How many inodes does a file, a folder take?

Question

After checking a lot of information on the Internet, it is still not quite clear for me how many inodes a file and a folder takes.

Also, I noticed that inodes are shown in kilobytes and not as cardinal numbers.

So how to count the number of inodes required for a file or a folder?

P.S. My bad, it seems that K and M in the output of $ sudo df -ih/ mean thousands and millions and not kilobytes and megabytes.

Answer 1

A file has one single inode since it is the inode that uniquely identifies the file. You can have several names/paths pointing to the same inode, this is called "hard links".

When you list a file with ls -l, a column is the number of references to the inode (i.e. the number of paths that lead to it). For files it is usually 1, unless hard links have been explicitly created:

drwxr-xr-x  2 root root  4096 Aug 20 12:11 ./
drwxrwxrwt 29 root root 86016 Aug 20 12:11 ../
-rw-r--r--  3 root root     0 Aug 20 12:10 linked1
-rw-r--r--  3 root root     0 Aug 20 12:10 linked2
-rw-r--r--  3 root root     0 Aug 20 12:10 linked3
-rw-r--r--  1 root root     0 Aug 20 12:10 single

Above, single is the name of a file with a single path to it, while linked* are three names for the same inode. If you do ls -il to show the inodes:

>>>ls -il *
24641901 -rw-r--r-- 3 root root 0 Aug 20 12:10 linked1
24641901 -rw-r--r-- 3 root root 0 Aug 20 12:10 linked2
24641901 -rw-r--r-- 3 root root 0 Aug 20 12:10 linked3
24641866 -rw-r--r-- 1 root root 0 Aug 20 12:10 single

This confirms that the three linked* files are actually the same inode. If we rm linked2, the reference count in the remaining names is decreased:

>>>ls -l
drwxr-xr-x  2 root root  4096 Aug 20 12:15 ./
drwxrwxrwt 29 root root 86016 Aug 20 12:15 ../
-rw-r--r--  2 root root     0 Aug 20 12:10 linked1
-rw-r--r--  2 root root     0 Aug 20 12:10 linked3
-rw-r--r--  1 root root     0 Aug 20 12:10 single

For directories the references count is normally 2 plus the number of subdirectories (AFAIK, the "normal" reference (from the parent), the . in the directory itself, and the .. in each subdirectory).

Otherwise, a file (or directory) can use several allocation blocks, depending on contents and this can change during the lifetime of the file.