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<H2><A NAME="SECTION00585000000000000000">Mounting and unmounting</A></H2>
<A NAME="subsecmount"> </A>
<P>
Before one can use a filesystem, it has to be <b>mounted</b>.
The operating system then does various bookkeeping things to
make sure that everything works. Since all files in UNIX are
in a single directory tree, the mount operation will make it
look like the contents of the new filesystem are the contents of
an existing subdirectory in some already mounted filesystem.
<P>
For example, figure <A HREF="node46.html#fighdmountroot">4.3</A> shows three
separate filesystems, each with their own root directory.
When the last two filesystems are mounted below <tt>/home</tt><A NAME="1582"> </A>
and <tt>/usr</tt><A NAME="1584"> </A>, respectively, on the first filesystem, we
can get a single directory tree, as in
figure <A HREF="node46.html#fighdmountall">4.4</A>.
<P>
<P><A NAME="1230"> </A><A NAME="fighdmountroot"> </A><IMG WIDTH=680 HEIGHT=99 ALIGN=BOTTOM ALT="figure1226" SRC="img7.gif"><BR>
<STRONG>Figure 4.3:</STRONG> Three separate filesystems.<BR>
<P>
<P>
<P><A NAME="1430"> </A><A NAME="fighdmountall"> </A><IMG WIDTH=371 HEIGHT=185 ALIGN=BOTTOM ALT="figure1233" SRC="img8.gif"><BR>
<STRONG>Figure 4.4:</STRONG> <tt>/home</tt><A NAME="1586"> </A> and <tt>/usr</tt><A NAME="1588"> </A> have been mounted.<BR>
<P>
<P>
The mounts could be done as in the following example:
<BLOCKQUOTE> <TT>
<code>$ </code> <I>mount /dev/hda2 /home</I> <BR>
<code>$ </code> <I>mount /dev/hda3 /usr</I> <BR>
<code>$ </code>
</TT></BLOCKQUOTE>
The <tt>mount</tt><A NAME="1590"> </A> command takes two arguments. The first one is
the device file corresponding to the disk or partition containing
the filesystem. The second one is the directory below which it
will be mounted. After these commands the contents of
the two filesystems look just like the contents of the <tt>/home</tt><A NAME="1592"> </A>
and <tt>/usr</tt><A NAME="1594"> </A> directories, respectively. One would then say that
``<tt>/dev/hda2</tt><A NAME="1596"> </A> <b>is mounted on</b> <tt>/home</tt><A NAME="1599"> </A>'', and similarly
for <tt>/usr</tt><A NAME="1601"> </A>. To look at either filesystem, one would look
at the contents of the directory on which it has been mounted,
just as if it were any other directory. Note the
difference between the device file, <tt>/dev/hda2</tt><A NAME="1603"> </A>, and the
mounted-on directory, <tt>/home</tt><A NAME="1605"> </A>. The device file gives access
to the raw contents of the disk, the mounted-on directory
gives access to the files on the disk. The mounted-on directory
is called the <b>mount point</b>.
<P>
Linux supports many filesystem types. <tt>mount</tt><A NAME="1608"> </A> tries to
guess the type of the filesystem. You can also use the
<TT>-t <I>fstype</TT></I> option to specify the type directly;
this is sometimes necessary, since the heuristics <tt>mount</tt><A NAME="1610"> </A>
uses do not always work. For example, to mount an MS-DOS
floppy, you could use the following command:
<BLOCKQUOTE> <TT>
<code>$ </code> <I>mount -t msdos /dev/fd0 /floppy</I> <BR>
<code>$ </code>
</TT></BLOCKQUOTE>
The mounted-on directory need not be empty, although it must
exist. Any files in it,
however, will be inaccessible by name while the filesystem
is mounted. (Any files that have already been opened will
still be accessible. Files that have hard links from
other directories can be accessed using those names.)
There is no harm done with this, and it can even be useful.
For instance, some people like to have <tt>/tmp</tt><A NAME="1612"> </A> and <tt>/var/tmp</tt><A NAME="1614"> </A>
synonymous, and make <tt>/tmp</tt><A NAME="1616"> </A> be a symbolic link to <tt>/var/tmp</tt><A NAME="1618"> </A>.
When the system is booted, before the <tt>/usr</tt><A NAME="1620"> </A> filesystem is
mounted, a <tt>/var/tmp</tt><A NAME="1622"> </A> directory residing on the root filesystem
is used instead. When <tt>/usr</tt><A NAME="1624"> </A> is mounted, it will make the
<tt>/var/tmp</tt><A NAME="1626"> </A> directory on the root filesystem inaccessible.
If <tt>/var/tmp</tt><A NAME="1628"> </A> didn't exist on the root filesystem, it would
be impossible to use temporary files before mounting <tt>/var</tt><A NAME="1630"> </A>.
<P>
If you don't intend to write anything to the filesystem, use
the <TT>-r</TT> switch for <tt>mount</tt><A NAME="1632"> </A> to do a <b>readonly
mount</b>. This will make the kernel stop any attempts at
writing to the filesystem, and will also stop the kernel from
updating file access times in the inodes. Read-only mounts
are necessary for unwritable media, e.g., CD-ROM's.
<P>
The alert reader has already noticed a slight
logistical problem. How is the first filesystem (called the <b>root
filesystem</b>, because it contains the root directory) mounted,
since it obviously can't be mounted on another filesystem?
Well, the answer is that it is done by magic.<A NAME="tex2html22" HREF="footnode.html#1275"><IMG ALIGN=BOTTOM ALT="gif" SRC="./foot_motif.gif"></A> The root filesystem is magically mounted at boot time,
and one can rely on it to always be mounted--if the root
filesystem can't be mounted, the system does not boot. The
name of the filesystem that is magically mounted as root is
either compiled into the kernel, or set using LILO or <tt>rdev</tt><A NAME="1636"> </A>.
<P>
The root filesystem is usually first mounted readonly. The
startup scripts will then run <tt>fsck</tt><A NAME="1638"> </A> to verify its
validity, and if there are no problems, they will <b>re-mount</b>
it so that writes will also be allowed. <tt>fsck</tt><A NAME="1641"> </A> must not
be run on a mounted filesystem, since any changes to the
filesystem while <tt>fsck</tt><A NAME="1643"> </A> is running <EM>will</EM> cause
trouble. Since the root filesystem
is mounted readonly while it is being checked, <tt>fsck</tt><A NAME="1645"> </A>
can fix any problems without worry, since the remount operation
will flush any metadata that the filesystem keeps in memory.
<P>
On many systems there are other filesystems that should also
be mounted automatically at boot time. These are specified
in the <tt>/etc/fstab</tt><A NAME="1647"> </A> file; see the <em>fstab</em><A NAME="1649"> </A> man page for
details on the
format. The details of exactly when the extra filesystems
are mounted depend on many factors, and can be configured
by each administrator if need be. When the chapter on booting
is finished, you may read all about it there.
<P>
When a filesystem no longer needs to be mounted, it can be
unmounted with <tt>umount</tt><A NAME="1651"> </A><A NAME="tex2html23" HREF="footnode.html#1431"><IMG ALIGN=BOTTOM ALT="gif" SRC="./foot_motif.gif"></A>. <tt>umount</tt><A NAME="1655"> </A> takes one argument:
either the device file or the mount point.
For example, to unmount the directories of
the previous example, one could use the commands
<BLOCKQUOTE> <TT>
<code>$ </code> <I>umount /dev/hda2 </I> <BR>
<code>$ </code> <I>umount /usr</I> <BR>
<code>$ </code>
</TT></BLOCKQUOTE>
See the man page for further instructions on how to use the
command. It is imperative that you always unmount a mounted
floppy. <EM>Don't just pop the floppy out of the drive!</EM>
Because of disk caching, the data is not necessarily written
to the floppy until you unmount it, so removing the floppy
from the drive too early might cause the contents to become
garbled. If you only read from the floppy, this is not very
likely, but if you write, even accidentally, the result may be
catastrophic.
<P>
Mounting and unmounting requires super user privileges, i.e.,
only <tt>root</tt><A NAME="1657"> </A> can do it. The reason for this is that if any
user can mount a floppy on any directory, then it is rather easy
to create a floppy with, say, a Trojan horse disguised as
<tt>/bin/sh</tt><A NAME="1659"> </A>, or any other often used program. However, it is
often necessary to allow users to use floppies, and there are
several ways to do this:
<UL>
<LI> Give the users the <tt>root</tt><A NAME="1661"> </A> password. This is obviously
bad security, but is the easiest solution. It works
well if there is no need for security anyway, which is
the case on many non-networked, personal systems.
<LI> Use a program such as <tt>sudo</tt><A NAME="1663"> </A> to allow users to
use mount. This is still bad security, but doesn't
directly give super user privileges to
everyone.<A NAME="tex2html21" HREF="footnode.html#1299"><IMG ALIGN=BOTTOM ALT="gif" SRC="./foot_motif.gif"></A>
<LI> Make the users use <tt>mtools</tt><A NAME="1665"> </A>, a package for manipulating
MS-DOS filesystems, without mounting them. This works
well if MS-DOS floppies are all that is needed,
but is rather awkward otherwise.
<LI> List the floppy devices and their allowable mount points
together with the suitable options in <tt>/etc/fstab</tt><A NAME="1667"> </A>.
</UL>
The last alternative can be implemented by adding a line like
the following to the <tt>/etc/fstab</tt><A NAME="1669"> </A> file:
<BLOCKQUOTE> <TT>
/dev/fd0 /floppy msdos user,noauto 0 0
</TT></BLOCKQUOTE>
The columns are: device file to mount, directory to mount on,
filesystem type, options, backup frequency (used by <tt>dump</tt><A NAME="1671"> </A>),
and <tt>fsck</tt><A NAME="1673"> </A> pass number (to specify the order in which
filesystems should be checked upon boot; 0 means no check).
<P>
The <tt>noauto</tt> option stops this mount to be done
automatically when the system is started (i.e., it stops
<tt>mount -a</tt><A NAME="1675"> </A> from mounting it). The <tt>user</tt> option
allows any user to mount the filesystem, and, because of security
reasons, disallows execution of programs (normal or setuid)
and interpretation of device files from the mounted filesystem.
After this, any user can mount a floppy with an <tt>msdos</tt>
filesystem with the following command:
<BLOCKQUOTE> <TT>
<code>$ </code> <I>mount /floppy</I> <BR>
<code>$ </code>
</TT></BLOCKQUOTE>
The floppy can (and needs to, of course) be unmounted with
the corresponding <tt>umount</tt><A NAME="1677"> </A> command.
<P>
If you want to provide access to several types of floppies,
you need to give several mount points. The settings can be
different for each mount point. For example, to give access
to both MS-DOS and ext2 floppies, you could have the following
to lines in <tt>/etc/fstab</tt><A NAME="1679"> </A>:
<BLOCKQUOTE>
<PRE>/dev/fd0 /dosfloppy msdos user,noauto 0 0
/dev/fd0 /ext2floppy ext2 user,noauto 0 0</PRE>
</BLOCKQUOTE>
For MS-DOS filesystems (not just floppies), you probably want to
restrict access to it by using the <tt>uid</tt>, <tt>gid</tt>,
and <tt>umask</tt><A NAME="1681"> </A> filesystem options, described in detail on the
<em>mount</em><A NAME="1683"> </A> manual page. If you aren't careful, mounting
an MS-DOS filesystem gives everyone at least read access to the
files in it, which is not a good idea.
<P>
<HR><A NAME="tex2html897" HREF="node47.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="./next_motif.gif"></A> <A NAME="tex2html895" HREF="node41.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="./up_motif.gif"></A> <A NAME="tex2html889" HREF="node45.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="./previous_motif.gif"></A> <A NAME="tex2html899" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="./contents_motif.gif"></A> <A NAME="tex2html900" HREF="node114.html"><IMG WIDTH=43 HEIGHT=24 ALIGN=BOTTOM ALT="index" SRC="./index_motif.gif"></A> <BR>
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<B> Previous:</B> <A NAME="tex2html890" HREF="node45.html">Creating a filesystem</A>
<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
</ADDRESS>
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