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Numeric Modes

File permissions are stored internally as 16 bit integers. As an alternative to giving a symbolic mode, you can give an octal (base 8) number that corresponds to the internal representation of the new mode. This number is always interpreted in octal; you do not have to add a leading 0, as you do in C. Mode 0055 is the same as mode 55.

A numeric mode is usually shorter than the corresponding symbolic mode, but it is limited in that it can not take into account a file's previous permissions; it can only set them absolutely.

The permissions granted to the user, to other users in the file's group, and to other users not in the file's group are each stored as three bits, which are represented as one octal digit. The three special permissions are also each stored as one bit, and they are as a group represented as another octal digit. Here is how the bits are arranged in the 16 bit integer, starting with the lowest valued bit:

Value in  Corresponding
Mode      Permission

          Other users not in the file's group:
   1      Execute
   2      Write
   4      Read

          Other users in the file's group:
  10      Execute
  20      Write
  40      Read

          The file's owner:
 100      Execute
 200      Write
 400      Read

          Special permissions:
1000      Save text image on swap device
2000      Set group ID on execution
4000      Set user ID on execution

For example, numeric mode 4755 corresponds to symbolic mode `u=rwxs,go=rx', and numeric mode 664 corresponds to symbolic mode `ug=rw,o=r'. Numeric mode 0 corresponds to symbolic mode `ugo='.

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