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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
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 POSIX.1‐2017, Section 4.5, 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 appropriate
privileges, shall be permitted to change the file mode bits of a file.
Upon successfully changing the file mode bits of a file, the chmod utility shall mark for update the last
file status change timestamp of the file.
OPTIONS
The chmod utility shall conform to the Base Definitions volume of POSIX.1‐2017, 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 POSIX.1‐2017, 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 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'
;
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 umask of the process when changing modes; systems conformant
with this volume of POSIX.1‐2017 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 POSIX.1‐2017. 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 POSIX.1‐2017 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 POSIX.1‐2017 does not require
mandatory locking (nor does the System Interfaces volume of POSIX.1‐2017), but does allow it as an
extension. However, this volume of POSIX.1‐2017 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 POSIX.1‐2017 indicates that implementation-defined restrictions may cause
the S_ISUID and S_ISGID bits to be ignored. This volume of POSIX.1‐2017 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 POSIX.1‐2017 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 Base Definitions volume of POSIX.1‐2017, Section 4.5, File Access Permissions, Chapter 8, Environment
Variables, Section 12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1‐2017, chmod()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 CHMOD(1POSIX)