ch08.htm

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

	<DD>

<HR>

<A NAME="Heading31<FONT COLOR="#000077"><B>WARNING: </B></FONT>As soon as a

	file is removed, it is gone! Always think about what you're doing before you remove

	a file. You can use one of the following techniques to keep out of trouble when using

	wildcards. 1. Run <TT>ls</TT> using the same file specification you use with the

	<TT>rm</TT> command. For instance:<FONT COLOR="#0066FF"></FONT>

</DL>





<DL>

	<DD><FONT COLOR="#0066FF">	darkstar:~$ ls *duck<BR>

		dead_duck    guiduck   lame-duck<BR>

		:~$ rm *duck</FONT>

</DL>



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





<BLOCKQUOTE>

	<P>In this case, you thought you wanted to remove all files that matched <TT>*duck</TT>.

	To verify that this indeed was the case, you listed all the <TT>*duck</TT> files

	(wildcards work the same way with all commands). The listing looked okay, so you

	went ahead and removed the files. 2. Use the <TT>i</TT> (interactive) option with

	<TT>rm</TT>:<FONT COLOR="#0066FF"></FONT>



</BLOCKQUOTE>



<PRE><FONT COLOR="#0066FF">	darkstar:~$ rm -i *duck

	rm: remove `dead_duck'? y

 	rm: remove `guiduck'? n

	rm: remove `lame-duck'? y

	darkstar:~$

</FONT></PRE>





<BLOCKQUOTE>

	<P>When you use <TT>rm -i</TT>, the command goes through the list of files to be

	deleted one by one, prompting you for the okay to remove the file. If you type <TT>y</TT>

	or <TT>Y</TT>, <TT>rm</TT> removes the file. If you type any other character, <TT>rm</TT>

	does not remove it. The only disadvantage of using this interactive mode is that

	it can be very tedious when the list of files to be removed is long.

<HR>





</BLOCKQUOTE>



<H4 ALIGN="CENTER"><A NAME="Heading32<FONT COLOR="#000077">Removing Directories</FONT></H4>

<P>The command normally used to remove (delete) directories is <TT>rmdir</TT>. The

syntax is <TT>rmdir &lt;</TT>directory<TT>&gt;</TT>.</P>

<P>Before you can remove a directory, it must be empty (the directory can't hold

any files or subdirectories). Otherwise, you see<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">rmdir: &lt;directory&gt;: Directory not empty

</FONT></PRE>

<P>This is as close to a safety feature as you will see in Linux!



<DL>

	<DT></DT>

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading33<FONT COLOR="#000077"><B>TIP: </B></FONT>This one might mystify

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

</DL>





<DL>

	<DD><FONT COLOR="#0066FF">darkstar:/home$ ls<BR>

	fido/    root/    zippy/<BR>

	darkstar:/home$ ls zippy<BR>

	core      kazoo     stuff<BR>

	darkstar:/home$ rm zippy/*<BR>

	darkstar:/home/zippy$ ls zippy<BR>

	darkstar:/home$ rmdir zippy<BR>

	rmdir: zippy: Directory not empty<BR>

	darkstar:~$</FONT>

</DL>



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





<BLOCKQUOTE>

	<P>The reason for the <TT>Directory not empty</TT> message is that files starting

	with <TT>.</TT> usually are special system files and are usually hidden from the

	user. To list files whose names start with <TT>.</TT>, you have to use <TT>ls -a</TT>.

	To delete these files, use <TT>rm .*</TT>:<FONT COLOR="#0066FF"></FONT>



</BLOCKQUOTE>





<DL>

	<DD><FONT COLOR="#0066FF">darkstar:/home$ ls -a zippy<BR>

	./   ../   .bashrc      .profile<BR>

	darkstar:/home$ rm zippy/.*<BR>

	rm: cannot remove `.' or `..'<BR>

	darkstar:/home$ ls -a zippy<BR>

	./   ../<BR>

	darkstar:/home$ rmdir zippy<BR>

	darkstar:/home$ ls<BR>

	fido/    root/<BR>

	darkstar:~$</FONT>

</DL>



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





<BLOCKQUOTE>

	<P>You will most often come across this situation in a system administrator role.

<HR>





</BLOCKQUOTE>



<P>Sometimes you want to remove a directory with many layers of subdirectories. Emptying

and then deleting all the subdirectories one by one would be very tedious. Linux

offers a way to remove a directory and all the files and subdirectories it contains

in one easy step. This is the <TT>r</TT> (recursive) option of the <TT>rm</TT> command.

The syntax is <TT>rm -r &lt;</TT>directory<TT>&gt;</TT>. The directory and all its

contents are removed.



<DL>

	<DT></DT>

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading34<FONT COLOR="#000077"><B>WARNING: </B></FONT>You should use

	<TT>rm -r</TT> only when you really have to. To paraphrase an old saying, &quot;It's

	only a shortcut until you make a mistake.&quot; For instance, if you're logged in

	as root, the following command removes all files from your hard disk, and then it's

	&quot;Hello, installation procedure&quot; time (do not type the following command!):

</DL>





<DL>

	<DD><FONT COLOR="#0066FF">rm -rf /</FONT>

</DL>



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





<BLOCKQUOTE>

	<P>Believe it or not, people do this all too often. Don't join the club!

<HR>





</BLOCKQUOTE>



<H3 ALIGN="CENTER"><A NAME="Heading35<FONT COLOR="#000077">File Permissions

and Ownership</FONT></H3>

<P>All Linux files and directories have ownership and permissions. You can change

permissions, and sometimes ownership, to provide greater or lesser access to your

files and directories. File permissions also determine whether a file can be executed

as a command.</P>

<P>If you type <TT>ls -l</TT> or <TT>dir</TT>, you see entries that look like this:<FONT

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

<PRE><FONT COLOR="#0066FF">-rw-r--r--   1 fido     users         163 Dec  7 14:31 myfile

</FONT></PRE>

<P>The <TT>-rw-r--r--</TT> represents the permissions for the file <TT>myfile</TT>.

The file's ownership includes <TT>fido</TT> as the owner and <TT>users</TT> as the

group.

<H4 ALIGN="CENTER"><A NAME="Heading36<FONT COLOR="#000077">File and Directory

Ownership</FONT></H4>

<P>When you create a file, you are that file's owner. Being the file's owner gives

you the privilege of changing the file's permissions or ownership. Of course, once

you change the ownership to another user, you can't change the ownership or permissions

anymore!</P>

<P>File owners are set up by the system during installation. Linux system files are

owned by IDs such as <TT>root</TT>, <TT>uucp</TT>, and <TT>bin</TT>. Do not change

the ownership of these files.</P>

<P>Use the <TT>chown</TT> (change ownership) command to change ownership of a file.

The syntax is <TT>chown &lt;</TT>owner<TT>&gt; &lt;</TT>filename<TT>&gt;</TT>. In

the following example, you change the ownership of the file <TT>myfile</TT> to root:<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:~$ chown root myfile

darkstar:~$ ls -l myfile

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

</FONT></PRE>

<P>To make any further changes to the file <TT>myfile</TT>, or to <TT>chown</TT>

it back to <TT>fido</TT>, you must use <TT>su</TT> or log in as <TT>root</TT>.</P>

<P>Files (and users) also belong to groups. Groups are a convenient way of providing

access to files for more than one user but not to every user on the system. For instance,

users working on a special project could all belong to the group project. Files used

by the whole group would also belong to the group project, giving those users special

access. Groups normally are used in larger installations. You may never need to worry

about groups.</P>

<P>The <TT>chgrp</TT> command is used to change the group the file belongs to. It

works just like <TT>chown</TT>.

<H4 ALIGN="CENTER"><A NAME="Heading37<FONT COLOR="#000077">File Permissions</FONT></H4>

<P>Linux lets you specify read, write, and execute permissions for each of the following:

the owner, the group, and &quot;others&quot; (everyone else).</P>

<P>Read permission enables you to look at the file. In the case of a directory, it

lets you list the directory's contents using <TT>ls</TT>.</P>

<P>Write permission enables you to modify (or delete!) the file. In the case of a

directory, you must have write permission in order to create, move, or delete files

in that directory.</P>

<P>Execute permission enables you to execute the file by typing its name. With directories,

execute permission enables you to <TT>cd</TT> into them.</P>

<P>For a concrete example, let's look at <TT>myfile</TT> again:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">-rw-r--r--   1 fido     users         163 Dec  7 14:31 myfile

</FONT></PRE>

<P>The first character of the permissions is <TT>-</TT>, which indicates that it's

an ordinary file. If this were a directory, the first character would be <TT>d</TT>.

There are also some other, more exotic classes. These are beyond the scope of this

chapter.</P>

<P>The next nine characters are broken into three groups of three, giving permissions

for owner, group, and other. Each triplet gives read, write, and execute permissions,

always in that order. Permission to read is signified by an <TT>r</TT> in the first

position, permission to write is shown by a <TT>w</TT> in the second position, and

permission to execute is shown by an <TT>x</TT> in the third position. If the particular

permission is absent, its space is filled by -.</P>

<P>In the case of <TT>myfile</TT>, the owner has <TT>rw-</TT>, which means read and

write permissions. This file can't be executed by typing <TT>myfile</TT> at the Linux

prompt.</P>

<P>The group permissions are <TT>r--</TT>, which means that members of the group

&quot;users&quot; (by default, all ordinary users on the system) can read the file

but not change it or execute it.</P>

<P>Likewise, the permissions for all others are <TT>r--</TT>: read-only.</P>

<P>File permissions are often given as a three-digit number--for instance, 751. It's

important to understand how the numbering system works, because these numbers are

used to change a file's permissions. Also, error messages that involve permissions

use these numbers.</P>

<P>The first digit codes permissions for the owner, the second digit codes permissions

for the group, and the third digit codes permissions for other (everyone else).</P>

<P>The individual digits are encoded by summing up all the &quot;allowed&quot; permissions

for that particular user as follows: 

<TABLE BORDER="0">

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT">read permission </TD>

		<TD ALIGN="LEFT">			<P>

		</TD>

		<TD ALIGN="LEFT">4 </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT">write permission </TD>

		<TD ALIGN="LEFT">			<P>

		</TD>

		<TD ALIGN="LEFT">2 </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT">





			<BLOCKQUOTE>

			<P>execute permission



			</BLOCKQUOTE>



			<P>

		</TD>

		<TD ALIGN="LEFT">			<P>

		</TD>

		<TD ALIGN="LEFT">





			<BLOCKQUOTE>

			<P>1



			</BLOCKQUOTE>



			<P>

		</TD>

	</TR>