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<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>
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<LI><A NAME="tex2html2011"
HREF="#SECTION001810000000000000000">15.1 Soft Links</A>
<LI><A NAME="tex2html2012"
HREF="#SECTION001820000000000000000">15.2 Hard Links</A>
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<H1><A NAME="SECTION001800000000000000000">
15. Symbolic and Hard Links</A>
</H1>
<P>
<A NAME="chap:links"></A>
<P>
Very often, a file is required to be in two different
directories at the same time. Think for example of a
configuration file that is required by two different software
packages that are looking for the file in different directories.
The file could simply be copied, but to have to replicate changes in more
than one place would create an administrative nightmare.
Also consider a document that must be present in
many directories, but which would be easier to update at one point.
<I>The way two (or more) files can have the same data is with links.</I>
<P>
<H1><A NAME="SECTION001810000000000000000">
15.1 Soft Links</A>
</H1>
<P>
To demonstrate a soft link, try the following:
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code> </code><br>
<code> </code><br>
<code> </code><br>
<code> </code><br>
<font size="-1"><code>5</code></font><code> </code><br>
<code> </code><br>
<code> </code><br>
<code> </code><br>
<code> </code><br>
<font size="-1"><code>10</code></font><code> </code><br>
<code> </code><br>
<code> </code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>touch myfile</code><br>
<code>ln -s myfile myfile2</code><br>
<code>ls -al</code><br>
<code>cat > myfile</code><br>
<code>a</code><br>
<code>few</code><br>
<code>lines</code><br>
<code>of</code><br>
<code>text</code><br>
<code>^D</code><br>
<code>cat myfile</code><br>
<code>cat myfile2</code><br>
</FONT></TD></TR></TABLE><P>
<P>
Notice that the <TT>
<FONT COLOR="#0000ff">ls -al</FONT></TT> listing has the letter
<TT>
<FONT COLOR="#0000ff">l</FONT></TT> on the far left next to <TT>
<FONT COLOR="#0000ff">myfile2</FONT></TT>, and the
usual <TT>
<FONT COLOR="#0000ff">-</FONT></TT> next to <TT>
<FONT COLOR="#0000ff">myfile</FONT></TT>. This indicates that
the file is a <I>soft</I> link (also known as a <I>symbolic</I>
link or <I>symlink</I>) to some other file.
<P>
A <I>symbolic link</I> contains no data of its own, only a
reference to another file. It can even contain a reference to
a directory. In either case, programs operating on the link
will actually see the file or directory it points to.
<P>
Try
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code> </code><br>
<code> </code><br>
<code> </code><br>
<code> </code><br>
<font size="-1"><code>5</code></font><code> </code><br>
<code> </code><br>
<code> </code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>mkdir mydir</code><br>
<code>ln -s mydir mydir2</code><br>
<code>ls -al .</code><br>
<code>touch ./mydir/file1</code><br>
<code>touch ./mydir2/file2</code><br>
<code>ls -al ./mydir</code><br>
<code>ls -al ./mydir2</code><br>
</FONT></TD></TR></TABLE><P>
The directory <TT>
<FONT COLOR="#0000ff">mydir2</FONT></TT> is a symbolic link to <TT>
<FONT COLOR="#0000ff">mydir2</FONT></TT>
and appears as though it is a replica of the original. Once
again the directory <TT>
<FONT COLOR="#0000ff">mydir2</FONT></TT> does not consume additional
disk space--a program that reads from the link is unaware
that it is seeing into a different directory.
<P>
Symbolic links can also be copied and retain their value:
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code> </code><br>
<code> </code><br>
<code> </code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>cp mydir2 /</code><br>
<code>ls -al /</code><br>
<code>cd /mydir2</code><br>
</FONT></TD></TR></TABLE><P>
You have now copied the link to the root directory. However, the
link points to a relative path <TT>
<FONT COLOR="#0000ff">mydir</FONT></TT> in the same
directory as the link. Since there is no <TT>
<FONT COLOR="#0000ff">mydir</FONT></TT> here, an
error is raised.
<P>
Try
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code> </code><br>
<code> </code><br>
<code> </code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>rm -f mydir2 /mydir2</code><br>
<code>ln -s `pwd`/mydir mydir2</code><br>
<code>ls -al</code><br>
</FONT></TD></TR></TABLE><P>
Now you will see <TT>
<FONT COLOR="#0000ff">mydir2</FONT></TT> has an absolute path. You can try
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code> </code><br>
<code> </code><br>
<code> </code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>cp mydir2 /</code><br>
<code>ls -al /</code><br>
<code>cd /mydir2</code><br>
</FONT></TD></TR></TABLE><P>
and notice that it now works.
<P>
One of the common uses of symbolic links is to make
<I>mount</I>ed (see Section <A HREF="node22.html#sec:mount">19.4</A>) file systems
accessible from a different directory. For instance, you may
have a large directory that has to be split over several
physical disks. For clarity, you can mount the disks as
<TT>
<FONT COLOR="#0000ff">/disk1</FONT></TT>, <TT>
<FONT COLOR="#0000ff">/disk2</FONT></TT>, etc., and then link the various
subdirectories in a way that makes efficient use of the space
you have.
<P>
Another example is the linking of <TT>
<FONT COLOR="#0000ff">/dev/cdrom</FONT></TT> to, say,
<TT>
<FONT COLOR="#0000ff">/dev/hdc</FONT></TT> so that programs accessing the device
file <TT>
<FONT COLOR="#0000ff">/dev/cdrom</FONT></TT> (see Chapter <A HREF="node21.html#chap:unixdevices">18</A>)
actually access the correct IDE drive.
<P>
<H1><A NAME="SECTION001820000000000000000">
15.2 Hard Links</A>
</H1>
<P>
U<SMALL>NIX</SMALL> allows the data of a file to have more than one name in
separate places in the same file system. Such a file with more
than one name for the same data is called a <I>hard-linked</I>
file and is similar to a symbolic link. Try
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code> </code><br>
<code> </code><br>
<code> </code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>touch mydata</code><br>
<code>ln mydata mydataB</code><br>
<code>ls -al</code><br>
</FONT></TD></TR></TABLE><P>
The files <TT>
<FONT COLOR="#0000ff">mydata</FONT></TT> and <TT>
<FONT COLOR="#0000ff">mydataB</FONT></TT> are
indistinguishable. They share the same data, and have a
<TT>
<FONT COLOR="#0000ff">2</FONT></TT> in second column of the <TT>
<FONT COLOR="#0000ff">ls -al</FONT></TT> listing. This
means that they are hard-linked <I>twice</I> (that there are
two names for this file).
<P>
The reason why hard links are sometimes used in preference to
symbolic links is that some programs are not fooled by a
symbolic link: If you have, say, a script that uses <TT>
<FONT COLOR="#0000ff">cp</FONT></TT>
to copy a file, it will copy the symbolic link instead of the
file it points to. <FONT COLOR="#ffa500">[<TT>
<FONT COLOR="#0000ff">cp</FONT></TT> actually has an option to override
this behavior.]</FONT> A hard link, however, will always be seen as a
real file.
<P>
On the other hand, hard links cannot be made between files on different
file systems nor can they be made between directories.
<P>
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