ cd test2/
~/test/test2$ touch testfile; echo "hello, world" > testfile
~/test/test2$ cat testfile 
hello, world
~/test/test2$ cd ..
~/test$ ln -s ./test2/testfile testfileln
~/test$ ls -l
total 8
drwxr-xr-x 2 xxxx xxxx 4096 2010-09-09 09:18 test2
lrwxrwxrwx 1 xxxx xxxx   16 2010-09-09 09:18 testfileln -> ./test2/testfile
~/test$ cd ..
~$ mv test/ testfoo
~$ cd testfoo/
~/testfoo$ ls -l
total 8
drwxr-xr-x 2 xxxx xxxx 4096 2010-09-09 09:18 test2
lrwxrwxrwx 1 xxxx xxxx   16 2010-09-09 09:18 testfileln -> ./test2/testfile
/testfoo$ cat testfileln 
hello, world

As for your first question, if you really want a link that will refer to the same file no matter what you do with the original location of the file, a hard link is probably what you want. A hard link is basically just another name refering to the same inode. Thus, there is no difference between the hard link and the "original file." However, if you need to link across file systems, hard links often do not work and you usually cannot make hard links to directories. Further, you will notice some differences when performing some file operations. Most notably, removing the original will not remove the file. The hard link will still point to the file and be accessible.

Best Answer

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Symbolic Links – How to Keep Track of Symbolic Links

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