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<TITLE>Developer.com - Online Reference Library - 0672311623:SAMS TEACH YOURSELF LINUX IN 24 HOURS:Handling Files</TITLE>

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<!-- ISBN=0672311623 //-->

<!-- TITLE=SAMS TEACH YOURSELF LINUX IN 24 HOURS //-->

<!-- AUTHOR=BILL BALL, STEPHEN SMOOGEN //-->

<!-- PUBLISHER=MACMILLAN //-->

<!-- IMPRINT=SAMS //-->

<!-- PUBLICATION DATE=1998 //-->

<!-- CHAPTER=21 //-->

<!-- PAGES=0313-0328 //-->

<!-- UNASSIGNED1 //-->

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<P><CENTER>

<a href="0323-0325.html">Previous</A> | <a href="../ewtoc.html">Table of Contents</A> | <a href="../ch22/0329-0332.html">Next</A>

</CENTER></P>



<A NAME="PAGENUM-326"><P>Page 326</P></A>







<P>to have. The chmod command also has a command-line form as follows:

</P>



<!-- CODE SNIP //-->

<PRE>

ugoa +-= rwxXstugo

</PRE>

<!-- END CODE SNIP //-->



<P>This book doesn't go into all the details of this notation (you can read the

chmod command's manual page for more details), but the next few examples duplicate

chmod's actions using the previous examples. You can protect a file from anyone else with

</P>

<!-- CODE //-->

<PRE>

# ls -l file1

-rw-rw-r--   1 bball    bball           0 Nov 23 13:50 file1

# chmod go-rwx file1

# ls -l file1

-rw------   1 bball    bball           0 Nov 23 13:50 file1

</PRE>

<!-- END CODE //-->



<P>As you can see, the file is now readable and writable only by you, because you have

specified that your group (g) and others (o) do not

(-) have read, write, or execute (rwx) permission. Now, you can protect your directory from prying eyes as follows:

</P>

<!-- CODE SNIP //-->

<PRE>

# chmod go-rwx temp

</PRE>

<!-- END CODE SNIP //-->



<P>And, to mimic the last example, to enable others to read files in the directory, but not

list the directory contents, you can use

</P>

<!-- CODE SNIP //-->

<PRE>

# chmod o+x temp

</PRE>

<!-- END CODE SNIP //-->



<P>You're now familiar with file and directory permissions, and using the

chmod command. The next section shows you how you can change ownership of files or directories using the

chown command.

</P>



<H3><A NAME="ch21_ 13">

Changing File Ownership with the chown Command

</A></H3>



<P>The chown (change ownership) command, found under the

/bin directory, is used to change, either permanently or temporarily, the ownership of files or directories. If you recall

the previous discussion of the /etc/group file in this hour, you'll remember that your users

can be assigned to different groups. Using the

chown command, you can assign ownership to different users or groups.

</P>



<P>For example, if you've created a text file, you can share it with members of your group

or others with the chmod command. By using chown, you can tell Linux specifically what

other users or groups can have access to your file. You can use the

groups command to find out what groups you belong to, for example:

</P>

<!-- CODE SNIP //-->

<PRE>

# groups

bball users

</PRE>

<!-- END CODE SNIP //-->



<P>This shows that the user, bball, belongs to two groups: bball and users. As

the root operator, you belong to at least seven groups, for example:

</P>



<A NAME="PAGENUM-327"><P>Page 327</P></A>





<!-- CODE SNIP //-->

<PRE>

# groups

root bin daemon sys adm disk wheel

</PRE>

<!-- END CODE SNIP //-->



<P>To find out who belongs to a group, look at the

/etc/group file, or use the name of a user, for example:

</P>

<!-- CODE SNIP //-->

<PRE>

# groups cloobie

cloobie : cloobie users

</PRE>

<!-- END CODE SNIP //-->



<P>This shows that you and cloobie belong to at least one group, called users. To assign

one of your files to the users group, and give cloobie access, you can use the

chown command's syntax of user:group, for example:

</P>



<!-- CODE SNIP //-->

<PRE>

# chown :users myfile

# ls -l myfile

-rw-rw-r--   1 bball    users           0 Nov 23 14:16 myfile

</PRE>

<!-- END CODE SNIP //-->



<P>You might think that to assign specific ownership, you can use the following:

</P>



<!-- CODE SNIP //-->

<PRE>

#  chown cloobie:users myfile

chown: myfile: Operation not permitted

</PRE>

<!-- END CODE SNIP //-->



<P>What happened? This shows why Linux has groups. You can assign access of one of

your files to a group, but unless you're the root operator, you cannot assign one of your files

to appear to have been either created by or owned by another user. Make sure you're

logged in as the root operator and use

</P>



<!-- CODE SNIP //-->

<PRE>

# chown cloobie:cloobie myfile

# ls -l myfile

-rw-rw-r--   1 cloobie  cloobie         0 Nov 23 14:16 myfile

</PRE>

<!-- END CODE SNIP //-->



<P>As you can see, even though the file myfile was created by the user bball, as the

sysadmin, you can assign ownership to any users and any group. If you just want to change the

group ownership of a file or directory, you can use the

chgrp command; if you want to change your users or your own group, you can use the

newgrp command.

</P>



<H3><A NAME="ch21_ 14">

Changing Groups and Ownerships with the chgrp and

newgrp Commands

</A></H3>



<P>The chgrp (change group) command, found under the

/bin directory, is used only to change group ownerships. In this regard, it is not as flexible as the

chown command, which can do both. The chgrp command accepts a group name or group id (GID), for example:

</P>



<!-- CODE SNIP //-->

<PRE>

# ls -l myfile

-rw-rw-r--   1 bball    bball           0 Nov 23 14:16 myfile

</PRE>

<!-- END CODE SNIP //-->



<P>This shows that the file belongs to user bball and group bball. To change the

group ownership and grant access to other members of the group, use

</P>



<A NAME="PAGENUM-328"><P>Page 328</P></A>





<!-- CODE SNIP //-->

<PRE>

# groups bball

bball : bball users

# chgrp users myfile

# ls -l myfile

-rw-rw-r--   1 bball    users           0 Nov 23 14:16 myfile

</PRE>

<!-- END CODE SNIP //-->



<P>Now, other members of the users group can access the file. Along with the

chgrp command, you'll find the newgrp command, which is found under the

/usr/bin directory. Although the chgrp command will change group ownership of one of your files or directories to a group

you belong to, (or if you're the root operator, any group), you can use the

newgrp to shift your current group membership, for example:

</P>



<!-- CODE //-->

<PRE>

# groups

bball users

# touch file1

# ls -l file1

-rw-rw-r--   1 bball    bball           0 Nov 23 14:53 file1

# newgrp users

# groups

users bball

# touch file2

# ls -l file2

-rw-rw-r--   1 bball    users           0 Nov 23 14:54 file2

# newgrp bball

</PRE>

<!-- END CODE //-->



<P>This shows that the user bball originally belonged to the default group bball. This

was verified by creating a file showing the current user and group ownership. Next, the

user bball changed to the users group, created a file, and verified that the created file has

the new group's access. Finally, the user bball changed back to the original group, bball.

</P>



<P>As you can see, Linux offers you a great deal of flexibility in assigning file ownerships

and permissions. By using different combinations of directory and file ownership

and permissions, you can organize your system along lines of types of work, types of users,

or types of files.

</P>



<P><CENTER>

<a href="0323-0325.html">Previous</A> | <a href="../ewtoc.html">Table of Contents</A> | <a href="../ch22/0329-0332.html">Next</A>

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