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

</TABLE>

Therefore, a file permission of 751 means that the owner has read, write, and execute

permission (4+2+1=7), the group has read and execute permission (4+1=5), and others

have execute permission (1).</P>

<P>If you play with the numbers, you quickly see that the permission digits can range

between 0 and 7, and that for each digit in that range there's only one possible

combination of read, write, and execute permissions.



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	<DD>

<HR>

<A NAME="Heading38<FONT COLOR="#000077"><B>TIP:</B> </FONT>If you're familiar

	with the binary system, think of <TT>rwx</TT> as a three-digit binary number. If

	permission is allowed, the corresponding digit is 1. If permission is denied, the

	digit is 0. So <TT>r-x</TT> would be the binary number 101, which is 4+0+1, or 5.

	<TT>--x</TT> would be 001, which is 0+0+1, which is 1, and so on.

<HR>



</DL>



<P>The following combinations are possible: 0 or <TT>---</TT>: no permissions at

all</P>



<P>4 or <TT>r--</TT>: read-only</P>

<P>2 or <TT>-w-</TT>: write-only (rare)</P>

<P>1 or <TT>--x</TT>: execute</P>

<P>6 or <TT>rw-</TT>: read and write</P>

<P>5 or <TT>r-x</TT>: read and execute</P>

<P>3 or <TT>-wx</TT>: write and execute (rare)





<BLOCKQUOTE>

	<P>7 or <TT>rwx</TT>: read, write, and execute



</BLOCKQUOTE>





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<A NAME="Heading39<FONT COLOR="#000077"><B>NOTE:</B> </FONT>Anyone who has

	permission to read a file can then copy that file. When a file is copied, the copy

	is owned by the person doing the copying. He or she can then change ownership and

	permissions, edit the file, and so on.

<HR>



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<A NAME="Heading40<FONT COLOR="#000077"><B>WARNING: </B></FONT>Removing write

	permission from a file doesn't prevent the file from being deleted! It does prevent

	it from being deleted accidentally, since Linux asks you whether you want to override

	the file permissions. You have to answer <TT>y</TT>, or the file will not be deleted.

<HR>



</DL>



<CENTER>

<H4><A NAME="Heading41<FONT COLOR="#000077">Changing File Permissions</FONT></H4>

</CENTER>

<P>To change file permissions, use the <TT>chmod</TT> (change [file] mode) command.

The syntax is <TT>chmod &lt;</TT>specification<TT>&gt; </TT>file.</P>



<P>There are two ways to write the permission specification. One is by using the

numeric coding system for permissions:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">darkstar:~$ ls -l myfile

-rw-r--r--   1 fido     users         114 Dec  7 14:31 myfile

darkstar:~$ chmod 345 myfile

darkstar:~$ ls -l myfile

--wxr--r-x   1 fido     users         114 Dec  7 14:31 myfile

darkstar:~$ chmod 701 myfile

darkstar:~$ ls -l myfile

-rwx----x   1 root     users         114 Dec  7 14:31 myfile

</FONT></PRE>

<P>This method has the advantage of specifying the permissions in an absolute, rather

than relative, fashion. Also, it's easier to tell someone, &quot;Change permissions

on the file to seven- five-five&quot; than to say, &quot;Change permissions on the

file to read-write-execute, read-execute, read-execute.&quot;</P>

<P>You can also use letter codes to change the existing permissions. To specify which

of the per- missions to change, type <TT>u</TT> (user), <TT>g</TT> (group), <TT>o</TT>

(other), or <TT>a</TT> (all). This is followed by a + to add permissions or a - to

remove them. This in turn is followed by the permissions to be added or removed.

For example, to add execute permissions for the group and others, you would type<FONT

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

<PRE><FONT COLOR="#0066FF">darkstar:~$ chmod go+r myfile

</FONT></PRE>

<P>Other ways of using the symbolic file permissions are described in the <TT>chmod</TT>

man page.

<CENTER>

<H4><A NAME="Heading42<FONT COLOR="#000077">Changing Directory Permissions</FONT></H4>

</CENTER>

<P>You change directory permissions with <TT>chmod</TT>, exactly the same way as

with files. Remember that if a directory doesn't have execute permissions, you can't

<TT>cd</TT> to it.



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<A NAME="Heading43<FONT COLOR="#000077"><B>WARNING: </B></FONT>Any user who

	has write permission in a directory can delete files in that directory, whether or

	not that user owns or has write privileges to those files. Most directories, therefore,

	have permissions set to <TT>drwxr-xr-x</TT>. This ensures that only the directory's

	owner can create or delete files in that directory. It is especially dangerous to

	give write permission to all users for directories!

<HR>



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<CENTER>

<H3><A NAME="Heading44<FONT COLOR="#000077">Miscellaneous File Commands</FONT></H3>

</CENTER>

<P>There are many Linux commands to manipulate files, directories, and the entire

file system. Many of these commands are used only by system administrators. You will

touch on a few that are also used by ordinary users. These and other important system

administrator commands are further detailed in Chapter 38, &quot;System Administration

Basics.&quot;

<CENTER>

<H4><A NAME="Heading45<FONT COLOR="#000077">Fear of Compression: The Zipless

File</FONT></H4>

</CENTER>

<P>Most Linux files are stored on the installation CD-ROM in compressed form. This

allows more information to be stored.</P>

<P>When you installed Linux, the installation program uncompressed many of the files

it transferred to your hard drive. However, if you look, you will be able to find

compressed files!</P>

<P>Any file ending in .gz--for example, <TT>squashed.gz</TT>--is a compressed file.

To uncompress this particular type of file, type <TT>gunzip &lt;</TT>file<TT>&gt;</TT>.

For this example, you would type <TT>gunzip squashed.gz</TT>. The <TT>gunzip</TT>

program creates an uncompressed file and removes the .gz extension. Therefore, you

would wind up with a normal file called <TT>squashed</TT>.</P>

<P>To compress a file, use the <TT>gzip</TT> command. Typing <TT>gzip squashed</TT>

would compress <TT>squashed</TT> and rename it <TT>squashed.gz</TT>.</P>

<P>Another type of compressed file you might see ends with the extension .zip. Use

<TT>unzip</TT> to uncompress these files. To create files of this type, use <TT>zip</TT>.

<CENTER>

<H4><A NAME="Heading46<FONT COLOR="#000077">How to tar Without Feathering</FONT></H4>

</CENTER>

<P>In almost any location with several Linux or UNIX systems, sooner or later you

will hear someone say, &quot;Put that in a <TT>tar</TT> file and send it over.&quot;</P>

<P>They are referring to the output created by the <TT>tar</TT> program. Although

<TT>tar</TT> stands for tape archive, it can copy files to floppy disk or to any

filename you specify in the Linux file system. The <TT>tar</TT> command is used because

it can archive files and directories into a single file and then recreate the files

and even the directory structures later. It's also the easiest way to place Linux

files on a floppy disk.</P>

<P>To create a <TT>tar</TT> file, you typically type <TT>tar cvf &lt;</TT>destination<TT>&gt;</TT>

<TT>&lt;</TT>files/directories<TT>&gt;</TT>, where files/directories specifies the

files and directories to be archived, and destination is where you want the <TT>tar</TT>

file to be created. If you want the destination to be a floppy disk, you usually

type <TT>/dev/fd0</TT> as the destination. This specifies your primary floppy drive

(A: in MS-DOS). You can use a floppy disk that's been formatted under MS-DOS.



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<A NAME="Heading47<FONT COLOR="#000077"><B>WARNING:</B> </FONT>When <TT>tar</TT>

	archives to a floppy disk, all the data already on the disk is destroyed. You have

	to reformat it to use it with MS-DOS again.

<HR>



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<P>To extract a <TT>tar</TT> file, you typically type <TT>tar xvf &lt;</TT>tar file<TT>&gt;</TT>.

For instance, to pull files from a floppy disk, you would type <TT>tar xvf /dev/fd0</TT>.



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<A NAME="Heading48<FONT COLOR="#000077"><B>NOTE: </B></FONT>Unlike <TT>zip</TT>

	and <TT>gzip</TT>, <TT>tar</TT> doesn't remove, delete, or rename the files it puts

	into the archive. However, when <TT>tar</TT> extracts archived files, it overwrites

	existing files with files of the same name from the archive.

<HR>



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<CENTER>

<H3><A NAME="Heading49<FONT COLOR="#000077">Important Directories in the Linux

File System</FONT></H3>

</CENTER>

<P>Most of the directories that hold Linux system files are &quot;standard.&quot;

Other UNIX systems will have identical directories with similar contents. This section

summarizes some of the more important directories on your Linux system.

<CENTER>

<H4><A NAME="Heading50<FONT COLOR="#000077">/</FONT></H4>

</CENTER>

<P>This is the root directory. It holds the actual Linux program, as well as subdirectories.

Do not clutter this directory with your files!

<CENTER>

<H4><A NAME="Heading51<FONT COLOR="#000077">/home</FONT></H4>

</CENTER>

<P>This directory holds users' home directories. In other UNIX systems, this can

be the <TT>/usr</TT> or <TT>/u</TT> directory.

<CENTER>

<H4><A NAME="Heading52<FONT COLOR="#000077">/bin</FONT></H4>

</CENTER>

<P>This directory holds many of the basic Linux programs. <TT>bin</TT> stands for

binaries, files that are executable and that hold text only computers could understand.

<CENTER>

<H4><A NAME="Heading53<FONT COLOR="#000077">/usr</FONT></H4>

</CENTER>

<P>This directory holds many other user-oriented directories. Some of the most important

are described in the following sections. Other directories found in <TT>/usr</TT>

include <TT>docs</TT> Various documents, including useful Linux information.</P>

<P><TT>man</TT> The man pages accessed by typing <TT>man &lt;</TT>command<TT>&gt;</TT>.





<BLOCKQUOTE>

	<P><TT>games</TT> The fun stuff!



</BLOCKQUOTE>



<CENTER>

<H4><A NAME="Heading54<FONT COLOR="#000077">/usr/bin</FONT></H4>

</CENTER>

<P>This directory holds user-oriented Linux programs.

<CENTER>

<H4><A NAME="Heading55<FONT COLOR="#000077">/usr/spool</FONT></H4>

</CENTER>

<P>This directory has several subdirectories. <TT>mail</TT> holds mail files, <TT>spool</TT>

holds files to be printed, and <TT>uucp</TT> holds files copied between Linux machines.

<CENTER>

<H4><A NAME="Heading56<FONT COLOR="#000077">/dev</FONT></H4>

</CENTER>

<P>Linux treats everything as a file! The <TT>/dev</TT> directory hold devices. These

are special files that serve as gateways to physical computer components. For instance,

if you copy to <TT>/dev/fd0</TT>, you're actually sending data to the system's floppy

disk. Your terminal is one of the <TT>/dev/tty</TT> files. Partitions on the hard

drive are of the form <TT>/dev/hd0</TT>. Even the system's memory is a device!</P>

<P>A famous device is <TT>/dev/null</TT>. This is sometimes called the bit bucket.

All information sent to <TT>/dev/null</TT> vanishes--it's thrown into the trash.

<CENTER>

<H4><A NAME="Heading57<FONT COLOR="#000077">/usr/sbin</FONT></H4>

</CENTER>

<P>This directory holds system administration files. If you do an <TT>ls -l</TT>,

you see that you must be the owner, r