86 lines
4.7 KiB
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86 lines
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<TITLE>3.9.1 Concepts of file permissions</TITLE>
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<B> Next:</B> <A NAME=tex2html3341 HREF="node126.html">3.9.2 Interpreting file permissions</A>
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<B>Up:</B> <A NAME=tex2html3339 HREF="node124.html">3.9 File Permissions</A>
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<B> Previous:</B> <A NAME=tex2html3333 HREF="node124.html">3.9 File Permissions</A>
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<BR> <HR> <P>
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<H2><A NAME=SECTION00591000000000000000>3.9.1 Concepts of file permissions</A></H2>
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<P>
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<A NAME=3174> </A>
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<A NAME=3175> </A>
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<A NAME=3176> </A>
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Because there are multiple users on a UNIX system, in order to protect
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individual user's files from tampering by other users, UNIX provides
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a mechanism known as <b>file permissions</b>. This mechanism allows
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files and directories to be ``owned'' by a particular user. As an example,
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because Larry created the files in his home directory, Larry owns those files,
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and has access to them.
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<P>
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UNIX also allows files to be shared between users and groups of users.
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If Larry so desired, he could cut off access to his files, such that no
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other user could access them. However, on most systems the default is to
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allow other users to read your files, but not modify or delete them in
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any way.
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<P>
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<A NAME=3178> </A>
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As explained above, every file is owned by a particular user.
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However, files are also owned by a particular <b>group</b>, which is a
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system-defined group of users. Every user is placed into at least
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one group when that user is created. However, the system administrator
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may also grant the user access to more than one group.
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<P>
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<A NAME=3180> </A>
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<A NAME=3181> </A>
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Groups are usually defined by the type of users which access the machine.
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For example, on a university UNIX system, users may be placed into the
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groups <tt>student</tt>, <tt>staff</tt>, <tt>faculty</tt> or <tt>guest</tt>. There
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are also a few system-defined groups (such as <tt>bin</tt> and <tt>admin</tt>)
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which are used by the system itself to control access to resources---very
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rarely do actual users belong to these system groups.
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<P>
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Permissions fall into three main divisions: read, write, and execute.
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These permissions may be granted to three classes of users: the owner
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of the file, the group to which the file belongs, and to all users,
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regardless of group.
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<P>
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<A NAME=3188> </A>
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<A NAME=3189> </A>
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<A NAME=3190> </A>
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<A NAME=3191> </A>
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<A NAME=3192> </A>
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<A NAME=3193> </A>
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<A NAME=3194> </A>
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<A NAME=3195> </A>
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<A NAME=3196> </A>
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Read permission allows a user to read the contents
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of the file, or in the case of directories, to list the contents of
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the directory (using <tt>ls</tt>). Write permission allows the user to
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write to and modify the file. For directories, write permission allows
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the user to create new files or delete files within that directory. Finally,
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execute permission allows the user to run the file as a program or
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shell script (if the file happens to be a program or shell script, that is).
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For directories, having execute permission allows the user to <tt>cd</tt>
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into the directory in question.
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<P>
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<BR> <HR><A NAME=tex2html3340 HREF="node126.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A> <A NAME=tex2html3338 HREF="node124.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A> <A NAME=tex2html3332 HREF="node124.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <A NAME=tex2html3342 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <A NAME=tex2html3343 HREF="node250.html"><IMG ALIGN=BOTTOM ALT="index" SRC="index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3341 HREF="node126.html">3.9.2 Interpreting file permissions</A>
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<B>Up:</B> <A NAME=tex2html3339 HREF="node124.html">3.9 File Permissions</A>
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<B> Previous:</B> <A NAME=tex2html3333 HREF="node124.html">3.9 File Permissions</A>
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<BR> <HR> <P>
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<BR> <HR>
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<P><ADDRESS>
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<I>Matt Welsh <BR>
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mdw@sunsite.unc.edu</I>
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