chmod

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



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(1P)