27.3 Numeric Modes

As an alternative to giving a symbolic mode, you can give an octal (base 8) number that represents the mode.

The permissions granted to the user, to other users in the file’s group, and to other users not in the file’s group each require three bits: one bit for read, one for write, and one for execute/search permission. These three bits are represented as one octal digit; for example, if all three are present, the resulting 111 (in binary) is represented as the digit 7 (in octal). The three special mode bits also require one bit each, and they are as a group represented as another octal digit. Here is how the bits are arranged, starting with the highest valued bit:

Value in  Corresponding
Mode      Mode Bit

          Special mode bits:
4000      Set user ID
2000      Set group ID
1000      Restricted deletion flag or sticky bit

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

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

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

For example, numeric mode ‘4751’ corresponds to symbolic mode ‘u=srwx,g=rx,o=x’, and numeric mode ‘664’ corresponds to symbolic mode ‘ug=rw,o=r’. Numeric mode ‘0’ corresponds to symbolic mode ‘a=’.

A numeric mode is usually shorter than the corresponding symbolic mode, but it is limited in that normally it cannot take into account the previous file mode bits; it can only set them absolutely. The set-user-ID and set-group-ID bits of directories are an exception to this general limitation. See Directories and the Set-User-ID and Set-Group-ID Bits. Also, operator numeric modes can take previous file mode bits into account. See Operator Numeric Modes.

Numeric modes are 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’. However, modes of five digits or more, such as ‘00055’, are sometimes special (see Directories and the Set-User-ID and Set-Group-ID Bits).