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<H4 ALIGN="LEFT"><A NAME="Heading22"></A><FONT COLOR="#000077">Managing Groups with <I>gpasswd</I>
</FONT></H4>
<P>To add users to an existing group, log in as the root user and enter the following command:
</P>
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<PRE>
[root@insoc /root]# gpasswd -a <I>loginname groupname</I>
</PRE>
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<P>where <TT><I>loginname</I></TT> is the login name of the user you want to add to the group <TT><I>groupname</I></TT>.</P>
<P>For example, to add the user vector to the group ittf, you would use the command:</P>
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<PRE>
[root@insoc /root]# gpasswd -a vector ittf
</PRE>
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<P>To remove a user from an existing group, log in as the root user and enter the following command:
</P>
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<PRE>
[root@insoc /root]# gpasswd -d <I>loginname groupname</I>
</PRE>
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<P>where <TT><I>loginname</I></TT> is the login that you want to remove from the group <TT><I>groupname</I></TT>.</P>
<P>For example, if you wanted to remove the user yoko from the group beatles, you would use the command:</P>
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<PRE>
[root@insoc /root]# gpasswd -d yoko beatles
</PRE>
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<P>For a full list of command-line options available for the <TT>gpasswd</TT> command, see the related <TT>man</TT> page.</P>
<H4 ALIGN="LEFT"><A NAME="Heading23"></A><FONT COLOR="#000077">Using the <I>chgrp</I> Command
</FONT></H4>
<P>To change the group of a file, use the <TT>chgrp</TT> command. Log in as the root user and enter the following command:</P>
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<PRE>
[root@insoc /root]# chgrp <I>groupname filename</I>
</PRE>
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<P>where <TT><I>groupname</I></TT> is the name of the group you want to change the file’s group setting to, and <TT><I>filename</I></TT> is the name of the file for which you want the group changed.</P>
<P>For example, if you wanted to change the group for the file <TT>index.html</TT> to <TT>www</TT>, you would use <TT>chgrp</TT> as follows:</P>
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<PRE>
[root@insoc /root]# <B>chgrp www index.html</B>
</PRE>
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<P>To change the group of a directory and all its subdirectories and files, you can use the <TT>chgrp</TT> command with the <TT>-R option</TT>. For example, to change the group on all the files in the <TT>htdocs</TT> directory to <TT>www</TT>, you would use:</P>
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<PRE>
[root@insoc /root]# <B>chgrp -R www htdocs</B>
</PRE>
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<H3><A NAME="Heading24"></A><FONT COLOR="#000077">Changing File Ownership and Permissions</FONT></H3>
<P>Two programs in the command-line arsenal help change file ownership and permissions. <TT>chown</TT>, or Change Ownership, lets you change a file (or group of files) to another owner. <TT>chmod</TT> lets you change the access permissions to individual files.</P>
<H4 ALIGN="LEFT"><A NAME="Heading25"></A><FONT COLOR="#000077">Using the <I>chown</I> Command
</FONT></H4>
<P>To change the owner of a file, you use the <TT>chown</TT> command. Log in as the root user and enter the following command:</P>
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<PRE>
[root@insoc /root]# chown <I>ownername filename</I>
</PRE>
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<P>where <TT><I>ownername</I></TT> is the login name of the user you want to change the file’s owner setting to, and <TT><I>filename</I></TT> is the name of the file for which you want the owner changed.</P>
<P>For example, if you wanted to change the owner for the file <TT>index.html</TT> to sshah, you would use <TT>chown</TT> as follows:</P>
<!-- CODE SNIP //-->
<PRE>
[root@insoc /root]# chown sshah index.html
</PRE>
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<P>To change the owner of a directory and all its subdirectories and files, you can use the <TT>chown</TT> command with the <TT>-R</TT> option. For example, to change the owner on all the files in the <TT>htdocs</TT> directory to sshah, you would use:</P>
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<PRE>
[root@insoc /root]# chown -R sshah htdocs
</PRE>
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<H4 ALIGN="LEFT"><A NAME="Heading26"></A><FONT COLOR="#000077">Using the <I>chmod</I> Command
</FONT></H4>
<P>Before we can explain the usage of the <TT>chmod</TT> command, you need to first understand file permissions.</P>
<P>In Linux, every file and directory has three sets of access permissions: those applied to the owner of the file, those applied to the group the file has, and those of all users in the system. You can see these permissions when you do an <TT>ls -lg</TT>. For example:</P>
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<PRE>
drwxr-xr-x 2 sshah sysadmin 1024 Feb 14 15:49
wedding_plans
-rw------- 1 sshah sysadmin 2465 Feb 5 19:22
index.html
</PRE>
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<P>The first column of the listing is the permissions of the file. The first character represents the type of file (ëd’ means directory, ël’ means symbolic link, and so on), and the next nine characters are the permissions. The first three characters represent the permissions held by the file’s owner, the second three are for the group the file is in, and the last three represent the world permissions.
</P>
<P>The following letters are used to represent permissions:</P>
<TABLE WIDTH="50%"><TR>
<TH WIDTH="50%" ALIGN="LEFT">Letter
<TH WIDTH="50%" ALIGN="LEFT">Meaning
<TR>
<TD>r
<TD>Read
<TR>
<TD>w
<TD>Write
<TR>
<TD>x
<TD>Execute
</TABLE>
<P>Each permission has a corresponding value. The read attribute is equal to 4, the write attribute is equal to 2, and the execute attribute is equal to 1. When you combine attributes, you add their values. See the following examples.
</P>
<P>The most common groups of three and their meanings are:</P>
<TABLE WIDTH="90%"><TR>
<TH WIDTH="30%" ALIGN="LEFT">Permission
<TH WIDTH="30%" ALIGN="LEFT">Values
<TH WIDTH="40%" ALIGN="LEFT">Meaning
<TR>
<TD>---
<TD>0
<TD>No permissions
<TR>
<TD>r--
<TD>4
<TD>Read only
<TR>
<TD>rw-
<TD>6
<TD>Read and write
<TR>
<TD>rwx
<TD>7
<TD>Read, write, and execute
<TR>
<TD>r-x
<TD>5
<TD>Read and execute
<TR>
<TD>--x
<TD>1
<TD>Execute only
</TABLE>
<P>Although other combinations do exist (for example: -wx), they are nonsensical, and the likelihood you’ll ever run across them is almost nil.
</P><P><BR></P>
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