ch09_01.htm

By Tom Christiansen and Nathan Torkington ISBN 1-56592-243-3 First Edition, published August 1998

><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>File is a symbolic link</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>-p</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>mode</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>File is a named pipe (FIFO)</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>-S</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>mode</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>File is a socket</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>-b</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>rdev</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>File is a block special file</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>-c</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>rdev</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>File is a character special file</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>-t</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>rdev</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Filehandle is opened to a tty</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
></P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>-u</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>mode</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>File has setuid bit set</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>-g</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>mode</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>File has setgid bit set</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>-k</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>mode</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>File has sticky bit set</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
></P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>-T</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>N/A</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>File is a text file</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>-B</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>N/A</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>File is a binary file (opposite of <CODE
CLASS="literal"
>-T</CODE
>)</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
></P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>-M</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>mtime</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Age of file in days when script started</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>-A</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>atime</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Same for access time</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>-C</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>ctime</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Same for inode change time (not creation)</P
></TD
></TR
></TBODY
></TABLE
><P
CLASS="para"
>The <CODE
CLASS="literal"
>stat</CODE
> and the <CODE
CLASS="literal"
>-X</CODE
> operators cache the values that the <EM
CLASS="emphasis"
>stat </EM
>(2) system call returned. If you then call <CODE
CLASS="literal"
>stat</CODE
> or a <CODE
CLASS="literal"
>-X</CODE
> operator with the special filehandle <CODE
CLASS="literal"
>_</CODE
> (a single underscore), it won't call <CODE
CLASS="literal"
>stat</CODE
> again but will instead return information from its cache. This lets you test many properties of a single file without calling <EM
CLASS="emphasis"
>stat </EM
>(2) many times or introducing a race condition:</P
><PRE
CLASS="programlisting"
>open( F, &quot;&lt; $filename&quot; )
    or die &quot;Opening $filename: $!\n&quot;;
unless (-s F &amp;&amp; -T _) {
    die &quot;$filename doesn't have text in it.\n&quot;;
}</PRE
><P
CLASS="para"
>The <CODE
CLASS="literal"
>stat</CODE
> call just returns the information in one inode, though. How do we get a list of the contents of a directory? For that, Perl provides <CODE
CLASS="literal"
>opendir</CODE
><A
CLASS="indexterm"
NAME="ch09-idx-1000004346-0"
></A
><A
CLASS="indexterm"
NAME="ch09-idx-1000004346-1"
></A
><A
CLASS="indexterm"
NAME="ch09-idx-1000004346-2"
></A
>, <CODE
CLASS="literal"
>readdir</CODE
>, and <CODE
CLASS="literal"
>closedir</CODE
>:</P
><PRE
CLASS="programlisting"
>opendir(DIRHANDLE, &quot;/usr/bin&quot;) or die &quot;couldn't open /usr/bin : $!&quot;;
while ( defined ($filename = readdir(DIRHANDLE)) ) {
    print &quot;Inside /usr/bin is something called $filename\n&quot;;
}
closedir(DIRHANDLE);<A
CLASS="indexterm"
NAME="ch09-idx-1000004345-0"
></A
></PRE
><P
CLASS="para"
>These directory reading functions are designed to look like the file open and close functions. Where <CODE
CLASS="literal"
>open</CODE
> takes a filehandle, though, <CODE
CLASS="literal"
>opendir</CODE
> takes a directory handle. They look the same (a bare word) but they are different: you can <CODE
CLASS="literal"
>open(BIN,</CODE
> <CODE
CLASS="literal"
>&quot;/a/file&quot;)</CODE
> and <CODE
CLASS="literal"
>opendir(BIN,</CODE
> <CODE
CLASS="literal"
>&quot;/a/dir&quot;)</CODE
> and Perl won't get confused. You might, but Perl won't. Because filehandles and directory handles are different, you can't use the &lt; &gt; operator to read from a directory handle.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="ch09-idx-1000004347-0"
></A
><A
CLASS="indexterm"
NAME="ch09-idx-1000004347-1"
></A
><A
CLASS="indexterm"
NAME="ch09-idx-1000004347-2"
></A
>The filenames in a directory aren't necessarily stored alphabetically. If you want to get an alphabetical list of files, you'll have to read all the entries and sort them yourself.</P
><P
CLASS="para"
>The separation of directory information from inode information can create some odd situations. Operations that change directory only require write permission on the directory, not on the file. Most operations that change information in the file's data require write permission to the file. Operations that alter the permissions of the file require that the caller be the file's owner or the superuser. This can lead to the interesting situation of being able to delete a file you can't read, or write to a file you can't remove.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="ch09-idx-1000004348-0"
></A
><A
CLASS="indexterm"
NAME="ch09-idx-1000004348-1"
></A
>Although these situations make the filesystem structure seem odd at first, they're actually the source of much of Unix's power. Links, two filenames that refer to the same file, are now extremely simple. The two directory entries just list the same inode number. The inode structure includes a count of the number of directory entries referring to the file (<CODE
CLASS="literal"
>nlink</CODE
> in the values returned by <CODE
CLASS="literal"
>stat</CODE
>), but it lets the operating system store and maintain only one copy of the modification times, size, and other file attributes. When one directory entry is <CODE
CLASS="literal"
>unlink</CODE
>ed, data blocks are only deleted if the directory entry was the last one that referred to the file's inode &nbsp;-  and no processes still have the file open. You can <CODE
CLASS="literal"
>unlink</CODE
> an open file, but its disk space won't be released until the last close.</P
><P
CLASS="para"
>Links come in two forms. The kind described above, where two directory entries list the same inode number (like <EM
CLASS="emphasis"
>vi</EM
> and <EM
CLASS="emphasis"
>nvi</EM
> in the earlier table), are called <EM
CLASS="emphasis"
>hard links</EM
><A
CLASS="indexterm"
NAME="ch09-idx-1000004349-0"
></A
>. The operating system cannot tell the first directory entry of a file (the one created when the file was created) from any subsequent hard links to it. The other kind, <EM
CLASS="emphasis"
>soft</EM
> or <EM
CLASS="emphasis"
>symbolic links</EM
><A
CLASS="indexterm"
NAME="ch09-idx-1000004350-0"
></A
><A
CLASS="indexterm"
NAME="ch09-idx-1000004350-1"
></A
>, are very different. A soft link is a special type of file whose data block stores the filename the file is linked to. Soft links have a different <CODE
CLASS="literal"
>mode</CODE
> value, indicating they're not regular files. The operating system, when asked to <CODE
CLASS="literal"
>open</CODE
> a soft link, instead opens the filename contained in the data block.</P
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="ch09-chap09_executive_0"
>Executive Summary</A
></H3
><P
CLASS="para"
>Filenames are kept in a directory, separate from the size, protections, and other metadata kept in an inode.</P
><P
CLASS="para"
>The <CODE
CLASS="literal"
>stat</CODE
> function returns the inode information (metadata).</P
><P
CLASS="para"
><CODE
CLASS="literal"
>opendir</CODE
>, <CODE
CLASS="literal"
>readdir</CODE
>, and friends provide access to filenames in a directory through a <EM
CLASS="emphasis"
>directory handle</EM
>.</P
><P
CLASS="para"
>Directory handles look like filehandles, but they are not the same. In particular, you can't use &lt; &gt; on directory handles.</P
><P
CLASS="para"
>The permissions on a directory determine whether you can read and write the list of filenames. The permissions on a file determine whether you can change the file's metadata or contents.</P
><P
CLASS="para"
>Three different times are stored in an inode. None of them is the file's creation time.</P
></DIV
></DIV
></DIV
><DIV
CLASS="htmlnav"
><P
></P
><HR
ALIGN="LEFT"
WIDTH="684"
TITLE="footer"><TABLE
WIDTH="684"
BORDER="0"
CELLSPACING="0"
CELLPADDING="0"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="228"
><A
CLASS="sect1"
HREF="ch08_21.htm"
TITLE="8.20. Program: laston"
><IMG
SRC="../gifs/txtpreva.gif"
ALT="Previous: 8.20. Program: laston"
BORDER="0"></A
></TD
><TD
ALIGN="CENTER"
VALIGN="TOP"
WIDTH="228"
><A
CLASS="book"
HREF="index.htm"
TITLE="Perl Cookbook"
><IMG
SRC="../gifs/txthome.gif"
ALT="Perl Cookbook"
BORDER="0"></A
></TD
><TD
ALIGN="RIGHT"
VALIGN="TOP"
WIDTH="228"
><A
CLASS="sect1"
HREF="ch09_02.htm"
TITLE="9.1. Getting and Setting Timestamps"
><IMG
SRC="../gifs/txtnexta.gif"
ALT="Next: 9.1. Getting and Setting Timestamps"
BORDER="0"></A
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="228"
>8.20. Program: laston</TD
><TD
ALIGN="CENTER"
VALIGN="TOP"
WIDTH="228"
><A
CLASS="index"
HREF="index/index.htm"
TITLE="Book Index"
><IMG
SRC="../gifs/index.gif"
ALT="Book Index"
BORDER="0"></A
></TD
><TD
ALIGN="RIGHT"
VALIGN="TOP"
WIDTH="228"
>9.1. Getting and Setting Timestamps</TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="684"
TITLE="footer"><FONT
SIZE="-1"
></DIV<!-- LIBRARY NAV BAR --> <img src="../gifs/smnavbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links"><p> <a href="copyrght.htm">Copyright &copy; 2002</a> O'Reilly &amp; Associates. All rights reserved.</font> </p> <map name="library-map"> <area shape="rect" coords="1,0,85,94" href="../index.htm"><area shape="rect" coords="86,1,178,103" href="../lwp/index.htm"><area shape="rect" coords="180,0,265,103" href="../lperl/index.htm"><area shape="rect" coords="267,0,353,105" href="../perlnut/index.htm"><area shape="rect" coords="354,1,446,115" href="../prog/index.htm"><area shape="rect" coords="448,0,526,132" href="../tk/index.htm"><area shape="rect" coords="528,1,615,119" href="../cookbook/index.htm"><area shape="rect" coords="617,0,690,135" href="../pxml/index.htm"></map> </BODY
></HTML
>