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(fileutils.info)Mode Structure

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Structure of File Permissions
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   There are three kinds of permissions that a user can have for a file:

  1. permission to read the file.  For directories, this means
     permission to list the contents of the directory.

  2. permission to write to (change) the file.  For directories, this
     means permission to create and remove files in the directory.

  3. permission to execute the file (run it as a program).  For
     directories, this means permission to access files in the
     directory.

   There are three categories of users who may have different
permissions to perform any of the above operations on a file:

  1. the file's owner;

  2. other users who are in the file's group;

  3. everyone else.

   Files are given an owner and group when they are created.  Usually
the owner is the current user and the group is the group of the
directory the file is in, but this varies with the operating system, the
filesystem the file is created on, and the way the file is created.  You
can change the owner and group of a file by using the `chown' and
`chgrp' commands.

   In addition to the three sets of three permissions listed above, a
file's permissions have three special components, which affect only
executable files (programs) and, on some systems, directories:

  1. set the process's effective user ID to that of the file upon
     execution (called the "setuid bit").  No effect on directories.

  2. set the process's effective group ID to that of the file upon
     execution (called the "setgid bit").  For directories on some
     systems, put files created in the directory into the same group as
     the directory, no matter what group the user who creates them is
     in.

  3. save the program's text image on the swap device so it will load
     more quickly when run (called the "sticky bit").  For directories
     on some systems, prevent users from removing or renaming a file in
     a directory unless they own the file or the directory; this is
     called the "restriction deletion flag" for the directory.