Advanced Programming in the UNIX® Environment

ptg10805159

Section 4.14 File Systems 115

•Because the i-node number in the directory entry points to an i-node in the same file

system, a directory entry can’t refer to an i-node in a different file system. This is

why theln( 1 )command (make a new directory entry that points to an existing file)

can’t cross file systems. We describe thelinkfunction in the next section.

•When renaming a file without changing file systems, the actual contents of the file

need not be moved—all that needs to be done is to add a new directory entry that

points to the existing i-node and then unlink the old directory entry.The link count

will remain the same. For example, to rename the file /usr/lib/foo to

/usr/foo,the contents of the file foo need not be moved if the directories

/usr/liband/usrareonthe same file system. This is how themv( 1 )command

usually operates.

We’ve talked about the concept of a link count for a regular file, but what about the

link count field for a directory? Assume that we make a new directory in the working

directory, as in

$mkdir testdir

Figure4.15 shows the result. Note that in this figure, we explicitly show the entries for

dot and dot-dot.

i-node array directoryblock directoryblock

directory blocks and data blocks

i-node

0

i-node

1267

i-node

2549

data block

data block

2549.

1267 ..

1267.

i-node

number ..

2549 testdir

Figure 4.15 Sample cylinder group after creating the directorytestdir

The i-node whose number is 2549 has a type field of ‘‘directory’’and a link count equal

to 2. Any leaf directory (a directory that does not contain any other directories) always

has a link count of 2. The value of 2 comes from the directory entry that names the

directory (testdir)and from the entry for dot in that directory.The i-node whose