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CHMOD(P) CHMOD(P)
NAME
chmod - change the file modes
SYNOPSIS
chmod [-R] mode file ...
DESCRIPTION
The chmod utility shall change any or all of the file mode bits of the file named
by each file operand in the way specified by the mode operand.
It is implementation-defined whether and how the chmod utility affects any alter-
nate or additional file access control mechanism (see the Base Definitions volume
of IEEE Std 1003.1-2001, Section 4.4, File Access Permissions) being used for the
specified file.
Only a process whose effective user ID matches the user ID of the file, or a pro-
cess with the appropriate privileges, shall be permitted to change the file mode
bits of a file.
OPTIONS
The chmod utility shall conform to the Base Definitions volume of
IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
The following option shall be supported:
-R Recursively change file mode bits. For each file operand that names a direc-
tory, chmod shall change the file mode bits of the directory and all files
in the file hierarchy below it.
OPERANDS
The following operands shall be supported:
mode Represents the change to be made to the file mode bits of each file named by
one of the file operands; see the EXTENDED DESCRIPTION section.
file A pathname of a file whose file mode bits shall be modified.
STDIN
Not used.
INPUT FILES
None.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of chmod:
LANG Provide a default value for the internationalization variables that are
unset or null. (See the Base Definitions volume of IEEE Std 1003.1-2001,
Section 8.2, Internationalization Variables for the precedence of interna-
tionalization variables used to determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of all the other
internationalization variables.
LC_CTYPE
Determine the locale for the interpretation of sequences of bytes of text
data as characters (for example, single-byte as opposed to multi-byte char-
acters in arguments).
LC_MESSAGES
Determine the locale that should be used to affect the format and contents
of diagnostic messages written to standard error.
NLSPATH
Determine the location of message catalogs for the processing of LC_MESSAGES
.
ASYNCHRONOUS EVENTS
Default.
STDOUT
Not used.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
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 por-
tions 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 sym-
bols shall represent the operation performed, as follows:
+ If perm is not specified, the ββ+ββ 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 ββ-ββ 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
IEEE Std 1003.1-2001.
If perm is not specified, the ββ=ββ 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-execu-
tion 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 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>s within the single argument being processed.
%start symbolic_mode
%%
symbolic_mode : clause
| symbolic_mode ββ,ββ clause
;
clause : actionlist
| wholist actionlist
;
wholist : who
| wholist who
;
who : ββuββ | ββgββ | ββoββ | ββaββ
;
actionlist : action
| actionlist action
;
action : op
| op permlist
| op permcopy
;
permcopy : ββuββ | ββgββ | ββoββ
;
op : ββ+ββ | ββ-ββ | ββ=ββ
;
permlist : perm
| perm permlist
;
perm : ββrββ | ββwββ | ββxββ | ββXββ | ββsββ | ββtββ
;
EXIT STATUS
The following exit values shall be returned:
0 The utility executed successfully and all requested changes were made.
>0 An error occurred.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
Some implementations of the chmod utility change the mode of a directory before the
files in the directory when performing a recursive ( -R option) change; others
change the directory mode after the files in the directory. If an application tries
to remove read or search permission for a file hierarchy, the removal attempt fails
if the directory is changed first; on the other hand, trying to re-enable permis-
sions to a restricted hierarchy fails if directories are changed last. Users should
not try to make a hierarchy inaccessible to themselves.
Some implementations of chmod never used the processβ umask when changing modes;
systems conformant with this volume of IEEE Std 1003.1-2001 do so when who is not
specified. Note the difference between:
chmod a-w file
which removes all write permissions, and:
chmod -- -w file
which removes write permissions that would be allowed if file was created with the
same umask.
Conforming applications should never assume that they know how the set-user-ID and
set-group-ID bits on directories are interpreted.
EXAMPLES
Mode Results
a+= Equivalent to a+, a=; clears all file
mode bits.
go+-w Equivalent to go+, go- w; clears group
and other write bits.
g=o-w Equivalent to g= o, g- w; sets group bit
to match other bits and then clears
group write bit.
g-r+w Equivalent to g- r, g+ w; clears group
read bit and sets group write bit.
uo=g Sets owner bits to match group bits and
sets other bits to match group bits.
RATIONALE
The functionality of chmod is described substantially through references to con-
cepts defined in the System Interfaces volume of IEEE Std 1003.1-2001. In this way,
there is less duplication of effort required for describing the interactions of
permissions. However, the behavior of this utility is not described in terms of the
chmod() function from the System Interfaces volume of IEEE Std 1003.1-2001 because
that specification requires certain side effects upon alternate file access control
mechanisms that might not be appropriate, depending on the implementation.
Implementations that support mandatory file and record locking as specified by the
1984 /usr/group standard historically used the combination of set-group-ID bit set
and group execute bit clear to indicate mandatory locking. This condition is usu-
ally set or cleared with the symbolic mode perm symbol l instead of the perm sym-
bols s and x so that the mandatory locking mode is not changed without explicit
indication that that was what the user intended. Therefore, the details on how the
implementation treats these conditions must be defined in the documentation. This
volume of IEEE Std 1003.1-2001 does not require mandatory locking (nor does the
System Interfaces volume of IEEE Std 1003.1-2001), but does allow it as an exten-
sion. However, this volume of IEEE Std 1003.1-2001 does require that the ls and
chmod utilities work consistently in this area. If ls -l file indicates that the
set-group-ID bit is set, chmod g-s file must clear it (assuming appropriate privi-
leges exist to change modes).
The System V and BSD versions use different exit status codes. Some implementations
used the exit status as a count of the number of errors that occurred; this prac-
tice is unworkable since it can overflow the range of valid exit status values.
This problem is avoided here by specifying only 0 and >0 as exit values.
The System Interfaces volume of IEEE Std 1003.1-2001 indicates that implementation-
defined restrictions may cause the S_ISUID and S_ISGID bits to be ignored. This
volume of IEEE Std 1003.1-2001 allows the chmod utility to choose to modify these
bits before calling chmod() (or some function providing equivalent capabilities)
for non-regular files. Among other things, this allows implementations that use the
set-user-ID and set-group-ID bits on directories to enable extended features to
handle these extensions in an intelligent manner.
The X perm symbol was adopted from BSD-based systems because it provides commonly
desired functionality when doing recursive ( -R option) modifications. Similar
functionality is not provided by the find utility. Historical BSD versions of
chmod, however, only supported X with op+; it has been extended in this volume of
IEEE Std 1003.1-2001 because it is also useful with op=. (It has also been added
for op- even though it duplicates x, in this case, because it is intuitive and eas-
ier to explain.)
The grammar was extended with the permcopy non-terminal to allow historical-prac-
tice forms of symbolic modes like o= u -g (that is, set the "other" permissions to
the permissions of "owner" minus the permissions of "group").
FUTURE DIRECTIONS
None.
SEE ALSO
ls , umask , the System Interfaces volume of IEEE Std 1003.1-2001, chmod()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating
System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C)
2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The
Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is
the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
POSIX 2003 CHMOD(P)
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