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

CHMOD(1P)                 POSIX Programmer's Manual                CHMOD(1P)

PROLOG         top

       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         top

       chmod — change the file modes

SYNOPSIS         top

       chmod [−R] mode file...

DESCRIPTION         top

       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.1‐2008, 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 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         top

       The chmod utility shall conform to the Base Definitions volume of
       POSIX.1‐2008, 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         top

       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         top

       Not used.

INPUT FILES         top

       None.

ENVIRONMENT VARIABLES         top

       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.1‐2008, 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         top

       Default.

STDOUT         top

       Not used.

STDERR         top

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES         top

       None.

EXTENDED DESCRIPTION         top

       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.1‐2008.

             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         top

       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         top

       Default.

       The following sections are informative.

APPLICATION USAGE         top

       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.1‐2008 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         top

                     ┌──────┬────────────────────────────────┐
                     │Mode  Results             │
                     ├──────┼────────────────────────────────┤
                     │a+=   │ Equivalent to a+,a=; clears    │
                     │      │ all file mode bits.            │
                     │go+−w │ Equivalent to go+,gow; clears │
                     │      │ group and other write bits.    │
                     │g=ow │ Equivalent to g=o,gw; sets    │
                     │      │ group bit to match other bits  │
                     │      │ and then clears group write    │
                     │      │ bit.                           │
                     │gr+w │ Equivalent to gr,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         top

       The functionality of chmod is described substantially through
       references to concepts defined in the System Interfaces volume of
       POSIX.1‐2008. 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.1‐2008 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.1‐2008 does not
       require mandatory locking (nor does the System Interfaces volume of
       POSIX.1‐2008), but does allow it as an extension. However, this
       volume of POSIX.1‐2008 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.1‐2008 indicates that
       implementation-defined restrictions may cause the S_ISUID and S_ISGID
       bits to be ignored. This volume of POSIX.1‐2008 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.1‐2008 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         top

       None.

SEE ALSO         top

       ls(1p), umask(1p)

       The Base Definitions volume of POSIX.1‐2008, Section 4.4, File Access
       Permissions, Chapter 8, Environment Variables, Section 12.2, Utility
       Syntax Guidelines

       The System Interfaces volume of POSIX.1‐2008, chmod(3p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                           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)