407 lines
7.3 KiB
HTML
407 lines
7.3 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>How it works</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Autodir HOWTO"
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HREF="index.html"><LINK
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REL="PREVIOUS"
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TITLE="Differences between Autodir and Autofs"
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HREF="x130.html"><LINK
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REL="NEXT"
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TITLE="Some definitions"
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HREF="x235.html"></HEAD
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><BODY
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CLASS="sect1"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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VLINK="#840084"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Autodir HOWTO</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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>Prev</A
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VALIGN="bottom"
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ALIGN="right"
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VALIGN="bottom"
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HREF="x235.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="sect1"
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><H1
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CLASS="sect1"
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><A
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NAME="AEN170"
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></A
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>7. How it works</H1
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><P
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><EM
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>Autodir</EM
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> uses modules to get specific functionality. The core <EM
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>Autodir</EM
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> implements generic functionality that modules can exploit and add specific functionality to.</P
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><P
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>At any moment only one module can be added to <EM
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>Autodir</EM
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>. If there are two modules, for example <TT
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CLASS="literal"
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>autohome</TT
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> and <TT
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CLASS="literal"
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>autogroup</TT
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>, then two processes of <EM
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>Autodir</EM
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> should be created so that each process can have one of the required modules attached to it.</P
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><P
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>For further explanation I chose the <TT
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CLASS="literal"
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>autohome</TT
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> module which handles transparent home directory creation.</P
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><DIV
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CLASS="note"
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><P
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></P
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><TABLE
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CLASS="note"
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WIDTH="100%"
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BORDER="0"
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><TR
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/note.gif"
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HSPACE="5"
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ALT="Note"></TD
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><TH
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ALIGN="LEFT"
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VALIGN="CENTER"
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><B
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>Assumptions</B
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></TH
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></TR
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><TR
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><TD
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> </TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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></P
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><UL
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><LI
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><P
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>The <TT
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CLASS="literal"
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>autohome</TT
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> module creates user home directories on demand if these do not exist already.</P
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></LI
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><LI
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><P
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>It is assumed that user accounts exists, but the accompanying home directories do not - either because these accounts were created with the <TT
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CLASS="literal"
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>-M</TT
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> option with <B
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CLASS="command"
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>useradd</B
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> or because these accounts were imported from LDAP, NIS or some other external database for which home directories are yet to be created.</P
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></LI
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><LI
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><P
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>It also assumed <EM
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>for this explanation only</EM
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> that all user home directories are expected to be in the <TT
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CLASS="filename"
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>/home</TT
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> directory.</P
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></LI
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></UL
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></TD
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></TR
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></TABLE
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></DIV
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><DIV
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CLASS="note"
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><P
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></P
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><TABLE
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CLASS="note"
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WIDTH="100%"
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BORDER="0"
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><TR
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/note.gif"
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HSPACE="5"
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ALT="Note"></TD
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><TH
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ALIGN="LEFT"
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VALIGN="CENTER"
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><B
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>KISS</B
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></TH
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></TR
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><TR
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><TD
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> </TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>Keep it Simple: Some fine details are intentionally kept aside to make the explanation easy to understand.</P
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></TD
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></TR
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></TABLE
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></DIV
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><P
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>First the autofs file system is mounted on the <TT
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CLASS="filename"
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>/home</TT
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> directory by <EM
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>Autodir</EM
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>. The Linux kernel is informed that <TT
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CLASS="filename"
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>/home</TT
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> is managed by a user space application, <EM
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>Autodir</EM
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>, from now on.</P
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><DIV
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CLASS="note"
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><P
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></P
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><TABLE
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CLASS="note"
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WIDTH="100%"
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BORDER="0"
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><TR
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/note.gif"
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HSPACE="5"
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ALT="Note"></TD
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><TH
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ALIGN="LEFT"
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VALIGN="CENTER"
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><B
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>Autofs?</B
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></TH
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></TR
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><TR
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><TD
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> </TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>Do not bother too much about the autofs file system if you do not understand about it. Just think of it as a special kind of file system, similar to memory based file systems but with some additional special properties.</P
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></TD
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></TR
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></TABLE
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></DIV
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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>
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+----------------+
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| Linux Kernel |
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+----------------+
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/ \
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/ \
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/ \
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/ \
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+-------------+ +--------+ +------------+ +-----------------+
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| Application |------>| /home |<----->| Autodir |<------>| autohome module |
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+-------------+ +--------+ +------------+ +-----------------+
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\ /
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\ +----------------+ /
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+-| /autohome |<------------------+
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+----------------+
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>Whenever an application or daemon needs access to a user's home directory, for example <TT
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CLASS="filename"
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>/home/userhome1</TT
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>, they directly enter into <TT
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CLASS="filename"
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>/home/userhome1</TT
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> to access it. The kernel, which notices this, informs <EM
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>Autodir</EM
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> if the <TT
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CLASS="filename"
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>userhome1</TT
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> directory does not yet exist already in <TT
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CLASS="filename"
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>/home</TT
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>.</P
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><P
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><EM
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>Autodir</EM
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>, in turn, passes this request to the <TT
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CLASS="literal"
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>autohome</TT
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> module. The <TT
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CLASS="literal"
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>autohome</TT
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> module does not touch the <TT
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CLASS="filename"
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>/home</TT
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> directory. Instead it manages <EM
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>real home directories</EM
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> somewhere else, for example in <TT
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CLASS="filename"
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>/autohome</TT
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> as shown in the above figure.</P
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><P
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>The <TT
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CLASS="literal"
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>autohome</TT
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> module creates a real home directory if it does not exist in the <TT
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CLASS="filename"
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>/autohome</TT
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> directory. After it is successfully created or failed to be created, whatever the outcome, <TT
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CLASS="literal"
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>autohome</TT
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> reports back to <EM
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>Autodir</EM
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>. When the directory creation task has completed successfully, the path to real home directory is provided to <EM
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>Autodir</EM
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>.</P
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><P
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>If the <TT
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CLASS="literal"
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>autohome</TT
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> module reports success, <EM
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>Autodir</EM
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> creates <TT
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CLASS="filename"
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>userhome1</TT
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> directory under <TT
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CLASS="filename"
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>/home</TT
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> and mounts the <EM
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>real home directory</EM
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> from <TT
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CLASS="filename"
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>/autohome</TT
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> on it. At the end of the process, <EM
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>Autodir</EM
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> informs the kernel whether the whole operation was successful or not. Accordingly, the kernel allows applications to enter the directory, or, in case of failure, it reports that no such directory exists.</P
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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CELLPADDING="0"
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CELLSPACING="0"
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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HREF="x130.html"
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>Prev</A
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WIDTH="34%"
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VALIGN="top"
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HREF="index.html"
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>Home</A
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>Next</A
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>Differences between Autodir and Autofs</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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> </TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>Some definitions</TD
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></TR
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