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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 alternate 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 process 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 directory,
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 internationalization 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 characters 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 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
IEEE Std 1003.1-2001.
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
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 ',' 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'
;
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 permissions 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 process' 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 concepts
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 usually set or cleared
with the symbolic mode perm symbol l instead of the perm symbols 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 extension. 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
privileges 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 practice 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 easier to explain.)
The grammar was extended with the permcopy non-terminal to allow historical-practice 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 .