Hard link not works under Mac OS X in GUI mode

Question

I faced a little strange behavior, while using hard links. From Terminal, I create a text file 1.txt and a hard link "to this file"

nano 1.txt
mkdir dir
ln 1.txt ./dir/

I check the resulting hard link and see that its contents are the same as of the original file.

less ./dir/1.txt

I change the initial file ...

nano 1.txt

... and see, that changes was reflected in hard-link

less ./dir/1.txt

I change content of hard-link (more correct, of course - file, being referenced with hard-link) ...

nano ./dir/1.txt

... and see, that changes are reflected in initial file

less 1.txt

Until now, all going well...

Now, I close Terminal and start playing with created files (1.txt and ./dir/1.txt) from Finder. When I change on this two files with TextEdit, changes are not reflected in another file.

The hard link is now broken. What's going on?

Answer 1

It's not about the GUI, it's specifically about TextEdit's strategy for saving changes: it does not write in place on the existing file, but rather it first writes a new one, and when that's completed it removes the old one and renames the new one to the old one's names. Many editors (programs that conceptually alter a file "in place"), GUI or not, use this strategy for safety purposes (you won't lose both the new and old versions if there's a crash at a very unfortunate moment just when the writing is taking place), but as you noticed it "breaks" hard links.

One example of a non-interactive, non-GUI editor program with this behavior is perl with the -i ("in-place edits") command-line option switch...:

$ touch za.txt
$ ln za.txt zo.txt
$ echo ciao >za.txt
$ cat zo.txt 
ciao
$ perl -i -p -e 's/a/b/' zo.txt
$ cat zo.txt
cibo
$ cat za.txt
ciao