XPost: comp.unix.internals, comp.unix.questions, comp.unix.shell   
   From: spamhater113+U050305054519@grymoire.com   
      
   "Eli Aran"  writes:   
      
   > hi there   
   > i am learning unix on my own   
   > i need some elaboration on the concept   
   > of file and directory permissions.   
   > i dont understand the concept of "others" (the third option regarding   
   > permissions)   
      
      
   Each file has a user ID and group ID associated with it.   
   The system:   
           checks the user ID of the person, and the file.   
           If the number is the same, then permission is granted.   
      
   OTHERWISE   
           The list of groups of the user is obtained, and if one of them matches   
           the group of the file, permission is granted.   
      
   OTHERWISE   
           The "other" permission is used.   
      
      
   So if you have  the following permissions (userID = 100, group ID == 200)   
           User   Group    Other   
           RWX    ---      R   
      
      
   User ID 	Group ID			Permission   
   100         (doesn't matter)    RWX   
   150         (200)               None   
   151         151                 R   
      
   Note that the "other" permission is used if UID != 100 AND GID != 200   
   If the GID matches, then "other" is never checked.   
      
      
      
   RWX in directories is a little different in what R, W and X means.   
      
      
   R = Read the contents of the directory   
   W = Create new files in the directory   
   X = use the directory in a path   
      
      
      
   So you can have a directory   
           /secret   RWX--X--X   
   and a file   
           /secret/file    RWXR-XR-X   
      
   "others" can use the file /secret/file, but thet cannot see any other   
   file in that directory. If /secret had RWXR-XR-X permissions, they can   
   see (read) the list of files in the directory.   
      
   If a directory has "write" permission, you can create new files and rename   
   files.   
   So if it was   
           /secret         RWXRWXRWX   
           /secret/file    RWXR-XR-X   
      
   you can rename /secret/file and replace it with your own.   
      
   So permission of directories are as important as permissions on files.   
      
   And it goes all the way up.   
      
   If the directory is in a directory with RWX permissions, you can rename the   
   directory.   
      
      
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