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><A
NAME="PERMISSIONS"
>Permissions</A
></H1
><P
>      Every file on your system has an <I
CLASS="EMPHASIS"
>owner</I
> &#8212; one
      of the users (usually the one who created this file), and a system of
      <I
CLASS="EMPHASIS"
>permissions</I
>, which regulate access to this
      file.
    </P
><P
>      For ordinary files, there are 3 types of access permissions:
      read, write, and execute (the latter only makes sense for
      executable files). They can be set independently for 3
      categories of users: the file owner, the users in the group
      owning the file, and everyone else. Discussion of groups of
      users goes beyond the scope of this document; the other two
      categories are self-explanatory.  So, if the permissions on a file
      <TT
CLASS="FILENAME"
>/home/sasha/talk.txt</TT
> are set to read and
      write for user sasha, who is the file owner, and read only for
      everyone else, only sasha will be able to modify this file.
      
    </P
><P
>      All newly created files carry some standard permissions, 
      usually read/write for user and read only for
      everyone else. You can view the permissions using the GNOME File
      Manager, by right-clicking on the file, choosing
      <B
CLASS="GUIMENUITEM"
>Properties</B
> in the pop-up menu, and then the
      <TT
CLASS="GUILABEL"
><B
>Permissions</B
></TT
> tab. Using this dialog, you can also
      change the permissions &#8212; just click on a square
      representing the permission to toggle it.  Of course, only the
      file owner or the system administrator can change the
      permissions of a file. Advanced users can also  change the default
      file permissions which are assigned to newly created
      files&#8212;see the manual pages for your default shell (usually
      <B
CLASS="COMMAND"
>bash</B
>,
      <B
CLASS="COMMAND"
>csh</B
> or <B
CLASS="COMMAND"
>tcsh</B
>) and look for the command
      <B
CLASS="COMMAND"
>umask</B
>.  
    </P
><P
>      A file can also have some special permission properties such
      as UID, GID, and "sticky" bit. They are for experienced users
      only &#8212; do not change them unless you know what you are
      doing. (If you are curious: these permissions are typically
      used on executable files to allow the user to execute
      <I
CLASS="EMPHASIS"
>some</I
>  commands
      which read or modify  files to which the user himself doesn't
      have access.) 
    </P
><P
> Similar to files, the directories also have
      permissions. Again, there are  3 possible
      permissions: read, write, and execute. However, they have
      different meaning: namely, "read" permission for a directory
      means permission to list the contents of the directory or
      search for a file; "write" means permission to create and
      remove files in the directory, and "execute" means permission
      to access files in the directory. 
    </P
><P
>      Note that the permissions granted to a file depend on the
      permissions of the directory in which the file is located: in
      order to be able to read a file, a user needs to have the read
      permission  for the file itself and "execute" permission
      for the directory that contains it. So, if user sasha doesn't
      want anybody else to see his files, he can achieve this by
      removing the execute permission on his home directory for all
      other users. This way, no one but himself (and, of course,
      root) will be able to read any of his files, no matter what
      the permissions of individual files are.
    </P
><P
> Detailed explanation of the permission system can be
      found, for example, in the info page for the GNU "File
      Utilities" package. You can view this info page using GNOME Help
      browser. 
      </P
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