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File Premissions

Permissions define the possible actions an entity in the system can do on the
file.
There are permissions for

• the owner of the file (the "user"),
• members of the group who owns the file (the "group"),
• and anyone else ("others").

The permissions have a slightly different meaning for files and directories:

• Files:

  • Read permission, grants the ability to open and read contents of a file
  • Write permission, grants the ability to alter the contents of a file.
  • Execute permission, grants the ability to run the file content as a program.

• Directories:

  • Read permission, grants the ability to list the files and directories
    contained in the directory in question
  • Write permission, grants the ability to add, rename or delete (rename/delete
    combined means also able to move) files in the directory.
  • Execute permission, grants the ability to change into the directory.
    Implies, that if any directory in a path has not the execute permission for
    the user, the user cannot use cd to change into the desired directory.

There are two ways to represent these permissions: with symbols
(alphanumeric characters), or with octal numbers (the digits 0 through 7).

• In general chmod commands take the form:

  > chmod [OPTIONS]  

• Change the permissions of file to octal, which can be found separately
  for user, group, and world by adding:

  > chmod  

  • Read (r):

    > chmod 444 

  • Write (w):

    > chmod 222 

  • Execute (x):

    > chmod 111 

  • Examples:

  > chmod 777

  Read, write execute for all:

  > chmod 755

  rwx for owner, rx for group and world:

• Change the permissions in symbolic way:
  The symbols for the code are two fold, one for

  • owner (u)
  • group (g)
  • other (o)

  and another for

  • read (r)
  • write (w)
  • execute (x)

  They are combined to the for the CODE below, either with + to add a
  permission or - to revoke a
  permission.

  > chmod  

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System Informations