ch18_04.htm

the unix power tools

<EM
CLASS="emphasis"
>file</EM
> and <EM
CLASS="emphasis"
>hardlink</EM
> is the same;
that's because they both refer to exactly the same file, with two links (names).</P
><P
CLASS="para"
>A symbolic link has an <CODE
CLASS="literal"
>l</CODE
> at the start of
the permissions field.
Its i-number isn't the same as the file it points to
because a symbolic link takes a separate
inode; so, it also takes disk space (which an extra hard link doesn't).
The name has two parts: the name of the link (here, <EM
CLASS="emphasis"
>symlink</EM
>) followed
by an arrow and the name the link points to (in this case, to <EM
CLASS="emphasis"
>file</EM
>).
The symlink takes just 4 characters, which is exactly enough to store the
pathname (<CODE
CLASS="literal"
>file</CODE
>) that the link points to.</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-7944-SECT-1.2"
>18.4.2 Links to a Directory </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-20311"
></A
><A
CLASS="indexterm"
NAME="AUTOID-20314"
></A
>While we're at it, here's a section that isn't about
linking to files or making symbolic links.
Let's look at the first two entries in the previous sample directory
in terms of links and link counts.
This should help to tie the filesystem together (both literally and in your
mind!).</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-20318"
></A
><A
CLASS="indexterm"
NAME="AUTOID-20321"
></A
>You've seen <CODE
CLASS="literal"
>.</CODE
> and <CODE
CLASS="literal"
>..</CODE
> in
<SPAN
CLASS="link"
>pathnames (<A
CLASS="linkend"
HREF="ch01_21.htm"
TITLE="Making Pathnames "
>1.21</A
>)</SPAN
>;
you might also have read an
<SPAN
CLASS="link"
>explanation of what's in a directory (<A
CLASS="linkend"
HREF="ch18_02.htm"
TITLE="What's Really in a Directory "
>18.2</A
>)</SPAN
>.
The <CODE
CLASS="literal"
>.</CODE
> entry is a link to the current directory; notice that its
link count is 2.
Where's the other link?
It's in the parent directory:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>ls -li ..</B
></CODE
>
total 2
 140330 drwxr-xr-x   2 jerry    ora      1024 Aug 18 10:11 sub
  85524 drwxr-xr-x   2 jerry    ora      1024 Aug 18 10:47 sub2</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Look at the i-numbers for the entries in the parent directory.
Which entry is for our current directory?
The entry for <EM
CLASS="emphasis"
>sub</EM
> has the i-number 140330, and so does
the <CODE
CLASS="literal"
>.</CODE
> listing in the current directory.
So the current directory is named <EM
CLASS="emphasis"
>sub</EM
>.</P
><P
CLASS="para"
>Now you should be able see why every directory has at least two links.
One link, named <CODE
CLASS="literal"
>.</CODE
>, is to the directory itself.
The other link, in its parent, gives the directory its name.
(Article
<A
CLASS="xref"
HREF="ch14_04.htm"
TITLE="How Does UNIX Find Your Current Directory? "
>14.4</A
>
has a picture of this.)</P
><P
CLASS="para"
>Every directory has a <CODE
CLASS="literal"
>..</CODE
> entry, which is
a link to its parent directory.
If you look back at the listing of our current directory,
you can see that the parent directory has four links.
Where are they?
Think for a minute.
(No fair peeking ahead!)</P
><P
CLASS="para"
>Bzzzzzzt... time's up.<A
CLASS="indexterm"
NAME="AUTOID-20342"
></A
>
When a directory has subdirectories, it will also have a hard link named
<CODE
CLASS="literal"
>..</CODE
> in each subdirectory.
You can see above, in the output from <CODE
CLASS="literal"
>ls&nbsp;-li&nbsp;..</CODE
>, that the parent
directory has two subdirectories:
<EM
CLASS="emphasis"
>sub</EM
> and <EM
CLASS="emphasis"
>sub2</EM
>.
That's two of the four links.
The other two links are
the <CODE
CLASS="literal"
>.</CODE
> entry in the parent directory,
and the entry for the parent directory (which is named <EM
CLASS="emphasis"
>test</EM
>)
in <EM
CLASS="emphasis"
>its</EM
> parent directory:</P
><P
CLASS="para"
><TABLE
CLASS="screen.co"
BORDER="1"
><TR
><TH
VALIGN="TOP"
><PRE
CLASS="calloutlist"
>
<A
CLASS="co"
HREF="ch16_08.htm"
TITLE="16.8 The ls -d Option "
>-d</A
> 
&#13;</PRE
></TH
><TD
VALIGN="TOP"
><PRE
CLASS="screen"
>
% <CODE
CLASS="userinput"
><B
>ls -dli ../. ../../test</B
></CODE
>
  85523 drwxr-xr-x   4 jerry    ora      1024 Aug 18 10:47 ../.
  85523 drwxr-xr-x   4 jerry    ora      1024 Aug 18 10:47 ../../test</PRE
></TD
></TR
></TABLE
></P
><P
CLASS="para"
>As they should, all the links have the same i-number: 85523.
Make sense?
This concept can be a little abstract and hard to follow at first.
Understanding it will help you, though&nbsp;- especially if you're a system
administrator who has to use strong medicine like 
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>clri</EM
> (<A
CLASS="linkend"
HREF="ch23_13.htm"
TITLE="Deleting Files with the Null Name "
>23.13</A
>)</SPAN
>.
For more practice, make a subdirectory and experiment in it the way
we've shown in this article.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-20361"
></A
>By the way, directories and their hard links <CODE
CLASS="literal"
>.</CODE
> and <CODE
CLASS="literal"
>..</CODE
>
are added by the <EM
CLASS="emphasis"
>mkdir</EM
>(2) system call.
That's the only way that normal users can create a directory (and the
links to it).
Article
<A
CLASS="xref"
HREF="ch23_18.htm"
TITLE="How Making and Deleting Directories Works "
>23.18</A
>
has even more low-level details.</P
></DIV
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>JP</SPAN
>, <SPAN
CLASS="authorinitials"
>ML</SPAN
></P
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