Is there a way for symlinks to have fallback? If the destination is not found, another one should be used.
I want to use a config file for a program that is different whether a drive is mounted or not. So I would have symlink to a config on that particular drive and if not found, it would use an alternative one.
I guess I could use a wrapper script that would check if a drive is mounted, link to a desired config and then run the application, but a wrapper script feels weird.
There isn't any way with standard symlinks. They always point to a single target path.
However, in your particular case, you could make use of how a drive shadows a mount: if you point the config file to /path/to/mount/config.cnf, then you have one that's on the parent filesystem (used when the mount isn't present) and one on the mounted filesystem (used when the mount is present).
But I wouldn't recommend that—for one thing, it's hard to open them both at once to e.g., compare them. I'd personally go for a wrapper script over that, many times over. There might also be program-specific ways, but you didn't name the program.
You can use a union mount for this. Showing one file if present in a certain place and another file if the first choice wasn't present is the whole point of union mounts. Symlinks won't help, except if to show the file in a different place from where the mount is.
mkdir ~/overridden
unionfs-fuse "/media/external:$HOME/overridable" ~/overridden
~/overridden shows a merge of the directory trees under ~/overridable and /media/external. When a file is present in both, the one from /media/external is shown.
See also Union mount on Linux (when someone answers it…).
