modify basic filestructure

It's unclear what's 'basic', so `basic/` notes have been mostly moved.
The remainder became `shell/`.

system/hard_links.md

@@ -0,0 +1,38 @@
+---
+title: "hard links"
+tags: [ "basics", "links" ]
+---
+
+A hard link is one file which exists in multiple locations.
+
+Each file has an ID, which is kept on the hard disk's partition.
+Each hard link has the same ID, because they are the same file.
+This ID is called the 'inode'.
+
+Create a file, and a hard link:
+
+```sh
+fortune > $file_1
+mkdir -p x/y/z/
+ln $file_1 x/y/z/$file_2
+```
+Have a long look at the file with the `-l` flag, and check the inode with `-i`:
+
+```sh
+ls -li $file_1 x/y/z/$file_2
+```
+
+Since they are the same file, you can make a change to one, and it changes both:
+
+```sh
+fortune | tee x/y/z/$file_2
+cat $file_1
+cat x/y/z/$file_2
+```
+
+# Danger Zone
+
+- hard links will not work on directories, only standard files and fifos.
+- `git` will destroy and remake files, so it will not respect hard links.
+- Files cannot have a hard link on another disk partition, because the inode is stored on each partition.
+