Unix files consist of two parts: the data part and the filename part.
The data part is associated with something called an ‘inode’. The inode
carries the map of where the data is, the file permissions, etc. for the
data.
.---------------> ! data ! ! data ! etc
/ +------+ !------+
! permbits, etc ! data addresses !
+------------inode---------------+
The filename part carries a name and an associated inode number.
.--------------> ! permbits, etc ! addresses !
/ +---------inode-------------+
! filename ! inode # !
+--------------------+
More than one filename can reference the same inode number; these files are
said to be ‘hard linked’ together.
! filename ! inode # !
+--------------------+
\
>--------------> ! permbits, etc ! addresses !
/ +---------inode-------------+
! othername ! inode # !
+---------------------+
On the other hand, there’s a special file type whose data part carries
a path to another file. Since it is a special file, the OS recognizes the
data as a path, and redirects opens, reads, and writes so that, instead of
accessing the data within the special file, they access the data in the
file named by the data in the special file. This special file is
called a ‘soft link’ or a ‘symbolic link’ (aka a ‘symlink’).
! filename ! inode # !
+--------------------+
\
.-------> ! permbits, etc ! addresses !
+---------inode-------------+
/
/
/
.----------------------------------------------'
(
'--> !"/path/to/some/other/file"!
+---------data-------------+
/ }
.~ ~ ~ ~ ~ ~ ~ }-- (redirected at open() time)
( }
'~~> ! filename ! inode # !
+--------------------+
\
'------------> ! permbits, etc ! addresses !
+---------inode-------------+
/
/
.----------------------------------------------------'
(
'-> ! data ! ! data ! etc.
+------+ +------+
Now, the filename part of the file is stored in a special file of its own
along with the filename parts of other files; this special file is called a
directory. The directory, as a file, is just an array of filename parts of
other files.
When a directory is built, it is initially populated with the filename parts
of two special files: the ‘.’ and ‘..’ files. The filename part for the ‘.’
file is populated with the inode# of the directory file in which the entry
has been made; ‘.’ is a hardlink to the file that implements the current
directory.
The filename part for the ‘..’ file is populated with the inode# of the
directory file that contains the filename part of the current directory
file. ‘..’ is a hardlink to the file that implements the immediate parent
of the current directory.
The ‘ln’ command knows how to build hardlinks and softlinks; the
‘mkdir’ command knows how to build directories (the OS takes care of the
above hardlinks).
There are restrictions on what can be hardlinked (both links must reside on
the same filesystem, the source file must exist, etc.) that are not
applicable to softlinks (source and target can be on seperate file systems,
source does not have to exist, etc.). OTOH, softlinks have other
restrictions not shared by hardlinks (additional I/O necessary to complete
file access, additional storage taken up by softlink file’s data, etc.)
In other words, there’s tradeoffs with each.
Now, let’s demonstrate some of this…
ln in action
Let’s start off with an empty directory, and create a file in it
~/directory $ ls -lia
total 3
73477 drwxr-xr-x 2 lpitcher users 1024 Mar 11 20:16 .
91804 drwxr-xr-x 29 lpitcher users 2048 Mar 11 20:16 ..
~/directory $ echo "This is a file" >basic.file
~/directory $ ls -lia
total 4
73477 drwxr-xr-x 2 lpitcher users 1024 Mar 11 20:17 .
91804 drwxr-xr-x 29 lpitcher users 2048 Mar 11 20:16 ..
73478 -rw-r--r-- 1 lpitcher users 15 Mar 11 20:17 basic.file
~/directory $ cat basic.file
This is a file
Now, let’s make a hardlink to the file
~/directory $ ln basic.file hardlink.file
~/directory $ ls -lia
total 5
73477 drwxr-xr-x 2 lpitcher users 1024 Mar 11 20:20 .
91804 drwxr-xr-x 29 lpitcher users 2048 Mar 11 20:18 ..
73478 -rw-r--r-- 2 lpitcher users 15 Mar 11 20:17 basic.file
73478 -rw-r--r-- 2 lpitcher users 15 Mar 11 20:17 hardlink.file
~/directory $ cat hardlink.file
This is a file
We see that:
- hardlink.file shares the same inode (73478) as basic.file
- hardlink.file shares the same data as basic.file
If we change the permissions on basic.file:
~/directory $ chmod a+w basic.file
~/directory $ ls -lia
total 5
73477 drwxr-xr-x 2 lpitcher users 1024 Mar 11 20:20 .
91804 drwxr-xr-x 29 lpitcher users 2048 Mar 11 20:18 ..
73478 -rw-rw-rw- 2 lpitcher users 15 Mar 11 20:17 basic.file
73478 -rw-rw-rw- 2 lpitcher users 15 Mar 11 20:17 hardlink.file
then the same permissions change on hardlink.file.
The two files (basic.file and hardlink.file) share the same inode and data,
but have different file names.
Let’s now make a softlink to the original file:
~/directory $ ln -s basic.file softlink.file
~/directory $ ls -lia
total 5
73477 drwxr-xr-x 2 lpitcher users 1024 Mar 11 20:24 .
91804 drwxr-xr-x 29 lpitcher users 2048 Mar 11 20:18 ..
73478 -rw-rw-rw- 2 lpitcher users 15 Mar 11 20:17 basic.file
73478 -rw-rw-rw- 2 lpitcher users 15 Mar 11 20:17 hardlink.file
73479 lrwxrwxrwx 1 lpitcher users 10 Mar 11 20:24 softlink.file -> basic.file
~/directory $ cat softlink.file
This is a file
Here, we see that although softlink.file accesses the same data as
basic.file and hardlink.file, it does not share the same inode (73479 vs
73478), nor does it exhibit the same file permissions. It does show a new
permission bit: the ‘l’ (softlink) bit.
If we delete basic.file:
~/directory $ rm basic.file
~/directory $ ls -lia
total 4
73477 drwxr-xr-x 2 lpitcher users 1024 Mar 11 20:27 .
91804 drwxr-xr-x 29 lpitcher users 2048 Mar 11 20:18 ..
73478 -rw-rw-rw- 1 lpitcher users 15 Mar 11 20:17 hardlink.file
73479 lrwxrwxrwx 1 lpitcher users 10 Mar 11 20:24 softlink.file -> basic.file
then we lose the ability to access the linked data through the softlink:
~/directory $ cat softlink.file
cat: softlink.file: No such file or directory
However, we still have access to the original data through the hardlink:
~/directory $ cat hardlink.file
This is a file
You will notice that when we deleted the original file, the hardlink didn’t
vanish. Similarly, if we had deleted the softlink, the original file wouldn’t
have vanished.
A further note with respect to hardlink files
When deleting files, the data part isn’t disposed of until all the filename
parts have been deleted. There’s a count in the inode that indicates how
many filenames point to this file, and that count is decremented by 1 each
time one of those filenames is deleted. When the count makes it to zero,
the inode and its associated data are deleted.
By the way, the count also reflects how many times the file has been opened
without being closed (in other words, how many references to the file are
still active). This has some ramifications which aren’t obvious at first:
you can delete a file so that no "filename" part points to the inode,
without releasing the space for the data part of the file, because the file
is still open.
Have you ever found yourself in this position: you notice that
/var/log/messages (or some other syslog-owned file) has grown too big, and
you
rm /var/log/messages
touch /var/log/messages
to reclaim the space, but the used space doesn’t reappear? This is because,
although you’ve deleted the filename part, there’s a process that’s got the
data part open still (syslogd), and the OS won’t release the space for the
data until the process closes it. In order to complete your space
reclamation, you have to
kill -SIGHUP `cat /var/run/syslogd.pid`
to get syslogd to close and reopen the file.
You can use this to your advantage in programs: have you ever wondered how
you could hide a temporary file? Well, you could do the following:
{
FILE *fp;
fp = fopen("some.hidden.file","w");
unlink("some.hidden.file"); /* deletes the filename part */
/* some.hidden.file no longer has a filename and is truely hidden */
fprintf(fp,"This data won't be found\n"); /* access the data part */
/*etc*/
fclose(fp); /* finally release the data part */
}
Nikolai's Diary 