coreutils: Numeric Modes
27.3 Numeric Modes
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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. ⇒Directory Setuid and Setgid. Also,
operator numeric modes can take previous file mode bits into account.
⇒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 (⇒Directory Setuid and Setgid).
Info Catalog
coreutils: Symbolic Modes
coreutils: File permissions
coreutils: Operator Numeric Modes