I have a couple of identical files stored in more than one place on my hard disk. I figure I can save a lot of disk space by hard-linking them to point to the same file. I am a little worried about possibly disastrous side effects.
I guess it does not affect permissions, as those are stored in the respective directories, just like the file name, right? (Update: Apparently, I guessed wrong, permissions are shared, as Carl demonstrates in his answer)
The biggest concern is changes to one file inadvertently also changing the other files. Read-only files should be safe then. And files that can be changed are also okay, if rather than updating within the existing file, a new file gets written. I believe most applications work that way, but probably not all.
Is there anything else to consider?
I am on OS X / HFS+.
Don't use hard links if you want changes to one file not to be reflected in other files. That's the whole point of hard links - multiple directory entries for the same file (same blocks on disk). Changing permissions on one of the names of a hard link changes them on both:
$ touch file
$ ln file link
$ ls -l
total 0
-rw-r--r-- 2 owner group 0 Nov 11 16:44 file
-rw-r--r-- 2 owner group 0 Nov 11 16:44 link
$ chmod 444 file
$ ls -l
total 0
-r--r--r-- 2 owner group 0 Nov 11 16:44 file
-r--r--r-- 2 owner group 0 Nov 11 16:44 link
From the ln man page:
A hard link to a file is indistinguishable from the original directory entry; any changes to a file are effectively independent of the name used to reference the file.