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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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CHMOD(1P) POSIX Programmer's Manual CHMOD(1P)
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.
chmod — change the file modes
chmod [-R] mode file...
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.
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.
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.
Not used.
None.
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.
Default.
Not used.
The standard error shall be used only for diagnostic messages.
None.
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'
;
The following exit values shall be returned:
0 The utility executed successfully and all requested changes
were made.
>0 An error occurred.
Default.
The following sections are informative.
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.
┌───────┬────────────────────────────────┐
│ 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. │
└───────┴────────────────────────────────┘
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'').
None.
ls(1p), umask(1p)
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(3p)
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(1P)
Pages that refer to this page: chgrp(1p), chown(1p), find(1p), ln(1p), ls(1p), mkdir(1p), mkfifo(1p), umask(1p), uudecode(1p), uuencode(1p)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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