ch38.htm

linux-unix130.linux.and.unix.ebooks130 linux and unix ebookslinuxLearning Linux - Collection of 12 E

compression required, the longer it takes to compress). To use <TT>gzip</TT>, specify

the filename to be compressed and the compression type:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">gzip -9 filename

</FONT></PRE>

<P>The <TT>-9</TT> option, which tells <TT>gzip</TT> to use the highest compression

factor, will probably be the option you use the most. Alternatively, leave this option

off and let <TT>gzip</TT> work with its default settings. A <TT>gzip</TT> compressed

file has the extension <TT>.gz</TT> appended, and the original file is deleted. To

uncompress a <TT>gzipp</TT>ed file, use the <TT>gunzip</TT> utility.

<H3 ALIGN="CENTER"><A NAME="Heading16<FONT COLOR="#000077">Using tar</FONT></H3>

<P>The <TT>tar</TT> (tape archiver) utility has been used with UNIX systems for many

years. Unfortunately, it's not very friendly and can be quite temperamental at times,

especially when you're unfamiliar with the syntax required to make <TT>tar</TT> do

something useful.</P>

<P>The <TT>tar</TT> program is designed to create a single archive file, much as

the ZIP utilities do for DOS. With <TT>tar</TT>, you can combine many files into

a single larger file, which makes it easier to move the collection or back it up

to tape. The general syntax used by <TT>tar</TT> is as follows:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">tar [options] [file]

</FONT></PRE>

<P>The options available are lengthy and sometimes obtuse. Files can be specified

with or without wildcards. A simple example of creating a <TT>tar</TT> archive file

is<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">tar cvf archive1.tar /usr/tparker

</FONT></PRE>

<P>which combines all the files in <TT>/usr/tparker</TT> into a <TT>tar</TT> archive

called <TT>archive1.tar</TT>. The <TT>c</TT> option tells <TT>tar</TT> to create

the archive; the <TT>v</TT> tells it to be verbose, displaying messages as it goes;

and the <TT>f</TT> tells it to use the filename <TT>archive1.tar</TT> as the output

file.</P>

<P>The extension <TT>.tar</TT> is not automatically appended by <TT>tar</TT>, but

is a user convention that helps identify the file as an archive. This convention

isn't widely used, though, although it should be as it helps identify the file.</P>

<P>The <TT>c</TT> option creates new archives. (If the file existed before, it is

deleted.) The <TT>u</TT> (update) option is used to append new files to an existing

archive, or to create the archive if it doesn't exist. This is useful if you keep

adding files. The <TT>x</TT> option is used to extract files from the archive. To

extract with the <TT>tar</TT> command all the files in the archive in the earlier

example, you would use the command<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">tar xvf archive1.tar

</FONT></PRE>

<P>There's no need to specify a filename, because the filenames and paths will be

retained as the archive is unpacked. It's important to remember that the path is

saved with the file. So if you archived <TT>/usr/tparker</TT> and then moved into

<TT>/usr/tparker</TT> and issued the <TT>extract</TT> command, the files would be

extracted relevant to the current directory, which would place them in <TT>/usr/tparker/usr/tparker</TT>.

You must be very careful to extract files properly. If you want to force a new directory

path on extracted files, a command-line option allows this.</P>

<P>The <TT>tar</TT> system does not remove the original files as they are packed

into the archive, nor does it remove the archive file when files are extracted. These

steps must be performed manually.</P>

<P>You can use <TT>tar</TT> to copy files to tapes or floppies by specifying a device

name and the <TT>f</TT> option as a device name. To archive files in <TT>/usr/tparker</TT>

to a floppy disk in the first drive, you could use the following command:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">tar cvf /dev/fd0 /usr/tparker

</FONT></PRE>

<P>This can cause a problem if the floppy doesn't have enough capacity, however,

so <TT>tar</TT> lets you specify the capacity with the <TT>k</TT> option. In this

case, the command for a 1.44MB floppy is as follows:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">tar cvfk /dev/fd0 1440 /usr/tparker

</FONT></PRE>

<P>If the floppy is full before the entire archive has been copied, <TT>tar</TT>

prompts you for another one. It's important to keep the arguments in the right order.

You see that the <TT>f</TT> is before the <TT>k</TT>, so the device name must be

before the capacity. All the argument keyletters are gathered together instead of

issued one at a time followed by their value, which is one aspect of <TT>tar</TT>

that can be very confusing. As a last issue for backing up to floppy, it is sometimes

necessary to tell the <TT>tar</TT> program about the blocking used (blocking identifies

how many blocks are used for each chunk of information on the device). A floppy usually

has a blocking factor of 4, so the command becomes the following:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">tar cvfkb /dev/fd0 1440 4 /usr/tparker

</FONT></PRE>

<P>A final problem with <TT>tar</TT> is that it can't always handle a generic device

such as <TT>/dev/fd0</TT>, and must be specifically told the disk type. For more

complete information on all the options used by <TT>tar</TT>, check the man pages

or, even better, a good system administration book. You can use <TT>tar</TT> to archive

compressed files, too, in the same manner. You can also compress a <TT>tar</TT> file

without any problems. In these cases, you might get filenames such as<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">filename.tar.gz

</FONT></PRE>

<P>which show that you should run <TT>gunzip</TT> first to recover the <TT>tar</TT>

file, and then run <TT>tar</TT> to extract the files in the archive. You can run

the commands together with pipes:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">gunzip filename.tar.gz | tar xvf -

</FONT></PRE>

<P>The hyphen as the <TT>tar</TT> filename after the pipe symbol is standard UNIX

terminology for taking the input from the pipe (<TT>stdin</TT>).

<H3 ALIGN="CENTER"><A NAME="Heading17<FONT COLOR="#000077">Backups</FONT></H3>

<P>The three rules of system administration are back up, back up, and back up. This

might sound silly and trite, but a backup can save you whenever you do something

silly to the file system, or when problems occur. With UNIX, most backups are made

to a tape device using <TT>tar</TT>, although many Linux users don't have tape units

available and have to resort to floppies.</P>

<P>Backups are made with the <TT>tar</TT> utility, as I mentioned earlier. The procedure

is exactly the same as I showed you earlier. To back up the entire system on floppy,

the command is<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">tar cvfbk /dev/fd0 1440 4 /

</FONT></PRE>

<P>To back up to a high-capacity tape device larger than the file system (and hence

not needing a capacity limit) called <TT>/dev/rct0</TT>, the command is<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">tar cvfk /dev/rct0 20 /

</FONT></PRE>

<P>In many cases, you won't want to back up the entire system, because it's easier

to reinstall off a CD-ROM. However, you should back up your user files by either

backing up the entire <TT>/usr</TT> directory or specifically backing up your own

home directory.</P>

<P>To restore a backup, you use the <TT>tar</TT> command again:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">tar xvf /dev/rct0

</FONT></PRE>

<P>This recovers all files from the tape device <TT>/dev/rct0</TT>. You can explicitly

restore specific files if you need to.</P>

<P>Several commercial products offer automated backups, although you can do this

quite easily with the <TT>cron</TT> command.

<H3 ALIGN="CENTER"><A NAME="Heading18<FONT COLOR="#000077">Setting Up Your

System</FONT></H3>

<P>You can perform several little tasks to tweak or optimize your Linux system, although

in many cases they are dependent on the version you are running and other applications

coexisting. We can look at a few of the miscellaneous tasks here.

<H4 ALIGN="CENTER"><A NAME="Heading19<FONT COLOR="#000077">Setting the System

Name</FONT></H4>

<P>The system name is contained in a file called <TT>/etc/HOSTNAME</TT>. It is simply

the name the system calls itself for identification, which is especially useful if

you are networking your Linux machine with others. You can call the system anything

you want.</P>

<P>To set your system name (also called a host name), you can either edit the system

files (which should be followed by a reboot to make the changes effective) or use

the <TT>hostname</TT> command. The following command sets the machine's name to <TT>hellfire</TT>:<FONT

COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">hostname hellfire

</FONT></PRE>

<H4 ALIGN="CENTER"><A NAME="Heading20<FONT COLOR="#000077">Using a Maintenance

Disk</FONT></H4>

<P>Every system should have a maintenance disk that enables you to check the <TT>root</TT>

file system, recover from certain disk problems, and solve simple problems (such

as forgetting your <TT>root</TT> password). The emergency disks, also called the

boot/root floppies, are created with the setup program in most distributions of Linux

when the configuration is changed. You can usually create an emergency boot disk

from the CD-ROM that the system came on, as well as obtain the necessary files from

FTP sites. After you have booted your machine with the emergency disk, you can mount

the disk partitions with the <TT>mount</TT> command.

<H4 ALIGN="CENTER"><A NAME="Heading21<FONT COLOR="#000077">Forgetting the root

Password</FONT></H4>

<P>This is an embarrassing and annoying problem, but luckily one easily fixed with

Linux. (If only other UNIX systems were so easy!) To recover from a problem with

the <TT>root</TT> password, use a boot floppy and boot the system. Mount the <TT>root</TT>

partition, and edit the <TT>/etc/passwd</TT> file to remove any password for <TT>root</TT>;

then, reboot from the hard disk.</P>

<P>After the system has booted, you can set a password again.



<DL>

	<DT></DT>

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading22<FONT COLOR="#000077"><B>WARNING: </B></FONT>This points

	out one major security problem with Linux: Anyone with a boot floppy can get unrestricted

	access to your system! On some systems, you can avoid this problem by setting the

	system BIOS to require a password before booting from a floppy drive.

<HR>



</DL>



<H4 ALIGN="CENTER"><A NAME="Heading23<FONT COLOR="#000077">Setting the Login

Message</FONT></H4>

<P>If you have more than one user on the system, you can display information about

the system, its maintenance, or changes in a file called <TT>/etc/motd</TT> (message

of the day). The contents of this file are displayed whenever someone logs in.</P>

<P>To change the <TT>/etc/motd</TT> file, use any text editor and save the contents

as ASCII. You can make the contents as long as you want, but readers usually appreciate

brevity. The <TT>/etc/motd</TT> file is useful for informing users of downtime, backups,

or new additions. You can also use it to give a more personal feel to your system.

<H3 ALIGN="CENTER"><A NAME="Heading24<FONT COLOR="#000077">Summary</FONT></H3>

<P>System administration is not a complicated subject, unless you want to get into

the nitty-gritty of your operating system and its configuration. For most Linux users

who use the operating system for their personal experimentation, the administration

steps explained in this chapter should be sufficient for most purposes. If you want

to get into more detail, check out a good UNIX system administration book.









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