Modes may be absolute or symbolic. An absolute mode is an octal number constructed by
or'ing the following values:
4000
set-user-ID-on-execution
2000
set-group-ID-on-execution
0100
execute (or search for directories) by owner
0070
read, write, execute/search by group
0007
read, write, execute/search by others
The read, write, and execute/search values for group and others are encoded as described for owner.
The symbolic mode is described by the following grammar:
mode ::= clause [, clause ...]
clause ::= [who ...] [action ...] last_action
action ::= op [perm ...]
last_action ::= op [perm ...]
who ::= a | u | g | o
op ::= + | - | =
perm ::= r | s | t | w | x | X | u | g | o
The
who symbols ``u'', ``g'', and ``o'' specify the user, group, and other parts of the mode bits, respectively. The
who symbol ``a'' is equivalent to ``ugo''.
The
perm symbols represent the portions of the mode bits as follows:
s
The set-user-ID-on-execution and set-group-ID-on-execution bits.
x
The execute/search bits.
X
The execute/search bits if the file is a directory or any of the execute/search bits are set in the original (unmodified) mode. Operations with the perm symbol ``X'' are only meaningful in conjunction with the op symbol ``+'', and are ignored in all other cases.
u
The user permission bits in the mode of the original file.
g
The group permission bits in the mode of the original file.
o
The other permission bits in the mode of the original file.
The
op symbols represent the operation performed, as follows:
+
If no value is supplied for perm, the ``+'' operation has no effect. If no value is supplied for who, each permission bit specified in perm, for which the corresponding bit in the file mode creation mask is clear, is set. Otherwise, the mode bits represented by the specified who and perm values are set.
-
If no value is supplied for perm, the ``-'' operation has no effect. If no value is supplied for who, each permission bit specified in perm, for which the corresponding bit in the file mode creation mask is clear, is cleared. Otherwise, the mode bits represented by the specified who and perm values are cleared.
=
The mode bits specified by the who value are cleared, or, if no who value is specified, the owner, group and other mode bits are cleared. Then, if no value is supplied for who, each permission bit specified in perm, for which the corresponding bit in the file mode creation mask is clear, is set. Otherwise, the mode bits represented by the specified who and perm values are set.
Each
clause specifies one or more operations to be performed on the mode bits, and each operation is applied to the mode bits in the order specified.
Operations upon the other permissions only (specified by the symbol ``o'' by itself), in combination with the
perm symbols ``s'' or ``t'', are ignored.