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<UL>
<LI><A NAME="tex2html1996"
HREF="#SECTION001710000000000000000">14.1 The <TT>
<FONT COLOR="#0000ff">chmod</FONT></TT> Command</A>
<LI><A NAME="tex2html1997"
HREF="#SECTION001720000000000000000">14.2 The <TT>
<FONT COLOR="#0000ff">umask</FONT></TT> Command</A>
<LI><A NAME="tex2html1998"
HREF="#SECTION001730000000000000000">14.3 Modification Times: <TT>
<FONT COLOR="#0000ff">stat</FONT></TT></A>
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<HR>
<H1><A NAME="SECTION001700000000000000000">
14. Permission and Modification Times</A>
</H1>
<P>
<A NAME="chap:permmodtimes"></A>
<P>
Every file and directory on a U<SMALL>NIX</SMALL> system, besides being owned
by a user and a group, has access <I>flags</I> <FONT COLOR="#ffa500">[A switch
that can either be on or off.]</FONT> (also called <I>access bits</I>) dictating what kind of access that
user and group have to the file.
<P>
Running <TT>
<FONT COLOR="#0000ff">ls -ald /bin/cp /etc/passwd /tmp</FONT></TT> gives you a
listing like this:
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code> </code><br>
<code> </code><br>
<code> </code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>-rwxr-xr-x 1 root root 28628 Mar 24 1999 /bin/cp</code><br>
<code>-rw-r--r-- 1 root root 1151 Jul 23 22:42 /etc/passwd</code><br>
<code>drwxrwxrwt 5 root root 4096 Sep 25 15:23 /tmp</code><br>
</FONT></TD></TR></TABLE><P>
<P>
In the leftmost column are flags which completely
describe the access rights to the file.
<P>
So far I have explained that the furthest flag to the left is either
<TT>
<FONT COLOR="#0000ff">-</FONT></TT> or <TT>
<FONT COLOR="#0000ff">d</FONT></TT>, indicating an ordinary file or directory. The
remaining nine have a <TT>
<FONT COLOR="#0000ff">-</FONT></TT> to indicate an unset value or
one of several possible characters. Table
<A HREF="node17.html#table:permissions">14.1</A> gives a complete description of file system permissions.
<P>
<BR><P></P>
<DIV ALIGN="CENTER"><A NAME="table:permissions"></A><A NAME="18523"></A>
<TABLE>
<CAPTION><STRONG>Table 14.1:</STRONG>
File and directory permissions</CAPTION>
<TR><TD><IMG
WIDTH="556" HEIGHT="707" BORDER="0"
SRC="img17.png"
ALT="\begin{table}{\small\begin{tabularx}{1.0\textwidth}{\vert c \vert p{9ex} \vert X...
...ode{\color{blue}{T}}} has no effect.
\\
\par\hline
\end{tabularx}}
\end{table}"></TD></TR>
</TABLE>
</DIV><P></P>
<BR>
<P>
<H1><A NAME="SECTION001710000000000000000">
14.1 The <TT>
<FONT COLOR="#0000ff">chmod</FONT></TT> Command</A>
</H1>
<P>
You use the <TT>
<FONT COLOR="#0000ff">chmod</FONT></TT> command to change the permissions of
a file. It's usually used as follows:
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code> </code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>chmod [-R] [u|g|o|a][+|-][r|w|x|s|t] <file> [<file>] ...</code><br>
</FONT></TD></TR></TABLE><P>
<P>
For example,
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code> </code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>chmod u+x myfile</code><br>
</FONT></TD></TR></TABLE><P>
adds execute permissions for the user of <TT>
<FONT COLOR="#0000ff">myfile</FONT></TT>. And,
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code> </code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>chmod a-rx myfile</code><br>
</FONT></TD></TR></TABLE><P>
removes <TT>
<FONT COLOR="#0000ff">r</FONT></TT><I>ead</I> and <I>e</I><TT>
<FONT COLOR="#0000ff">x</FONT></TT><I>ecute</I> permissions
for <TT>
<FONT COLOR="#0000ff">a</FONT></TT><I>ll</I>--that is, user, group, and other.
<P>
The <TT>
<FONT COLOR="#0000ff">-R</FONT></TT> option, once again means <I>recursive</I>,
diving into subdirectories as usual.
<P>
Permission bits are often represented in their binary form,
especially in programs. It is convenient to show the
<TT>
<FONT COLOR="#0000ff">rwxrwxrwx</FONT></TT> set in octal, <FONT COLOR="#ffa500">[See Section <A HREF="node5.html#sec:binocthex">2.1</A>.]</FONT>where each digit fits
conveniently into three bits. Files on the system are usually
created with <I>mode</I> <TT>
<FONT COLOR="#0000ff">0644</FONT></TT>, meaning
<TT>
<FONT COLOR="#0000ff">rw-r--r--</FONT></TT>. You can set permissions explicitly with an octal
number, for example,
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code> </code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>chmod 0755 myfile</code><br>
</FONT></TD></TR></TABLE><P>
gives <TT>
<FONT COLOR="#0000ff">myfile</FONT></TT> the permissions <TT>
<FONT COLOR="#0000ff">rwxr-xr-x</FONT></TT>. For a full list
of octal values for all kinds of permissions and file types, see
<TT>
<FONT COLOR="#0000ff">/usr/include/linux/stat.h</FONT></TT>.
<P>
In Table <A HREF="node17.html#table:permissions">14.1</A> you can see <TT>
<FONT COLOR="#0000ff">s</FONT></TT>, the <I>setuid</I> or
<I>setgid</I> bit. If it is used without execute permissions
then it has no meaning and is written as a capitalized <TT>
<FONT COLOR="#0000ff">S</FONT></TT>.
This bit effectively colorizes an <TT>
<FONT COLOR="#0000ff">x</FONT></TT> into an <TT>
<FONT COLOR="#0000ff">s</FONT></TT>,
so you should read an <TT>
<FONT COLOR="#0000ff">s</FONT></TT> as e<B><TT>
<FONT COLOR="#0000ff">x</FONT></TT></B>ecute
<I>with</I> the setuid or setgid bit set.
<TT>
<FONT COLOR="#0000ff">t</FONT></TT> is known as the <I>sticky</I> bit. It also has no
meaning if there are no execute permissions and is written as a capital
<TT>
<FONT COLOR="#0000ff">T</FONT></TT>.
<P>
The leading <TT>
<FONT COLOR="#0000ff">0</FONT></TT> can in be ignored, but is preferred for
explicitness. It <I>can</I> take on a value representing
the three bits, <I>setuid</I> (<TT>
<FONT COLOR="#0000ff">4</FONT></TT>), <I>setgid</I>
(<TT>
<FONT COLOR="#0000ff">2</FONT></TT>), and <I>sticky</I> (<TT>
<FONT COLOR="#0000ff">1</FONT></TT>). Hence a value of
<TT>
<FONT COLOR="#0000ff">5764</FONT></TT> is
<FONT COLOR="#0000ff"><!-- MATH
$101~111~110~100$
-->
<IMG
WIDTH="112" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img18.png"
ALT="\bgroup\color{blue}$101~111~110~100$\egroup"></FONT> in binary and gives
<TT>
<FONT COLOR="#0000ff">-rwsrw-r-T</FONT></TT>.
<P>
<H1><A NAME="SECTION001720000000000000000">
14.2 The <TT>
<FONT COLOR="#0000ff">umask</FONT></TT> Command</A>
</H1>
<P>
<A NAME="sec:umask"></A>
<P>
<TT>
<FONT COLOR="#0000ff">umask</FONT></TT> sets the default permissions for newly created files; it
is usually <TT>
<FONT COLOR="#0000ff">022</FONT></TT>. This default value means that the permissions of any new
file you create (say, with the <TT>
<FONT COLOR="#0000ff">touch</FONT></TT> command) will be
<I>masked</I> with this number. <TT>
<FONT COLOR="#0000ff">022</FONT></TT> hence <I>excludes</I> write
permissions of <TT>
<FONT COLOR="#0000ff">g</FONT></TT>roup and of <TT>
<FONT COLOR="#0000ff">o</FONT></TT>ther. A <TT>
<FONT COLOR="#0000ff">umask</FONT></TT> of <TT>
<FONT COLOR="#0000ff">006</FONT></TT>
would exclude read and write permissions of <TT>
<FONT COLOR="#0000ff">o</FONT></TT>ther, but would allow read
and write of <TT>
<FONT COLOR="#0000ff">g</FONT></TT>roup. Try
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code> </code><br>
<code> </code><br>
<code> </code><br>
<code> </code><br>
<font size="-1"><code>5</code></font><code> </code><br>
<code> </code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>umask</code><br>
<code>touch <file1></code><br>
<code>ls -al <file1></code><br>
<code>umask 026</code><br>
<code>touch <file2></code><br>
<code>ls -al <file2></code><br>
</FONT></TD></TR></TABLE><P>
<TT>
<FONT COLOR="#0000ff">026</FONT></TT> is probably closer to the kind of mask we like
as an ordinary user. Check your <TT>
<FONT COLOR="#0000ff">/etc/profile</FONT></TT> file to see
what <TT>
<FONT COLOR="#0000ff">umask</FONT></TT> your login defaults to, when, and also why.
<P>
<H1><A NAME="SECTION001730000000000000000">
14.3 Modification Times: <TT>
<FONT COLOR="#0000ff">stat</FONT></TT></A>
</H1>
<P>
In addition to permissions, each file has three integers associated with
it that represent, in seconds, the last time the file was accessed
(read), when it was last modified (written to), and when its permissions
were last changed. These are known as the <I>atime</I>, <I>mtime</I>, and
<I>ctime</I> of a file respectively.
<P>
To get a complete listing of the file's permissions, use the <TT>
<FONT COLOR="#0000ff">stat</FONT></TT>
command. Here is the result of <TT>
<FONT COLOR="#0000ff">stat /etc</FONT></TT>:
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code> </code><br>
<code> </code><br>
<code> </code><br>
<code> </code><br>
<font size="-1"><code>5</code></font><code> </code><br>
<code> </code><br>
<code> </code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code> File: "/etc"</code><br>
<code> Size: 4096 Filetype: Directory</code><br>
<code> Mode: (0755/drwxr-xr-x) Uid: ( 0/ root) Gid: ( 0/ root)</code><br>
<code>Device: 3,1 Inode: 14057 Links: 41 </code><br>
<code>Access: Sat Sep 25 04:09:08 1999(00000.15:02:23)</code><br>
<code>Modify: Fri Sep 24 20:55:14 1999(00000.22:16:17)</code><br>
<code>Change: Fri Sep 24 20:55:14 1999(00000.22:16:17)</code><br>
</FONT></TD></TR></TABLE><P>
The <TT>
<FONT COLOR="#0000ff">Size:</FONT></TT> quoted here is the actual amount of disk space
used to store the directory <I>listing</I>, and
is the same as reported by <TT>
<FONT COLOR="#0000ff">ls</FONT></TT>. In this case it is
probably four disk blocks of 1024 bytes each. The size of a
directory as quoted here does <I>not</I> mean the sum of all
files contained under it. For a file, however, the <TT>
<FONT COLOR="#0000ff">Size:</FONT></TT> would
be the exact file length in bytes (again, as reported by <TT>
<FONT COLOR="#0000ff">ls</FONT></TT>).
<P>
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