The mode operand shall be either a symbolic_mode expression or a
non-negative octal integer. The symbolic_mode form is described
by the grammar later in this section.
Each clause shall specify an operation to be performed on the
current file mode bits of each file. The operations shall be
performed on each file in the order in which the clauses are
specified.
The who symbols u, g, and o shall specify the user, group, and
other parts of the file mode bits, respectively. A who consisting
of the symbol a shall be equivalent to ugo.
The perm symbols r, w, and x represent the read, write, and
execute/search portions of file mode bits, respectively. The perm
symbol s shall represent the set-user-ID-on-execution (when who
contains or implies u) and set-group-ID-on-execution (when who
contains or implies g) bits.
The perm symbol X shall represent the execute/search portion of
the file mode bits if the file is a directory or if the current
(unmodified) file mode bits have at least one of the execute bits
(S_IXUSR, S_IXGRP, or S_IXOTH) set. It shall be ignored if the
file is not a directory and none of the execute bits are set in
the current file mode bits.
The permcopy symbols u, g, and o shall represent the current
permissions associated with the user, group, and other parts of
the file mode bits, respectively. For the remainder of this
section, perm refers to the non-terminals perm and permcopy in
the grammar.
If multiple actionlists are grouped with a single wholist in the
grammar, each actionlist shall be applied in the order specified
with that wholist. The op symbols shall represent the operation
performed, as follows:
+ If perm is not specified, the '+' operation shall not
change the file mode bits.
If who is not specified, the file mode bits represented by
perm for the owner, group, and other permissions, except
for those with corresponding bits in the file mode creation
mask of the invoking process, shall be set.
Otherwise, the file mode bits represented by the specified
who and perm values shall be set.
- If perm is not specified, the '-' operation shall not
change the file mode bits.
If who is not specified, the file mode bits represented by
perm for the owner, group, and other permissions, except
for those with corresponding bits in the file mode creation
mask of the invoking process, shall be cleared.
Otherwise, the file mode bits represented by the specified
who and perm values shall be cleared.
= Clear the file mode bits specified by the who value, or, if
no who value is specified, all of the file mode bits
specified in this volume of POSIX.1‐2017.
If perm is not specified, the '=' operation shall make no
further modifications to the file mode bits.
If who is not specified, the file mode bits represented by
perm for the owner, group, and other permissions, except
for those with corresponding bits in the file mode creation
mask of the invoking process, shall be set.
Otherwise, the file mode bits represented by the specified
who and perm values shall be set.
When using the symbolic mode form on a regular file, it is
implementation-defined whether or not:
* Requests to set the set-user-ID-on-execution or set-group-ID-
on-execution bit when all execute bits are currently clear
and none are being set are ignored.
* Requests to clear all execute bits also clear the set-user-
ID-on-execution and set-group-ID-on-execution bits.
* Requests to clear the set-user-ID-on-execution or set-group-
ID-on-execution bits when all execute bits are currently
clear are ignored. However, if the command ls -l file writes
an s in the position indicating that the set-user-ID-on-
execution or set-group-ID-on-execution is set, the commands
chmod u-s file or chmod g-s file, respectively, shall not be
ignored.
When using the symbolic mode form on other file types, it is
implementation-defined whether or not requests to set or clear
the set-user-ID-on-execution or set-group-ID-on-execution bits
are honored.
If the who symbol o is used in conjunction with the perm symbol s
with no other who symbols being specified, the set-user-ID-on-
execution and set-group-ID-on-execution bits shall not be
modified. It shall not be an error to specify the who symbol o in
conjunction with the perm symbol s.
The perm symbol t shall specify the S_ISVTX bit. When used with a
file of type directory, it can be used with the who symbol a, or
with no who symbol. It shall not be an error to specify a who
symbol of u, g, or o in conjunction with the perm symbol t, but
the meaning of these combinations is unspecified. The effect when
using the perm symbol t with any file type other than directory
is unspecified.
For an octal integer mode operand, the file mode bits shall be
set absolutely.
For each bit set in the octal number, the corresponding file
permission bit shown in the following table shall be set; all
other file permission bits shall be cleared. For regular files,
for each bit set in the octal number corresponding to the set-
user-ID-on-execution or the set-group-ID-on-execution, bits shown
in the following table shall be set; if these bits are not set in
the octal number, they are cleared. For other file types, it is
implementation-defined whether or not requests to set or clear
the set-user-ID-on-execution or set-group-ID-on-execution bits
are honored.
┌─────────────────┬──────────────────┬──────────────────┬──────────────────┐
│Octal Mode Bit │ Octal Mode Bit │ Octal Mode Bit │ Octal Mode Bit │
├─────────────────┼──────────────────┼──────────────────┼──────────────────┤
│4000 S_ISUID │ 0400 S_IRUSR │ 0040 S_IRGRP │ 0004 S_IROTH │
├─────────────────┼──────────────────┼──────────────────┼──────────────────┤
│2000 S_ISGID │ 0200 S_IWUSR │ 0020 S_IWGRP │ 0002 S_IWOTH │
├─────────────────┼──────────────────┼──────────────────┼──────────────────┤
│1000 S_ISVTX │ 0100 S_IXUSR │ 0010 S_IXGRP │ 0001 S_IXOTH │
└─────────────────┴──────────────────┴──────────────────┴──────────────────┘
When bits are set in the octal number other than those listed in
the table above, the behavior is unspecified.
Grammar for chmod
The grammar and lexical conventions in this section describe the
syntax for the symbolic_mode operand. The general conventions for
this style of grammar are described in Section 1.3, Grammar
Conventions. A valid symbolic_mode can be represented as the
non-terminal symbol symbolic_mode in the grammar. This formal
syntax shall take precedence over the preceding text syntax
description.
The lexical processing is based entirely on single characters.
Implementations need not allow <blank> characters within the
single argument being processed.
%start symbolic_mode
%%
symbolic_mode : clause
| symbolic_mode ',' clause
;
clause : actionlist
| wholist actionlist
;
wholist : who
| wholist who
;
who : 'u' | 'g' | 'o' | 'a'
;
actionlist : action
| actionlist action
;
action : op
| op permlist
| op permcopy
;
permcopy : 'u' | 'g' | 'o'
;
op : '+' | '-' | '='
;
permlist : perm
| perm permlist
;
perm : 'r' | 'w' | 'x' | 'X' | 's' | 't'
;
